Rough idle while idling hot


4 replies in this topic | 2,369 reads

I definitely don't have a problem getting from point A to point B with my 1968 442, but there is a small issue with the idle I want to see if I can get help with. In park, I have no problems. The car sounds great and idles just fine (hot or cold). However, after I'm driving around for a bit and if I get stuck at a red light, I notice that the car almost has the feeling of pulsating occasionally, like the engine is trying to jolt forward. This is not constant, and seems to maybe happen once every 5 seconds, and more often the longer I am in idle. I does seem like giving it a little gas (while braked) helps, but you can tell its just not perfect. The engine has not completely died on me.

Does my carb maybe just need some adjustments with the hot idle? Maybe a vacuum leak?

Picture of the carb for reference:

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The float may be too high. If fuel is dripping off the boosters at idle, it could cause the surging.


I would definitely check for a vacuum leak as well as the float level. Depending on how many miles are on the car, could be the throttle shaft wearing out baseplate holesif it's never been bushed, a gasket somewhere, the secondary air valve, etc... I'm assuming you've also checked/set the timing?


I'm betting on the vacuum leak. If you own a compressor and can follow directions a carb rebuild is relatively easy too. (assuming you don't have to bush the shaft)


As all the have already stated, vacuum leaks. But you didn't mention though the climactic conditions at which this occurs July 90 def F 80% RH, or Jan 0 deg F @ 5% RH, or anytime its hot.

You might also, with the aid of an assistant of course, use tried and true method of spraying propane gas [unlit] around the carb while everything is hot & it is 'acting up' and see if this smooths it out. If it does you definitely have vacuum leaks somewhere.

GLWS, Brett

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