Lack of low down power..

[Zaphod]

More questions now I can drive my excel! The engine does not seem fully happy in the lower RPM range. It idles nicely, however when pulling away after waiting for some time at a junction or traffic lights it can seem to loose power and hesitates before pulling away, and at anything under 2000 rpm is often feels hesitant. Above that it starts to take off, but as 500 miles from a rebuild it is still running in. Getting the idle mixture sorted out improved it a lot, the ignition timing seems to be correctly set, 10 BTC at idle Mech and vac advance seem to be behaving. I have read a few posts regarding the idle jets and emulsion tubes, is this worth investigating?
I should add that the car seems to be returning 26.16 mpg (imperial) on a mix of a and b roads

[bash]

Remind me, did you rebuild the carbs ?, cos Im thinking pump diaphram. If these arnt working properly it will cause hesitation. Some others on here are much more conversant with this. Mpg sound good but they are a bit flat low down normally and will feel moreso if you are used to fuel injected cars.

Bash

[Zaphod]

The carbs were fully rebuilt by Eurocarb about 2 years ago now

[Zag]

For the £20ish it'll cost , assuming your cam timing is correct too, I'd recommend changing the 55 idle jets to 58s (idle mixture will need resetting).

[Lotus-e-Clan]

If it were mine I'd do this; Check the plugs after prolonged idling. If not rich, then put a few degrees into the static (say 12 to 15 degs or so via a dizzy swing ) and retest for hesitation. If it improves then either vac advance or mech advance isn't sufficient off throttle or the progression circuit is suspect on fuel delivery. If plugs are rich then weaken idle screws and retest hesitation before temporarily advancing static. Weak part throttle mixtures respond to more advance, hence temporarily advancing ignition to refine diagnosis.
Don't assume the 4 butterflies are balanced until you visually check them (don't rely on vacuum balancers). If the bypass screws have been used to balance the carbs then re visit the settings and start from ground zero. Often bypass screws are incorrectly used with vacuum gauges to compensate for physical butterfly asynchrony which can otherwise be corrected mechanically. Only after eliminating ignition, carb balance, and air leaks of any kind, should you mess with idle jets and holders and definitely don't change emilusion tubes on an unmodified engine. Emulsion jetting is linked to engine tune, cam selection/timing particularly.

[tonypoll]

For the £20ish it'll cost , assuming your cam timing is correct too, I'd recommend changing the 55 idle jets to 58s (idle mixture will need resetting).

I agree, replacing the idle jets can make a difference, usually around 2,000 RPM.
Lotus made the originally jetting marginal so that it passed emission test. If everything is perfect then the 55 idle jet is fine. However, after some miles the 55 can causes hesitation at around 2,000. Fitting the 58 jet usually fixes this and make for a much better driving experience.

[MetBlue]

Have you checked all correct jets are fitted as detailed in the service notes.
A recognised Lotus service centre had 'tuned' my jets before I bought the car because it wasn't running right rather than find the real fault. Two wrongs didn't make a right!
I'm sure many other cars have suffered this fate over the years.

[Zaphod]

It seems to vary with the weather at the moment! which could be a clue?

[Hawaiis0]

Your car is a late model. Does it have the later air intake with preheat pipe? Is that a possible area to look at?

[Zaphod]

It does have a more angular air cleaner housing than most I have seen, with a vac operated valve in the inlet pipe that attached to the cold air intake. I guess its a hot air intake but there is no sign of any pipework to the exhaust or anything to attach it to. But as you know this was all in bits when I got it!. It also has the later F pipes on the DHLAs attached to a vac take off on each barrel (the valve is shown on the diagram of the vac system)

[MetBlue]

I guess its a hot air intake but there is no sign of any pipework to the exhaust or anything to attach it to.

That's exactly how my car is. I've never had a connection pipe between the air box and exhaust, but seems to make no difference.
What ever is does, it would only do it for the first. Mile or so whilst the engine warms up. - unless your valve is stuck but that's relatively easy to check.
.

[Zaphod]

Well.. Replacing the emulsion tubes as per Tim Engles advice elsewhere has transformed things. This may have just mitigated the issue but if that's the case then it should be a clue as to what is going on?

[DavidOliver]

Zaphod. You mention a recently overhauled engine and less recent Eurocarb refurb.
Not knowing your engine overhaul, I ask do you have the cams accurately timed, an inlet cam a tooth retarded could produce your symptoms.
On carbs, did Eurocarb refurb the carbs individually, off the inlet manifold. If so check the synchronisation (Vacuum guage) between carbs and that the flexible mounts are flexible and not leaking.
Then spark timing, and I believe your car has a vacuum advance on the distributor so check advance with a timing light when raising rev range.
And then you can start changing carb jets/tubes but not by much at a time.

Dave the cog.

[Zaphod]

Hi Dave,
Yes the engine has just been rebuilt after a somewhat less than ideal rebuild by the cars previous owner.. (He used standard OD mains in an engine that should have the +0.015" OD mains.. this was less than ideal! and the valve clearances were 'interesting' the only bit I have not taken off is the head, however it has healthy compression even across all 4). I do know the cam timing is slightly off, but by less than a tooth, to get bang on would seem to require the use of new pulleys which I plan on doing The dot lines up perfectly when the crank is the width of the bottom pulley groove beyond the TDC mark. When fitting the belt all marks were lined up, they move when the tensioner is adjusted I have already tried adjusting by one tooth but that only makes it worse not better(I should add it was not started in this condition, just that the marks were way off), it's about 1/4 of a tooth off above and below on both sides by eye. The carbs now have 700 miles from a rebuild by Eurocarb. If there is a way to get the cam pulleys closer without resorting to a set of vernier pulleys I'd love to know! if not all will be quiet till the festive season is over. Then I might ask many questions about how to deal with the vernier pulleys... I think the idea is to get the cams to the correct position remove the belt, move the crank to tdc, take the pulleys off fit the new adjustable pulleys with the outer loose, fit belt, tension belt then tighten up the verniers after checking the cams have not moved.. how this will work in practice I don't know! they are not something I have had to deal with. Changed many belts though and seen mild deviations after a head skim, which I think the previous owner did have done

Stewart

[DavidOliver]

A head skim does in fact produce a slight movement of the cams relevant to the TDC crank mark, both effectively will rotate anticlockwise from cam opposing dot position when tensioning the belt to take up the slack in the belt between exhaust cam and crank pulley. This is very small indeed, almost just theoretical.
In my opinio there is no need to go to vernier cam pulleys to get adequate low end torque.
From your description I am not sure whether you are setting crank and cams correctly. The crank TDC is 0º position on the advance scale. Then set the cams to have dots opposing one another.
Then tension the belt and see whether the dot positions have moved. (You might select bottom gear to help avoid crank movement). The inlet cam dot needs to be at least advanced to the exhaust to ensure lower torque. One tooth will not bend valves/puncture pistons.

You do not mention if you have checked flexibility of carburettors/manifold, an air leak here will seriously reduce lower torque. So will carbs when spindles are not co-ordinated, check with an air volume guage.

How did you find out that the big-end bearings were undersize for the job, any engine use like this will knock hell out of the crank.

Dave the cog.