Lotus Elan

Weber carbs - idling at 2500rpm all of a sudden

PostPost by: RichardS » Wed Aug 25, 2010 2:30 pm

My twincam has been running fine until yesterday when, out of the blue, switch on and engine fires fine and then revs up to 2500 with no touching of the accelerator [cable fine, and Pete I have looked through the archives!]. The mixture screws have not been touched.

Take the plug cable off no 3 or 4 and idle reduces so I am presuming something has gone amiss in this carb. They were fully rebuilt 1000 miles ago so hopefully nothing drastic but am presuming something has "stuck" somewhere and hence too much fuel from this carb. I did take the top cover off and the float seems fine.

Can someone advise me on what to try next? My carb knowledge is, as yopu can see, rather limited.

Thanks

Richard
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PostPost by: twincamman » Wed Aug 25, 2010 2:49 pm

check the carb mounting nuts for tightness -check the inner throttle cable isn't frayed and hanging up inside the sheath or fouled elsewhere - I assume the return spring is attached to the actuating leaver and doing its job ---or the peddle shaft may need lubing and see the return spring is attached also check the garage for gremlins -and remember this --CARBURETTOR IS FRENCH FOR 'LEAVE IT ALONE ' carbs do not change on their own --POINTS- NUTS -O RINGS all can fail ----but never the settings --ed
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PostPost by: types26/36 » Wed Aug 25, 2010 3:35 pm

Richard, as you have a Sprint I assume your carbs are Delloto's, as Ed says check the return springs.
Delloto's have a spring to return the cable which is hooked in the linkage and hooked into the backplate at the bottom....check the spring is still hooked in or has not broken.(only one spring)
They have a second return spring (one on each carb) and can be seen from the cam cover side, check both these springs.
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SPRING ON CABLE.JPG and
SPRING ENG SIDE.JPG and
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PostPost by: RichardS » Wed Aug 25, 2010 4:25 pm

Ed/Brian

I have Webers - return spring and cable fine. Will check the carb nuts - am I correct in thinking if carbs loose more air gets in and so more combustion?

I haven't touched anything in the carbs or the various screws - honest!

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PostPost by: paddy » Wed Aug 25, 2010 4:39 pm

If it's idling that fast I think the only possibility is that the throttle isn't fully closing.

Take the airbox off and shine a torch down the barrels and hopefully you should see that it's not fully closed.

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PostPost by: archigator » Wed Aug 25, 2010 4:43 pm

Those return springs eventually wear out, even though they may look okay visually, the metal can fatigue. I had the same issue with "something sticking" in the carbs, and found it was the large return spring that attaches to the airbox... it had weakened over time. Just a thought.

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PostPost by: types26/36 » Wed Aug 25, 2010 5:06 pm

Webbers also have a secondary spring (separate from the cable spring) to return the linkage to rest, its cannot be seen with out taking the carb off as it is behind a plate and more difficult to get to.
Is the linkage returning to the closed position? i.e. is there a "gap" between the linkage and throttle screw? (idle speed adjusting screw) the linkage should be resting on the end of the screw.
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PostPost by: RichardS » Thu Aug 26, 2010 5:56 pm

twincamman wrote:check the carb mounting nuts for tightness -check the inner throttle cable isn't frayed and hanging up inside the sheath or fouled elsewhere -d



Thanks everyone - Ed wins the prize for correct diagnosis!

A little embarrassing - I solved the problem this afternoon. The outer of the accelerator cable had got hitched up in the ferrule where it goes through the bulkhead, which had the effect of slightly pulling on the throttle linkage!! Interestingly there was slightly more "pull" on the left hand carb throttle which is why it appeared that there was a problem with this carb.

Anyway - a couple of seconds to fix rather than a carb dismantling job.

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PostPost by: billwill » Thu Aug 26, 2010 11:24 pm

Put some silicone sealer on the end caps/ferrule to stop them moving relative to the outer cable.
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PostPost by: elanbaby » Fri Aug 27, 2010 10:34 am

RichardS wrote: ...Interestingly there was slightly more "pull" on the left hand carb throttle which is why it appeared that there was a problem with this carb...

Richard


Yes, interesting. Maybe your carbs are not really "working together" as they should? When pulling the accelerator cable both carbs should work absolutely identically. Worth checking I?d say.

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