Low Speed Missfire

[Greg Vaka]

I have a 1956, 2x4 that has developed a low speed miss that only happens after the engine has warmed up. When it is cold it runs fine at both idle and higher rpm's. The miss will go away after it reaches about 2000 rpm. Can anyone give me some ideas of what to look for.

Thanks in advance for your responses.

Greg

[James F.]

Re: Low Speed Missfire

Greg,
The easiest first, replace all the spark plugs, even if they were new. AC45S are a pretty good place to start. Next, this "miss" might be a lean cylinder due to a maniflold to head minor leak. Lastly, measure ignition wire resistance. Hope some of this helps. Regards,

[Clem Z.]

Re: Low Speed Missfire

if you mean when the choke is on there is no low speed miss i would guess that you have a lean condition after the choke goes off. i would look for a vacuum leak somewhere.

[Greg Vaka]

Re: Low Speed Missfire

Living in hot Palm Desert, Ca we don't use the choke. It is set to be open all of the time. Plugs are new as is the wires. Do not get any reading showing a problem with points on the dwell meter. Any other ideas?
I will check for any manifold leeks, but that has not been a problem in the past. The missfire conditon just started recently.

Thanks

[Dan Pepper]

Re: Low Speed Missfire

This is a long shot, but I experienced the same problem with my '66 BB. In the process of tracking down the miss/roughness, I checked for vaccuum leaks, had the dist. rebuilt, changed plugs/wires, etc. As it turned out, I had a broken valve spring that'd make her idle rough up to about 1500 RPM, then it'd smooth out. Seems this was a known problem with early BB valve springs. Don't think it was a problem with SB's. If all else fails, pop the valve covers and take a look. Just a thought.

[Les Jacobs]

Re: Low Speed Missfire

Another long shot: I had a British car with a carb that ran rich when it was hot. Symptoms sounded like yours. New seat and needle and it was fine. To trouble shoot, I warmed it up until it ran rough, then packed ice around the carb and voila it smoothed out. Good Luck Les