1979 F150 4x4
400 C6 NP205
slightly older Holley 4-barrel 600cfm carb
CompCams XTreme Energy camshaft with custom grind

I have been having issues with my carb overflowing since I droped this 400 in several months back. A co-worker of mine had built it up for his '80 but sold it to me when he bough a used '02 F350 diesel. I finally got around to throwing my cheap-o pressure gauge on there and the Mr Gasket replacement fuel pump (80gph/8psi) was reading 10psi (it might have been higher but the gauge doesn't read higher than 10). At the suggestion of my local speed shop I swapped the fuel pump out for the stock one I had on the 351M I had pulled out of the truck. I checked the stock pump and it is also reading 10psi. I figured the gauge was inaccurate so I asked a friend to bring his gauge over. The new gauge only went to 5 psi and it pegged immediately. We plugged the pump and ran the carb directly off of a gas can. Brought the truck to temp, check timing and air mixture screws. Everything seems to be running great. The truck will begrudgingly idle down to almost 200 without stalling. We hooked the fuel pump back up and almost instantly the front vent started overflowing. What could cause this? My friends suggestion was to block the pump port and go electric. Is this the right thing to do? I would really like to understand how a stock fuel pump could push 10psi.

 

I think the problem is more likely in the carb fuel inlet valve. It oly takes a small piece of debris to keep it from seating. A gas can would not have any pressure to speak of. Your fuel pressure should be between 7 and 9psi. To check your gauge you would need another one preferably that ranges to 15psi.

 

I have had the carb off several times in recent days including a full rebuild a couple of weeks ago. Although not impossible, I would find it unlikely that something slipped in.

 

Just put a regulator on it...I know that the pump is supposed to be self regulating, but at least you would have some more adjustability.

 

Would it be better to add a regulator rather than an electric pump? Cheaper perhaps

 

Whatever gets you to 8 psi.

anything more than 10 psi you will have fuel problems.

 

You might double check the part number on the fuel pump to make sure you didn't get a 15 psi unit that would require a regulator. I personally prefer electric pumps and regulators, preferable with a return line. I helps with vapor lock problems.