Fuel problem on SBC
 

I've bin chasing fuel issues on a 260 merc for most of the summer. the problem I have right now is I can only get 3 pounds of fuel pressure threw the the rpms. I've changed the fuel pump with a new Sierra unit no change in pressure I've replaced the anti siphon valve and tried without it as well. new fuel/water separator and tried with it bypassed. Fuel pick up is clear any ideas are welcome. And to go along with the low fuel pressure I'm running lean and can't turn a 19p over 4000. Set up right now is a 260horse bored .40 over with a Holley 600cfm mech secondary with #74 jets on the primary and #81 on the secondary side on a edelbrock EPS intake. All is new and I'm out of ideas

Have you replaced the rubber line from the tank. They collapse under suction

This might sound screwy but you might have a vacuum lock being formed in the vent line. Check the vent screen on the thru hull vent along with the hose leading to it from the tank to make sure there are no kinks or obstructions. A quick check for this would be to run a short distance with no gas cap on the fill neck of the tank.

Let us know how this turns out.

I should have mentioned that I tried running off a bucket of fuel so that rules out the tank vent and when I did that I used a new short section of line. The line from the tank was replaced the end of last season

And all pressure readings are being taken at the carb inlet with a new auto meter gauge. Is it possible that my set up needs a larger than stock fuel pump? Holley says to have 7psi at idle and a min of 4psi wot

Mechanical or electric pump?

It's mechanical

By running it out of the bucket you have eliminated everything but the fuel pump. I would switch to an electric pump to be sure you dont have something wrong with the cam in relation to the push rod for the mechanical pump.

1) What size fuel line are you running?
2) Did you check fuel pump push rod to make sure that it is in tact, Also check the cam loab for the fuel pump to make sure it is not bad this would casue your problem as well as the pump would not be getting a full stroke.
3) Lastly check your connections at the carb to make sure there are no restrictions like teflon tape for bad banjo gasket.

It's all3/8 line most connections are tape less I use a paste thread sealant. The pump push rod appears to be all there nothing hiding in the hole and I'm assuming that the cam loab is fine it's a reman engine with 97hrs on it. I should mention this engine has never bin right I've always had fuel issues. Is it possible there is different length push rods I think I've seen that mentioned on here