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

I was thinking about switching to a Holley red electric pump 67gph at 5 psi

Pulled the pump push rod and compared it to a305 and it's the same 5.75in rod and checked on summit racing that's what it should be. So is my new fuel pump junk or to small???

Since you are looking for all possibilities, here is a long shot....I once purchased a boat and had the same problem... The problem turned out to be that the marine fuel pump weep hole nipple tubing was hooked up to a vacuum port on the carb instead of being hooked up to the flame arrestor. So every time I would accelerate it would pull a vacuum on the top of the mechanical fuel pump diaphragm which resulted in weak fuel psi.. It made the motors run very lean causing what I call lean pops at 4000 rpm's. I could run it at 3000 rpm's but that was it until I put the tubing in the correct position.. Simple problem to fix if this is it....Good luck...r

The weep tube is on the nipple that dumps into the top of the secondary side of the carb where it should. So it's not receiving manifold vacuum

It seems like you have ruled out everything but the pump. I would try another one if you can. My experience is mechanical is very reliable and simple I would stay mechanical for simplicity. My block is a 98 so its not drilled so I have an electric pump. Fwiw mine is a summit 95gpm and have never had any issues knock on teak

I would try another but I'm having trouble believing that both pumps I've got are bad. I'm thinking of going electric but if I don't have to ill save my money for some stainless risers. If it is the cam loab I really won't know till I pull the cam which I'm planing to do this winter anyway going bigger so maby the electric is the better route

Even tho the line is going to the flame arrestor make sure the flame arrestor is vary clean and is big enough for the engine..if the fins are cluttered with crap it can suck and act like a vacuume port..also get the mallory red marine electric fuel pump..the holly one sucks and is vary noisy if you do decide to go electric..me personally i hav a electric pump on my donzi and would not go with out one..

You could get an indicator extension for a dial indicator and run it against the cam and turn the engine over by hand to see what the travel is. It's unusual to see 2 bad pumps with identical issues, unless you got them both from the same batch. Does your fuel distribution to your carb have an internal filter, that could be plugged?

Took out the carb filters that I installed when it was new a couple of months ago they were clean any how. Onto the pumps one was the original that came withe the boat when I bought it and then transferred to the new engine the second was bought a week ago. I'm starting to wonder if there is a reason that the smallest mechinal pump from Holley is110gph free flow. I'm going to try to get a pump only pressure to see if I can get the 5.5-6.5 the replacment pump is rated for, that should be a good indacator if its pump defect or lack of flow. If I do check the cam loab what should it measure at? This was a reman jasper and I don't know if they grind cams or not. And not to forget the spark arrester it is clean and I did a short run without it.

if not the pump would have to be the cam for the pump I would think..

Finely got a chance to do some more testing of the pump today I disconnected the weep tube for the heck of it no improvement. I installed a tee and some hose between the pump and carb with another gauge, both gauges reed the same 4.5 psi but when I clamp off the line to the carb I won't get any more than 5 psi from the pump and the gauge at the carb will drop to 0 as it should. Starting to think its the cam loab since I have two pumps with low pressure and I can't get the rated pressure out of the new one. Any thoughts?

The SBC fuel pump eccentric should have a lift of .345"-.334". This came directly from Crane tech support.

how about the metal tube going from filter to pump, have seen gunk build up in them.

many marine engine remaners regrind the cams they used for the build. You either have the fuel pump cam lobe too small or the incorrect fuel pump rod (tad too short) that is under stroking the fuel pump causing your low fuel pressure.

We see these new mechanical fuel pumps that cause too much fuel pressure from being over stroked - the fix for that is thicker gaskets and or a proper spacer to obtain the proper fuel pressure - this has been a very common problem for the last 2 years (at least 20 to 25 apps) even buying OEM Mercruiser mechanical fuel pumps.

IMO the fuel pump arm might be the problem as it is under stroking causing your low fuel pressure issue. Make sure you are using the correct mechanical fuel pump for your app also (just saying). What Sierra part # being use for your appd. I just had one that would not pump at all for a 2008 - 3.0 Mercruiser.