Yamaha 15hp 2 stroke bogging

[Cheechako]

I had that issue with a Yam 15, it was the main bearings. Above about 1/2 throttle it would get rough and not go faster rpm. Fortunately I was in Trini and a guy I met was an ex OB mechanic, he helped me do it in the boat yard and it took us about 1.5-2 hours after I got the parts.
I also had the prop hub come apart, but in that case it would rev up but not go up on plane.

[ColinIOM]

Hi, firstly I don't know this particular engine however, I am convinced this is a “lean” issue IE, its short of fuel at that point of revs.
If it was “rich” (to much fuel) then it would “four stroke” (keeps running but fires every other stroke hence its called “four stroking”) if it was timing, It would still run, just not as well as it should.
To prove its lean, you can try driving it into the bogged area, then operate the choke (or slightly block the intake with your hand) if it get better slightly that proves its lean.

So your issue in my opinion is almost certainly a shortage of fuel. Finding the cause is the difficult part. You need to backtrack, when did it start? If it has just creeped up on you? and if the engine has not been tampered uwith, it could just be wear in the carb spindle, (new carb) Have you changed anything? the jets? Are they OEM? or copies? anything, has someone else changed something? Is this an engine you just bought without knowing its history?
I don't think its a float bowl height issue because when the little needle wears it usually causes fuel leaking and flooding or enriching so unless someone has been tampering with it, I doubt thats the issue. I don't think its a read valve issue or it would be spitting back out the carb.
The other guys talking about carb, fuel line cleaning and fuel pump checks are all correct, I know you said you changed the pump diaphragm, is the vacuum hose in good condition with good connections, make sure the vacuum takeoff from the engine is clear so the pump sees the full vacuum as required.
Is there an air filter? Is there supposed to be one? Is it fitted correctly and in good condition?
I had an issue a long time ago with an inline paper filter that was effected by the two stroke oil on a racing engine, it actually caused a seize, the filter cost about £2, the damage was much more expensive however!
Try feeding the carb by gravity from a small completely different source and trying it. Try to separate the fault.
The idle screw is mainly for when its idling but it DOES have a continuing effect throughout the whole throttle range.
You should start by setting this at a basic setting and barring any better IE manufactures info, then screw it all the way in (gently) then back out 1.5 turns.
I hope this is of some help, good luck, please remember to post the outcome.
All the best
Colin

[P3sailor]

Originally Posted by camerondueck

the jets are clean, see light through the holes and i can spray carb cleaner through all the passages

Though your jets may appear to be unobstructed, they may only be permitting a small amount of fuel to pass through. Try cleaning the jets with a strand of copper or brass wire, you'd be surprised how much larger the jet appears when you're finished. Carb cleaners and similar products usually won't clean the build-up of crud that collects in the jet orifice.

[matari]

I had same problem for 2 1/2 years . There’s no mention in these posts of replacing the diaphragms in the carburetor fuel pump. I’m surprised Yamaha don’t mention these as a routine maintenance. They harden up and become brittle and will eventually crack and diminish fuel flow. . For $10 or less it’s an easy fix and motor runs like new. In my case bottom plug kept oiling up as well and gobs of oily fuel would appear in water. Cheers from Fiji

[Wldrup]

If all else fails check your exhaust path What goes in must come out and a plugged exhaust will limit rpm. I've had it happen twice and it's a simple and free check.

[Thumbs Up]

He did say that he replaced the diaphragms. You guys are all saying it is a lean issue. I say eroded jets/ passing to much gas (running rich). Brass jets get eroded. It's a common issue. The hole is too big We might have lost the OP. Try plugging the main jet a little with a small loop of thin copper wire. Or order a new jet.

[Cheechako]

Originally Posted by undercutter

Well if it isn't a fuel problem then maybe it is a load problem. Bottom end or even a small line wrapped around the shaft?


If it will rev out in neutral how can it be an ignition problem?

Sounding more and more like my earlier post: Main bearings. That OB mechanic took about 5 mins to diagnose it.

[a64pilot]

Originally Posted by Cheechako

Sounding more and more like my earlier post: Main bearings. That OB mechanic took about 5 mins to diagnose it.

Did yours make a gravelly, rattling type of sound?

[Cheechako]

Originally Posted by a64pilot

Did yours make a gravelly, rattling type of sound?

It's been a while, but in gear it shook a bit like it was going into some sort of harmonic balance issue if you tried to rev it up. Ran great at lower rpm though. No load it would rev up but vibrate a bit more than usual.
Mine was a 90's engine and the spec'd mix was 100:1, I've often wondered if the bearing failure was because they spec'd that so lean.

[a64pilot]

I understand why they spec lean mixes and why oil injection is used, but I still prefer pre-mix at 50 to 1, and will run the first few hours at 25 to 1.
But I believe my two stroke days are most likely behind me.

I’m also a fan of the better synthetics especially if run rich as they don’t seem to foul plugs, in fact I’m not so sure the good synthetics even burn.
They seem to have a whole lot less carbon build up.

Ever run Castor (bean) oil? Boy that stuff was a mess.

[Wldrup]

Did you check for plugged / obstructed exhaust. I know there's no muffler but maybe something growing, bent or collapsed in the exhaust path. If it can't exhale it can't inhale and it shows as a loss of upper end rpms.

[donradcliffe]

I'm chasing a similar problem with a 4 cylinder Yamaha 100. It bogs and at least one cylinder misses for a while when you try to plane, but eventually gets enough speed up and then the engine runs fine. The spark plugs all look clean and dry.

I'm in the high voltage breakdown camp myself--as you apply more throttle at lower speeds the voltage across the spark gap goes up and marginal coils/high tension lines break down. I have bought an adjustable ignition spark tester, and will be looking for at least a 1/2 inch spark on each cylinder the next time I take it out.

[a64pilot]

Originally Posted by donradcliffe

I'm chasing a similar problem with a 4 cylinder Yamaha 100. It bogs and at least one cylinder misses for a while when you try to plane, but eventually gets enough speed up and then the engine runs fine. The spark plugs all look clean and dry.

I'm in the high voltage breakdown camp myself--as you apply more throttle at lower speeds the voltage across the spark gap goes up and marginal coils/high tension lines break down. I have bought an adjustable ignition spark tester, and will be looking for at least a 1/2 inch spark on each cylinder the next time I take it out.

Also as you apply more throttle the pressures in the cylinder go up quite a lot and this high pressure atmosphere, it’s much more difficult for a spark to cross the gap on a spark plug.
One way to help diagnose a weak spark is to temporarily reduce the plug gap and if it works, then your onto something.

Although it’s very unusual for a four cylinder motor to have individual coils, usually two cylinders will share a coil, a four stroke fires on exhaust and it’s called “lost spark” ignition, but a two stroke of course fires every time the piston is close to TDC.

So if it’s only one cylinder, it might not be an ignition issue, unless of course you have one with four coils

[Jimmyhenry]

Last time we had this problem it was a blockage in the fuel tap at the tank end. We took it to bits, cleaned it and problem solved.

Pete

[vk4hr]

did we find a solution?