Hot Rods Backfire through carb - possible damage?

Rebuilt a Holley double pumper and put on a rebuilt SBF motor. Set timing according to TDC using my thumb over the spark plug hole on #1 cylinder technique. Motor fired off nicely and idled well. However, soon noticed an oil leak and figured it was easiest to pull the motor to fix it.

Reinstalled motor. Set distributor using thumb over #1 spark plug hole again. Must have been a little off and got a pretty significant back fire with a foot of flames coming out through the carb. Double checked for #1 at TDC, realized the amount of miss, and reset the distributor to get it correct.

Now the motor is struggling to idle and have to rev it up to 3000 rpm’s to keep it running. No amount of distributor/timing adjustment helps.

Wondering if I damaged the carb with the large backfire and if so, what would be a common failure point within the carb that I could check?


Thanks in advance -

Bantam

 

Power valve could be damaged.


Sent from my iPad using The H.A.M.B. mobile app

 

^^^what he said^^^

 

Yep...
Replace it, even if you "see"...no damage.

Mike

 

Do all of the above and also check the float level in the fuel bowls.....

 

Holley once sold a "backfire kit" to prevent this problem. Basically you drill a hole in the carb body and install a one way check valve.

 

I believe that's for the throttle plate.... I could be wrong....

 

Backfires can blow out (rupture) a vacuum diaphragm power valve. Not sure of the double pumpers, but other Holleys use them.

 

Geeze...really.
A quick lookup and wonder of wonders...
https://www.holley.com/products/tools/carburetor_tools/parts/125-500

And NO, it is not (?) FOR the throttle plate. I don't understand that comment...
It "becomes"...part of the throttle plate.
It helps protect the power valve from backfires, thought it's not 100%, it's much better than nothing.

Mike

 

Actually ,it's called the throttle body. This is a throttle plate:

 

Ok, my bad.... Sheeeesh!

 

^^^^^^^^ I knew what you meant!! I just don't understand why Mike VV got so upset and then called it wrong too.

 

No biggie.... I just try to keep the peace....

 

Thanks for initial burst of feedback. I’ll check the power valve today.

But let me make sure I understand the impact of a blown power valve: it is essentially allowing a vacuum leak because the diaphragm on the valve is damaged, correct? And thus, I am struggling to maintain an idle due to the leak?

Thanks in advance for continuing to educate your fellows HAMB members. Always helps to know the “why” behind the issue rather than just part swapping.

Bantam

 

When the diaphragm ruptures, it will draw fuel through it and cause a over rick idle.

 

I think you should start looking for a fairly large vacuum leak- carb base gasket, intake gaskets. Is the exhaust blowing out black smoke? Does it try to idle any better if you partially cover the butterfly?

 

If I recall correctly, after about '92 Holley's came equipped with a built-in check valve, though the ball gets lost a lot when they get rebuilt. Never hurts to replace the check valve, they're cheap, and if we're honest, there's probably one that will help with your fuel economy and throttle response too.

 

I've blown a good number of power valves due to backfire (usually when accelerating too hard before engine is warm and choke is off). The check valve doesn't work all that well, at least it hasn't for me.

A backfire induced power valve rupture can range from a slightly poor idle to an engine that will barely run at all except at higher RPM. If you suspect it at all, replace it.