Yamaha Pro 50 problems.

[jakelogsdon]

I have been having a few issues with this motor lately and I am getting ready to sink it in the river! The other day while heading to the duck blind the motor toke a dump on me. I was running full speed when it slowly started losing power and came to a idle with the throttle still hammered. I thought it felt like a dropped cylinder so i checked the plugs. The top plug's gap was closed and looked like it had been that way for a little while. I fixed the gap and the motor ran fine. when i got it back to the house, just for curiosity i checked the compression, 130 psi all cylinders. This morning it ran fine all the way to the blind, when i killed the motor and tried to start it back up to go park it, it acted like the battery was dead. I knew this wasn't the case. Brand new battery! I took the hood off to try to pull start it and the flywheel was extremely hard to turn it would turn about a quarter to half a turn like it should then it would get harder to turn. I put the motor in gear and turned on the prop just cause it was easier to crank on. It seemed way too difficult to turn. I dicked around barley turning it over and over, tried to start it up and it fired right up! I know this has to be something internally in the motor but have no idea what it could be.. Please Help!

 

[99yam40]

Re: Yamaha Pro 50 problems.

may need to check to see if it is getting enough oil and make sure it is not running too lean of a fuel/ air ratio. lack of oil will cause bearing problems, and lean fuel will cause piston and cylinder scoring.

I have also seen lower units lock up or bind the motor so you may want to check that. The drive shaft runs directly from the power head into the lower unit

 

[qwackhead]

Re: Yamaha Pro 50 problems.

sounds like a gaulded piston when its cool it will turn over fine when it heats up it will start to sieze had a nissian that had good compresion would run great for about a mile or so then it would just start to slow down and die you could barley turn it over with the crank let it cool for a bit it would fire back up and run good till it got hot again.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I put brand new plugs in this motor two days ago and when i pulled them today the bottom one looked like it should, but the top two had little metal particles welded to them looked like light welding slag. still got good compression. I don't really know about the oiling, but the injection system has been removed on this motor, so i always mix the fuel 50:1. I am not going to try to run this motor any more until i figure this problem out. Thanks for the replies guys!

 

[99yam40]

Re: Yamaha Pro 50 problems.

sounds like you have some pistons melting from lean fuel is my guess

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

Should i tare into the motor? I have been very religious when mixing fuel. could 50:1 be too lean?

 

[99yam40]

Re: Yamaha Pro 50 problems.

I would think you need to pull the heads to see where the metal is coming from.
When I said running lean I meant fuel/air not oil. carb related
restricted jets/passages maybe
Wrong plugs maybe
wrong timing maybe

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I will post results when the head comes off. Thanks for the help, I really appreciate it.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I took the cylinder head off today, and the top piston looked a little beat up looked like it had something in it a one point. the cylinder walls looked fine. I dont think it could have been a broken piece of ring cause it had 130 psi compresion?????

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

If something was rattling around in the top cylinder which closed the plug gap and now you have found metal deposits on top and middle plugs, the crank bearing between those cylinders may be coming apart. Might explain the tight flywheel until you worked it around a little, now that the heads off, do you hear any death noises when you rotate the flywheel by hand? Should spin freely

The more likely senerio has already been mentioned, running lean and in your case, less lubrication due to dirty carbs, or maybe just running too hot because the impeller hasn't been changed in awhile.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

No noises when turning the flywheel, and it does spin freely. I'm going to tear it down tomorrow. I'll post pics of the results

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

Hold on.... I wouldn't tear into the motor just yet, I read through your previous posts on your motor and it seems like you had starter trouble a couple of weeks ago at which time you removed the top plug to get it started. Was the plug closed up then? Was it dropped and then replaced? Did you ground the plug lead while it was warming up? Lots of questions but we only know what you type. I agree with 99yam40 that you may be running lean and therefore hot due to dirty carbs, pre-ignition from over-heating will also cause damage to the top of the piston. For the cost of a head gasket and carb rebuild kits you might be out on the water this weekend, take your time on the carbs, they're known to be hard to keep clean.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I thought it was a weak starter until i tried to pull start it and almost broke the rope while attempting that. I considered that the carbs might need to be adjusted, but if that were the case i would assume that the motor wouldnt idle just right. But when it does run it runs good, no misses or anything. Plus that doesnt really explain where the small metal particles on the top two plugs is coming from???

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

Just trying to give you some options before opening the crankcase. If the motor over-heats just shy of siezing it will indeed be hard to turn over. If you're going to tear into it, start with the carbs, if you find them gunked up with the main jets partially clogged, I would try that first, 2-strokes produce more power running slightly lean, they just can't survive. I would next remove the exhaust cover to expose your pistons/rings, it will tell you a lot about the health of your motor. The bits of metal might be from your pistons, does the middle cylinder have any erosion on it's crown also?

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I really appreciate the feedback and the help! The picture i posted doesnt really show the damage to the top piston that well, but the damage that looks like pitting is actually from something that got in the cylinder and hammered around for a while (most likely when the spark plug got smashed down the first time). The other two cylinders look perfect. I will break the carbs down first tomorrow like you said, just to make sure it wasnt something easy and quick. I know It is really hard to go by what im typing, and i know there must be some small things i am leaving out, but from all the things that have happen with this motor in the past month I am leaning strongly towards some of the internals being screwed up!

 

[99yam40]

Re: Yamaha Pro 50 problems.

hopefully when they disconnected the oiling system they did it right, other wise it may cause major problems to the crank from what I have read.
It always worries me when someone changes the way a motor was designed to run

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

Sounds like you have a plan, while you have the carbs off for cleaning, shine a light in the intakes to verify that all your metal reed valves are intact. Remove the intakes next if it's hard to see all the reeds.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

Broke down the powerhead today. The top bearing was shot. The top piston was scored up a little, but the damage was isolated to the top only! I guess I got a little lucky.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

Ok. New dilemma! I was getting ready to order parts and ran into some discrepancies. The model number of the motor is P50tlrp or that is what i assumed. The stamped plate is really hard to read. If it is a p50tlrp it is a 1991 and the pistons have two rings, problem is, the pistons in my motor all have three rings??? The only three ringed pistons i can find on a pro 50 are from the 1988 and 1989 models? WTF? I am really starting to scratch my head about this motor.

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

Bummer on the bearing, amazing it still ran at speed sometimes. The parts websites I've looked at show 3 rings on the 88 and 2 on the 89, I have an 89 parts motor so I checked it and sure enough it has 2 rings. They also changed cdi's that year, if you post a picture of the cdi I can at least verify that it's 88 or older.