[jims68ragtop]

I have a 1970 Firebird 400 engine that has been rebuilt - Edelbrock aluminum manifold - Edelbrock 750 performer carb. It is in a '68 Lemans factory 4-speed. The engine is built up to around 400 hp by being bored, beefy crank and cam, and ported heads. The trans has also been rebuilt. The car starts and idles well. Acceleration is great. Dwell, timing and rpm are properly set. The car jolts or misses when trying to run it at an even speed at 30 to 40 mph. Runs great at 50 to 60 mph. The only main parts remaining from the original 350 Lemans engine is the generator and the distributor. Any ideas on the jolts and misses? Thanks.

[2manyT/A'S]

I'd suggest too much camshaft? What is the operating RPM?

What RPM does the problem occur & what rear axle ratio are you running?

[70RAlll]

Originally Posted by jims68ragtop

I have a 1970 Firebird 400 engine that has been rebuilt - Edelbrock aluminum manifold - Edelbrock 750 performer carb. It is in a '68 Lemans factory 4-speed. The engine is built up to around 400 hp by being bored, beefy crank and cam, and ported heads. The trans has also been rebuilt. The car starts and idles well. Acceleration is great. Dwell, timing and rpm are properly set. The car jolts or misses when trying to run it at an even speed at 30 to 40 mph. Runs great at 50 to 60 mph. The only main parts remaining from the original 350 Lemans engine is the generator and the distributor. Any ideas on the jolts and misses? Thanks.

Sounds like this could be in your carb. At 30-40 mph, accelerating up would most likely open up your secondarys ,even if you are taking it easy on the throttle pedal. You could be having a momentary lean or rich situation. Or could be in your distributor..It is my opinion, from the description you have given, it is not internal engine problems.Rarely do internal engine problems cause intermittent speed/rpm issues...I could be wrong, but if it was my car, I would lock out the secondary and see if the problem is still there. if so, check your secondary release spring, or the choke rod that drops the secondary lock out clip on top passenger side of quadrajet..it could be sticking and not allowing secondary air valve to open until throttle pedal is depressed further..

[jims68ragtop]

Thanks for your replies. Following is the feedback:

- I am not sure about the operating RPM. I don't have a tach in the car. I have an tach on my test set to set the idle rpm. I have it around 1000.

- When the car is missing at 35 mph, I would say that the rpm's are not real high. I would guess them at 2000.

- The cam is not crazy. It is 231/240 @ .050. The block has been bored to 4.160. The GM crank has been ground .010/.010.

- The carb is an Edelbrock Performer 750. I got rid of the Quadrajet because it was loading up and the cold idle was messed up.

- The engine builder suggested that I change the coil which I did. This did not make any difference.

- The engine builder suggested that I install a timing advance kit in the distributor. I have not done this yet, because I don't understand how this would affect the slower running RPM's. I may do this anyway because it is a cheap experiment.

- I will probably need to get with Edelbrock techs to see how to lock out the secondary on the new carb..unless anyone here knows how???

Thanks,
Jim