Self destructing yamaha

[AGENT 37]

I just can't seem to win. My latest endeavor has been a 1999 Yamaha T50 4-stroke. The motor wouldn't run when I got it. I couldn't turn the crank either. I pulled the cyl head off to check out the inside. The cyls looked good and the crank would turn when the head was removed. I found the head gasket blown between #2 and #3 and found the exhaust valve seized partially open in the #4. I put some penetrating lube in the head and let it sit for a few days and then broke it free with a light hammer tap. The valve train was then working fine. I replaced the head gasket and cracked water pump impeller (that's what caused all the trouble to begin with, it got overheated by looking at the paint). I put it back together and ran it on the hose for about an hour. Then I took it for a test run. I idled out of the harbor and throttled up to get on plane and in a few seconds, around 4500 RPM it "blew up" (cloud of oil smoke, oil out the carb into pan and draining out leaving a puddle in the cowl and a sheen on the water) it stalled and died. No knocking our other terrible noise.... So maybe the rings gave out after sitting for a few years or that valve un-assed itself. I'll take it back apart later and see. I didn't loose too much oil; after it cooled off the dipstick was reading full.

 

[drewmitch44]

Re: Self destructing yamaha

how about that.

 

[AGENT 37]

Re: Self destructing yamaha

The cyl head is off now and I see no damage or malfunctioning valves. I didn't do a compression test before I disassembled it because I didn't want to risk scoring the walls if there was debris. I am not set up to do a leak down test. Everything looks perfect; cyl walls, pistons, valves, gasket, no drop-out spots in the connecting rods (bearings and wrist pins are fine). I haven't taken the crank case apart to pull the pistons out and check the rings though. There are also no metal particles in the oil. I'm considering that I possibly overfilled the oil and it purged out the relief port and blew into the front of the carbs soaking the plugs. I was coming up out of the hole when it happened so the motor was on a sloped plane which could have aided in the blowout and with the vacuum of the high RPMs, oil could have been sucked into the intake quickly. I guess that I can put it back together and do a compression test now..... "teat on a boar hog"..... but if it comes up good. Blown rings or an obliterated valve sleeve should show on a compression test. What do you guys think?

 

[rodbolt]

Re: Self destructing yamaha

a compression test is a teat on a boar hog in your case.
teat on a boar hog is what us rednecks consider usless.
occasionally we see ring gaps line up which wont show up on a compresion test but will allow the excess crankcase presures to blow oil up the crankcase vent tube into the airbox.
how did you check any bearings or wristpins without removing the piston/rod assy?
myself, until you do proper predissasembly inspection,teardown and inspection of the head nd reciprocating assy with the correct measuring tools I think your chashing your tail.
this is a very compact high performnce engine that is deigned to run all day above 5000 RPM and will do it willingly IF all the subsystems are in correct shape. if not it will rapidly selfdestruct,again.
wonder how many nascar engines are asked to race after being freed up with mystery oil and a hammer ?
not trying to bust your chops,just trying to get you to see the absurdity of doing half the job and expecting 100% when done.

 

[AGENT 37]

Re: Self destructing yamaha

I haven't ever had the crank case disassembled on this motor. I have no reason to believe that the rods, bearings, or wrist pins are bad. There have been no symptoms of failure of these components. I simply pushed on each of the pistons throughout their respective range of travel as I changed the flywheel position to see if I could detect any play. Ring gaps lining up..... you're right. I just wonder if it was as simple as overfilling the oil and would like some comment on that matter. This has become a hobby for me. I did a complete rebuild of my F100 and finally got it perfect. But this T50 was free and I don't have much to put into it. I was pretty thrilled when I got it to just run with as little as I did. It just goes to show that they are that high performance; if just one component is malfunctioning they don't work. I put the cyl head back on and will reassemble and see if it runs. Thanks for the feedback.

 

[rodbolt]

Re: Self destructing yamaha

if the cylinder walls look ok and there is no reason to suspect anything else then have the head serviced, best to find a motorcycle shop or similar machine shop used to dealing with small heads.
yamaha sells the seat cutters and guide installers but does not reccomend a valve be refaced. if the valve is bad its a replacement item.
have the head dye checked for crcks. if all is well and the cyl and head surfaces are flat within specs install the head with a new gasket.
then before reasembling the valve train do a leakdown test to check the rings.

unless it was several quarts overful I cant see enough presure to puh oil up the crankcase vent tube into the airbox and thats about the only way to get oil into the airbox.
only other posible route is past an intake valve. if the intake valve was that stuck you would notice as that cylinder would be dead and that crb would be blowing gas and backfireing.

 

[AGENT 37]

Re: Self destructing yamaha

The valves all looked great and operated smoothly when I turned the CAM by hand and I had already put the head back on with a new gasket before you replied. The other thing that I thought pointed to an overfill as a possible culprit was the amount of oil in the breather tube when I took it off; it came pouring out. I don't know if I accidentally referenced the wrong capacity or if there was a misprint in the book on the oil capacity. The dipstick was wet all the way up beyond the full mark. There isn't a capacity labeled on the motor. I will watch the level as I fill it next time. I believe that it's supposed to take roughly two and a half qts. Is that right? If I have any more trouble I think that I'll get a replacement for the cyl head rather than have the current one overhauled. I can get one that has been completely serviced with a complete valve train for around $350. But before putting the cart ahead of the horse, I think I'll put the valve cover and carbs back on this thing and see what happens. After all, from the back of the boat it takes less than half an hour for the cyl head to come off from this motor to check things out. It is an easier motor to work on than the twin cams.

 

[AGENT 37]

Re: Self destructing yamaha

Runs perfect. I filled the oil 2/3 up the dipstick measure mark this time; approximately 2.0 quarts, 2.2 is capacity.