Yamaha 2 stroke issue

[hauntedbywaters]

Hello - new poster (and, of course, a problem).

2000 Yamaha 130 2 stroke
Every once in awhile (3x this season), tearing down the lake at 2/3 to full throttle, the rpm's will suddenly drop to 3-4k. Slowing down, then shifting to neutral, then re-powering up to full throttle (1-3x) and then engine runs fine. Outside of these intermittent issues, the motor starts, idles, *****s and runs great.

Brought into mechanic: checked filters (no sediment), coil, plugs (replaced), gas in tank (one quart out and let sit over night for separation - none), extensive testing on dyno, compression was right on 120# on all 4 cylinders, ran full bore on lake test for 40 min. and as was said to me - 'we checked everything that we could think of ...'
No issues. (Of course, no anomalies of any kind reared their heads during all of these diagnostic tests. )

Any thoughts, ideas on what I could check/try next?
Thanks ....

[Jason2tpa]

90% of the time an engine issue is related to the fuel system. What did the mechanic do when addresing this system? The only fuel system components you mentioned checking were filters and water in the tank. What about the carbs?

[rodbolt17]

fuel pumps last forever.

[Captn Dave]

fuel pumps last forever.

Sometimes the easiest is overlooked. The lift pumps on my brothers OX 66 250 didn't last 3 years.

[hauntedbywaters]

Thanks for the responses,

Jason2tpa:
Yes, they did check the carbs - sorry I did not mention that.

Rodbolt17:
Could you explain for the cheap seats? - thanks.

It is so hard to diagnose something when one cannot duplicate
the problem. I spoke with the service manager (whom I know fairly well) and the mechanic - both were as frustrated (well, almost) as I.

[Kaysea99]

I once was asked to work on a similar bogging issue on a Carbed 200 HP Yamaha after the other mechanic took a shot at rebuilding all 3 carbs.
I disassembled the carbs to check his work, found tiny bits of deteriorating fuel line in each. Replaced al the engine mounted fuel lines. Thinking we were good to go I took it on another test run, still boggin at 3500. I pulled the air box and sprayed fuel (or WD40) into each Carb throat to prove it as a high speed fuel starvation issue also ran it on a separate 6 gal tank with 3/8 hose.
Finally I replaced the fuel line from the pumps to the carbs with a clear vinyl hose. Bingo!! at 3500RPM I could see a tiny stream of air bubbles flowing from the clear plastic fuel strainer cup into the fuel line. After close examination of the cup I saw someone had used a pair of pliers to unscrew the cup and cracked it. It didn't leak until the fuel pump suction was enough which apparently was at 3500rpm.
Never assume, a clear test fuel line can show a lot.

[hauntedbywaters]

Kaysea99:
Interesting - but, would this behavior not show up much of the time, then?
My motor runs great at all rpms, until these 'intermittent' episodes.

[boscoe99]

Kaysea99:
Interesting - but, would this behavior not show up much of the time, then?
My motor runs great at all rpms, until these 'intermittent' episodes.

A crack or a pinhole in a fuel line or fuel filter cup is a dynamic thing. With vibration from the motor the crack or hole can open up or close. A hole might not let air in during times of low demand for gasoline but then open up at times of high demand. Say when the motor is running at high RPM under a load.

As noted, at the point where the motor starts to bog down have someone mist a bit of gasoline into the intake system. If it picks up and runs you know it is a fuel supply issue.

Rodbolts comment is cynical in saying that fuel pumps live forever. They don't. They are a known failure issue. He is saying these should be checked.

Some tools can be used to help you trouble shoot. As noted, a clear fuel hose can be plumbed into the system to check for air bubbles/signs of a leak. A fuel pressure gauge on the output of the pumps will tell you if pressure is being maintained at all RPM's.

A 3/6 gallon tank plumbed directly in front of the fuel pumps can isolate the rest of the fuel system. If it runs good on the portable tank, check upstream all the way to the vent in the boat fuel tank. If it does not run well on the portable tank then check downstream. Could be the pumps or hoses to and from.

Some will even plumb a portable tank directly to the inlet of the carburetors. The tank will need to be held above the top carburetor so that fuel can flow by gravity to the carburetors. That or use a primer ball in the fuel line from the portable tank to the carburetors to make sure the carburetors are supplied with gasoline at all times. If the motor still bogs down it is in the carburetors. Or downstream from there.

[hauntedbywaters]

Thanks boscoe99.
This will give me some things to verify with the mechanic.

[rodbolt17]

that is the problem with irregular/intermittent issues.
if you test it when it is running correctly I can tell you what the results will be.
could be fuel could be a loose connection.

[throrope]

I'm with bosco99. Your description is enough for me to suspect a hose leak that pulls air into the fuel line. May get enough air to starve the top carb and the subsequent hose flexure corrects the issue until next time.

If you're not sure how old your lines are, I suggest replacing all from tank to outboard connection even if that's not your issue. Much cheaper than a failed trip and no matter what the label says, our ethanol fuels eventually eat them.

I replaced all with 3/8" ACR soft copper tubing for less than the cost of fuel line, except for short lengths at the under console tank and outboard. I used flare to barb fittings and flare to threaded fittings at the water separating filter. Now in ten minutes for ten bucks I can renew the short lengths every other year.

Good luck and keep us posted.

[hauntedbywaters]

I think replacing the fuel line sounds like a plan. The lines are at least 5 yrs. old and I need to start somewhere ....

[hauntedbywaters]

Yeah, i go out of my way to use ethanol-free. Must be the marine environment, because my harley does not seem to mind as long as I use 91+ octane.

[hauntedbywaters]

Decided to continue using (short trips) - so far, working great - until the issue rears its' head one more time (which will likely be any time). At that point, will bring in to the mechanic and share what you guys have told me and tell him to 'go for it'. Will cost me, but, needs to be done, I like the motor and still cheaper than a new one.

Thanks to all for the help and ideas !!!

[hauntedbywaters]

Well, that did not last long (not surprised, though). Yesterday worked fine, today worked fine for several hours; then, bogged down above 4k rpms (Ran fine under that).
But, told myself 1 more time, so, made a service appointment for Monday. Starting with carbs and maybe fuel line. Also, checking connections.

Guy also checked it today for 30 min. - checked gas, filter, ran it with earmuffs, disconnected line from engine and pumped bulb watching for leaks, disconnected one plug at a time while running and did a quick visual inspection.

Should I replace fuel pump as well or can that be 'checked' for proper function?
Anthing else I should/could run by him?

Thanks ...