I've got an 80457 600 cfm Holley on my 390 in my '74. Engine is bone stock except for the 4 barrel. Truck sat for ~5 years. I got it running and rebuilt the carb. Had some issues with the electric choke, but got those resolved. The newest thing is while driving down the road about 50-55 mph it will run fine for a 1/2 mile or so then start cutting out and backfiring. If I downshift to second and wrap it up it sometimes comes out of it, but sometimes won't. Only way is to slow down to 35-40. I can then maintain that speed, but I can't speed back up. It doesn't matter if I ease into it, or hammer it. I've verified the float settings twice - just level with the bottom of the plug when running.

Now, would going to the center hung floats help since the bowls are bigger? Or do I need a larger needle and seat (not sure what size is in there now - have to check when I get home from work). I'm about ready to junk it and get a 670 Street Avenger, as the one on my warmed up 390 runs awesome and I've had ZERO trouble with it.

Fuel pump has been replaced. Fuel tank cap as been replaced as it wasn't venting correctly. This problem first showed up with pulling a 24' gooseneck stock trailer. Now it shows up just running down the road with no trailer. The trailer does magnify it severely though!

Thoughts?

Thanks!
-Jake

 

Have you changed the fuel filter? If you can, next time it does that just shut it off and coast to a safe place and pull the plug and see where the fuel level is when it was running.. If you still hate the carb i'll pay shipping so you don't throw it away.. But check that filter and make sure no lines got bent or kinked almost shut and there may be a screen behind where the fuel line attaches and maybe not.. thats where I would start

 

Is it backfiring through the carb or the exhaust?

 

Through the carb. I replaced the power valve and that helped, but didn't totally solve the problem. I need to put my 670 on it and see how it runs.

 

yeah check the fuel system from the tank forward and Check your ignition timing man!

 

had something similar happen chased carb related problems for days turns out in my case the little wire going from coil to dizzy was loose and when the wind blowing through the front at 65 mph would blow it around to where it missed like crazy but if i had my radiator blocked off i could get 80-90 mph before it did it fixed wire no issues since..... its always the little things

 

Verify that the carb has unrestricted vents for the float bowls, that the PV isn't blown again, and also that the secondaries are opening/closing properly. What you are describing sounds like a fuel shortage to me, verify all that you can before swapping parts. Is your secondary circuit clear? Could it have restricted passages in the metering somewhere?

 

Two things I've dealt with that caused backfiring through the carb is timing and vacuum leaks. But in your case, if it was the timing I would think it would duplicate the symptom at start up unless your vacuum advance is causing it. I can't imagine what would cause a vacuum leak all of a sudden at 55-mph. Of course there are many other malfunctions that could cause this but usually it would happen all the time. So before tearing stuff apart or replacing parts, I would go back through and double check everything you touched and search for a self induced problem. And if you have a spacer between the carb and manifold, double check those gaskets. If you had any issues with backfiring throught the carb they can get saturated with fuel and also shred.

 

Power valve. Test drove it today and it was okay. Hooked gooseneck on to go get some stuff. Ran pretty good. Timing was a little hot but I controled it with my right foot. Then a vacuum line came off. Blew power valve again. I may install the protection kit. I think I will be good to go after that and getting my timing set for pulling.