Pure Energy

Fuel lines are supplying procharged 502s via Aeromotive 1000 fuel pumps. I am experiencing loss of rpm. More specifically, the motors will reach 4500 rpm, but when I push the throttles to WOT, nothing happens. Fuel pressure does read 14 lbs though, under boost.

The fuel tanks were just replaced and before that the motors topped out at 5300 rpm. The tanks now have 3/8" pick up tubes, lines to the pump are 5/8". I think the old pick ups were 1/2". Could this be the cause of my problem? Are the motors not getting fuel? I know Aeromotive recommends 1/2" lines to the pump, but again, I am getting enough fuel pressure.

We are going nuts trying to find out what is wrong. The motors ran great before replacing the tanks. The tanks were the only thing done to the boat, everything else is the same.

Dennis Moore

Check out the vent hoses and make sure they are working ok.

Do you think that the pickups could be full of dirt? Sometimes new fuel tanks are dirty from the manufacturing process or get debris inside them when they get installed. Maybe your engine picked up some dirt from the tanks and the filters are plugged.

Could the pickups be to close to the bottom of the tank?
Dennis Moore

Pure Energy

I've checked and double checked all hoses, connections, pulled pick up tubes out, checked to make sure they are not too close to bottom of tanks, new filters, new fuel pumps, regulators, senders, rewired all connections with new wire and clean terminals, checked throttle linkage, etc, etc.

The marina is going to send me the pick ups from the old tanks to see if they are larger. If that is not the answer then next is to pull the carbs. Maybe some dust from grinding out the fiberglass has clogged them. It is strange that both motors are the same. Or maybe one is holding back the other, I don't know.

Thanks

mopower

3/8" line isn't very much when your making big horse power .Now add to this , anti siphon valves and it gets even worse. Drilled 90* fittings anywhere in the fuel system are also flow restrictive. If you must use a 90* fitting , use a formed 90* as opposed to a drilled 90*. It has a larger radius and is less restrictive. This is partiularly so AFTER the fuel pump , because liquid being pushed tend to go in a straight line and when it hits a drilled 90* tends to really back up. Whereas being sucked through a 90 it will tend to pull it in that direction. Since the trouble started with the new tanks ,I'd keep looking there. The only strange part is you still have 14 lb fuel pressure. Is this at the carb or some place prior to that. And is that at idle or WOT.
I'm sure you've thought of that by now but I just thought I'd throw it in "just in case"

homan

Iwas told by someone to go 1/2 of which I replied what does it matter if it's half or not if the pickup is 3/8"s. of which they explained it to me as even if pickup is 3/8 switch to 1/2 hose fitting because volume wise it's easier to suck fuel with only 12" of 3/8 and 6' of 1/2 than 7' of all 3/8.

Pure Energy

Fuel pressure is measured right at the output of the fuel pump.

mrv8outboard

there is a way to test this. make a tee fitting, in and out straight through your line size 1/2 inch, out the tee a reduced nipple for a vacuum hose 3/16 or 1/4 inch. use a vacuum gage that has a scale of 0-15 in.

now put this in the fuel line, you can start just before your pump and work backward , (but the only thing that is different is your pickups then you could start there.) so put it in line right at the pick up and run it. you should not be over 4" of vacuum. above that the fuel will boil in the line. to check for air in the fuel just use a piece or clear hose to test,on the output side of the test tee.

1/2" pick down to 3/8" pickup could be your problem.test it,know whats wrong and why,then fix it knowing it was fixed right the first time.

mopower

Ok, now is that a direct read guage or is it a remote guage that is read at the pump outlet which you can see while running WOT. That's what really counts...a reading at idle is almost useless
You engine is only sipping fuel at an idle. a bad pump , retricted or small line or clogged filter could all have good pressure then yet WOT when volume is needed the pressure can drop drastically.

Gary Anderson

If it's the same problem on 2 engines with 2 different tanks, and that's the ONLY change made, it's probably the pickup or tank vents. Go with the bigger pickup, it's a cheap and easy change.
One other question, where are your elictric pumps located with respect to the tank and pickup? Distance and height?
Gary

HPJunkie

Make sure the blower belts are tight! The same symtoms will occur. Do the motors blubber at that 4500 rpm? Check the plugs. I've there on the dyno. Greg