Lotus Elan

Full Throttle Stumble

PostPost by: ftsoft » Tue Jun 23, 2020 8:25 pm

HI guys

I just got my S2 back from having its valve guides replaced and it runs really well as long as I don't give it full throttle from around 3000 rpm's or so. It stumbles pretty badly and then rights itself and pulls up to red line. I noticed this as soon as i got it back and so re-balanced and tuned the carbs. I'm certainly not a Weber expert, but I have been tuning these carbs for 53 years and have always had good luck. I have an electric fuel pump set at 2 psi. I'm kind of stumped. Also, it might be my imagination, but it seemed ok with the air box removed. Probably just my imagination. Thanks.

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PostPost by: mbell » Tue Jun 23, 2020 8:29 pm

Are the pump jets clear and pump circuit working?
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PostPost by: ftsoft » Tue Jun 23, 2020 8:40 pm

That would seem like a good place to start. I was looking for something that might have changed because of removing the head and reinstalling. What would the symptoms of carbs not tightened correctly be?

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PostPost by: SENC » Tue Jun 23, 2020 8:57 pm

Could the floats have been banged around a bit and fuel level no longer correct? Or crud in the fuel left as sediment in the internal filter or float valve restricting fuel intake? I'd also look at ignition/timing.
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PostPost by: mbell » Tue Jun 23, 2020 9:02 pm

ftsoft wrote: I was looking for something that might have changed because of removing the head and reinstalling. What would the symptoms of carbs not tightened correctly be?


You'd get an air leak, which would like cause a lean mixture which might be enough yo cause a stumble. Not sure how likely a air leak at higher rpm and opening throttle would be. Worth checking the carbs are mounted correctly.
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PostPost by: wotsisname » Tue Jun 23, 2020 10:36 pm

off the wall suggestion... are the vacuum take offs leaking or have any muck in them [I am assuming the set up to be pretty much the same as the +2]..
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PostPost by: LaikaTheDog » Wed Jun 24, 2020 3:07 pm

My recent experience with similar symptoms, was an airleak from cracked misab gasket.
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PostPost by: ftsoft » Wed Jun 24, 2020 6:37 pm

Laika--What is a misab gasket?
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PostPost by: pharriso » Wed Jun 24, 2020 6:56 pm

ftsoft wrote:Laika--What is a misab gasket?

These:
misabs.jpg and
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PostPost by: LaikaTheDog » Wed Jun 24, 2020 6:59 pm

yes, those are the rings that go between webers (and dellorttos?) and the inlet manifold to reduce vibration in the carb.
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PostPost by: ftsoft » Wed Jun 24, 2020 8:41 pm

Laika How did you determine that one was cracked? Could you describe your symptoms?
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PostPost by: Craven » Wed Jun 24, 2020 11:19 pm

It should be noted that the Lotus/Weber setup does not use this type of plate.
If you have the correct carb manifold interface you will have a metal plate with grooves on both faces that accept free ‘O’ rings, 4 plates 8 ‘O’ rings in total.
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PostPost by: awatkins » Thu Jun 25, 2020 4:58 pm

“ If you have the correct carb manifold interface you will have a metal plate with grooves on both faces”

When you say “correct” do you mean “as originally built” or do you mean “works properly?”. If the latter, what is wrong with the above and other common methods used to reduce vibration?
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PostPost by: Craven » Thu Jun 25, 2020 6:44 pm

Reference to poster S2, has/had this type as original equipment, in my experience the only sure way, when used, again with original type thackery washers and locknuts, to ensure a good seal and correct compliance.
Nothing in my reply suggests alternative are not suitable but I have used most and have after a period gone back to original equipment arrangement.
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PostPost by: oldchieft » Fri Jun 26, 2020 2:02 pm

Small amount of petrol brushed around the joints, with the engine on a smooth tickover would show any leaks as a change of engine speed.

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