Lotus Elan

Rough running at low throttle opening.

PostPost by: saildrive2001 » Wed Oct 24, 2018 3:38 pm

My car is a UK spec Stromberg engine that was rebuilt 6 years ago & now has 8,500 miles on it. The car has been running great until a couple of months ago when I started to have some driveability issues. Engine starts & idles with no problems but when I am running below about 2500 rpm on a steady light throttle it appears to be hunting. Above that rpm it runs smoothly. If I open the throttle at this rpm to accelerate it picks up cleanly and runs fine at higher rpm. I don't believe it is an ignition problem because again if I press the accelerator hard in top gear from 2500 rpm it doesn't hesitate like it would if the ignition or plugs were a problem. It will also hesitate when starting off if I only give it a small amount of throttle. I have a petronix system on the car & I have replaced the plugs, the dashpot oil level is correct. The carbs were rebuilt with the engine with new jets & needles plus of curse new diaphragms. I really have not changed anything in the last six years so am a little puzzled as to what the problem is, an suggestions gratefully received.
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PostPost by: RogerFrench » Wed Oct 24, 2018 4:40 pm

I don't have a Stromberg Elan, but a Europa TC so fitted.
Check that the connections between the throttle shafts haven't slipped, and that you're getting equal opening on each carb. Sounds like you may have one opening a little earlier than the other.
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PostPost by: saildrive2001 » Wed Oct 24, 2018 8:11 pm

I checked the throttle shafts & nothing has slipped and they are both opening together.
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PostPost by: Concrete-crusher » Thu Oct 25, 2018 5:52 am

It might be worth checking the valve timing , maybe the chain has jumped a tooth

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PostPost by: rgh0 » Thu Oct 25, 2018 10:33 am

Check also the rubber diaphragms that control the piston opening, as one may have split. Also check that float fuel valves as if one is leaking you may get issues. Problems can also occur with the "choke" auxiliary valves and temperature compensators

Last but not least most Carburetor problems are electrical :lol:

cheers
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PostPost by: LaikaTheDog » Thu Oct 25, 2018 10:50 am

Had exactly this symptom with a very slight twist in my spindle shaft.. causing an imbalance.. more likely to be loss of synch in the linkage though
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PostPost by: oldelanman » Thu Oct 25, 2018 12:28 pm

A couple more suggestions.....apologies if you've already tried them.
Check the air valves are not sticking - with the dampers removed check the pistons lift and drop freely and at the same rate.
Check the breather hole in the damper cap thread is not being obstructed by the o ring or washer.
Diaphragms can stretch and get 'baggy' overtime - not obvious until you compare with a new one.
Check for air leaks at the large O rings on the manifold adaptor plates and on the ends of the balance tube.
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PostPost by: saildrive2001 » Thu Oct 25, 2018 5:24 pm

Thanks to everyone for their suggestions, some of them are easy to check, others like the float valve will require me to remove the carbs. I do notice a small amount of something has leaked under the rear carb onto the footwell. I thought it was a bit of oil blowby from the oil breather, but it could be a small amount of fuel that evaporates, which might be one of Rohan's suggestions with the level valve. I will try to check the easy ones first before the winter sets in here in Ontario. Any other suggestions please send them on.
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PostPost by: Concrete-crusher » Thu Oct 25, 2018 6:25 pm

Actually I had a problem with faster rough running and found air leaking into my glass type fuel filter , I only noticed because I could see there was a fuel leak. I have since replaced the glass type filter as opinion seems to be that they are a hazard

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