Random carb backfire

[Richardy]

--------------UPDATE---------------

CHECKED:
CARB base - carb was not tightened down enough.
Vacuum - very steady and good vacuum
Choke and idle solenoid rewired (were not connected at all before)
Accelerator pump linkage adjusted and pump shot of fuel is going in
Timing was off - reset to 10 BTDC per emissions label


Have not checked yet:
Sparplug and compression, getting a compression gauge today.

Now I am getting a rapid popping the went up and down with rpm in 2nd and 3rd gear that will cause the engine to shut off, but it does restart easily with one press of the gas pedal. A couple of times it will go away for a minute then come back. I drove it almost a mile around the neighborhood, no faster than 40. The popping noise almost sounded like thumping and the vacuum lines did not come off this time. This only happens while driving, I can not get it to pop while sitting in the driveway.

Thank you all for the help so far.

 

[HotRodPC]

Don't check the distributor cap for cracks or carbon fouling cuz that just might be the problem.


That pretty much includes the rotor cap too since you're there. Like for a loose tab that's moving around.

 

[Obwonkonobe]

I don't mean to insult anyone's intelligence by recommending something so basic, but seeing as how it hasn't been mentioned I'd go back and double check my firing order..

 

[Richardy]

great things to check. i am willing to go as basic as it takes to figure this out. it will be a couple of days before i can get back with an update.

 

[shiftpro]

Burnt valve sounds close but...no. If the valve was burnt it would be backfiring all the time.
But... intermittent could very well be a valve starting to stick.

 

[highdesertrange]

a back fire through the carb is a almost always something in the valve train or a cross fire in the ignition. highdesertranger

 

[Richardy]

Sorry for the long delay in an update. Work and Life have gotten in the way. I just got the truck running properly just today. It ended up being a bad carburetor even after a rebuild , or it was out of adjustment. I ended up ordering a remanufactured carburetor from http://nationalcarburetors.com/. I was able to get a carb that was rebuilt and tuned and adjusted on an engine before shipping, and a Rochester Varajet 2 only cost me $179 plus shipping. Most local parts stores wanted $350 or more plus core. Thanks to everyone that helped me figure this problem out.

 

[Richardy]

Been awhile since an update. Life and work just keeps getting the way. Mostly work. New carb installed, timing set to 10 degrees BTDC, per the sticker on the truck, with the vacuum advance disconnected. Checked the cap for cross fires or arcing and I do not see anything. New cap, rotor , Ignition Coil, Wires and plugs (HEI factory system), verified firing order. Still backfiring through the carb, so bad it blew the foam air filter I had on it in half and blew off three vacuum caps. It does not does this until it is fully warmed up but I have not verifed whether the choke is fully open when it backfires. With the old carb it did not do this but the choke and idle solenoid were not connected at all. Not sure if this makes a difference and I have not tried to run it without those connected, I just thought about that. The new carb was supposed to have been tested on a running engine and tuned to factory specs. I pulled the plugs and found an interesting issue, along with the plug gaps being closed about 20 thousands from factory 44 thousands to about 22 thousands. Plugs 1 , 2 , 5, and 6 where clean as in the top picture below which is from #1 cylinder but plugs 3 and 4 were black and smelled like gas as in the second picture which is cylinder # 3. Valves were mentioned in an earlier post,. How do I check for a stuck valve, and could this be a cause of the backfire issue?

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[Rusty Nail]

There ya go!

Y nobody says bad gas?

Cool thread bro.

 

[78C10BigTen]

Someone may have already stated this as i didnt read the whole thread, check that your wires are on the right plugs. My old 350 was carb backfiring after an engine transplant and it turned out i had 2 wires in the wrong order.

 

[Rusty Nail]

Well that bottom plug is oil fouled.
The other one is lean as can be!
It was running fine you say OP?

 

[73c20jim]

x2, Noticed the difference also.

 

[Rusty Nail]

It may be the camera flash but looks like ASH on the electrode top pic. Bottom is for sure oil, bad valve guide fouling the plug or something and every once in a while it tries to fire making the pop?