Reply With QuoteCheck/replace the filter first. Pump may be on its way out. Tank vents OK? Anti-siphon check balls removed?
I'm running an 89 Eliminator Tahoe with a 7.4LX / Bravo I. I switched to a Holley 850 Carb on an Edelbrock intake. I'm also running a set of Powerflow exhaust manifolds. I changed the original stock fuel pump out for an edelbrock mechanical 130 gph fuel pump a few years ago.
The motor will only turn 4400 rpm with a 21 pitch prop. My fuel pressure is running 5-6 psi at idle. I have a mechanical gage at the end of the dual fuel line connector for the carb. The pressure drops to 2-3 psi at WOT.
The fuel system is two saddle tanks at 30 gallons each. They are plumbed to an IMCO super fuel valve (electrically actuated ball valve) for tank selection. The line then runs to an aftermarket fuel / water seperator, to the fuel pump inlet and then to the carb. The lines are 1/2" ss braided. The total length of the lines is about 12'
The rpm increases about 50 when I go down to a 19 pitch and decreases about 100 when I go up to a 23 pitch. I think that I'm running out of gas at wide open throttle.
Do I need to replace the mechanical fuel pump with a new / higher flow model or should I put an electric fuel pump on the transom. If I put the electric in, do I plump it to the mechanical or do I eliminate the mechanical and go straight to the carb?
Any help would be appreciated.
Check/replace the filter first. Pump may be on its way out. Tank vents OK? Anti-siphon check balls removed?
Beware there may additional filters in the carb, and you are reading pressure prior to these. Unless they have been removed, there were small bronze colored filters behind the inlet fittings. You should also be monitoring pressure at WOT, as this is the area that will give you performance problems related to fuel pressure/volume..........MP
Too small fuel pick-up lines in the tank?
Thanks for the suggestions guys. I'll take another look at the filters.
I'm still concerned that the problem is the mechanical pump on the motor having to suck through all of the valves and filters.
I've heard of nightmares with electric fuel pumps in boats going out. Is that still the case or are they doing better these days?
If I were to go with an electrical fuel pump on the transom, do I plumb it to the mechanical pump to pressure feed it or do I just eliminate the mechanical pump?
My other question is about the fuel pressure. I've heard that I should be running around 9 psi at idle and 4-5 psi at wide open throttle. Is that correct?
Thanks for the responses.
I'd aim for 7-8 max at idle. Above 8 will make some needle valves leak although some carbs will take up to 9 or so.
"If" your fuel system is of adequate size and your filters are not creating a restriction a "good" mechanical pump like the Clay Smith or equiv is plenty. Keep in mind a fuel circuit will only pass as much fuel as the smallest point in it. If you have a couple fittings somewhere that are not of adequate size you can run a fire hose everywhere else and it won't fix it. Your 2-3 psi at WOT is definitely a problem, about 5 should be absolute minimum.
As far as elec vs mech you can argue that one till the cows come home and never get a consensus. There are many good pumps out there of both styles that should be more than adequate "IF" your fuel system is adequate to flow the fuel.
Beware there may additional filters in the carb, and you are reading pressure prior to these. Unless they have been removed, there were small bronze colored filters behind the inlet fittings. You should also be monitoring pressure at WOT, as this is the area that will give you performance problems related to fuel pressure/volume..........MP
Excellent point and I've seen those filters totally plugged even though they "appear" clean. If you have good high volume filters elsewhere they should be removed for perf applications.
Posting Permissions