Trouble with F75

[Mid-Lake Rock]

I have a 2004 Yamaha F75 four stroke, that is carbed. Motor has run like a top and I take good care of her. Used it through the end of last November, and winterized it the first weekend of December. I took it out two weeks ago and it ran fine for half the day. Later, while at WOT it sounded like it was misfiring, then it started to speed up, slow down, speed up, slow down with the throttle in a fixed position. I figured I might have a fouled plug and replaced all four plugs with OEM plugs. Took it out on Sunday and it was still fussy. Sounded like it was misfiring at times and would hesitate, speed up, hesitate, speed up, etc. At other times it ran fine. It seemed to settle down and when I went to WOT and it was running about 5 mph slower than normal. When I checked the primer bulb it was solid and when I squeezed it it did not change anything. It starts fine, idles fine, runs well at lower speeds/RPMs. My only problem is when I start to get up on plane or am running at higher RPMs.

I have changed plugs, it has fresh gas, I've checked the fuel lines. Anyone have a guess? I read a prior post and CJAC seemed to have the same problem and it was a clogged injector.

My guesses are:

1. Clogged injector/carbs need cleaning.

2. One cylinder not getting spark.

3. Fuel pump (however, when I prime the bulb it makes no difference).

I'm just wondering if someone has a really good guess of what the problem may be? I'd like to give it a shot before taking it to a marina. My other question, deals with the carbs. My brother and I have cleaned carbs on all types of small motors and an older 2 stroke outboard. However, I've never cleaned carbs on a four stroke. Am I safe to clean these and put back? Is synching difficult?

Thanks in advance. I always get good advice on this forum.

[marine_man]

Sounds like a plugged high speed jet to me, especially given the proximity to winterizing.

When you winterized did you run any fuel stabilizer through the motor?

Doing carb's on a 4 stroke isn't too bad, especially if you take your time and bottom out the jets fully and reset them exactly the same when you're done.

marine_man

[Mid-Lake Rock]

MM, thanks for the response. Yes, I did run stabilizer through the engine prior to winterizing. I'm going to tackle the project later this week or over the weekend. I'll report back. One more question, do you think running a shock treatment would be helpful or is this a situation where the jet needs to be cleaned with carb cleaner and an air compressor? Again, thanks for the advice.

[boatfixer]

I agree with MM

For the best results, taking them apart and cleaning them is best. Shock treatments are more for cleaning carbon out of the cylinders.