[krman68]

I have a 68 428 with a 4bbl Holley Carb. The carb is a rebuilt one with 2000 miles on it now. I've been noticing a slight bucking/hesitation when I'm cruising at less than 2500 rpm. Its fine when I have the throttle opened up more. It also tends to backfire once in a while when I get off the gas...I think its running to rich and maybe my jets....any suggestions?

Thanks
Jack

 

[Guest]

Had a similar problem with a holley 4 barrel on a 302. First try adjusting the jets one at a time all the way in then out to 1 and 1/2 turns. If that does not fix it, have it checked for an internal gasket rupture. I wound up going through a few carbs till I figured it out. Better luck for you I hope.

 

[krman68]

Thanks - I will try that

 

[dailydriver]

The idle air mixture screws are only for idle. Once you step on the accelerator and open the throttle plates, that circuit is bypassed. So, adjusting the mixture screws will do nothing for your low cruise hesitation.

Perhaps isn't the carb at all. Could be timing, vacuum advance, or some other portion of the ignition system not functioning properly. The backfire while slowing down sounds timing related, or it could possibly be a lean condition in the carb. The only way to richen/lean the carb is by swapping out jets.

The best way to determine if it's too rich, or too lean, is to get a "read" on the plugs after running it at cruise RPM. Dark, sooty is too rich, white is too lean, brown is just right. Hope this helps!

 

[krman68]

Thanks Ken...I'll try that too.