Lotus Elan

S4 Rear Carb not sucking Air?

PostPost by: Temmck » Wed Sep 14, 2022 5:28 pm

With the float bowl seal trimmed on the front carb, the fuel flowing out the intake of the carb is solved but the engine now stutters at constant speed. A quick check showed strong suction from the front carb when blocking with gloved hand but nearly no suction from the rear carb. Lifting the front carb piston slightly results in engine rpm dropping. Lifting the rear carb piston results in no change in rpm.

I plan to dig into this issue his weekend but I welcome any ideas and things to check/look for as this seems rather peculiar. Last I check, I had excellent compression on all cylinders. My plan of attack it to recheck compression, check for any major air leaks, then pull the carb.

I'm running Stomberg's with blanked off crossover tubes and no secondary butterflies.
Terry
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PostPost by: snowyelan » Wed Sep 14, 2022 7:18 pm

Hi Terry,

Have you synchronized the linkage between the 2 carbs? I initially set my carbs by adjusting the idle stops equally to get it running. To set them properly the flow through each carb needs to be synchronized by loosening the linkage and adjusting each carb idle stop separately to balance the flow rates while maintaining idle RPM. Only after that is done can the linkage be tightened up.
Scott
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Hawkestone, On, Ca
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PostPost by: Temmck » Wed Sep 14, 2022 8:44 pm

I had last set the balance about two months ago and the engine has been running fine until last week. I did check and the linkage has not loosened. But there is no way to balance these right now since I am getting very little airflow through the rear carb. I can raise the piston in that carb and there is no change in rpm. I'm thinking there has to be a massive air leak somewhere.
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PostPost by: Temmck » Thu Sep 15, 2022 5:19 pm

Checked the compression last night and all is good 180-190. Checked for any potential leaks around the carb mounts, crossover tubes, etc. - no gaps, nothing loose. I removed the rear carb and did a full teardown inspection - nothing out of the ordinary. Reinstalled and started the engine - everything was running fine with good airflow through the carb. I took it for a drive and everything was back to normal. I have no idea what caused this problem and like all good intermittent problems, I fully expect to see it again.
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