cars been sitting in garage and I start it occasionally except last week it was running very rough and barely stayed running, now it wont start at all and after trying to start it a few minutes it backfires out the exhaust like an explosion. I pulled carb. and cleaned inside the bowls. it has spark and its getting fuel. the timing is close enough it should try to start. tried a different distributor and points. Nothing was changed before it started running bad. the motor was rebuilt 12 years ago and has never been on the road 4-5 hours run time. its an olds 350. thanks

 

Engines need air, fuel, and timed spark to run, your missing something. How old is the fuel your running?

 

Float needle valve stuck open or propped open by a piece of crud,
engine flooding, raw fuel going out exhaust, BANG.

- Eric

 

My '59 did a similiar thing but while driving. It expanded the mufflers when it backfired. In my case it was the condenser. Replaced it several years ago and no more problems.

 

I put in a breakerless point set and it still does the same. I'll take carb off and clean it out good. Its a holley, ive heard you can have a problem with the power valve? I will let you know how I make out. thanks

 

The motor has only 4-5 hrs of run time in 12 years of sitting?? The holley carb would be dried up and crud will be moving thru it for sure. Pushing a lot of fuel thru their to get an explosion. Try tapping on the carb with a hammer to settle the needles back in .

 

Yes backfire can cause the power valve to blow out.

 

I installed my distributor 180 degrees out once, tried to start the car in the garage, blew both muffles inside out, i thought i died!

 

Lmao... Ohhh that would rock ya

 

LOLX2! That was funny. I could see that in my mind's eye.

 

well after taking the carb apart and cleaning it, installing breakerless points, and an accel super stock coil. After removing the spark plugs and checking all for spark, 4 of them had a week spark. Replaced them and car runs like a top. Go figure! about $100 later and 2 fatter mufflers thanks for your help

 

Good to hear you have it running, now you'll have to watch for chipmunk's burying their nuts in your fatter mufflers.