Yamaha Pro 50 problems.

[jakelogsdon]

Re: Yamaha Pro 50 problems.

Well, dilemma solved. It seems that the motor has been rebuilt before and the cylinders were bored .050. Luckily you could still barely make out the .050 stamped in the piston itself. Also verified it is a 1991 motor by the serial number stamped into the power head. So I am spit balling here. but i'm guessing the o/s Yamaha .050 pistons have three rings. Did you see the picture of the bearing i posted? It was missing three balls!

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

Just saw the photos you uploaded, I just had a couple red x's before so I had to mess around with my computer. It's amazing that the debris didn't do more damage, and with that piston I'm amazed that you still had 130psi. Not sure what your rebuild plan is but I'm wondering if the shop that did the first rebuild used o/s pistons from a previous year, just something to keep in mind if you're only replacing one. If you phone in the order maybe you can ask them to check on the number of rings.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I ordered from boats.net today. I do not know if the o/s piston is three ringed. However it was the only .050 piston i could find, to go with that year model, I really didnt want to buy pistons for a 1988 model . Wiseco only went up to .040! Do you think it would hurt to put a two ring piston in the top with the other two being three ringed??? I talked to a local shop and they didnt seem to think it would hurt.

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

Yamaha offers an o/s .50mm piston which is roughly the same as a .020in wiseco, best to stay with Yamaha. I'm guessing that piston weight regardless of size or manufacture has to be exactly the same as long as the crankshaft counterweights were not changed to match a new 2 ring piston design in 89. Just checked boats.net and they have the same crankshaft part no. for 88 and 91 so hopefully you can just clean it thoroughly, put a nice hone on it and go hunting. Still need to really clean those carbs during this downtime, since you're mixing, if the carbs are dirty and not delivering the right amount of fuel, they are also not delivering the right amount of oil.

 

[99yam40]

Re: Yamaha Pro 50 problems.

but the top two had little metal particles welded to them looked like light welding slag.

Did you find out where the metal on the middle cylinder spark plug was coming from?
There are seals between the different cylinder crank areas, so I would not think it could get there from the top bearing

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

That's a great question, given the amount of schrapnel and metal debris that has been flying around in the top crankcase, the seal may have been damaged, but I agree with your theory of running lean or hot, even a small amount of scuffing on the middle piston could leave deposits on the plug. A close up photo of the plugs would help.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

Thanks for all the help guys. I am definitely going to go through the carbs. when we pulled the cylinder head off the middle and bottom cylinder had good oil standing in the bottom of the cylinders, but the top cylinder was a little dry. As far as the metal goes, I have no clue?? Everything seemed to be in good shape in the other two cylinders. There wasn't much metal on either plugs but the top plug was worse. Could it have made it through the exhaust or intake somehow? I have no explanation for that at all.
I will try to get some close up pics of the plugs tomorrow.

 

[99yam40]

Re: Yamaha Pro 50 problems.

The exhaust maybe but I would not think it could through the intake as all those are separate. What did the middle piston look like when you pulled it?

Next question is, does the top bearing get lub from the fuel mix ?
if so maybe the seal went 1st and that caused the top bearing to not get the lub it needed.

I guess the top seal going out along with the bearing would cause the top cylinder to suck air in and lean out that cylinder or the carb has some plugging, one or the other.

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I am still waiting on parts to get here, but I have been debating on what to do if the piston I ordered has two rings on it. I guess I should replace the other two pistons as well. I am thinking that the individuals who worked on this motor previously put 1988 or earlier pistons in it!

 

[99yam40]

Re: Yamaha Pro 50 problems.

I have heard of the more rings the better the heat transfer from the piston to the cylinder walls for dissipating heat to the water jackets, but I have no idea if that is a true statement.

Are you replacing the seals between the different crankcases?
Or how did you check them to make sure they are good?

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I don't think there are actual seals between crankcases..

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

Here we go....

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

here we go what? I guess I just don't know what seals you are talking about?

 

[99yam40]

Re: Yamaha Pro 50 problems.

Are there different carbs for each cylinder?
How do you think they can work properly if they do not feed the individual cylinders?
The older units that ran 1 carb for 2 cylinders worked that way but not the ones with individual carbs for each cylinder.
The more I think about it even those had to have seals as each cylinder crankcase has to be sealed from the other to make the motor to work properly

 

[mphelle8vld]

Re: Yamaha Pro 50 problems.

There we go....

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

Yes there are three carbs, yes i understand how the oiling process works, and no I have no clue where the seals could be nor have I seen any on a motor like this. Could you please tell me where they should be?? Perhaps point them out on a parts list or schematic! I have been nice and thankful up to this point, but you need to gently remove your tampon, or have a snickers..

 

[99yam40]

Re: Yamaha Pro 50 problems.

there are bearings on the crank between the cylinders and the labyrinth seals are built into them or with them when the cranks are put together and are not shown separately. Only sold as a complete crank assembly, so do not show up on parts just like those bearings.
I did not say anthing about the oiling, but you are right, fuel and oil are together, just that each cylinder has its own crankcase . Not a common one with no seal between, otherwise the fuel from each individual carb would not hit the individual cylinder and 2 strokes do not function that way.

I guess there in no way to really inspect or test that I can think of, so I should not have even brought it up about inspecting. But the metal on the middle plug is still a mystery that need to be looked into.

maybe make sure all of those bearings are all OK as there was a lot of stuff floating around in the top crankcase and could have got to the bearing and seal between.

Not trying to push any buttons. I thought that the chocolate chip cookies and cold beer I had was as good as a snickers but apparently I was wrong

 

[jakelogsdon]

Re: Yamaha Pro 50 problems.

I inspected the crankshaft and all connecting parts the best that i could. However if one of the inner bearings or circ seats did get jacked up, I would have to replace the whole assembly. right around $900, I would not waste my time on this motor if that were the case. I am already not happy about having to replace the parts that i am. Especially since it appears to have been rebuilt in the not too distant past. I do have a lot of faith in these motors, but only if they are serviced and maintained correctly. I don't think that either of the two were the case with this particular motor.