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It’s my first time working on an outboard motor with a fuel pump.The first question is,can a bad pump work very well at low speeds,but as speed approaches 1/3 -1/2 throttle cause the motor to repeatedly slow and then speed up?I’ve checked every other possibility twice and think the pump is the culprit,but wanted to check here before I bought a pump.Second question is what is a good aftermarket fuel pump?The motor is a Johnson QD24.
I believe your problems could be fuel pump related, but the same thing will happen
if the fuel filter is partially plugged. Of course, if the inlet to the fuel pump is sucking
air, that can be an issue as well.
Not sure what’s the best replacement fuel pump, but there should be plenty
of information if you do a search on this site.Prepare to be boarded!
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I’ve found quite a few of the original pumps used on the early sixties 5.5 and 10 horse motors are quite durable and still work well. But like anything else, they can get tired with age and will need replacing sooner or later. Rebuilding them isn’t an option anymore as I believe the rebuild kits are unavailable now.
Aftermarket square pumps as used on the later 6 and 9.9 horse motors are readily available and will work just fine on your ten horse. Search for Sierra 18-7350 and you should gets lots of results from Ebay, Amazon etc. One advantage of the square pumps is the inlets and outlets can be clocked in amy position to suit your needs. Doing it requires the instruction sheet so everything goes back together orrectly.
When the square pumps get old, the diaphragm inside gets really hard and brittle. Symptons are similar to yours, the motor will keep stalling at low speeds due to lack of fuel but might run just fine at high speeds.
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One way to verify a fuel pump diagnosis is to pump the primer bulb when the faltering condition starts to happen. If the engine picks up and smooths out, the pump is a very strong possibility. If this temporarily solves the problem, I’d just go ahead and try a new fuel pump. They don’t cost a lot. However, it can also be from air leaking into the fuel line, anywhere from the pick up in the tank, to the filter on the engine. You would likely see a fuel LEAK, too, if you pumped the primer ball and the air leak was anywhere beyond the primer bulb. An air leak before the bulb can be a little trickier to find.
Long live American manufacturing!
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What these guys are saying about the fuel pump, air leaks, squeezing the primer bulb when it misbehaves, etc. are all things to check first. But I’ll throw this out there only because of the symptom you describe as repeatedly slowing down and speeding up. There are a couple variations of that symptom but one of the causes can be electrical.
Some guy sent me a video of what his ’63 Evinrude 10 HP was doing and when I posted it here a while back a couple guys immediately said, “that looks like an electrical problem.”
So I popped the flywheel and saw a black mark on the armature plate where there was some arcing going on. So to avoid any hassle I just replaced the coil and plug wire and the motor ran great after that.
Not saying this is your problem, just passing it along as an outside chance that your slowing down and speeding up is an electrical issue somewhere.
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