I'm hesitating!!!

[MickG]

Going back to your original post I too had the same problem with popping on overrun albeit on Webbers. Sounds sporty but not good. I am almost certain that you are running too lean on one or more cylinders. I retuned mine a little richer on the idle screws and cured it immediately.
Regards Mick G

[types26/36]

Popping back through the exhaust often has its origins in a leak in the exhaust system....manifold to head gasket...crack in the manifold...pipe fitting.....hole in the silencer....etc.

[Elanconvert]

Before you go start changing O Rings on the carbs, Do a little squirt test using engine start fluid (sparlingly) around each carb O ring/manifold when engine its at idle. If rpm pick up you have a Vac leak, and O-rings need replacing. Good luck James

now that's a useful tip........I'm adding 'easy start' to shopping list..........thanks james
fred

[Elanconvert]

Going back to your original post I too had the same problem with popping on overrun albeit on Webbers. Sounds sporty but not good. I am almost certain that you are running too lean on one or more cylinders. I retuned mine a little richer on the idle screws and cured it immediately.
Regards Mick G

maybe!!! would also help explain hesitation......colortune should reveal all!!!! thanks mick
fred

[Galwaylotus]

ok I'm being picky now......
the workshop manual that I have just consulted [1970 ed] states that the carbs should be mounted so that the gap between flange - spacer - flange i.e. the space taken up by the 'O' rings when compressed, should be 0.040" [1mm approx]
this seems more sensible and accurate than measuring the gap in the thackery washer coils, which I suppose could vary in compressive strength?
what do you reckon?

As is typical for Lotus, the manual is not always straightforward. There seems to be a conflict of information between pages Pages 15 & 17, the former indicating the gap between the coils of the spring washers (see attached page from Lotus Elan Workshop Manual) and the latter, as you indicated, between carburettor and spacer. The drawing on page 16 indicates a gap of 0.04 inches (1.02mm) between adjacent coils of the spring washers so that is what I have been using.

[Elanconvert]

yep.....just checked ... you are right! and I hadn't even read either of those pages!!!! I was looking at page 68 which deals with dellortos........however, just from my own 'common sense' point of view, I think I would prefer to trust a feeler gauge in the flange/spacer gap than trust that the thackeray washers are totally uniform......
although I suppose I'm then trusting that the 'O' rings are uniform!!
I have had another quick look at my set-up, and despite the large difference in thackeray washer coil gap, the flange/spacer gap looks about right for each choke [can't find my feeler gauge at the mo to check accurately]
fred

ps interestingly, my manual doesn't have the thackeray washer illustration;

[MickG]

Hi Fred,
Just to add a bit of confusion my manual (circa 1972) states .050 inches, Although I've always set them to .040,
Check the adapter flanges also, they corrode where the o ring seals fit. Hold them up to the light and check. Two of mine had holes all around the sealing ring. Bought new ones from QED.
Mick G

[RogerFrench]

ok I'm being picky now......
the workshop manual that I have just consulted [1970 ed] states that the carbs should be mounted so that the gap between flange - spacer - flange i.e. the space taken up by the 'O' rings when compressed, should be 0.040" [1mm approx]
this seems more sensible and accurate than measuring the gap in the thackery washer coils, which I suppose could vary in compressive strength?
what do you reckon?

The same manual also says, in the flexible mounting paragraph, that there should be a .040" clearance between the washer coils.
Maybe with new washers and 'O' rings that's what happens, but it is confusing.

[Elanconvert]

ok I'm being picky now......
the workshop manual that I have just consulted [1970 ed] states that the carbs should be mounted so that the gap between flange - spacer - flange i.e. the space taken up by the 'O' rings when compressed, should be 0.040" [1mm approx]
this seems more sensible and accurate than measuring the gap in the thackery washer coils, which I suppose could vary in compressive strength?
what do you reckon?

The same manual also says, in the flexible mounting paragraph, that there should be a .040" clearance between the washer coils.
Maybe with new washers and 'O' rings that's what happens, but it is confusing.

yes I had read that.....hence my having maybe to decide between flange/spacer gap, or washer coil gap....as far as my set up goes, it seems 0'040" certainly can't be achieved on both!!! It could well be that if spacers/'O' rings are re-used, or from different suppliers, some may be harder than others, or slightly different dimensions? If it turns out that there is a vacuum leak, I will go for new ones[washers and 'O' rings + spacers] anyway as they are not expensive, and they should all then match; that way, I will hopefully be able to shed some light on things! thanks for all the input, guys.....I'll keep you posted!

[billwill]

I don't think it is really that critical, that you should need to get feeler gauges out. The objective is to press the O-rings a little bit flat, but not too flat and to make sure that when the carbs tilt relative to the cylinder head during vibration, no part of the O-rings lift off any of the flat surfaces, causing an air leak.

Given that it is really hard to see the thackery washer on the underside of the rear carb, and given that alternative fasteners exist in the form of rubber bushes instead of springs, my feeling is that the gap between flanges is the priority factor, so once you know what the 0.040" roughly looks like, you do up the carb securing nuts until
(a) it looks like that clearance and
(b) the intermediate holder, (whether it is the combined single piece unit or the 3 piece incorporating 2 o-rings) looks parallel to the flanges and
(c) at rest, the flange of the cylinder head looks parallel to the flange of the carb.

[Elanconvert]

hi bill
thinking aout it.....it seems that the thackeray washers are keeping the seal under compression when the carbs flex or vibrate, hence the recommended gap to give the right amount of pressure on the O-ring. I think this must be their primary role, since it would be possible and easy to get the 'correct' O-ring gap with ordinary nut and plain washer.
anyone used/tried the rubber bush/bobbin alternative?
fred

[rgh0]

i use the recommended Dave Bean arrangement

Rubber on top because the Thackerays break on top
Thakerays on the bottom because fuel leaks rot the rubber.

cheers
Rohan

[billwill]

hi bill
thinking aout it.....it seems that the thackeray washers are keeping the seal under compression when the carbs flex or vibrate, hence the recommended gap to give the right amount of pressure on the O-ring. I think this must be their primary role, since it would be possible and easy to get the 'correct' O-ring gap with ordinary nut and plain washer.
anyone used/tried the rubber bush/bobbin alternative?
fred

Oh, yes, I did not mean above that you should not use compressible fixings, such as Thackery washers, I merely meant that the spacing of the gaps in the thackery washers is less important than the spacing between the flanges.

The springy washers whether steel or rubber are what allow the carbs to vibrate less when the engine vibrates.

And yes, I have the rubber versions of the mounting washers on my own car. They were put there by Rob Morley for me some time ago when he did some engine work. Since he normally does racing engines, I'm confident that they are at least as good and possibly better than steel thackey wasers for this job.

~~~~~~~


I see that Rohan has explained that rubber on top and steel underneath might be even better.

[Elanconvert]

quick up-date...
tried the 'easy-start' check on the flanges/O rings, and no increase in engine speed....... so, if that test is reliable, there is no air leak......
noticed that tick-over was not quite as smooth as before, so removed plugs - which were quite sooty/ slightly oily.
car has been sitting for a week so maybe not surprising after only a few minutes running.....anyway, checking service record, plugs appear to have been replaced 4k miles/4 years ago, so new set going in, before messing about with colortune......

[holywood3645]

Sounds like you have ruled out the o rings. Next check ignition timing, individual air flow, and balance. However I would replace spark plugs and HT leads first. (4 years) long time.
Have you checked compression?
James