Backfiring thru the carb

[Caseyrhe 650]

Well, today was the day. Rebuilt the carb, primed it, and good lord willing she fired right up. Idled her up to 2500, temp ran at 185, 70 psi on the oil pressure, all with no leaks. Purred like a kitten, ran smooth, and no misses or back firing

When idled her down to 800, started back firing thru the carb. Timing good, points, rotor, cap, and wires all new, but went ahead an replaced again anyway. No vac leaks. Tried adjusting timing +/- and still backfiring. As you increase the rpm's, back firing thru the carb increases.

Plan is to pull the valve covers, turn over by hand and make sure all the p rods spin freely when each piston at tdc and rockers are moving.

Sounds like a rounded lobe.

390, 40 over, 390GT heads, 780 Holley, New Mellings SYB-22 (428cj specs)cam and lifters  

Any other suggestions would be greatly 

[barnett468 418]

Need more info.

Were you breaking in a new cam?

Is it a roller cam?

A flat lobe will typically cause a ticking sound.

Free spinning push rods doesn't mean it does not have a flat lobe etc.

[Caseyrhe 650]

Yes, breaking in the cam. Lower end was not touched. Heads rebuilt with new valves and springs-from what I was told and could see, no other info on them.

Original rocker arms, stock push rods, non roller cam.

[barnett468 418]

5 hours ago, Caseyrhe said:

Yes, breaking in the cam. Lower end was not touched. Heads rebuilt with new valves and springs-from what I was told and could see, no other info on them.

Original rocker arms, stock push rods, non roller cam.

Ok, one way to determine if it has a flat lobe is to remove the rocker arms then check the amount of lift each lobe has by measuring how much the push rod moves up and down when you rotate the engine. Another way is to remove the intake and look at them.

[Cruzzar 22]

Backfiring thru the carb usually indictes a lean condition if timing and valve settings are correct. Is it possible that the idle circuits are plugged and not getting enough fuel?  If it runs fine at a higher  rpm then I would not think of a flattened cam. Just $,02 worth of suggestion. 

[barnett468 418]

25 minutes ago, Cruzzar said:

Backfiring thru the carb usually indictes a lean condition if timing and valve settings are correct. Is it possible that the idle circuits are plugged and not getting enough fuel?  If it runs fine at a higher  rpm then I would not think of a flattened cam. Just $,02 worth of suggestion. 

From his first post.

"As you increase the rpm's, back firing thru the carb increases."

[Caseyrhe 650]

Thanks

I guess one good thing is, the hood is still off and won’t break my back to bad lifting off the intake

[barnett468 418]

34 minutes ago, Caseyrhe said:

Thanks

I guess one good thing is, the hood is still off and won’t break my back to bad lifting off the intake

lol, there ya go, always look on the bright side!

[ralt962 11]

I had a flattened #1 intake lob.  I did get a bit of backfire but not much.  It idled rough but it ran pretty well.  

As Barnett says, check the lift of the push rod with no rocker.  Mine went from roughly 1/2" movement to less than an 1/8"  

[fvike 173]

Drain the oil and see. You'll need to do that anyway when breaking in a cam.

[1969_Mach1 333]

What oil were you using to break in the cam?

One method I saw to check for a bad cam lobe is to remove the valve cover and mark the push rod(s) for the suspected bad cam lobes.  Disable the ignition system and crank the motor.  Watch the push rod(s) you marked while cranking to see if they are rotating.  If they are rotating, the cam lobe is still okay.

If you are lucky you might have an intake valve with a lifter that is preloaded a little too much.  Did you install adjustable rocker arms on this motor?  After machine work on cylinder heads, resurfacing them, etc. the lifter preload with non adjustable rocker arms might not be ideal.