Home Forums Hunting & Shooting Hunter's Campfire Two-stroke outboard experts....

My 2004 Yamaha 90 hsp 2-stroke has an issue. Runs fine from at low/moderate throttle to wide open, but doesn't want to idle or run really slow. If I run it down really slow, just above idle, or try to idle it, it will quit. Fuel prime bulb will have gone soft. If I pump up the fuel bulb and hit the starter, it will start right up again and go. I'm pretty certain I have a fuel problem and I have a couple of ideas as to what it might be, but I'd rather hear from those who know a lot more about it than I do before steering the conversation in any particular direction. For the record, I only run ethanol-free fuel and the fuel filter is clean and clear. Also, it happens with either of my tanks (which are pricey, Yamaha OEM tanks, not $35 Walmart jobs.) Thoughts?

I had a small 4-stroke years ago that exhibited the same type of issue. Turned out the fuel line had a 'lining' inside the hose that started to separate and would starve the engine for fuel. Just an odd-ball thing that might be worth checking.

You might have some debris from deteriorating fuel lines plugging the low speed or idle circuit. A good carb service and fuel line replacement and fresh gas is where I would start.

Yes…that can happen. First thing I’d check is the fuel tank ventilation.

Some tank filler caps need to be unscrewed a bit for proper ventilation

If the fuel bulb is going soft in a short amount of time you probably have a bad check valve in the bulb and or the fuel pump on the engine is getting weak.

Keep in mind that a lot of those fuel pumps work off of the fluctuation of pressure in the crankcase, make sure you check the hose from the crankcase to the fuel pump for leaks (If it has one)

This was my first thought. (I've taken off before without opening the tank vent.) I usually unscrew the filler cap just a little, just to be sure.



I was wondering if I had a diaphragm going bad in the fuel pump and maybe sucking hard enough to compensate when under some throttle, but not at idle or low speed. Hadn't thought about the tube.

All good thoughts...Kind of confirmed my intended direction of inquiry. Thanks all.

Same last year w my Yamaha 90 two stroke. Turned out it was my anti siphon valve from fuel tank. Although outboards don’t require them, it seems all built in fuel tanks have the from tank mfgs.

I have watched so much TarylFixesAll I should know this.

put the 2 smoker ob in the back of your truck back up to the dump bin and deposit. Buy a 4 stroke. Sorry, that's all i got

Here is your correct answer.

It runs fine at WOT so the fuel supply isn't an issue. More fuel is used at WOT than idle so that's out of the equation. I'm assuming some trash in the bowls or plugged idle jets. They are much smaller than the main jets and more susceptible to getting partially plugged.

Drop the bowls and check for sludge or water.

Probably injected?

2004 90 HP 2 stroke is carbureted.

Had a nissan outboard give me trouble turned out the motor ran perfect when I installed new hose/bulb.

I doubt it… a bad check valve in the bulb wouldn’t allow the bulb to ever get hard… Is there a way to check the ignition module???

Not necesarily true. Fuel pump on that motor is a diaphragm mechanical pump which gets it's signal from crankcase pressure. When compression gets low in the motor or diaphragm's get weak then the fuel pump can keep up when the motor is wound up but not when it is at low rpms.

That motor don't have an ignition module.

Just put some Sea-Foam in it, that seems to be the consensus from all the Fire marine experts who don't know sheit. 😁😁😁