Popping out the exhaust

Hello there I have a 454 pretty much stock in a mud drag pickup. I have been to a couple of races this year but my last one it started making the popping noise at about mid range and higher rpm. My first couple runs it ran really strong never missed a beat. Then we started elimination and I had to run back to back races. Thats when it started. It was about 95 degrees out. It seemed when the temp of the motor got over about 150 it started popping when I raced it. I could rev it up at the line and wouldn't here popping it just seems when you put it under load it would happen. I pretty sure the I'm not floating the valves it starts popping at about 3500 rpm. I have a 700 double pumper holley. New plugs, wires, Mallory ignition, timing chain and cam. My first race of the year it was only about 70 degrees and it ran awesome but I don't think I had the motor totally warmed up the whole time though. Confused. Any advice is appreciated thanks.

Might be possibly that when the engine is hot the valves are expanding from the heat and are not closing fully therby allowing some mixture to escape into the exhaust, just a thought. Is it effecting the performance of the engine?

Sounds like the motor is running too rich. When you heat up the engine, the mixture changes due to heat in the chamber. As such, less fuel is needed for an effective burn. the popping as you put it would be fuel burning when it contacts fresh air in the exhaust. Check your choke for opening, or lean back the carb.

Sounds like a broken valve spring. An exhaust valve is floating at low RPM.

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Originally posted by R Pope
Sounds like a broken valve spring. An exhaust valve is floating at low RPM.

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I agree.

Thanks for the advice guys. I'm still trying to understand. So when its cold it needs a richer mixture right and when its warm its too rich. I should be able to tell by the plugs too shouldn't I. Some people have told be it might be a cracked base plate or a bad bushing in the throttle shaft. Could this be true, is there any way to test this out. Cause I probly won't be able to see it with a naked eye. Also how do I check the power valve and how to check if I have valve binding.

Maybe I should just try a vacuum secondary carb. Would that be less fuel for the engine compared to my double pumper carb. Would I lose any power with a vacuum secondary. Is there smaller jets in those carbs. As you can tell I don't really know that much about the differences in this carbs.

Maybe your lobes are getting worn they say big blocks have a tendancy to that just a thought

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Originally posted by 383 chev
Maybe your lobes are getting worn they say big blocks have a tendancy to that just a thought

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The cam has about 300 miles on it. It only starts popping when it gets warm,

oh my bad u said it was stock

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Originally posted by 383 chev
oh my bad u said it was stock

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Sorry about that, the guy I bought it from just replaced the cam because it had flat lobes. But the cam he put in there was small .475 ex475 intake.
I have another queston I have my vacuum advance plugged. What should i set my timing at when its idling. Where should it be at around 2500 rpm.

do u know what ur compression is if its low try anywhere from 8-10 at idle and if its high try around 20-22 at idle if ur running about 22 right now u should only be using ur mech. adv. if u are running low u should try both vacc. and mech. adv.

I think I have low compression ratio so I timed it to 10 degrees at idle. I got the popping figured out I swamped carburators with another 700 double pumper. But I think now its running rich. I don't know if theres different squirters or jets in this one I didn't have time to compare with the other 700. When I floor it off the line it kind of stumbles then some black smoke rolls out of the pipes. I have pipes coming out the hood so I can tell this. It goes pretty good after that. But then even through the RPM range a little black smoke rolls out. Should I try going with some smaller squirters for off line problems and some smaller jets for the black smoke. If so how much smaller should I go.

your off line stumble and rich idle have nothing to do with the jets its your air/fuel mixture screws you are to far out lean them out more then you should have to turn ur idle down cause as u lean the a/f screws idle will pick up. now if it keeps burning rich after you open up the carb and idle curcuit is not being used then check your power valve make sure it is tailored to your vaccum needs. if that all seems good then start with your jets. but make sure your timeing is set where you think it runs the best cause timing can alter lean,rich conditions.