carb backfire

I just got the newly installed '55 235 fired up in my 37 chev towtruck and my rochester model B is giving me a little trouble. The engine starts right up and idles fine but when driving it I get a pop though the carb when i stomp down on the throttle from low RPM. No flames or anything. I retarded the timing a little with no effect. I've been reading a little bit and im getting the idea that maybe my accelerator pump isnt giving the engine fuel for quick accelration? I just did a fresh rebuild on the carb and replaced the accelerator pump so maybe i have a problem in there. What do you guys think?

 

Advance the ignition.

 

Was the replacement accel pump the exact same diameter as the original? Some kits have the wrong pump. Also low timing can cause a lean pop. peace

 

Hone the barrel for the pump plunger if it's scored. A ball type wheel cylinder hone works well. Try to find a neoprene pump to replace the leather that comes in the kit.
Make sure the vacuum advance is working - you'll see the dist. move when you wind it up.

 

vacuum advance is working, i know what you mean i didnt like the leather pump washer either. The cast in the carb was a little funky so maybe im not getting a good seal in the plunger barrel ill have to check it out n see what happens.

 

Need to advance timing. Spark knock,retard timing.

 

There should be 2 check valves in the pump circuit. One could be a ball and one could be like a needle valve. One shuts when you push the plunger down, and the other one opens to let the gas go out the squirter. I'd bet one is stuck.

When the plunger is coming back up, one shuts so air can't come in through the squirter, and the bottom one opens to let gas come into the pump well.

 

With the engine not running, remove the air cleaner, make sure the choke is open, look down in the carb while you actuate the throttle to full open. Do you see a nice long squirt of fuel, beginning as soon as you move the throttle? If not, fix the accelerator pump.

Retarding timing makes it worse, not better. Did you disconnect the vacuum line to the distributor before setting the timing? you did use a timing light, right? plug wires are all in the correct order? new points and condensor?