FC eng. carburetor backfire

Thanks for the suggestions, I believe I have solved the problem, I have fabricated some new larger main carburetor jets, giving a richer crusing mixture to help prevent the lean backfire, drove it about 10 miles and seems OK----keeping my fingers crossed it doesn't happen again

Charvey wrote: Help! I'm running out of ideas to fix this. My 812SC has an intermittet carb. backfire while accelerating through 2000 rpm or higher, usually if the acceleration is quick. I have rebuilt the carb., set the accel. pump rod on the longer stroke. Checked fuel pump output, new fuel filter, drained the tank & installed fresh gas. I went through the entire ignition sys. New plugs, wires, points & cond. cap & rotor, checked advance weights, set timing. Compression check is good (within 10 lbs of each other). Manifold vacuum is 15.5 @ idle with a steady needle. Checked for vacuum leaks. Any ideas? I greatly appreciate your help

Hello,

first of all, my english is not perfect and I hope what I try to express will be understandable.

As the others posters wrote before it could have many reasons that the engine fires back.

But as you discribed the problem (backfire appears during "quick/hard" acceleration) I would guess it could be a problem with the ignition distributor ( advance weights and/or its retract-springs).

On higher (increasing) rpm an engine needs an earlier ignition-timing than in idle-running, the (centrifugal-) advance-weights manage to adjust the timing.
On acceleration the rotating advance weights "move out" and put the ignition timing to an "earlier timing level" and when the RPM are reduced (to idle running), the retract-springs pull the weights back into the idle-running position (-> "later" timing).
If there is something wrong with the weight-mechanism (e.g.: lame/old retract springs) the ignition would go to fast on the earlier ignition time with the consequence -> Ignition on open intake valves.

Matt

Had the exact symptom of backfiring on a Chevy, did everything, exhaust intake gaskets, carb, points, everything possible. I asked a friend what could I do and he instantly said get a new distributor. Mine looked fine but got and installed a new one. Guess what, no backfire. It must have had a bad bronze shaft bushing that caused a wobble. Worth a try.

Could a sticky intake valve be hanging open from time to time ?
Possibly a weak intake valve spring; or dirty valve guide or valve seat ?

How about a low-weak, or lack of fuel flow/ fuel pump issue, to your carb from the auto's fuel tank ?

Low carb float level setting ?

You seemed to have already checked-out the carb & ignition systems ?

A lean fuel/air mixture, or improper ignition timing, can also cause this sort of backfire issue.

just a few thoughts~

Help! I'm running out of ideas to fix this. My 812SC has an intermittet carb. backfire while accelerating through 2000 rpm or higher, usually if the acceleration is quick. I have rebuilt the carb., set the accel. pump rod on the longer stroke. Checked fuel pump output, new fuel filter, drained the tank & installed fresh gas. I went through the entire ignition sys. New plugs, wires, points & cond. cap & rotor, checked advance weights, set timing. Compression check is good (within 10 lbs of each other). Manifold vacuum is 15.5 @ idle with a steady needle. Checked for vacuum leaks. Any ideas? I greatly appreciate your help