Misfire under load

As many other guys have already stated, I'd be checking a few electrical things first (cuz its cheap). run it at night to check for spark plug/spark plug wire arcs. replace the distributor cap/rotor. get a new condensor. Possibly new points if that new of a motor uses points. All those things are cheap, and take a lot of age off of your motor when replaced. As said before, get a timing light and make sure mark lines up. Idle your motor for a while, when its good and warm listen for lifter noise (which may be caused by several things). Another cheap check is a vacuum gauge. They kick butt, they come with a visual diagram on how to diagnose valve train and carbuerator related problems.

If you had the carb rebuilt to solve this problem, and the problem still exists as it did before, and all the electrical/vacuum checks stated above check out, you most likely have a valve train related problem. Man, lets not forget it could even be an elevation problem at the fathest reaches (where do you live :D) Its always next to impossible to do a dead on dignosis of a problem without being there.

Good luck.

you should check you exhaust manifold water could be leaking into the exhaust port of the motor causing preignition. which will come out of the carb and seem like a bad miss. hope this helps with your problem.

I have cured this problem in the past by advancing the timing. Put a timing light on and turn your dist. clockwise to advance.

                          -Brad-

My name is de raaff from the netherlands Europe. I've searched for a long time before I solved my back-fire problem on a '94 ski-nautique.
The problems was a bad connection in a connector. The engine was running OK on all rpm's except sometimes on accelating from stationary. Sometimes he didn't backfire at all sometimes he did.
When i wired directly from the main connection to the power of the igntion unit it was all over that's the way I found the problem. Maybe it ca be the same situation by U.

The factory springs and not the same so what you are seeing is correct. Since your RPM is so low I would say your issue is strictly a timing issue only. If you didn't replace the power valve during the carb rebuild you should do it now.

Have u checked the timing?