VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Bucking or missing at light throttle

Hello Lachlan31,
Not speaking for SteveEC but I'm thinking that a miss at idle is when the vehicle is not moving and only the engine is the suspect and you can hear it miss. When put in gear and moving, the whole drive train could be suspect. If the engine misses in drive while slowly accelerating it would transmit a shudder or bucking pulse through the drive train and be felt throughout the vehicle. You could try coasting in neutral with engine at idle to eliminate anything other than the engine causing a buck.

Hi Brian - The bucking or miss that i have is not severe, just not smooth, like ether the fuel or spark is intermittently interrupted or something else is off, especially at light throttle (holding a constant speed on a level road or slightly letting off the throttle). I also have rough idle, like one cylinder is intermittently missing. A timing light on each plug shows a consistent flash, with nothing that lines up with the miss. A light shot of cleaner into the carb, or at the base of the carb before replacing the gasket, smoothed the idle and higher revs out.

Good luck! And let me know if you find something, I'm running out of ideas.

I think it is either a vacuum leak (sometimes impossible to find!) or the electrolock shorting out.

I test the electrolock by removing the electrolock cap from the coil and disconnecting that wire. Then I run a jumper wire from the starter post to the post on the coil.

Steve has a 1931 and the electrolock that year went to the side of the distributor and not to the top of the coil. The first year for the electrolock to go to the top of the coil was 1933.

I just read this all through and no one mentioned the air bleed or idle mixture screw on the side of the carb. The problem is a miss at low speed and there was a mention of it running better with additional fuel added to the carb. My 31 will miss and studder at low speed if that screw is not adjusted right. Possibly the screw is out too far causing a lean idle. I know I have to run mine differently than the recommended setting for it to idle smoothly. I think I’m only out a half turn or so.

As the throttle shaft wears in the carburetor body more air enters past the shaft. This air leans out the idle mixture. So as the carburetor ages it may be necessary to adjust the idle mixture screw inward. I have several carburetors that idle best with the screw all the way closed. Not a problem as long as you get a smooth idle. If that is not possible then bushing the carburetor body and/or replacing the throttle shaft may be necessary. A relatively expensive fix and only for the best rebuilders or machinists.

Thanks guys. The idle screw is all the way in, with anything else resulting in the idle becoming worse. I am beginning to lean towards carb related though, but trying to rule out any other possibilities since messing with the carb is not for the average home mechanic from what I'm hearing.

One test you can do is to put a little heavy oil or grease on the throttle shaft. If that noticeably improves the idle then there is too much wear on the throttle shaft.

Great thought, I'll try that this weekend. Thanks