Twin Dellorto Dilemma: flat spot

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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Oct 19, 2017 3:10 pm

I haven't, but probably should -- best way to get the most out of it. Evidence based, and thorough too.

(Truth is, it's running so nicely now, I'm reluctant to mess with it...)
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Thu Oct 19, 2017 5:42 pm

bazzateer wrote:Only just seen this and this may be a bit superfluous now but, have you tried taking it to a rolling road?
And where is the challenge in that? :shock:

Some folk LIKE to problem-solve and thrive on their victories ..and gain strength from their defeats! :)

If I didn't know you better BAZ (in a forum sense), I'd say you were trying to wind us up ... :lol:

Anyway, rolling roads are just an approximation of a real road ... and the fun is all over in an hour or so. :wink: Clan004 8)
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Oct 19, 2017 6:31 pm

Who says it better than Lotus-e-Clan? (With all my defeats strung together, I should be as strong as an ox -- and nearly as smart)

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Dave ' Linwood ' Lane » Thu Oct 19, 2017 7:43 pm

Took me 6 months to get my first set of twin SUs running right 10 odd years ago - got a real buzz when i finally cracked it :)
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Re: Twin Dellorto Dilemma: flat spot

Post by bazzateer » Fri Oct 20, 2017 12:23 pm

I was only asking a question.............................. :(
:D
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Re: Twin Dellorto Dilemma: flat spot

Post by ImpManiac » Fri Oct 20, 2017 1:49 pm

Some rolling roads do a proper job and set an engine up to perform at its best, in a given application. A great many, though, from what I read and hear, chase headline power figures, often at the expense of driveability. :roll: I think, compared to that, you would be better off doing your own set up and gradually tweaking and refining your fuelling and ignition settings until you achieve the performance that you want. 8) It must be very difficult to achieve that within the time constraints of a rolling road session. :?

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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Fri Oct 20, 2017 2:08 pm

Actually, the previous owner of my Ginetta told me he did have the car on a rolling road.

But when I got the car (I imported to where I live in Winnipeg, Canada), it's drivability was very poor for my kind of use.

He had it set up for racing. But I don't race. It's really just my summer toy -- and I love it!

When I got it, it wouldn't idle and had a huge flat spot on acceleration. But it was great at high rpms.

For better or worse, most of my driving isn't at really high rpms. That's just me...

Now, the car is far better suited to me -- an old guy who loves having some fun with a unique car.

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Re: Twin Dellorto Dilemma: flat spot

Post by ImpManiac » Fri Oct 20, 2017 3:07 pm

I rest my case. :D

IM 8)
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Nov 02, 2017 6:21 pm

Guys, my car now sleeps (a Canadian winter is nearly upon us -- ugh!)

I'm thinking of getting me Lucas 23D (no-vacuum) distributor rebuilt and "curved' properly.

Been talking to a specialist in the U.S., who has asked me a bunch of specs (cam duration, lift and so on), which I've provided him.

But he also prefers to know the compression ratio for my car, which I simply don't know.

Again, r22 cam, twin dellortos, engine built for racing, but car raced only twice.

Given how our engines tend to be set up for racing, is there an educated guess as to what the compression ratio would be?

(I've read could be anywhere from 10 to 11.5 at the more extreme end?)

Any thoughts or suggestions about this?

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Thu Nov 02, 2017 8:24 pm

You can't guess :| .... because many a tuner in the past paid too little attention to matching the correct comp ratio to the cam inlet timing/valve overlap.

IF ... (a big IF), they did it right (used an SCR /CAM calculator), then an R22 cam should have a static comp ratio (SCR) close to 11.7:1.

The 11.7 SCR is necessary in order to acheive a dynamic CR (DCR) close to the 9:1 of the std IMP engine. Maintaining the STANDARD IMP ENGINE DRC of 9:1 will optimise low end torque for road use without causing issues when using ordinary road fuel (95 to 99RON).

Of course for RACING, you could increase the SCR (and therefore DCR) a lot further and yet prevent pinking/destructive detonation of a very high DCR by using VERY high octane racing fuels >100RON.

A lot less than 11.7 (say 10.7:1) then the engine will be lack-luster off-cam (below 4000 rpm) using an R22 no matter how much you try to compensate with jetting and mechanical ignition curves.

If fact ALL of the CURVE of the mechanical ignition for an R series cam operates BELOW 4000 rpm! So don't expect optimum on-road drivability from an R22 with a low SCR/DCR.

And you will always be better off with a vacuum dizzy for the road unless you don't care about performance /driveabilty at part-throttle. A purely mechanical ign curve would be at best a compromise when it comes to getting optimum ignition timing for both part, and full, throttle.
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Nov 02, 2017 8:58 pm

Wow! Lots to absorb.

Since guessing about the compression ratio is inadvisable, and given my driving is mostly at mid rpms rather than higher,

is this "recurving" even feasible or advisable?

With the information this U.S. expert has about the cam and my car, am I wasting my time (and maybe money), because he likely doesn't have enough specs to improve drivability?

Maybe even make it worse?

What do you think? Do you think I should simply stand pat?

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Thu Nov 02, 2017 9:24 pm

If you go with the recurved non-vacuum dizzy the BEST you will get is this :

No flat spots off-cam, or on-cam. That is, IF you learn NOT to open the throttle more than 15% when off-cam (say below 3500rpm) .

The most ignition advance you will get with a purely mechanical curve at say 2000 rpm (a typical flat-spot area) is 20 degrees BTDC (typically 19 degrees @2000rpm) ...When in reality you will be at part-throttle... just when you could do with the 50% more advance you would enjoy with a vacuum dizzy ...ie about 32 degrees BTDC @ 2000 rpm. On top of that, it will pop and bang on the overrun (off throttle) at high rpm too ... great, if you like that sort of thing.

You will also have to run MORE fuel @ 2000rpm to compensate for the lack of advance.
Racers who use non-vacuum dizzies don't care about using more fuel.

With 32 deg BTDC @ 2000 rpm you can run LESS fueling AND have no flat-spots, a cleaner burn and MORE TORQUE at low rpm.... and with 50 deg advance BTDC @ high rpm on the overrun it won't be popping and banging so much.

You pays your money ...
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Nov 02, 2017 9:34 pm

My friend, I'm hearing a strong recommendation for the 25D vacuum dizzy.

I like what you're telling me!

However, I know my car has it's vacuum port plugged.

Is there an easy way around this, without pulling the engine?

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Thu Nov 02, 2017 10:12 pm

What vacuum port? :)

You run DHLAs ....so you have 4 vacuum ports across the two carbs that were originally meant for TUNING the idle-sir-bypass system (ie to fine tune the vacuum balance across all four throttles).

You need to find 4 screw-in tails (tiny plastic or steel things) that will tap into these 4 ports. You then join, with tubing (about 4 or 5mm ID thick-wall tubing), the 4 ports into 2 then into 1 tube which you use to operate the dizzy vacuum capsule.

Simples. :D
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Nov 02, 2017 10:19 pm

You're a genius -- and I'm a little thick!

Your cheque is in the mail...

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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Thu Nov 02, 2017 10:25 pm

renaldo wrote:You're a genius -- and I'm a little thick!

Your cheque is in the mail...

Ron
Not true either -way.

I'm just old and been around the block! :lol:

If I can remember to do it, I'll try to get a picture of the 4 ports on the carb ...my DHLAs still have the tails and vacuum pipes attached (I think) despite having converted to mapped ignition (which uses TPS, not vacuum).
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Nov 02, 2017 10:59 pm

Wonderful, if you can post some photos.

By the way, will pretty much any decent used 25D serve the purpose?

Or should I go with one of the new "electronic" ones you see on E-Bay?

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Fri Nov 03, 2017 12:32 am

renaldo wrote:Wonderful, if you can post some photos.

By the way, will pretty much any decent used 25D serve the purpose?

Or should I go with one of the new "electronic" ones you see on E-Bay?

Ron
As long as you can acheive the required advance curve and have a means of increasing advance linked to engine load, it doesn't matter if it's via mechanical 'analogue' springs or via interpolated digital programming....but maybe you should be aware that some dizzies are limited within software to a max rpm of 8000 rpm whereas an R22 imp engine still has useful power between 8000 and 10,000 rpm. If the digital dizzy still works above 8000 rpm 'all-in advance' then no worries. If the dizzy is programmed to fail above 8000 rpm than you will have an issue with a R22 engine.

Better still, if you can find a universally programmable dizzie that uses TSP for setting load rather than vacuum, then it is easier to programme extra part-throttle advance liked to Throttle Posiston (using a TSP bolt-on for DHLAs from Eurocarb) rather than rely on vacuum for load.
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Re: Twin Dellorto Dilemma: flat spot

Post by Dave in Oz » Fri Nov 03, 2017 12:49 am

Hi Ron,

If it was me (and given that your G15 is generally running "OK"), I would probably get the 23D reconditioned as-is, rather than attempting to get it re-curved.
An imp engine with R22 and DHLA's is probably difficult to "curve" without using a rolling road.
Then you can spend your money on playing with an alternative, e.g. one of those 25D's with electronic gadgetry :-)
You'll then have one dizzy that is reasonably OK, and can hope to improve things with an alternative ignition system but can revert if necessary.

Throttle Position is definitely a better approach for an R-cammed Twin 40's imp engine, rather than vacuum, which is why I've got a mapped ignition system (Canems) like Pete ;-)
If there is a Dizzy that can use throttle position input rather than vacuum then that is a wonderful thing!
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Re: Twin Dellorto Dilemma: flat spot

Post by benwick3 » Fri Nov 03, 2017 12:57 pm

I'm currently running one of my Clans with a R22 cam and twin Weber 40's using an Accuspark programmable ignition module and a locked 25D distributor. The results are very good but I had to make sure that I had the software that goes to 10,000 rpm. It's a basic 2D system that allows you to set the ignition curve you want. I'm not sure, however if this is still available

An alternative to this is the Aldon Amythyst ignition system which again uses the distributor as the trigger along with either inlet manifold vacuum or a throttle position sensor and is a fully 3D system, This is more expensive than the above but cheaper than the systems from Canems etc. It also comes with some base maps that can be modified. Check out the following for more details http://www.aldonamethyst.co.uk/

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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Fri Nov 03, 2017 1:39 pm

Dave and Pete, thanks for your advice.

This stuff has my head spinning. Not entirely sure at this point which way to go.

But I have had a chat with Martin at the Distributor Doctor.

When I mentioned the 25D to him, with the recommended advance curve and springs, he said he could do it for me.

However, he did ask me for the "vacuum unit rating", which I don't know. D-Clan??

I'm still weighing all of this stuff, trying to read up on it, so that I can feel a little better informed.

I appreciate all the expert advice. Thank you everyone!

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Fri Nov 03, 2017 3:38 pm

Yes the vacuum unit rating sets the minimum and maximum operating vacuum plus the amount of distributor degrees shifted within that range:

From The Dist Doctors site:

"Sold by application or by "grading" e.g. 2-6-3, starting to work @ 2" of Mercury vacuum, finishing @ 6" of Mercury , & yielding 3 degrees of distributor advance. "

Vacuum taken from the twin DHLA's is low and relatively short in range so you want a low start and moderate finish point in terms of Inches of Mercury, with quite a bit of advance within that range in order for you run learner burns at part throttle on an R22.

This is what I learned from my in-dash vacuum gauge ...


When you open the throttle by 10 -15 % the vacuum will collapse to 2 - 5 "Hg.

I cruise at 9-10% throttle opening @ 60 mph (4500 rpm on close ratio gearing) with the vacuum around 4 or 5 inches and I'd like to have an extra 4 or 5 deg advance on top of the 36 deg (mechanical) which means I want to cruise with 40 -42 crank deg of advance @ 4500 rpm on 10% part-throttle.

But at idle (1300 rpm) the vacuum is 11 inches and I'd want no more than 5 or 6 crank degs meaning 12 + 5 (or 6) = approx 17 to 18 degs at idle.

At 15 inches of Hg on the overrun (throttle closed at high rpm) I'd want as much advance as possible ideally 46 -50 degs advance total. That's 36 mechanical plus 10-14 extra degs advance from the vacuum.

So to summarise.
If the capsule starts at 2" and ends at 12" and gives 10 extra CRANK degs at 12 inches then ....
  • My idle example at 11 inches will give about 9 extra degs on top of the 12-13 static which means 21-22 degs ( a tad too much advance at idle - you want no more or less than 17-18).
    But on the over run we will have 46 degs (36 + 10 from max Hg which is good.
    However at part throttle cruise (5 in Hg) we would have about 3 or 4 extra which is around the 40 degs mark which is OK.
So the only issue is too much advance at idle and it would be Ok else where. BTW This is why Lotus take their vac off the DHLAs at a special THROTTLE-EDGE PORT which means no extra advance at idle but when you move the throttle off idle then the vac kicks-in.

It's all a compromise with vacuum on DHLAs I'm afraid . But I'd still have a go with with a vac capsule spec of 2-12-5 ...that is: it starts at 2 inches Hg finishes at 12 inches Hg and yields 5 DISTRIBUTOR degs over the range (=10 crank degs) and see how it goes ..you can always change the capsule later if it needs tweaking.
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Fri Nov 03, 2017 3:48 pm

Thanks, my man.

Further discussion with Martin: he seemed quite taken aback by the 25D, in that he thought Dellortos had no vacuum ports! (I plead guilty on that one)

Based on that, he had recommended I go with a 23D, built to the advance curve you outlined.

But just to repeat and confirm: you recommend the 25D with the curve you outlined, and the vacuum of 2-12-5. :)

I'm sure he'll do exactly what I ask of him.

(Wow, is there a lot of different advice out there! :o )

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Fri Nov 03, 2017 3:57 pm

Yes but these are all ball park areas ....a basis on which to tweak. :)
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Fri Nov 03, 2017 4:31 pm

Well, it's all arranged!

Martin will do the job. I'm to supply him with a 25D, which I don't have at the moment.

Otherwise, he'll charge 50 to supply it.

Found a couple of decent looking ones on E-bay for less...

Another adventure.

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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Sat Nov 04, 2017 1:15 pm

Oh, oh! I feel another dumb question coming on! Here goes:

To get the necessary vacuum using the 25d distributor, must all 4 vacuum ports in the Dellortos be in use?

The reason I ask is the far carb on the G15 isn't easy to get at. It's nearest vacuum port might be ok, but its farthest one could be a problem.

And since the car now sleeps in storage some distance from me, I can't look at it more closely...

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Sat Nov 04, 2017 8:19 pm

1. if you only use one port the vacuum will PULSE at idle in particular. Also the vacuum will be weak. By using all four ports joined together you will smooth out pulses and increase the vacuum.

2. I used adaptors from a haematology analyser (I am a haematologist by profession) that operates via vacuum lines and switches. But there should be adaptors available for carb diagnostics ... I'll photo the carbs sometime next week when I get the time and head space.

3. If you ask this question on the forum you will get the overwhelming response that the R series cams work so much better on the road if you account for engine load within ignition timing... whether you do it through vacuum or by throttle position (which is better). Yes it will work without vacuum advance but you will use more fuel and you will lose every day refinement. Choice is yours.
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Thu Nov 16, 2017 6:24 pm

Curious about the mechanical advance built into a distributor.

I've been reading from Des Hammill's book and he mentions the possible benefits to high-performance, modified engines, of lower built-in mechanical advance.

Does this apply to our engines, with the R camshafts?

(My 23D has "15" mechanical advance built-in, which I believe translates to 30 degrees, plus the static/idle advance -- according to Hammill)

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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Thu Nov 16, 2017 7:32 pm

You want a 13 degree stop to give 26 +10 static = 36 crank degrees max.

Your 15 degree stop is too much. It will only leave room for 6 degrees Static and that means not enough early advance.

Race Cams (R series) need rapid early advance and a maximum of 36 degrees terminating at 4000 rpm. It's all been done. N o need for Des's book.
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Fri Nov 17, 2017 10:25 pm

I checked my parts bin and found a 25D, which must be from my previous life as an early MG Midget owner.

Or who knows?

Anyway, looked at the cam plate and it's stamped "10".

I realize this is not the correct mechanical advance, that being 13. But is it a better choice than the 15?

And what are the sources for a 13 cams, or even 14 as you mentioned previously? (Distributor Doctor I imagine?)

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Fri Nov 17, 2017 11:03 pm

You have an easy solution to the problem! File a smidge off your 10 degree stop to acheive I3.

Then fit the two springs that are specified in my other thread on this topic. That will get you a workable adavnce curve for little money. Dizzy springs available from the Dizzy Doctor online.
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Sat Nov 18, 2017 12:52 am

Lotus E, excellent!

The 25D I've got, seems to be pretty good. It was working ok in the Midget, when I replaced it with an electronic one.

Rather than have Distributor Doctor redo the whole thing, I'll ask him for the needed springs.

Then, I'm almost there! This may be less complicated and expensive than I thought...

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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Mon Nov 20, 2017 4:38 pm

Latest: discussion with Martin about advance springs.

He can only supply springs based on distributor part number, not on detailed spring specs, as outlined to him from our discussion.

I gave him the distributor number, and I've asked him if he can supply some springs, which are at least reasonably close to the description in the specs.

Waiting for his reply...

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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Mon Nov 20, 2017 6:28 pm

Distributor Doctor must be doing too well. He sent me a selection of ten springs close to an estimated dimension set. It is better to have a selection within a range for fine tuning. He must be too busy to bother these days.
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Mon Nov 20, 2017 8:06 pm

Lotus, I suspect you're right. I don't think he'll supply me a bunch of springs.

I also doubt he'll agree to "screening" whatever springs he will send me, at 14.95 for a set (ouch!)

From my exchange with him, he doesn't seem to be a fan of amateurs (like me) working with springs.

I'm quoting now:

"There's no substitute for dynamic testing..."

In the end, I guess sending him my 25D to rework, may be the best alternative. (Buying springs in pairs will cost a fortune.)

And he gets rave reviews for his work, by all accounts...

Ron
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Re: Twin Dellorto Dilemma: flat spot

Post by Lotus-e-Clan » Mon Nov 20, 2017 8:56 pm

He said same to me years ago!

I told him, leave me alone,I know what I am doing!

Send him the advance curve data (rpm v Crank degrees advance) and if it doesn't work then can keep sending it back until it's right.
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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Mon Nov 20, 2017 9:06 pm

Lotus, yes, still have some great specs to work with.

When I asked him before if he could rework my distributor, according to your specs, he said he could.

By the way, I just bought an Amethyst ignition system -- the idea of programming in my own advance curve sounds great. Hope it works. (And I like to have a number of choices to work with -- expensive, but that's just me.)

So, getting the 25D redone now may be overkill...


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Re: Twin Dellorto Dilemma: flat spot

Post by renaldo » Fri Nov 24, 2017 9:19 pm

After a phone chat with Martin at Distributor Doctor, I was persuaded to send him the 25D.

He assured me he will do it up properly, to the specs outlined here.

(And I'll have an Amethyst as well. Oh well, you only go around once...)

Ron
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