Car in bits again...
AO Silver Member
Join Date: Feb 2007
Location: Hampshire, UK
Help! twin-carb diagnosis required!
Hi guys - here's the symptoms; '82 Sud Green Cloverleaf, 1700 miles from new. Engine runs great, and at the correct temperature, but after a few miles of normal driving, or if I am stuck in traffic for any length of time, the engine idle speed increases appreciably (like 2500-3000 rpm), particularly if I just **touch** the accelerator. It takes maybe 15-20 seconds for the revs to drop down to a rapid idle (maybe 1200-1500 rpm), if at all. Not only is this highly embarrassing, but its playing merry Hell with my already tired 1st gear synchromesh!
because I'm mid-engined, I have been concerned about cold air induction to the carbs, but this goes beyond that. Things I have done so far -
1. Opened up both carbs, checked and cleaned all jets, float levels and confirmed satisfactory operation of both needle valves
2. Reduced the fuel pressure to 1.5 psi at hot (I've got a Malpassi regulator and an electric pump)
3. Checked for air-locks in the cooling system (none found)
4. Ducted cold air into the engine bay from the body shoulder intakes
5. Checked the air filter for cleanliness and even risked a run without (sounds great, but the NOISE!), no improvement in the condition.
The four vacuum hoses to the crankcase breather are all in good condition and don't appear to be leaking, neither is the vacuum advance pipe and the vacuum servo take-off is blanked off. That doesn't mean they aren't leaking, there's just no evidence that they are.
I appear to have one of two problems - either I have an induction leak from one or both inlet manifolds, or the carbs are dribbling fuel into the venturis even with the throttles closed. But which is it? And why? I can't be sure - if the vacuum leak was in one manifold only, wouldn't the engine idle unevenly instead of too fast?
I've been told that Weber carbs have springs and a ball-bearing in part of the choke assembly that is only held in place by the top cover. I did not find this when I opened my carbs. Is there something missing? They are Weber 36 IDF's, 95 bhp spec.
This problem is really starting to annoy me. I can make it to Auto Italia at Brooklands as it is, but trips further afield (like the Stoneleigh Kit Car show on May 6th) are looking doubtful unless I can figure this out.
Has anyone had this kind of problem before? What can I do about it (preferably in the next 4 days)?
A Desperate Spacenut