Engine Surges at WOT

[waterbill]

My '94 90TLRS Yam 90HP just started surging at WOT. It runs fine at 46-4800 RPM. Recently, I pumped all the 2-month old ethanol-laden fuel out of the tank and replaced with fresh. That's when this started happening. Is this possibly a partially plugged screen at the tank pickup? If that's not the problem, what else should I check for? My Fuel/water separator is new, and the filter on the engine is clean. Do fuel pumps act like this when they are going bad?

 

[cousinabe]

Re: Engine Surges at WOT

is the primer ball collapsed during this condition? this would indicate a restriction from the ball back to the tank.

if it's not collapsed, what happens when you pump it during the condition?
when was the last time the fuel pump was changed?

 

[waterbill]

Re: Engine Surges at WOT

Can't get to bulb while driving boat. Also, original fuel pump.

 

[cousinabe]

Re: Engine Surges at WOT

find a mate to help you. check if the ball is collapsing under this condition.

what preventative maint has been done on this motor?
I change my fuel pumps every 3-4 years.

 

[waterbill]

Re: Engine Surges at WOT

Thanks for the tip, Cousinabe. It makes sense. I'll get a helper to drive. Actually, I have only done waterpumps, a new head gasket, gearcase oil changes, spark plugs and greasings to this 15 year old engine since I bought it five years ago. It probably has the original fuel pump. Almost bought a new 2-cycle 90 when I was told the powerhead had to come off in order to replace a leaky exhaust plate, but Rodbolt saved me a bundle by advising me to try to get it off-- and it came off without removing the powerhead. I appreciate the advice and the advisors on this forum.

 

[James R]

Re: Engine Surges at WOT

If you can run the motor from an auxilliary tank, a jug of gas, the same gas that you have in the tank, you can quickly eliminate your tank or gas as the culprit.
With the recent fuel crisis there seems to be a greater incidence of fuel problems. Could be due to more Ethanol in the gas or more likely water.
My advice would be to fit a watertrap filter, a big one.
By draining your tank you could have released any shelac coating from the gas that often forms on the sides of tanks. Have one tank that had to be pumped out, fuel fitered through nylon stockings, and fuel pumped back real fast to try to scour the tank. Did this several times and then fitted a big fuel/water trap filter. Since that time no problem with the motor faltering.
Used a chemical pump driven from my compressor to avoid fire or explosion.