90 HP Yamaha Two Stroke Trouble

[Novurania]

I have a 1996 90 HP Yamaha 2 stroke. I have had the engine for two years. Here is the problem. When the engine is cold, it appears to need to run under load (low speed) for about ten minutes before it can attain any rpm’s beyond 2K, almost as if it needed to warm up. If not warmed up, it just bogs at anything over 1.5K to 2K rpms. If you leave the throttle WOT while it is bogging, it usually clears in under a minute or so and then runs great up to 5K. In other words, once she clears, it is perfect. A little rough at idle even when clear but that is not a big issue.

So far, I have changed the plugs, the fuel water separator, emptied the carb bowls, changed, the fuel filter on the engine and run Startron through the engine for about two hours. No change. What is very frustrating is that it does not always happen but does bog when cold most of the time. I have not had the carbs rebuilt or replaced, the idle adjustment screw check or any of the electrical part of the engine.

So, before throwing a new fuel pump at it which might just be a waste, I was hoping someone might have some ideas. Any help is greatly appreciated. Thanks.

[mygrady1]

sounds like carbs.
Have you ever replaced the fuel pumps? If not, then that is part of your maint.
Post your model #

[Novurania]

I have not replaced the fuel pump. Looks really easy to do. I am also embarrassed to say that I not know the model number of the engine other than I know it is a two stroke, of 1996 vintage, oil injected, trim/tilt and does not feed Yamaha digital gauges but rather analogue gauges. It is a motor on a RIB that is a tender. Could a leak in the diaphram of the fuel pump cause this problem? If so, I can replace that. I am not able to rebuild the carbs. I would need a mechanic mechanic for that.

[robert graham]

Next time it's bogging try pumping the primer bulb, which as you may know is a small fuel pump....if it opens up the RPM's then maybe the fuel pump is bad....you can test your fuel pump by removing it from the motor by those 2-10MM hex head screws, but leave the fuel hoses hooked up...then pump the primer bulb pand check for fuel coming from the small port hole on the back of the pump....there should be none....if the pump is bad then just replace it...it's about $50 and they do wear out those diaphragms after time...the pump is operated by crankcase vacuum and pressure from that crankcase area...

[Novurania]

Thanks. I think I might just replace the fuel pump in any event. Never thought about the priming pump as a fuel pump. Funny thing. The only time I seem to have had the problem lately is when I pumped the priming ball before starting the motor. I never do that when the engine is warm and never seem to have a problem with bogging absent pumping the ball. Any connection?

[southpac]

I can only agree with replacing the fuel pump really. Maybe the ball too? A good check for any air leaks tank to engine would be good too. It definitely sounds like fuel flow to me. Have to say check water trap/filter if fitted before line reaches motor. If you don't have one then I would fit one.

[Novurania]

I just verified that pumping the ball does not stop the bogging. Does that rule out the fuel pump as a problem? All other fuel filters are new. Leaning back toward a carb issue.

[robert graham]

Easy enough to just replace that primer bulb, just to be sure....

[rodbolt17]

you guys are stupidly funny about the primer bulb.
most don't even realise why grannys 70 impala grocery getter DOES NOT have a primer bulb.
it aint the primer bulb.
only the cheap ask west maine and sierra shheeet primers fail in 4 months.

ALL that primer does is fill the carbs with fuel.
nothing more nothing less.

if you have a bad fuel pump then you can use the primer to totally fill a cyl with liquid gasoline.

if granny parked her chebby on the grill taillights up every night, grannys chebby would have had a primer too.

sorry Charlie, but once a 2 strokes fuel system is nasty.
the only way to fix it is the ENTIRE fuel system

[Novurania]

Maintenance aside, does this sound like it is a fuel pump issue? It may be that the mechanic can not get to this problem for a few weeks. In the interim, I was curious whether replacing the fuel pump might be the cure. I know it cannot hurt but will it help?

[throrope]

Maintenance aside, does this sound like it is a fuel pump issue? It may be that the mechanic can not get to this problem for a few weeks. In the interim, I was curious whether replacing the fuel pump might be the cure. I know it cannot hurt but will it help?

Been there done that got the teeshirt.

Rodbolt as always is on the money.

No free lunch and no interim solution. If the fuel system is mucked up, anything less than a full front to back job will have you back in the shop and not on the water. Fuel pump unlikely to be the problem. Topside tanks, filters, fuel hose, connectors and bulbs easily puchased online with free shipping and less costly than a failed trip. If your handy and willing to patiently spend time, the carbs aren't difficult and gaskets are available here.

A sure fire solution is Joe's marine service in Wanchese NC.

I empty all fuel between trips and haven't had issue since.

[Novurania]

Mechanic did a link and synch. He could not get it to bog. Since then she continues to run great once warmed up, after bogging for the first ten minutes, and then runs great until I reduce speed when docking and she coughs and stalls. So, I still need help. Here are the exact symptoms. Starts fine, idling trying to increase rpm's beyond 2k she bogs for the first ten minutes. Then, as if another cylinder is kicking in, she runs rough as if she is blowing out crap for ten seconds and runs perfect after that until I reduce speed and just put her in gear to dock and she stalls. Something is preventing her from working fine until she gets warmed up. Any further help would be greatly appreciated. Thanks.