Backfiring

S

Shoat

Member
Popping back through carb . When accelerated.
Bogs down when accelerated and pops. Is this a
Carb, intake manifold, or firing issue?
 
Agreed, too lean.
many be just needs enriching via GAV. but if consistently poor then also consider,a) poor fuel flow( rust covering exit from tank, rust blocking jets in carb, no vent in tank, incorrect float level etc etc. b)manifold leak
 
Not for the sake of disagreement, but I think a lean condition, whatever the cause would backfire through the exhaust. I would guess that the problem is either valves or camshaft . A burnt valve is a real possibility.
 
Soft cams were common on some GM cars is the 70's and a flat lobe would make them spit back in the carb once in a while. A weak spark can cause loss of power when accelerating, but that usually shows as a backfire in the exhaust. Checking spark is easy, just hold the coil wire 1/4" from a head nut and see if you have a good blue spark. Of course even a hot spark can still leak away through a dirty coil tower, bad rotor, or bad distributor body, which seems to usually leak between #3 and #4.

As Tbird said, try turning the GAV a bit richer, or pull the choke a little to see if that helps. If it does, then you know it's fuel related. If not, then look for other causes, such as sticky valves, burnt valves, weak spark. A vacuum gauge can help show a sticky valve. A compression tester will show a burnt valve.
 
Thanks guys. Found that the spark is not strong as it should be. Installed coil, points, and condenser but still not as strong as my other car that runs correctly. Any other idea of what could cause weak firing? car runs, idles rough and has low power. backfires out the carb. when accelerated and will not go to full speed. Opening the choke or adjusting the air adjustment on the carb. makes no difference. Will check the gas flow.
 
In my "thinking", I think the most common causes of coughing BACK through the carb on acceleration are too lean and/or timing too retarded. Of course, many of the "not so common" things, mentioned here, could cause it also. (Also, is your gas "OLD" & poopy?) Bill W.
 
Check the voltage feeding the coil. Also make sure the distributor has a good ground. A good clip lead is handy to supply a good distributor ground, or directly supply 6 volts from the starter switch to the coil.
 
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i once had a modern style wireless dist plate cause weak and int spark. as tom said ck powers and grounds also it could be the coil itself.
 
I had a similar problem and it turned out to be the distributor bottom plate. It was so dirty and crudded up that the ignition switch wasn't making good contact - that spring loaded brass tip on the faux repop switch also needed cleaning. I ended up replacing the bottom plate entirely, because the wire going to the top plate was frayed.
 
Could also be that one of the new ignition parts may be at fault. New does not mean good. Or could be an issue with the distributor body or rotor - you didn't mention replacing those. Suggest switching things out one at a time so you know what caused the problem. Good luck!
 
Turned out to be carb. Installed another cab. after checking compression on each cylinder and ran good. Plenty of power. Old carb was wrapped and sucking air. Thanks for everyone's help.
 
also with the carb if your float does not allow the pin to open fully to fill the chamber with gas you may run fine until you accelerate and than consume more gas than is being supplied thus a lean situation develops a lot like if you shut of the gas valve to run out your gas from the carb when parking the car may backfire when the gas runs out
 
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