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. 😁😁😁

My 4-stroke had a similar problem:

If replacing a faulty fuel pump, remove the protective rupper caps from BOTH intake (done) and outflow nipples before attaching the fuel lines.


Don't ask.....

Doh!

If the hose/bulb isn’t at fault the low speed jets mentioned by others is a good possibility. If you have any Kroil on hand saturate the carbs. Turn the motor a few cranks then spray it again. Let it sit a day then run. Cured a badly gunked merc V6.

I am NOT an outboard motor guy, but see very good responses above:

dirt in idle circuit (maybe first try some Seafoam or other cleaner in fuel system before opening it)

if your fuel pump is vac operated (from crankcase?) and there is ANY leak in that area (bulb/hose?) or a check valve problem, the pump may not be getting enough vac at low rpms

pump may be wearing down enough to be weak at low rpm

best wishes

I had the soft bulb issue last summer and running problems with a 50hp two stroke, problems as yours. I’d never had real solid bulbs on motors of which I’ve had plenty.

Took the motor in to a really super guy who tested inside and out then grabbed a new hose and bulb from the wall. He chewed my ass for buying hoses and bulbs from fleet farm, Cabelas etc.
His was nearly 80.00, really good quality. That did it. Issues solved.
That thing once pumped up stays as hard as a rock.
I know the new hose/bulb sounds expensive but compared to the other things mentioned here it’s not.

Osky

I'd vac test the impulse line to the fuel pump. If that holds, I'd pop the bowl and check that pilot is clear....if all that (10 minutes time to do the above) passes, fuel line, bulb and any quick fittings get the peek.

Best bet is to buy a service manual and go from there… usually the first thing I do when buying a new or used boat motor.

This!!!

Yep, same for the fuel line connector at the motor. Had an off-brand that would act like it was clipped on and open to the engine...but it wasn't. The schitty materials or specs were off just enough to give inconsistent attachment. Replaced the cheap connector with an OEM and solved the problem.

2 strokes are incredibly simple. Pretty much if it has compression, spark and the seals are good.....it'll run.

2004 is pre ethanol right? Any of your internal fuel lines could be suspect, in addition to fuel lines outside the motor. A little fuel line hardening quickly leads to high rpm air leaks. Also those "lined" fuel lines can cause all sorts of problems and likely were required by the epa as a way to frustrate people into buying new fourstrokes....

Did you read the guys issue, or just see "boat motor problems" and just give your diagnosis?

Obviously neither of you have ever had to push/drag an 18 foot john boat loaded with decoys, guns and gear through 6” of water/wet grass. I like my 2-strokes for a few reasons and have no interest in motors that are heavier, more complicated and higher maintenance than what I’ve got.

#2 consensus- Check the ground wire

Check you fuel vent hose for muddobber stoppage. We have these little black bastids that will plug anything with a ⅛ to ⅜" hole in it.

What the he'll do you know about outboards, especially a Yamaha.

I'm gonna agree with CCCC about the idle air circuit being gummed or plugged. The fuel pump on that motor sucks from the tank with a single hose, there is no pressure in the primer bulb and it won't stay hard when running. There is a U-Tube video explaining the idle circuit, with a diagram of how the air in that circuit pushes fuel into the carb at low speed. The timing linkage needs to be set precisely, as well. I have a 2004 60 hp and a factory manual, which covers the 70 and 90 also.

😁😁😁

I replaced the fuel pump diaphragms twice in 2000 hours of running on my Pro-V 150 due to ethanol laced fuel pinholing the diaphragms. Same symptoms, bulb would go soft, hard starting and poor throttle response. Pretty inexpensive and easy to do.

Bad/weak spark plugs can also cause some funky issues.

That's where I'd start. Cheap. Easy.

Not. I've run oil slickers for 50 years with fewer (if somewhat repeatable and i know those!) problems up here in remote country, than my Yamaha f-40 has had in the last 9 ( exclusive of self-caused problems) And Yamahas are supposed to be the best 4 strokes going. I'm still learning about 4-strokes, too- mostly that they are heavy as hell and require shop service. Mostly. I'm still learning 4-stroke, admittedly.

I have such trust in my f-40 which has failed to get me to my remote cabin (700 miles OW, driving and boating) the last two times, that next year the reliable '96 2-stroke will be in the bottom of the boat in case I need it to get back up the main river to the landing. And I can work on that myself, even on a sandbar, without all this new electronic crap adding to the weight and unreliability. I'll just stash it in the brush before proceeding the 15 miles up tributary to my cabin. Don't need the 125 lb handicap in weight for that portion.

As mentioned in my OP, only ever run ethanol-free gas, engine starts like a champ, throttle response is fine, just dies at idle.

Spark plugs are brand new, fewer than 5 hours.

Anybody that thinks a carbureted two stroke is more reliable than a fuel injected four stroke is smoking some good stuff. They might be lighter and easier to work on but the trade off is that you do have to constantly work on them.

My post was in jest but the fact shouldn’t be lost that those claiming two strokes are so reliable are on a thread started about another broken down two stroke.

Everything else is arguable, but four strokes have brought a huge leap in reliability to outboards.

NEVER... I wrote the company years ago ...the new bottles have a no use in 2 stoke ...red one is OK...

Lol I been down this road before I got a four wheeler that will start and run like no tomorrow and then it won't start you'll check fuel you'll check here you'll check everything guess what you put a new plug in it and runs like a top until it does it again even went to my four wheeler and my outboard repair guys and they said yep too they say after a couple hours of pulling on a big old sled that you learn faster to just change the plug... the third time of 6 months that it happened to me I even have the NGK plug sitting on my desk at work to ship back to them and ask them why .......and no it ain't nothing else on my wheeler that's broke you can give it ethanol you can check everything change the plug boom it'll start .....put the other plug back in won't start put the new plug back in boom it'll start unbelievable but....

Maybe a slight air leakage somewhere? Check all your hose clamps and fittings. Or above suggestions. Could be at a higher rpm it is suckng enough gas to overcome.

I once had a similar (opposite) problem, running wide open down the middle of the river. The motor started missing and losing power. Turned out the rubber gas-line feed from the hose connector toward the carb had broken a plastic clamp (old and deteriorated), and was sucking air. Idled OK- enough to get me to shore and find the problem.

I had cleverly left my SS wire out of the tool box, but found a piece of white store string in the bottom of the boat. That got me to my cabin, where I did have some wire, replaced with a hose clamp when I got home.

]
This!!![/quote]

[quote=Crow hunter

Not.

I've had fewer problems (but lots of repeats - water pumps, fouled plugs mostly) with 50 years of oil-slicker 2 stroke use than I have had in 9 years of 4 stroke. Not counting self inflicted wounds... . 100+ pounds lighter, and I can work on them, even on a river bank, rather than the nearly mandatory shop service and 2 month wait for my 2005 Yamaha f40. Plus, it is readily removable from the boat if need be. A 260 lb 4 stroke is not. Oh, I coluld do it in the bush , if I had to.

That remote steering and shift on the 4 stroke Yamaha is admittedly nicer on neck, shoulder, arm than my 30hp 2-stroke '96 tiller Evinrude , which will be a passenger in the bottom of my boat on my next try for my remote cabin, 700 miles OW from here. It is reliable, and does as well with the prop as the 40 does with the jet (making it about a 30), said jet being handy during low water off the main river. The Yamaha F40 has failed me on the last two attempts, near the far end of the trip. 1400 miles RT for nothing pisses me off!. 30 gets brush cached at the mouth of the side river 15 miles downstream from the cabin in case I need motive power for the upstream portion either river.

I don't need that 130 lbs of extra weight (Motor, tanks, hoses, water pump, plugs) on the final segment to the cabin- I can (and have) floated/rowed back to the mouth. I really don't like the extra weight, but it would not have been "extra" on the last two attempts.

This guaranteeds the F'kin-40 will run like a top all the way!

I see people running 40 year old 2 strokes where I live all the time. I think you are sucking air through a connection or have some trash in the fuel system somewhere.

Had the same thing happen to one of mine. Hasbeen

Got to have fuel too!


Jerry

Idle fuel circuit in the carb is plugged as a couple others mentioned. Easy and cheap fix with some carb cleaner and a pin to get it cleaned out. Possibly even free if you don’t tear the carb gasket.

The fuel line connections are frequently a problem if it idles for a little while and then dies. Any little air leak from a stiff hose or a chewed up o-ring will cause the pump to suck air instead of fuel. A weak pump makes it worse.


Jerry

Are you running the fuel line through a fuel/water separator? If so, bypass it and see if there is any change. New fuel line and a bulb would be the best start after thst.

Lots of good suggestions (and a few that only rate participation awards) which support and expand on my own thoughts. Thanks to all. My plan, going forward, is 1. replace fuel line (it’s almost 20 years old anyway and owes me nothing.) 2. If that doesn’t do it get a fuel pump rebuild kit and replace diaphragms. Meanwhile, of course, check for any possible air leaks in system. I’ll report back .

2 strokes are pretty simple
I have never had to work on my 2 stroke outboard much only the 4 stroke
Is it carb or fuel injection

I'd bet dollars to bullets that you have crap in your carbs. One other thing could be your carbs could be out of sync. They'll run when you put the screws to them but won't when putzing around at the ramp.

I thought that I was the only person who wasn't an expert on two-stroke motors.

I had a similar problem on my 2000 Johnson 25hp and just adjusted a screw on the carburetor. It's a good idea to change the hose/bulb every so often too.

Problem has been solved and it was very simple.

I figured I'd start with a new fuel line, so off to the local Yamaha dealer, got a new fuel hose and went back to try it out. It didn't fit my tank. It had the wrong type of connector at the tank end.
Back to the dealer: "This hose doesn't work for me, wrong connector on the tank end."
Parts manager: "What kind of tanks do you have?"
"Yamaha tanks."
She took me around to look at all the hoses they had with connectors for Yamaha tanks: "Nope, nope, nope....come on out, I've got the old one in the truck."
Soon as she saw the old one, "Oh, those were recalled in 2014. Yamaha would provide replacements for your fuel pick up and give you a new fuel line as well."

We went back inside and she called Yamaha. They said give then the motor serial number and, even though long out of warranty, they would send replacement parts (they didn't have any in stock at the dealer. Only problem was, they were sending from Japan and it would currently be about 2 months. I don't want to wait two months. She went online and found that, regionally, there were three in stock, one in Savannah and two at a dealer in Columbia. Great, I have to go to Columbia Thursday anyway. She gave me the number of the dealer in Coumbia as well as the part number. I called, yep, they had them. Offered my CC number to hold for me until Thursday. "Just give me your name and I'll hold them for you."

Picked up the new fittings in Columbia yesterday. Refitted my tanks with them this morning, tried it out....runs like a champ. Will idle forever now.

Apparently, the old style, when they got a few years on them would get kind of aged and wouldn't seal well. That's why under throttle, there were no problems, the fuel pump was sucking hard enough to keep the connection tight enough that id didn't leak. Get down to an idle, and it would loosen up and start sucking air.

I like simple solutions....no carburetor, fuel pump, timing, spark plug, reed valve or other issues...just replace the old connectors with the new style.
Again, thanks all for thoughts and suggestions.

You should probably order another set and throw them in your possibles box for the next time.

Appreciate the follow up, people often over think the simplest of matters.
Fuel quality, lines and filters are the most common problem and the simplest of corrections.