Hot Rods Throttle "Blip" Backfire...Why?

What causes a backfire through the carb when you try to "blip" or "rev match" while downshifting? Cruising along, shift from third to second, tap the throttle before releasing the clutch and "POP..." I'm pretty sure it's through the carb. Lean? Rich? Been chasing it for awhile.
Holley 600 (1850-3).

Thanks.

Google and YouTube have been ZERO help.

 

Usually, a carb pop is a lean condition or not enough timing at that point in the rpm sweep, but it could be a bunch of things. What are your initial and total timing numbers?

-Abone.

 

First thing I would do is check compression, bad intake valve will do that.

 

Are header gaskets well sealed to the heads?

 

I can't imagine it's a bad valve. Everything moves just fine when I re-checked the lash a few months ago, and it runs great. I think it's more of a timing/ carb issue... Seemed to get better when I jetted up last time.

I think the exhaust manifolds/gaskets are good and snug.

I probably should check the intake, now that I'm thinking about it.

Total timing is about 37, if I recall. I never bothered checking the initial. Seems better if I rev it up "slower" rather than just stabbing the throttle.

 

I would check the accelerator pump operation, it's there to richen up the mixture when the throttle is given a quick blip, or opened quickly while driving, otherwise the ''lean mix'' (excess air) ****ed in on opening will cause a backfire out the carb..

 

LEAN

 

Good place to start.

 

Power valve. My Olds eats them for breakfast after a winter of storage. One backfire and it's time to tear the front bowl off and replace the "blow proof" power valve. I stock them like bacon strips.

 

I'm no stranger to a blown power valve in a Holley, but a bad power valve will make it too rich rather than too lean. No new information here I know, but on deceleration, shifting from third down to second could result in too much un-burned fuel in the exhaust and cause it to backfire.

 

Why not install a blow out preventer ?

 

Air leak between carb and head.

 

Re-torqued the intake bolts today and the ones in the "middle" (Y-Block) required a little more to get up to 25 lb/ft.
Also ordered the next size up on the accelerator pump discharge nozzle. Fingers crossed.

Thanks gents.

 

I tired the no blow valves. They blew. I have actually had this one in for over two years now. Running clear fuel seems to help.

 

Never heard of a" no blow valve" ?

 

I thought the blow out prevention went in the base plate, and the valve's were the same...

 

Exactly !