383 popping and backfiring thru carb/exhaust

[chargerhunter]

Carb is fine, fuel pump is fine, new distributor etc.

Timing is set correctly...

Could the rusted valley pan have pin holes near the intake and cause this??? Bought ready to shoot it! Have checked and double checked everything we can think of..... Stuck valve?? Ran motor flush thru the motor and no change.

Motor sat covered for about 2-3 yrs without being started.

HELP!!!!!!! 

[beedees]

Not to be a smart-alec, but check the simple things first.....plug wires and firing order.

[chargerhunter]

No offense taken. That was the first thing we checked. Motor ran pretty good when we first dropped it in, there is a significant amount of rust on the valley pan, this is what has made us lean to a vaccum leak. Won't a bad vaccum leak cause the popping. Runs smooth at idle, try to rev the motor and it starts spitting and popping thru the carb. It is 9 times out of 10 something simple, we just haven't been able to find it yet. It has a new Pertronix kit in the dist. We first thought it was a points issue, and tried this. Rebuilt the carb, new wires, plugs and distributor. Set the timing with a light. Guys kept telling us it must be a stuck valve causing the problem, if you guys think it could be that it would sure narrow alot of things down.

[Silver R/T]

possibly a vacuum leak between carb/intake/heads. I had same problem with my 440 then I used gasket sealant between intake/valley pan and carb and it runs much better now

[Big Lebowski]

  Mr. timing chain is strecthed or you have some burnt valves. Bad Intake valves pop through the carb, and bad Exhaust valves pop through the back end.

[COKE]

I had those  troubles until  i bought a timming light and adjusted the firing order.
Due you have this done and you have suspects about the vacuum leaks ,i would spray a bit of fuel on those areas to check if the engine RPMs goes up.
That can be a way to check this.

[deputycrawford]

     You can also spray some carb cleaner or brake cleaner in the area of the rust. If the engine reacts you have found a problem area. Valley pans are around $20 and take only 30 mins to install. Change it anyway.

[chargerhunter]

Yes, but is it possible a rusted through valley pan could cause this?

[deputycrawford]

     I guess anything is possible. You will get some of the same results if you have a vacuum leak at the carb base gasket. Just move down alittle and you have a valley pan. Everything connects to everything else. If it idles at normal idle speed and is, for the most part, responsive then I would spray the carb vleaner around and go back to timing issues. It couls be advancing too fast and causing it. I would have to hear it run and start from easy to hard. You could even have the distributor 180 degrees out. It would start and run but sound funny and back fire through the carb. Keep an open mind and start from scratch again.

[chargerhunter]

While ideling it runs fine, no popping, try to WOT and motor tries to die, back firing thru carb, exhaust. Would you guys think the best thing to do is pull the heads, do a valve job, replace valley pan and hope for the best? It would not take much more to pull the timing chain cover and replace the timing chain. Thanks for the tips.

[resq302]

Are you sure that the float is set correctly?  A low set float could possibly make it run good at idle but when it demands more fuel, the bowl will not have enough to make it run at a higher rpm.  Also you say when you give it WOT, that makes me think that the accelerator pump might be bad and not giving it enough fuel to make up the difference it needs.  An easy fix to see if you have a vacuum leak is to take one of those small propane torch type bottles with the fitting on the end and go around the suspected area slowly with the valve just cracked open to let some propane out.  This is less messy and will also not ruin your paint on your engine and will let the rpms increase if you find your vacuum leak area.

[chargerhunter]

A mechanic friend came over and took the top of the carb off and had it running, I was shocked that it would even run like that. He said the accelerator pump was fine and he checked the float, said it was fine too. I'll do the propane trick and see if I can find a vaccum leak. It is a stock 2bbl 383. I wish I could find another 2bbl that I know works for sure and try that...