Yamaha 15hp 2 stroke bogging

[bruceb]

Another note- a typical hose assembly has four connections- two at the ends and two at the fuel bulb, and most engines have at least two and some times four under the cowl, so really check them all!
B

[rbk]

Originally Posted by bruceb

Like RBK suggested, check for an AIR leak around any/all of the fuel connections. Current fuel connectors are very suspect after even one season. We used to get many years out of them but I am finding the seals cracked to be one of the most common problems these days, even when non alcohol fuel is used. The high speed cutout is the typical symptom.

B

I had issues last year with my river boat. The tank connection O ring had been rouged up a little in one spot which was enough to compromise the seal. wasn't cracked, broken or torn, just slightly mangled on one side.

[Billnpr]

Had an Evenrude that had a rough spot on the fuel line barb at the engine. Put on new connecter on the hose and that did not correct the problem. Finally took off the engine barb and ran the tank line to the fuel pump. Corrected the problem and still had the disconnect at the gas tank.

[Diesel Bill]

Last time I had this problem on an outboard the 90 deg. fuel line fitting going into the carb was partially plugged. Blew it out with an air nozzle through the inlet needle seat. The plug blew out so fast we never got a chance to I.D. what it was. The motor ran great afterwards

[Scott Berg]

As noted, I'm going with a stuck spark advance (sounds like you've ruled out fuel problems)

[PassePort III]

Chech the little square box on the side of the engine, it's the controller, it's called the ''ITD'' I think. It has something to do with the coils.
Also check your supposedly sealed connectors coming from that little box, they used some kind of seals but they are not as efficient as the shrink wrap we usually see.

Have fun.

Bob.

[camerondueck]

Originally Posted by undercutter

Does it rev up to WOT with no load?

Yes, that's exactly what it does

[PaulBck]

If it is old it may not have enough crankcase pressure to get the fuel/air mixture to the top.....or it sound like the CDI box is not advancing the ignition timing.

[TassieBloke]

Did it just start doing it one day, or has it gradually became worse?

Bunch of great suggestions above, particularly those that relate to sucking air in line fuel lines/gaskets etc.

Given how you've appeared to have cleaned the fuel system, and it 'bogs down', it seems the air flow is also ok. Before getting a CDI box, try doing a compression test to get a handle on what happening inside the combustion chamber. If those numbers are ok, then it can only really be the timing advance. Having said that, if it has gradually deteriorated, then it isn't really going to be the timing...

[FlyingScot]

You already mentioned the remote fuel tank vent, but are you sure it works? Tried running with the fuel cap off? Obviously be careful when doing this.

[camerondueck]

Originally Posted by FlyingScot

You already mentioned the remote fuel tank vent, but are you sure it works? Tried running with the fuel cap off? Obviously be careful when doing this.

yes I have.

[camerondueck]

Originally Posted by TassieBloke

Did it just start doing it one day, or has it gradually became worse?

Bunch of great suggestions above, particularly those that relate to sucking air in line fuel lines/gaskets etc.

Given how you've appeared to have cleaned the fuel system, and it 'bogs down', it seems the air flow is also ok. Before getting a CDI box, try doing a compression test to get a handle on what happening inside the combustion chamber. If those numbers are ok, then it can only really be the timing advance. Having said that, if it has gradually deteriorated, then it isn't really going to be the timing...

Wouldn't pumping the bulb at least temporarily fix this and give the engine a burst of speed? I have pumped it while it runs, no difference.

[Reefmagnet]

Have you checked the float bowl level in the carby is set to spec?

[camerondueck]

Originally Posted by Reefmagnet

Have you checked the float bowl level in the carby is set to spec?

I'm not sure there's much adjustment possible...is there?

[Reefmagnet]

Originally Posted by camerondueck

I'm not sure there's much adjustment possible...is there?

Dunno, don't own one but it there is a metal tang anywhere between the float and needle valve it will be adjustable.


Also, being a twin cylinder, have you tried starting and running the engine on a single cylinder by disconnecting alternate spark plug leads? One mightn't be running too well and a dead cylinder can go unnoticed until the engine is placed under load.