I recently bought a 68 Firebird convertible for my wife. The car has a 69 400ci engine with a new Edelbrock AVS2 carb. New plugs, new MSD plug wires, new fuel filter, new alternator, new battery, engine in very good shape however once the car is warmed up it doesn’t want to go over 50 or 55 mph. Before it is warm it is strong at/above 60. When it hesitates and I try to get it to 60 it even backfires every now and again. At 40/45 or slower the car runs like a top. Idles great too. Just getting above 50/55 once warm is the challenge. I am banging my head against the wall trying to find the issue. Thoughts? Thanks in advance for your input.

Engine compartment image.


Car image

Only two things can do it. Spark or fuel. If you want to save yourself a lot more head banging get a loaner carb and a loaner distributor and swap them out one at a time. Take it out for a ride after each( make sure it’s warmed up) and see which one gets rid of the problem.
It’s probably something as simple as your mechanical advance weights are stuck. But it could be a few other things as well.

Vapor lock, fuel boiling.

Try taking the fuel filter off and running a hose instead. I had a plugged filter one time that exhibited the same symptoms. Doesn't take long for them to plug up with a 51 y/o fuel tank.

Ignition has a compromised ground when warm or hot - Losing spark at higher RPM or speed.

Thank you for the information. The gas tank is brand new and the sending unit was replaced along with the tank. I thought the same thing regarding the old tank with a new carb. I also checked the fuel line to make sure there were no kinks, dents or other visible reasons for compromised fuel flow. Appeared fine. After all that I put installed the new fuel filter and still experienced the issue. I will check the ignition ground per your suggestion. This gives another item to check out and make sure it gets addressed. I will let you if it solves the issue.

Thank you. I have had the carb checked out by a professional already and it was good. The next thing I can look at besides the ignition ground mentioned in the other reply would be the distributor. Thanks for your suggestion and once I get this figured out I will post the cause and solution.

What happens when you accelerate hard? Does it bog with higher RPM or fuel needs then? I would expect that it would. Cruising at 55 uses less fuel that hard acceleration. And not for nothing but carbs frequently don't give up their secrets. More than one new carb has been bad or had an issue.

When I punch it it winds up like it should, feels like it’s going to take off then has the same issue. It’ll take me some time but once I get it fixed I will report back what actual issue was and what took care of it.

Sounds like a timing issue. Vac advance not working? Put a vac gauge on her and mount on the windshield and see if there is any deviation in vac when it's cold and hot.

Maybe take some pics of how thinks are setup. Would that be some sort of governor installed to control the speed of car?

Also check to see if the throttle opens up completely with the foot pedal.

Okay. Got it figured out. It appears at some point in this distributor's history it had gotten some moisture/water in it, and as a result, once it heated up it created issues with the control module and the rotor. Rotor plastic had a crack in it as well. Not sure that was really causing any issue as it was pretty minor. Pics of old compromised parts attached. If you look at the contact on the rotor the color indicates an issue. Both parts have been replaced and early test drives show much better performance, strong acceleration north of 50 mph and smoother general acceleration and idling. Before parts were replaced, once warm it had a little "shake" during idle and that is now gone as well. If this issue comes back I will report it, but I think we figured it out. Thanks again for all your input.

Good to here you found the problem. I find it helpful to do a full maint on the engine when you first get the car or when you discover new issues. Sometimes problems go away with a routine maint (oil, plugs, filters, rotor, cap, electronic ignition module, points, condenser)

That's beautiful blue bird!

Thanks Gus. I agree re: the maintenance when you first get the car. Most of it has been done, except what we just fixed, of course. It's running great now, though. Take care.

Most times when a module has a hot running problem it was because the tech. that installed it used to little or no dielectric grease on the backside. eventually burning it out totally.