Newly Built BIG BLOCK Popping- Causes?

[blklblsk8tr13]

I just dropped a 462 in my 71.

~ .620 lift Hyd. roller cam,
~1.7'' roller rockers
~compression around 10:1 (I think....).
~850 CFM Holley double pumper
~2'' primary tube headers with 3.5'' collectors
~3'' bent exhaust with Spintech mufflers


For some reason this motor pops on one of the cylinders (I don't know which, but sounds like its the same one every time) when I put it under load. All header bolts are tight. I put 112 octane in it to rule out detonation. jets on carb are 80 front and back. Ignition, plugs, wires are new. It does have a Pertronics kit in distributor for a stronger spark. 25 Autolite plugs.

Dyno showed it is running slightly rich to 5400 RPM. I have no idle fuel mixture due to the machine not being able to pick up a signal at idle, I assume because of a lack of pressure through the tubes.

It also sneezes air and fuel back up through the carb every once in a while, I don't know if that is related.


ANY KNOWLEDGE IS APPRECIATED. I'm looking to put my 100 shot on soon, and I cant do that in good faith with something popping.

Thanks,
Eric

[geezer#99]

You don't have any idle fuel mixture because your primary throttle has uncovered the transfer slot completely.
Cure it by opening the secondaries a bit and closing the primary.
Or add more timing.
What's your timing curve like?

[cableguy0]

Im gonna say its probably timing related. It also very well be a lean pop. Generally popping through the intake is timing/lean condition and pops through the exhaust are excessive fuel igniting in the pipe or an exhaust leak allowing fresh air into the pipe which ignites the exhaust gasses.

[blklblsk8tr13]

We dialed in the timing on the dyno, but every once in a while it still sneezes out the carb. I was missing a header bolt that fell out but I replaced it and tightened them all and it still is doing it but its not as pronounced.

Thanks for the ideas.

[blklblsk8tr13]

Excessive fuel may be a good reason. It's still running rich.

[geezer#99]

So, what is your initial timing?

[DMT]

Double check to make for absolute certain that the plug wires are on the correct plugs.

If that checks out make sure that you did or didn't purchase a 4/7 swap cam. If you did you may have the number 4 and number 7 plug wire mixed up?

You didn't say what kind of heads you have so IF they have pressed in studs then with that kind of lift, the spring pressure needed to control that lift will pull out the pressed in studs. The reason I mentioned this is because you wrote that you have a roller cam and what you are describing sounds like a flat tappet cam with a wiped out lobe. It can't be that being that you are running a roller cam that's why I thought about checking this for a valve not opening and closing. Popping thru the carb is more likely an ex. valve not opening if this is the problem?

Another area to check is if you distributor cap is on all the way and not at an angle which can reek havoc on your spark timing. Also check to make sure your wires aren't causing any cross firing, if you read this tonight that would be easy in the dark by looking at the engine running with no lights on you might be able to see the spark jumping. This still could be the problem if you can't see it with the lights off at night because the spark could be jumping a very small gap shielded by something. Make sure the wires aren't bunched up where they connect to the dis. and if you have wire looms make sure the wires aren't arching there as well. GL



Derek

[bigmac73]

Originally Posted by geezer#99

So, what is your initial timing?

X2 answer the question......

[cableguy0]

Originally Posted by DMT

Double check to make for absolute certain that the plug wires are on the correct plugs.

If that checks out make sure that you did or didn't purchase a 4/7 swap cam. If you did you may have the number 4 and number 7 plug wire mixed up?

You didn't say what kind of heads you have so IF they have pressed in studs then with that kind of lift, the spring pressure needed to control that lift will pull out the pressed in studs. The reason I mentioned this is because you wrote that you have a roller cam and what you are describing sounds like a flat tappet cam with a wiped out lobe. It can't be that being that you are running a roller cam that's why I thought about checking this for a valve not opening and closing. Popping thru the carb is more likely an ex. valve not opening if this is the problem?

Another area to check is if you distributor cap is on all the way and not at an angle which can reek havoc on your spark timing. Also check to make sure your wires aren't causing any cross firing, if you read this tonight that would be easy in the dark by looking at the engine running with no lights on you might be able to see the spark jumping. This still could be the problem if you can't see it with the lights off at night because the spark could be jumping a very small gap shielded by something. Make sure the wires aren't bunched up where they connect to the dis. and if you have wire looms make sure the wires aren't arching there as well. GL



Derek

Its a big block they had screw in studs and guideplates from the factory because of the valves being offset.

[DMT]

Originally Posted by cableguy0

Its a big block they had screw in studs and guideplates from the factory because of the valves being offset.

I was thinking about that after I posted it, its been a while since I played with a BBC. I have been building a lot of sbc lately for people, would be nice to have something different in the mix once and a while. Thanks for clearing that up.


Derek

[geezer#99]

Originally Posted by geezer#99

So, what is your initial timing?

Could you clear up this question too??

[70CHEVYBB]

Why is your compression ratio a question. How could you build a motor and not know exactly what it is. I hope you have the correct springs and retainers i had that problem years ago. Young and dumb and ruined an engine