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

 

Things I would check Fuel pressure and filters, timing , advance working , other than the accelerator pump I don't think its a carb issue my 2 cents

 

I'm starting to think it's the accelerator pump, clogged jet, possibly power valve sticking shut? Hopefully I get time to look at it this weekend. In my head float level was all it could be. Now I see it can be a whole bunch of things!

 

. Yes, clogged filter, failing fuel pump, rusted pinholes in steel fuel lines, cracked rubber hoses, or mysterious rust/rubber crap clogging inlet valve would be first guesses... modern gas can leave a clear waxy buildup that is hard to see at first when sitting around too long...

 

I agree, it is starving for fuel. I would replace the rubber line from the tank to the metal line on the frame. They tend to dryrot and even though they don't leak fuel, they will allow air to be sucked in, which makes it starve for fuel. If it sat a long time, the sock in the tank could be clogged up with old, varnished up gas.

 

. If this is one of those "one-piece" body Holley carb.s they made for a while, they were known for metering fuel very poorly even when new, so once you get the fuel supply issues resolved, look up how to make them meter/respond properly... I know Hot Rod magazine had an article on that, although I don't know if it was new enough to be put online... there are prolly other articles available, also... Holley may even offer a 'fix kit' with instructions for them...