Twin Dellorto Dilemma: flat spot

Information / Questions and Discussion -
Imps and Imp based Motorsport stuff from the olden days and current

Twin Dellorto Dilemma: flat spot

Postby renaldo » Sun Aug 07, 2016 4:46 pm

Guys, I'm a new member of the Imp club and I'm afraid I need some advice regarding the twin Dellortos on my 1970 Ginetta G15, 998, with R22 cam.

Runs very well, but a stubborn flat spot continues to be a problem, at about 3700 rpm

I'm no Dellorto expert, but I'm thinking the jetting on my car may need some fettling.

I've looked at the pump jets and they are 40s, which as I understand it is pretty high for the size of engine. (I have Des Hammill's book on Webers and Dellortos.)

Wondering if it makes sense to try pump jets one size lower (say 38s) -- if the problem is bogging from too much fuel.

Or, if trying a pump jet size higher (42s), might help -- if the problem is too little fuel on acceleration.

Any suggestions or advice would be greatly appreciated.

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Sun Aug 07, 2016 7:18 pm

Forget the pumps for the meantime (I use 38 pumps on a 1040 which work well) until you establish you have the right emulsion tube (No6 for R series cams), idle and mains that suit the choke size (30's are best for torque).

If all is well with chokes jets and emulsion sizes then look to the mechanical ignition curve - get this wrong and you will have a flat spot off cam (publish your advance curve data too if you can) ....you also need to specify when the flat spot occurs - is it at part throttle opened very slowly of more at WOT opened rapidly?

Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Sun Aug 07, 2016 9:17 pm

Haven't confirmed sizes of all jetting yet, but I have checked the emulsions: they are 7772.6

What main jets would likely be appropriate?

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Mon Aug 08, 2016 5:52 am

Correct emulsions (No6).

Main size depends on choke size. (C = 30 then M~112 to118) OR (C=32, then M~118 to125)
Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Mon Aug 08, 2016 5:05 pm

Choke size I haven't yet determined, but mains are 125, air correctors are 148.5

Idle jet holder is 7850.1 and idle jet is 50

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Mon Aug 08, 2016 7:14 pm

Well you'd better hope the chokes are at least 32 or the idle and mains are way too rich. :shock:

What about the ignition curve? Do you know what the No deg advance are for 1500, 2000, 2500, 3000, 3500 and 4000 rpm?b

And is the flat spot at slow part throttle or at WOT?
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Mon Aug 08, 2016 10:46 pm

I'll need to do some more substantial dismantling to get at the chokes.

But I can tell you the flat spot doesn't occur at wide open, but at partly open.

Basically when I'm trying to get the car to accelerate smoothly and steadily under partly open throttle, from about 3400 through to the start of its power zone at around 4400 or so.

Once it hits about 4400, its pulling strongly.
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Mon Aug 08, 2016 11:52 pm

Like most high revving cams, when off-cam, the R22 will need less fuel and more ignition advance to ignite the weaker mixture ...and that's why the No6 emulsion is profiled to have a weak bottom and a rich top end to suit high revving cams...but that's only going to work as it should if the choke and jets sizes are matched

On part-throttle the pump circuit shouldn't need to operate as the revs rise so this is why more part-throttle ignition advance is needed - but like most I guess you will be running a dizzy without any vacuum advance making it difficult to get enough advance to light a weak part throttle mixture.....but my guess is your 125 main and 50 idle with quite a small air corrector (148) means it's likely you are quite rich off cam - even if you have 32 chokes fitted.
The DHLA has a relative weak pump lever spring that will sacrificially compress rather than operate the lever when opened very slowly - but rapid throttle movements will transfer more directly to the pump levers and inject extra fuel to bridge point before before the air starts to move more strongly through the choke when it will drag fuel from the main circuit.

So there are two areas explained above where you could have too much fuel on part throttle - the rich 125 main and 50 idle plus a badly set pump lever spring that operates on part throttle would be a double-whammy. If this is the case you should see blacker plugs than optimum ..is that the case?

If it's not rich and you are sure it's running weak during the flat spot then it could be you need more ignition advance at that point (3700 rpm) OR if the chokes are large (32) it might be that the air flow stalls at 3700 whilst the R22 is still off-cam and it's not pulling enough fuel on part throttle air flow at that point ..if so, longer RAM tubes might help to keep the air momentum higher in the midrange. I use 45mm RAM tubes on 30 chokes which means the air flow is fast and consistent even when off cam - also my static compression ratio is 11.8:1 which helps ignite weaker mixtures off cam (my mains are 115 with a 45 idle and 160 airs -these are a perfect match for 30 chokes).

As ever the devil is in the detail ... it's easy to bark up the wrong tree without knowing if the issue is too weak, or too rich, too little advance, too little or too much air flow, SCR too low for the cam (yours with an R22 should be about 11.6:1) etc.. Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Tue Aug 09, 2016 12:50 am

Absolutely yes to the blackened plugs. Whenever I've pulled them, which is quite often, they do tend to be at the very least dark brown, all the way to black.

No sign whatsoever of lean overall running -- the opposite in fact.

Do you think it might be worth trying slightly smaller idle and main jets?

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Tue Aug 09, 2016 6:29 am

You need to find out your choke size ..the jetting revolves around that...
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Tue Aug 09, 2016 5:31 pm

Just for interest sake, I contacted Eurocarb and here's what they recommended for 32 chokes:
    135 mains
    180 airs
    7772.6 emulsions
    7850.1 idle holders
    55 idles
    35 pumps

For 30 chokes, they recommended 120-125 mains and 50-52 idles. Same otherwise.

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Tue Aug 09, 2016 8:09 pm

renaldo wrote:Just for interest sake, I contacted Eurocarb and here's what they recommended for 32 chokes:
    135 mains
    180 airs
    7772.6 emulsions
    7850.1 idle holders
    55 idles
    35 pumps

For 30 chokes, they recommended 120-125 mains and 50-52 idles. Same otherwise.

Ron


Well I'm pretty sure they are wrong ( a bit of an approximation) as far as getting the best real-world performance out of a tuned imp is concerned. :?

I run my 1040 BVH GB72 cam (like an R 21/frazer B) with a permanent AFR dash meter on board so have loads of supporting AFR evidence for DHLA choke and jets variations. With 30 chokes 115 mains 160 airs (and 45 idle) the AFR is 13 -13.5 WOT @ 4000 rpm and above (35-37 deg ign advance all-in) which gives best torque with golden brown plugs :D. Pete Richards also ran his race 998 on 115 on 30 chokes for years before I did my own evidence-based engine developments (PR originally supplied the jetting/choke info and the on-board AFR data and real-world seat-of-pants performance supports his spec as a good one. Mapped ignition helps you have more confidence in the jetting too because the ignition is easily optimised so you have one less variable to think about.

I have tried 118, 120, 125 and 130 mains on 30 chokes and they're all too rich. Also had 120 and 125s on 32 chokes which was OK above 6500-10,000 rpm but lost power and torque 4000-6500 rpm compared to 30 chokes which just where you want her to 'pick-up her skirts and run to the red line. 130 mains on 32s are definitely a tad rich, so 135s must be too .... :shock:

There are rolling road data sheets in the members section which supports the notion that the Imp produces more power and torque with AFRs 13-13.5 and starts to bog at AFRs below 12.8 WOT.


Clan004

BTW the idles can be richer than 45 IF you are running emissions-type DHLAs which have more air bleeds in the idle circuit according to Des Hammill.
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Tue Aug 09, 2016 9:40 pm

For a novice (like me), this stuff gets pretty intimidating.

I don't doubt your experience/expertise with the Imp engine. I think Eurocarb's advice is unfocused -- just general recomendations.

I think having the Air Fuel meter is a great idea -- otherwise, so much is guesswork. It's something I'll investigate.

Consistancy seems to exist on the air correctors though.

Mine are 148.5 and everything I'm hearing so far indicates I might be better off with a higher number than that (closer to 180), whether my chokes are 32 or 30s.

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Wed Aug 10, 2016 12:06 am

And I should also say, based on your past experience, I'm going to try some leaner main and idle jets.

Finding help with Dellortos in my neck of the woods (Winnipeg, Canada), is pretty unlikely.

Most mechanics here would likely just give me a puzzled look and change the spark plugs.

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby MMCPJONES » Wed Aug 10, 2016 11:10 am

Hi,

These settings worked perfectly for me:


Chokes 32mm
Emulsion Tubes 77772.5
Main Jets 132
Air Correctors 200
Pump Jets 40
Idle Jets 7644.52 (might need next size up)

On a 998, bvh, R17 etc.

Martyn
MMCPJONES
 
Posts: 96
Joined: Sun Sep 26, 2004 12:03 pm
Location: South Wales/Herefordshire border
Club Member: Yes
Car Model: Davrian Mark 8, 1969 Sunbeam Stiletto (resto)

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Wed Aug 10, 2016 1:25 pm

Thanks Martyn. Always good to hear what has worked for others...

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Wed Aug 10, 2016 1:33 pm

MMCPJONES wrote:Hi,

These settings worked perfectly for me:


Chokes 32mm
Emulsion Tubes 77772.5
Main Jets 132
Air Correctors 200
Pump Jets 40
Idle Jets 7644.52 (might need next size up)

On a 998, bvh, R17 etc.

Martyn

Image

If you look at Emulsion No5 you will see that the profile is different to the No6. No 5 is has a reduced diameter to half way down the tube allowing more sustained fuel /air emulsification during low to midrange throttle openings compared to the No 6 -which suits an R17 which comes on cam earlier (3500 rpm) than hotter race cams. The second (holeless) half of No5 provides a quicker, richer progression as the throttles are opened (when the revs rise and the emulsion tubes empties completely) just like No6 (and the R17 can rev!!). Your VERY big air correctors (200) fitted to the top of the emulsion tube are needed to weaken the 132 main jets throughout the delivery and particularly during the final rich (holeless) phase of the No5 AND these big air correctors are essential to keep the AFR on target once the emulsion tube has emptied at WOT - otherwise the 132 mains would be too rich with smaller air correctors.

ie No5 emulsions are close bretheren to No6 but provides more sustained delivery of relatively weaker emulsified fuel - No 6 has a big hole at the top to weaken the VERY bottom end for race cams when off cam and empties more quickly than No 5 because it hold less emusified fuel so you get on to pure main jet metering much earlier than with the No 5

Compare to No 10:
I use No 10 emulsions (twin DHLA40) on a std 875 Imp sport cammed engine. Again a reduced diameter to half way with MANY holes sustains weaker emulsified fuel for longer which suits the progression of the sport cam which has limited rev capacity which means the engine is drawing less air so needs less fuel at the top end compared to race cams. Btw the twin 40 DHLAs on the sport engine work well and give 50 mpg (30 chokes No10 emulsion, 112 mains 180 airs, 40 idles),
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby ImpManiac » Wed Aug 10, 2016 8:15 pm

I'm learning loads from this discussion! Thank you, guys! :D I have some Dell'orto 40DHLAs that I could use in my car, when that time comes. It's either those or Webers.

IM 8)
Paul Harrison
imp: a small demon : FIEND : a mischievous child : URCHIN
maniac: Raging with disordered intellect: affected with mania: MAD
User avatar
ImpManiac
 
Posts: 9247
Joined: Tue Feb 01, 2005 3:48 pm
Location: Houghton Regis, Bedfordshire
Club Member: Yes
Car Model: Singer Chamois

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Fri Aug 12, 2016 2:51 am

After a little dismantling, I've now confirmed I have 32 mm chokes.

Lotus-e-clan, could you give me your jetting recommendations, given I'm experiencing a flat spot as mentioned, and some signs of running rich.

998, with r22 cam.

Thanks so much!

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Fri Aug 12, 2016 2:37 pm

renaldo wrote:After a little dismantling, I've now confirmed I have 32 mm chokes.

Lotus-e-clan, could you give me your jetting recommendations, given I'm experiencing a flat spot as mentioned, and some signs of running rich.

998, with r22 cam.

Thanks so much!

Ron


You'll have to consider ignition issues with leaning-off the jetting. Leaner mixtures need a FAT spark at the right time - usually with more advance because it takes time to burn leaner mixtures completely. - especially at part throttle.

It might be that I encourage you to go leaner only for you to find the flat spot still exists and you lose confidence in further advice. So let's consider part-throttle ignition advance before you do a rejet -otherwise you might end-up thinking going leaner was fruitless. :wink:

Look at the ignition map below and bear in mind the following:
    1) left axis = throttle position sensor data (TPS) where 100% = wide open throttle (WOT) and 0% = closed throttle.
    2) The ignition advance curve @100% TPS is the BASE MAP and equivalent to a mechanical dizzy with NO vacuum advance
    3) Typical part-throttle when cruising is equivalent about 8% to 15% TPS. Note there is significant extra advance added to the base map BELOW 20% TPS - you can't do this if you are running a competition dizzy with no vacuum advance!
    4) Above 50% -100% TPS the extra advance isn't so important -I've left the ECU to interpolate advance at anything over 50% throttle. Those who run a mechanical competition dizzy with no vacuum advance usually report no issues above 4000 @ 50 -100% throttle because there is little variation required and it's easy to set the correct advance mechanically
CanemsIgMap22.jpg
CanemsIgMap22.jpg (166.25 KiB) Viewed 4635 times


So ...
Basically - [the flat spot occurs] - when I'm trying to get the car to accelerate smoothly and steadily under partly open throttle, from about 3400 through to the start of its power zone at around 4400 or so.

... for a mechanical dizzy (see 100% TPS base map) you SHOULD have 32 - 36 degrees advance during your flat spot (if you actually have less than this then you should correct this first) ..compare that to the map at part throttle below 20%TPS where 40- 45 degrees advance is typical - the extra 8 degrees advance helps to burn the part-throttle mixtures more efficiently.

So you can see that if we weaken the jetting we want to be sure it's going to burn ..you've been warned!

If it were me I'd put the 32 chokes to one side for now and invest in a set of 30 chokes with 115 mains 160 airs, No 6 Emulsions. Try the smallest pump jets 33 -35 (40 and above definitely NOT necessary). For idles it's more tricky - depends on your DHLA type- I can get away with 45 idles (especially with mapped ignition) but you might need 50 if your DHLAS are emission-type carbs and you can't add extra ignition advance to burn leaner mixtures.

If you stick to 32 chokes I'd try 120 mains to begin - You should invest in a range of air correctors (160, 170, 180) to try with the mains and ONLY increase the mains size IF your current smallest air corrector (148) leaves the plugs looking too lean (I mean white!) directly after WOT full-power road work, obviously stick with the No6 emulsion (correct for cam), 38 pumps (max), - for idles with the bigger 32 chokes the 50 is probably do-able (again depends on carb-sub-type and the amount of ignition advance you can add at part throttle -bear in mind going richer on idles may be masking part-throttle ignition insufficiency.

Hope that helps - just don't neglect the ignition aspect - fueling is too often blamed for ignition issues at part throttle .

..If you want to invest in an AFR meter look up the Australian budget TechEdge 2J2 O2 ECU (or equivalent) and use their in-dash LA1 display - all very reliable and robust well worth the investment and you will have the AFR data at your fingertips to prove you are going in the right direction ..
Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Fri Aug 12, 2016 4:06 pm

Thank you very much for sharing your expertise. It's a great help. (I don't think I could find anyone locally who had this kind of background.)

You've given me lots of things to consider. Should be fun trying to get this right!

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Fri Aug 12, 2016 5:13 pm

renaldo wrote:Thank you very much for sharing your expertise. It's a great help. (I don't think I could find anyone locally who had this kind of background.)

You've given me lots of things to consider. Should be fun trying to get this right!

Ron


Also consider that if you get the jets right - you should need CHOKE for cold starts - once warmed it should run without obviously! :)
Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Fri Aug 12, 2016 5:16 pm

Interesting! Right now, if I pump the throttle 4 or 5 times, I get instant starting, no choke.

Another sign of richness, perhaps?

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby The Nun » Fri Aug 12, 2016 5:28 pm

Pumping the accelerator actually squirts fuel into the carb from the accelerator pump jet, pump it enough and you'd probably flood it, that's how I used to start mine when cold, press the pedal twice and off you'd go, I.never had the choke connected.
Peter Nunn...ey! Im member no.33
Singer Chamois? I know them, my wife's got one of their sewing machines.
The Nun
 
Posts: 6171
Joined: Tue Jan 23, 2007 7:02 pm
Location: South Devon
Club Member: Yes
Car Model: Singer 998 B1 Sport Chamois 1966

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Fri Aug 12, 2016 7:02 pm

The Nun wrote:Pumping the accelerator actually squirts fuel into the carb from the accelerator pump jet, pump it enough and you'd probably flood it, that's how I used to start mine when cold, press the pedal twice and off you'd go, I.never had the choke connected.


Yes that works, but IF you can immediately drive away without choke @ below 10C ambient then you are bound to be a tad rich on the progression circuit - OR your pump circuit is operating at smaller throttle openings than necessary (lever setting or pump jets too big) and consequently wasting fuel when FULLY warmed-up. Twin 40s should acheive 45 mpg (875 998 or 1040) overall if set-up properly or even better - 50-55 mpg with a light foot + tweaked very lean on progression - basically you should aim for choke usage - then you know you're as lean as you can be when fully warmed-up ... :)
Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby Dave in Oz » Fri Aug 12, 2016 11:46 pm

renaldo wrote:
You've given me lots of things to consider. Should be fun trying to get this right!

Ron


Keep good records of your various trials/jet configurations/timing Ron, so that you can always go back to what it currently is (which works ok much of the time). It's easy to get carried away and try to change too many things at once.
Before doing anything with the carbs, tweak the ignition timing advance a bit to see if you can reduce the flat spot (with decent fuel, with your engine specs, you might be able to push the max advance to 38 degrees without causing pinking). Timing strobelight essential.
Dave George
Australia
Ginetta G15
Dave in Oz
 
Posts: 200
Joined: Thu Aug 19, 2004 10:23 am
Location: Darwin Australia
Club Member: Yes
Car Model: Ginetta G15

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Sat Aug 13, 2016 12:10 am

Thanks Dave. Sage advice as always...

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Tue Aug 16, 2016 7:01 pm

I've ordered a air/fuel ration gauge kit, as suggested, which I think will help with tuning.

I've also ordered some different jetting based on some of the suggestions here.

Just for the sake of curiousity, today, I swapped out my 148.5 air correctors with 180s.

If I understand correctly the 180s should be quite a bit leaner.

Of course, without measurements of an A/F ratio gauge, pretty hard to say what's wishful thinking and what's real.

But it seemed to me the hesitation/flat spot was less pronounced.

Do you think I'm likely engaged in some self delusion?

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Tue Aug 16, 2016 9:53 pm

Yes the 180 airs will make the main circuit leaner.

A change of airs could well be noticeable around your flat spot zone on part-throttle - but bear in mind that the idle/progression circuit (including the idle mixture screw settings!) will still be in play too. The biggest, single, purest effect from the change in airs will be at sustained WOT when the emulsion tubes have emptied - and here you have to be careful not to go too big (leaner) on airs just to get a positive mid-range effect - because you could fix the flatspot in the part-throttle midrange and go too lean under sustained WOT.

See what AFR you get when you get your gauge hooked-up - test both the 148 and the 180 airs see exactly where the effects are ...

Once you see the AFR data you won't look back - and will soon become expert at predicting what changes produce what effect!
Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby IMPerative » Wed Aug 17, 2016 3:13 am

Just a little anecdote - when I was young and stupid (as distinct from now, where I am old and stupid) a mate bought a second hand MGB. The previous owner(s) must have thought themselves tuning experts and left their mark on the original SU's. As a result the vehicle would not hold a tune from one day to the next. Together we deduced the most appropriate fix was to fit a single 45 Weber. The weber was duly ordered from a reputable place -jetted to suit along with all of the necessary hardware. We fitted it one saturday morning and to our huge disappointment it had the largest flat spot just off idle - so large in fact, the car was all but undriveable. For two days we played with it to no avail. We were at the point where we were considering removing the weber and returning to the retailer with a very strong message for him to "put it where it fitted best" and it just so happened we met an acquaintance who had some weber experience. We described our problem to him and the immediate response was "give it a couple of degrees advance". We did that and that car went on for many years needing no more setting of the weber, merely plugs and points to keep it in tune. Remember - the problem is not always what you think is the obvious.
IMPerative - must have one
Aussie MKIII
User avatar
IMPerative
 
Posts: 162
Joined: Sun Jun 01, 2008 11:44 am
Location: Australia
Club Member: No
Car Model: Aussie Imp MkIII

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Sun Aug 21, 2016 5:00 pm

I know I should be waiting to get the A/F ratio gauge kit installed. But I have a few different jets and I just can't resist trying them out. So simple to swap them in and out.

Today, I put in 122 main jets, to replace the 125s. And 160 air correctors to replace my 148.5s

Interesting effect: before, with the 125s, the car would pull away from cold without any protest, sputtering or popping.

With the 122s in on a lovely summer day, the car popped and sputtered a little while pulling away from cold. But that lasted for only a minute or two.

Once warmed up, my impression is the flat spot has improved (while still there momentarily). But transition to higher rpms seems smoother.

Again, I recognize subjective impressions are a little dangerous, and the A/F gauge will be the way to tell exactly what's going on.

But so far, I've gone a bit leaner overall, with what seems to be some improvement in the flat spot. Slightly higher, smoother idle too.

I have some 120 mains arriving very soon, so I may try them next.

Any thoughts, comments or advice?

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Sun Aug 21, 2016 5:53 pm

OK
You can keep ignoring the ignition curve factor if you are so inclined - especially if it seems all just a bit of 'black magic' to you and you'll probably end up refitting the richer jets come winter time.

The weaker mixtures need more ignition advance.

So using the 122 mains /160 airs -which are SIGNIFICANTLY weaker than 125/148, the cough and splutter would probably be resolved by more advance during the warm-up phase (OR the use of choke until warmed-up).

My Lotus 912 engine has twin DHLA 45s and uses more ignition advance when cold by virtue of a 'Cold Enrichment Valve (CEV)' fitted to the coolant system. When below 60C the CEV opens to direct full inlet manifold vacuum to the dizzy vacuum capsule thus providing more vacuum advance during the warm-up phase. When the CEV is above 60C the vacuum to the dizzy is restricted to prevent pinking under load when the engine is hot. The whole point of the CEV valve is to allow relatively lean jetting that suits the engine tune only when hot -thus saving fuel and improving emissions.

I would reconnect the choke OR get a handle on your ignition advance curve before committing to, or rejecting the leaner jetting.

Devil is in the detail - which includes ignition (and choke) ..you can ignore it for only so long... depends how sophisticated you want to get - (eg over-rich jetting solves some combustion issues but is an unsophisticated solution to a complicated problem) wink:
Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Sun Aug 21, 2016 6:06 pm

You're right. I have been afraid to deal with the advance issue, because I know so little about it, other than setting my timing during idle.

As I understand it, there's much more to it than that.

Be happy to be educated on the subject though!

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Sun Aug 21, 2016 6:28 pm

OK
You start the process by marking-up your crank pulley beyond (I mean before) the TDC mark (notch). IIRC, a set of dividers set to 10 mm will give you a 10 degree Before Top Dead Centre (BTDC) mark. But ideally you want to mark-up the pulley (with white paint/tipex) with 10, 20, 30, and 40 degrees BTDC marks as a minimum (you can guess the the intermediates during the timing process).

If you want to do this accurately, measure the pulley diameter and draw a circle (to that diameter) on a piece of paper. Then get a protractor and mark off the 10, 20, 30, and 40 degree positions. Use your dividers to transfer the distance from TDC (notch) to each BTDC mark on to the pulley and highlight with a fine white paint mark.

Now run your engine with an ignition stroboscope (buy one) connected. Note the number of degrees advance BTDC at idle, 1000, 2000, 3000, 4000 and 5000 rpm. With an R21 cam the ignition advance should stop (plateau) before 5000 rpm (usually 4000 -4500 rpm).

Once you have that data, publish it on this thread. ie tabulated data comprising number of degrees advance against engine rpm - (obviously you will need a rev counter or an automobile multi-meter that reads rpm).

That's for starters .. :)
Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs

Re: Twin Dellorto Dilemma: flat spot

Postby renaldo » Sat Sep 03, 2016 4:40 pm

Guys, working with a neighbour who is much more savvy on technical car-issues than I am.

He's agreed to install the A/F ratio gauge and he'll help me with the technical side of ignition advance. He's doing the A/F gauge today.

Meanwhile, I have an interesting "random" result to report.

Out of curiosity, I installed a set of richer idle jets to see the effect on flat spot -- if any. Replaced my 50s with 55s. All else the same.

The result was pretty stunning. Flat spot completely gone.

Now it may be I'm running too rich somewhere along the line.

But since I don't race the car (it's my summer toy), for most of my driving, its overall drivability is now very friendly indeed. Pulls the best it ever has -- very clean.

Still interested to see what the A/F gauge, and "advance" investigation, tells us...

Ron
User avatar
renaldo
 
Posts: 20
Joined: Mon Apr 20, 2015 10:50 pm
Club Member: No
Car Model: Ginetta g15

Re: Twin Dellorto Dilemma: flat spot

Postby Lotus-e-Clan » Sat Sep 03, 2016 5:22 pm

55 jets are a tad heavy-handed but given you have 32 mm chokes they are not so bad - especially if the type of carb you have turns out to be an 'emissions' variant with extra air-hole drillings in the progression circuit.

But wait until you plot your ignition advanced curve because enrichment of idle jets is a classic cover-up for sub-optimal ignition ..
It will all become clear as you gather data ..and real data acquisition (AFR and ignition advance) is the key to getting it sorted without pushing up your petrol bill or washing the bores down with rich mixtures.

Here is some technical info I wrote way back on a DIY (R series cam) ignition advance curve for your neighbour to peruse.... it uses a std 25D dizzy

..note the on-road experimental phase where you can iron out flat spots by changes in ignition advance alone ...jets/AFRs are set very much on the lean side of what you have/ are proposing - the ignition is tuned to make the leaner jets work (but you will have to use CHOKE on a cold start!! :)


You can convert a 25D dizzy. :D

Just dug out my tuning journal from August 2010 when I did a 25D conversion for a 1040 GB72 cammed engine. OK the Gb72 is not an R17 -it's a bit more like three-quarters full race - ie a more torquey George Bevan cam. Anyway I expect the following dizzy mods to work OK with an R17 - and it won't cost you an arm and a leg to make subtle changes if necessary.

During Aug 2010 I bought 20 different dizzy springs from the Distributor Doctor and spent a whole day measuring and recording their physical attributes! I then used a spring rate calculator to assign a relative spring rate number to each spring - this gave me an order of magnitude to make selecting springs easier during the development phase.

Base 25D Dizzy Spec: 41122 A 765 MkII Imp. Vac module: 4-14-8 5441807

BTW. The mechanical advance stop is marked 14 degrees which is near enough to the ideal 13 degree stop for an R series cam.

Long story short, I finally selected from my spec list spring numbers 9 (as a primary spring) and 20 (as a secondary).

These are the important physical attributes you NEED to note down....

Spring No 9 (calculated) used as a Primary:

Wire Diameter: 0.29
Free Length: 0.662
No Coils: 7
Coil diameter: 0.209
Symmetry: symmetrical
Relative spring rate: 1.682 (calculated)


Spring No 20 (calculated) used as a Secondary:

Wire Diameter: 0.29
Free Length: 0.643
No Coils: 5
Coil diameter: 0.233
Symmetry: symmetrical
Relative spring rate: 1.700 (calculated)


NB. These primary and secondary springs are different in appearance/conformation but the calculated rates are very close.

You can phone the dizzy doc and ask for identical physical specs - but not the calculated rate - he won't understand that 'cos it's my relative rate calculation!. I'd also ask him to include a couple of variants close to these specs to account for any physical difference in your own 25 D dizzy.

The selected primary and secondary springs gave me the following mechanical advance curve:
Mechanical advance curve (rpm versus crank degrees):

Static: 10 degrees
1300 = 14
2000 = 20
3000 = 29
3500 = 33
4000 = 36


25D Vacuum Advance Module spec: 4 - 14 - 8:
Using Twin DHLA's with the GB72 cam I only achieved a vacuum of about 11 -12 inches Hg around tickover - which is not a lot by all accounts! This will vary according to your own engine build and will be greatly influenced by the R17.
Anyway this meant that an extra 14 degrees of vacuum advance was available a tickover but due to the cam characteristics this rapidly tails off when you open the throttle - by the time you have got to 2500 rpm the vacuum has dived to about 2 inches Hg! Even so, it's extra advance that's worth having, and does contribute to the tractability in real world driving. BTW.The dizzy doc can supply a different vac module for the 25d that will provide more advance at low Hg numbers - I never bothered but it is something you can develop further to improve part throttle tractability.

On road tuning/adjustment:
The GREAT thing about he 25D is that you have a vernier adjustment of mechanical ignition advance! A third of a turn = 1 crank degree. - So if you suffer a flat spot at say 2000 rpm you can shift the whole advance curve up and down on the road and then retest to see if things improve at 2000 rpm (during this test ignore the effect elsewhere in the curve!). You can then fine tune your primary spring selection to iron-out the flat spot at 2000 rpm. I had an advantage here - I used AFR numbers to show me the effect of the on road vernier changes- always working toward getting an appropriate AFR for the throttle opening and load (I used a quiet local 1:10 hill to do these tests).

Hope this helps everyone - let me know how it goes if you try it out - this 25D conversion mod amounts to VERY cheap ignition tuning and gives a great deal of satisfaction when you get results! :D

EDIT: you will want to experiment with a high tickover using an R17 - 1100 rpm as start point - I use 1300 rpm - but you'll need a good cooling system to cope with a high tickover in traffic - I use an EWP but that's another story!

Clan004
74 Clan Crusader 1040 Red 190 bhp/ton
Members & partners who like to knit go to http://knitowl.co.uk
User avatar
Lotus-e-Clan
 
Posts: 3175
Joined: Thu Aug 10, 2006 9:08 pm
Location: North Yorkshire
Club Member: Yes
Car Model: 75 Clan 1040 BVSSH GB72 mapped ignition twin DHLAs


Return to Motorsport Forum

Who is online

Users browsing this forum: No registered users and 2 guests