Weber carbs, fuel pressure drops as motor warms up

[hank]

I have a stock block 240 that I installed triple Webers and headers on. After rebuilding them and figuring out the correct jets and venturi tubes, I now find the fuel pressure drops as the engine warms up. It starts off around 3 PSI and goes to zero, but the motor still runs. Is the fuel cooking in the bowl?

I tried replacing the fuel pump, regulator and pressure gauge...$$.

I also wrapped the headers with header wrap which made a little difference.

Nothing like trouble shooting with the wallet.

The water temp gauge says water temperature is normal.

Anyone else have this problem? These carbs came out of a junkyard 260Z. They had no heat shield but I have noticed many of the Weber carb pictures on this site do not have shields.

Any advice out there?

signed stumped

[d240zx2]

I run an old Stewart-Warner clicker (thumper?) fuel pump mounted at the tank, bypassing the mechanical unit completely....it's removed and the mounting hole is blocked off.

I've set the pump for 4 - 6 psi, which is a bit on the high side for Mikuni carbs, but it seems to work quite well. I've also blocked off the fuel return line to insure adequate fuel supply to the three fuel suckers under full throttle. I've never had any problem with vapor lock or fuel starvation in turns at the track, so it must be working pretty well.

Frank

[Zs-ondabrain]

My triples only show fuel pressure when the car is not running, especially when the heat from the headers reaches the carbs and fuel lines (due to the lack of a moving fan)

It'll show about 5 pounds when I turn the key on but after start up, the gauge drops down to 1 pound or less. I don't know if that's typical but that's what mine does. I figure that as long as the carbs are getting fuel, you're fine. When the jets start dumping fuel, it won't hold a constant pressure. Almost like on an air tank, you hit the nozzle and the needle drops, same situation??

Quit spending money unless something breaks. "if it ain't broke........."

Dave.

[gmr714]

I have a similiar problem with my 260 that has Hitachi carbs. It runs great but when I shut it off and try to restart it while hot it either turns over and wont start or it starts but runs very weak like it is starved for gas. It is an intermittent problem and I was thinking it was vapor locked but I am unsure of how to proceed. Fuel pump pressure is around 3psi hot. Perhaps we have the same problem. I was ready to change the mechanical fuel pump.

[d240zx2]

"I was ready to change the mechanical fuel pump."

I don't think changing the pump will fulfill your objective. Higher fuel pressure and/or lower temps prevents vapor lock. I'd suggest adding an electric unit either at the tank or in line before the mechanical unit to increase pressure at all times. Reducing underhood temps is a metal-bending endeavour with louvers, etc.

My 2 cents.

Frank

[bjhines]

The factory fuel rail has a restricted bypass that was there to aid in purging the vapors out of the lines during hot starts.