Rapid fire pops out the carb, new cam time?

[Paul_G]

Low compression 1976 440. Has been rebuilt using stock pistons, new rings and bearings. 452 heads with unknown springs. It has a smallish Comp XE256H cam, 212/218 @50, .447/.455, 110LSA. I had this cam and lifters so they went in the engine.

The car this engine went in has an F1 Procharger in it making about 10 PSI max. Now here is the problem. Just over 5000 RPM, full throttle, the engine has a rapid fire pops out of the carb. Otherwise it performs really well. Runs smooth, pulls hard. Idles like stock.

The trans wont shift yet at 5000, so I cant really flat foot the throttle. Been all over the thing trying to figure out the popping. AFR at WOT in the mid 11's. Timing from 2500 on up is 29°. Best I can come up with is weak valve springs lifting off the seat.

 

[RemCharger]

My buddy had the weak spring pop in his buick t type. GN. Boost pressure vs spring pressure.

 

[WileE]

What is your AFR throughout the RPM range ?

 

[icetech]

"unknown springs" <---- are they possibly stock springs?

 

[Bird 426]

What do the plugs look like? Disregarding the AFR. Ignition? Boost does affect valve spring pressure. Stock cast pistons? Did we open the ring gaps up for boost?

 

[icetech]

I would put money on stock springs... while that cam isn't huge it has much more aggressive lobe profiles, mix that with 50 year old springs or too weak of springs and you will get fun times at higher RPM.

 

[Paul_G]

What is your AFR throughout the RPM range ?

Idles in mid 13 range cruises high13 to low 14. Give it the beans and it is in the low 12's as rpm climbs, then high 11's.

What do the plugs look like? Disregarding the AFR. Ignition? Boost does affect valve spring pressure. Stock cast pistons? Did we open the ring gaps up for boost?

Plugs are clean. Timing color change is still above the center electrode. Running the fbo box and their coil. Stock pistons. Ring gaps are at the high end for boosted application.

 

[Demonic]

Bad intake valve seal? Maybe a leakdown test is in order.

 

[dvw]

If it runs fine below 5000 I doubt it has a valve sealing or bad cam lobe issue. My bet would be weak valve springs.
Doug

 

[snakeoil24]

Pops could be lean running or bad valve

 

[Geoff 2]

I think it is weak ign. Try reducing plug gaps to 0.010" & see if you get a little more rpm before the misfiring starts.

 

[dvw]

Rapid pops are usually combustion pressure being forced into the intake port. If the sound repeats like a machine gun either the exhaust valve couldn't open long enough. Or the intake is hanging open. Especially since it runs fine below 5000.
Doug

 

[Geoff 2]

The Chrys ign boxes have fixed dwell. Their main advantage is ign timing remains constant for many miles because there is nothing to wear [ points ]; it still uses the inefficient canister coil. GM with HEI introduced varaible/adaptive dwell, that went a long way towards providing maximum coil primary current when a single coil that is to fire a large number of cyls, runs out of time to charge the coil. GM also used the much more eficient E core coil.

 

[1 Wild R/T]

The old standard flat exhaust lobe test is a brief WOT snap, if you get poppoppop the cam is suspect... Strangely with a bad exhaust lobe you can bring throttle in slowly & while the engine may run rough it rarely backfires... But snap the throttle in neutral & you get poppoppop....