Lotus Elan and Lotus Elan Plus 2 Forums

I'm having trouble synchronising the weber carbs on my twin cam which is the big valve version. The tool I'm using is a Crypton Synchro-Check.

At around 1000 RPM the front carb (cylinders 1 and 2) gives a steady reading of 3 on both barrels and this reading increases smoothly as the throttle is opened.

On the rear carb (cylinders 3 and 4) the reading on both barrels is all over the place, fluctuating wildly, seemingly in time with the intake strokes.

Disconnecting plug leads 3 and 4 results in a small drop in revs. Disconnecting plug leads 1 and 2 results in a large drop in revs and leads to the engine eventually stopping. Increasing idle speed with plug leads 1 and 2 disconnected makes no difference, the reading still fluctuates.

The smell and look of the exhaust suggests rich running.

Checked so far:-
Static ignition timing spot on at 12 degrees BTDC.
Dwell angle spot on at 60 degrees.
All spark plugs clean and correctly gapped.

Valve timing correct ie marks on sprockets in line with cylinder head at TDC.
Compression good on all cylinders (checked with gauge).

Carb jetting all correct as per Lotus spec.
All mixture screws about 5/8 of a turn out.
Choke pistons seating correctly.
Synchronisation set approximately correctly by observing movement of throttle spindles and position of throttle plates in relation to progression holes.

Vacuum take off for brake servo on cylinder 4 blanked off, but makes no difference.

Any ideas anyone?

Thanks.

Well, the first observation, which is probably not very helpful, is that the factory settings are/were for leaded petrol and unleaded petrol runs much leaner.
Second observation is have you checked the fuel pressure? I found on fitting a new pump that the pressure was too high and it affected one of the Webers far more than the other (but I still don't know why). I just fitted a pressure regulator.

?Synchronisation set approximately correctly by observing movement of throttle spindles and position of throttle plates in relation to progression holes.?
Double check this, if done carefully is quite reliable and you can get very close, If you have recently removed the carbs this effect can be caused by air leaks on the engine side of carbs, O rings etc.

Just out of interest which Weber carbs are you using? ........what is the number on the lid i.e. 40 DCOE 18/31/151 etc.....?

Carbs are both 40DCOE31 as per the spec.

Air leak on carb for cylinders 3&4?

Could be one of a few issues that give those symptoms....... assuming you have disconnected things like Headlight vacuum and Servo vacuum to eliminate external leaks.

Also assuming you have done a compression test / leakage test to make sure the valves are shutting correctly (valve lash isn't too tight)

Dry/bad leather end seals (surprisingly common)

Bad seal between the carb-Engine o-rings (try a thin coat of RTV on the o-ring as you assemble as many new O-rings are simply too hard so don't compress properly)

Slightly twisted throttle shaft (you need to check that with some shims)

Bad throttle shaft inspection plate cover seal (some people drill holes in these inspection covers but all that does is introduce air leaks as the inner shaft holes are not sealed)

Also after using quite a number of Sync tools i was recommended the SK tool by Keith Francks and i'd agree they are definitely very good.

Over the last 30yrs I've found the most effective and reliable way to balance the carbs is to shine a light up them. Remove the airbox cover and progression hole screw plugs. Position the light to shine up the carb and view the trottle plate vs progression holes.
Advance the the side with the throttle stop screw to set one side vs the holes and then use the balance adjuster screw adjust the other side. Simples!

I had a similar problem last year on a 1962 Lotus 7 with Weber carbs. It turned out to be the seals between the carbs and head allowing an air leak as mentioned by Chrispy. Once fixed I found it more accurate to balance at higher revs. I think I used 3000 revs. Balancing where you?re more likely to run is better than at idle.

europatek wrote:Over the last 30yrs I've found the most effective and reliable way to balance the carbs is to shine a light up them. Remove the airbox cover and progression hole screw plugs. Position the light to shine up the carb and view the trottle plate vs progression holes.
Advance the the side with the throttle stop screw to set one side vs the holes and then use the balance adjuster screw adjust the other side. Simples!

This works but under the assumption there is no air leaks, twisted throttle shafts or worn throttle plates. I've just been through all this with a slightly bent Throttle shaft that appeared to be bang on when looking through the progression holes but was causing the over run to shoot off into the 18's. My carb's were rebuilt by a well known specialist and looked nice but i had nothing but problems with them. So my advice would if your doing it this way to make sure the throttle plates are correctly set up first..

Thanks for all your replies. It does sound like the problem is an air leak or leaks.

Today I removed the carbs from the engine. The 'O' rings looked to be in good condition, but I will replace them with new ones.

With the carbs, which have 79 deg 30 min throttle pales, on the bench I checked out the covers over the middle of the spindles, There are no holes in these. They are fitted with a gasket which has just been greased around the edges. I'll fit new gaskets and use some jointing compound to get a good seal.

A bit of sucking round the end of the spindles showed that there are air leaks there and not just on the rear carb. I've removed the nuts etc. from the end of the spindles to expose the brass bearing covers. According to Passini these covers can't be removed without damaging them. However I'll make up a tool as described in this thread:-

viewtopic.php?f=40&t=43962

and see how I get on.

Once the covers are removed I'll know what replacement parts to order.

Graham.

I got to a similair point with the carbs that came with my car, struggling to get when to balance and run well. I decided they needed new shafts, bearings and seals at minimal. I decided just to buy replacement 40 DCOE 151 instead, as I felt like I was just throwing good money after bad with them and had some concerns about the quality of the replacement parts and my ability to fit them.

I think this was the right decision in the end for me and suggest you carefully exam the condition of the carbs before considering repair v replace.

If you do replace the leather seals Keith Francks told me not to use the stock weber items, make your own!!

Get some 2mm leather and soak it for a week in mink oil (you may need to warm it up a few times), when it is suitably soaked through knock out 4 circles with a punch and pierce the center with a sharp awl (don't punch the hole out in the center as it makes a better seal on the shaft that way)

If you try to soak the stock weber seals they distort and go weird shapes so never quite seal properly.

You could of course just skip the lot and fit sealed bearings....... But!! i've been told be a few people now the fuel breaks down the rubber seals as they are designed to keep the grease in not fuel out.

mbell wrote:I got to a similair point with the carbs that came with my car, struggling to get when to balance and run well. I decided they needed new shafts, bearings and seals at minimal. I decided just to buy replacement 40 DCOE 151 instead, as I felt like I was just throwing good money after bad with them and had some concerns about the quality of the replacement parts and my ability to fit them.

I think this was the right decision in the end for me and suggest you carefully exam the condition of the carbs before considering repair v replace.

I genuinely wish i had gone this way with mine!! the amount of money i've spent getting mine working as intended would have bought a pair of new 40's. I'd warn you to be careful of specialists too, many just give your carbs a really good clean without really doing much more than fitting a service kit.