I recently put in a carbed 350 in my previously MPFI V6 berlinetta. I'm using the old return line as my new supply line to the new mechanical pump on the block. I capped off the old supply line and hooked the vent line back up to the charcoal canister (which is not connected to anything else). Every time I take the gas cap off a whole lot of pressure is released. Should I be doing something differently with the vent line? What damage could this have caused to the pump? Could this effect how rich the engine is running (increasing fuel pressure?)? Thanks.

 

No.None.No.

Carbs are only very slightly sensitive to fuel pressure. They will work at incredibly low pressures, as long as there's enough to keep th ebowls full. They're not like FI where fuel delivery increase with the fuel pressure. A carb will deliver the same amount of fuel regardless of pressure, up to the point where the pressure is great enough to force the needle valve open even when the bolw is already full.

 

I wouldn't have thought you could use the return line as your supply line- I didn't think it reached to the bottom of the tank so it could draw fuel. Regardless, that won't affect what you're seeing.

And what you're seeing is perfectly normal. The fuel tank system is designed to hold some pressure to prevent fuel vapors from escaping the system and becoming pollution. There is a pressure vent at the end of one of the lines coming from the gas tank to prevent building up TOO much pressure and rupturing something. Believe it or not, the limit of the valve is ~1 PSI- that's it. Some cars use vented gas caps, but the 3rd gen does not- it uses a separate valve.

When you hooked the vapor line back up to the charcoal canister that's when you sealed the system again and allowed it to build pressure. Remove it from the canister and you'll never build any pressure. Either way it's not hurting anything- just one of those weird little things.

 

I know pressure building up in the tank is normal, but it was an excessive amount when I took off the cap. That's the only reason I was concerned.

I did not know about the relief valve, though. Is that located near the tank or in the charcoal canister? Thanks for the info.

sofakingdom,
When the engine is running, the fuel pressure gauge consistently bounces between 6-10psi. I understand the typical range for carb pressure should be 4-7psi. Is 10psi enough "to force the needle valve open even when the bowl is already full?"

 

yes. 9 PSI can force it open. You should not be playing with 10 ever. If it is doing that, you shoul dhave fuel spilling out and making your car smell like gas. Are you sure you are jumping all the way to 10 PSI?

I may be wrong though. I keep my carbed 385 at 7 PSI

 

Well it's running really rich and there is fuel in the oil. So i'm not driving it around anymore until I figure out what is causing this. After sofakingdom said that about opening the needle valve I was thinking that the fuel pressure may be why it's running so rich.

I have the summit pressure gauge that hooks onto the carb rail. I'm positive that the needle bounces between the 6psi line and the 10psi line while it's running.

Any other thoughts?

 

Is the in-tank electric pump still operating?

 

Well, it shouldn't be. I don't have a computer anymore. Although I didn't disconnect anything going to it from the ignition now that I think about it.

 

well then first do that.

 

Yes, i'll look at that, but I don't see how that's really going to effect the problem of it being too rich. I've capped off the old supply line (since i'm now running fuel through the old return line) and there isn't any fuel leaking out of it. Maybe if it's still operating it's having some sort of effect on the pressure in the tank. Thanks.

 

exactly