81vetteman

I have an 81 with a fresh motor just put back in and now at about 55 mph its starts to break up but if I accelerate it clears up.It also breaks up at high rpm when you put your foot in it.

Smokehouse69

I'm guessing that you are still running your computer controlled Q-jet and computer controlled distributor. So I'd suggest trying to check the ECM codes and see if it throws up any error codes. It could be a problem with the O2 sensor since the miss clears up at WOT, which should be full rich.
Or it could be something as simple as a plugged fuel filter. Has your carb been rebuilt lately?
Sorry not to be more help, but I have very little experience with the computer control system on 81's.
Good Luck!

81vetteman

No computer it is all been taken out.

Smokehouse69

Fuel pump, fuel filter it could be several other things. I think if you gave some more information on your setup, what carb are you running, distributor (standard vacuum advance HEI ?) etc. You could get some more suggestions on what's causing the problem.
Good Luck.

81vetteman

Its standard vacum advance Hei distributor with a carter 625 or 650 cfm carb with mechanical fuel pump

81vetteman

The fuel filter is just the inline clear filter.

Noel Carboni

A remote possibility, but one I learned from experience:

At light throttle the spark advance is large. This tends to place the tab on the distributor rotor some distance away from each of the tabs in the cap at the instant that the spark voltage is generated. When you go to full throttle the spark is retarded and the tabs are closer when the spark fires.

So, the long and the short of it is that the spark may be a bit weaker at part throttle. So... Check your cap, plug wires, and plugs.

Hope this helps.

-Noel

gerry72

Further on the vacuum advance fronted by Noel, if you have too much total advance, you will have what amounts to the engine breaking up. You'll possbily feel this initially as a low-speed surge under light throttle application but turns into a break-up as speed increases. It tends to go away under throttle application since signal to the vacuum advance drops when you put some throttle into it. First, find out how much advance your vacuum can is adding (some of the emissions distributors put 20 or more degrees of advance into the timing. Ten to 12 degrees is a more managable advance). Check your centrifugal advance curve. It's possbile that too much is coming in too early. Third, check your initial advance. It's harder to go much higher than 10-degrees initial with a quick centrifugal or a high vacuum advance.