Missing, backfiring 350 SBC

Got a bellybutton 350 SBC, its missing at idle, under accleration wide open throttle, its also backfiring out of the carb. I got all new plugs, new wires, new HEI,fuel pump, filters, rebuilt carter AFB 4 bbl. also tried swapping with and another carb and no luck. I was thinking cam getting worn or bad valve, whaddya guys think?

 

check your spark plug wires, and make sure you've got the right wire going to the right cylinder....

maybe?...

 

not possible, it was running fine for a while, plug wires have always been in the correct firing order.

 

Pull the valve covers and watch for valves that don't move....sounds like the typical no lobe cam.

 

Check the plug wires anyway. Somebody might be screwing with you.

 

try this one on, too much vacuum advance. When the dizzy goes too far, this is exactly what you get.

Simple test too, just disconnect it and don't forget to plug the vacuum into the carb too

 

Back firing where? Carb or exhaust?

 

bad intake valve...Do a compression test.

 

OH! What's a "bellybuton 350"?

 

Second F&J......flat cam

 

Check for a broken valve spring, too. Also- double check for a rocker stud pulling out, unless you've got screw-in studs.

~Scotch~

 

backfiring outta the carb means cam or valves most of the time.

 

What did you gap the plugs? I had a similar problem with bad miss and I had plugs gap too close. With HEI you can go .050.

 

old timer is right, out the carb is cam/valves, out the exhaust is elect. plugs/wires/timing ect.

 

Good idea!

 

I was thinking the same, just wanted to make sure I did not possibly overlook something simple as I sometimes do.

 

Had a 350 in a pickup once that developed a pop every now and then out the exhaust on one side.Drove it that way for a year or so when i was a broke teenager ,till it got so bad i just pulled the plug wire off and drove it that way for another month.When i got the money to rebuild it finally ,it turned out to be a split exhaust valve that was shaped like a V from being slammed up in to the valve seat .I dont know what keeped the keepers on the stem ,cause i could almost take the spring off with my hand since the valve was getting pulled higher and higher from the split.I would sure check it out if you plan on using the motor .If it has a valve like mine did on the intake side(sounds like intake popping through the carb) it could drop the keepers and valve .A simple compression check with a cheap gauge will tell you witch cylinder is doing it...

 

Sounds like Flat cam on Exhaust Lobe
Happens on those usually about 40-60 thousand
Check with Vacume gage if fluctuating
Pull valve covers look for rocker not moving
Or look for push rod off rocker or pulled rocker stud
Ray