Thread: why is my 350 makeing that noise

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i rebuilt a 350 from a 1976 chevy pickup and put it in my 83 monte carlo it sounds great and idles fine but after it warms up it starts backfireing it sounds like it is comeing from the passenger side exhaust any ideas?

Need more info. How much did you putinto the rebuild? Bored, new heads, manifold, etc. Whats the timiing set for?

Dear ..Motors Repair Manual.....1976 edition...

I have a 350 , quadrajet,(rebuild kit just installed), headers, new lifters, new fuel pump, transmission just rebuilt, runs great but at low idle it has a knocking noise like it is almost a bolt or nut hitting something metallic...Anybody experienced this?

Hmm, check your 3 torque converter bolts. If they have loosened up, tighten them with locktight

check your valve adjustment

Thanks for the advise......Need to check the convertor....That is the only thing I did not personally do. the valves I loosened until they clanked, tightened to quite, and then 1/2 turn extra. Is that right with hydralic lifters?...CR

bigtruck... since the noise appears after warm up I would have to think the valves are set too tight. metal expands when heated so clearances shrink. now, that may not be the problem... hard to guess without more info. I had a rebuild once with some milled heads and domed pistons and it would knock the tip off the plug once warmed up.
81 chevytruck, you might also look at your flexplate. they can also develop hairline cracks next to the crank bolts. also, make sure the flexplate bolts aren't too long. they can stick out the inside of the crank and nick the back of the block.

Thanks......

Thanks......

Just a wild guess based on the limited info and not being able to actually hear the noise, but it sounds like a blown (leaking) exhaust gasket or exhaust manifold (header) leak. Usually this would sound more like a valve tapping (tic,tic,tic) than a back fire.

Hey, disregard my previous reply! I just read all the follow-ups including your added info. I did a HP build up on a small block in a Jeep a few years back and had the same problem. It turned out to be a bolt in the torque convertor, and only happened after warm up.

Actually the noise is at idle, but as you rev the motor the noise "disapears". I had a guy scope it and he said it sounded like a lifter on the 1 or 3 cylinder. After replacing them it still makes the sound. I am about to check the torque converter again. What about the fuel pump rod? Will it sound like that? Will let you know what I find, damn thing runs good, just the metallic knocking. I appreciate ya'lls response. CR

Did you install the little flap between the exhaust manifold and exhaust pipe. i.e., E.G.R. heat stove door. They stay closed during warm up but when hot sometimes the spring is worn and lets them flop. I've had the same thing happen and I pulled that little flapper door out of the exhaust by the manifold and it went away. The door is on the passenger side exhaust manifold of GM 305s and 350s from something like 75-85.

It could also be like doc al said with the flex plate cracks. I have a camaro and I thought the front pump was going out of the trans and it made a funny little clatering, tapping, grinding, hard to friggin explain sound and it was the flywheel. Cracked at the crank bolt holes on every hole except one. I'M LUCKY IT DIDN'T GIVE ON THE ROAD! Good ol' Chev brought me home though.