melted PTO piston.. let's find out why!

C

corcan1

New member
Joined
Dec 27, 2009
Messages
39
Age
54
Location
Sudbury, Ontario
well.
i bought these 2 machines a few months ago. made 1 out of the 2 and also bought a 2nd engine off my nephew. theres a mix of 89 and 92 exciters here but both engines are out of a 89.

drove the sled, ran ok.. few problems. new ski skins.. adjust track.. nothing major till last saturday. burnt about 1 tank of fuel. melted the PTO piston on the exhaust port. badly.....

i'll post pics (theres lots!) then post more to the story.

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Sorry to hear. That looks nasty.I have an extra jug and head if yours are no good-89. Actually might have a used piston as well.
 
ahh thanks for the offer. but i do have a spare engine that i'm thinking of swapping in.... but would like to know why this happened first.

think it's just running waaaaay to lean and is a carb issue?
or perhaps a crank seal problem?
 
more info.

plugs that were in the motor were BR9ES. and it was a little on the warm side the day this happened. -5C or there abouts. plugs came with the motor, didn't bother to put new ones... probably should have.

just came off a quick rip on a plowed ice drag strip on the lake, had her goin pretty fast when it did this. was driving around for about 1hr, around lake and in the bays... on the shore and where the beach was just takin a few little jumps..

sled is setup to run on mixed gas. oil injection system was removed. also running a little on the heavy side on oil in the gas, probably 45:1

stock exhaust system and breather. nothing aftermarket.
both cyl's had an even 120psi.
 
unknow how many miles are on this engine. probably alot.
when i got the piston out i noticed 1/4 of the top ring is gone. probably all the marks onthe head and top of the piston... rest must have got blown out the exhaust.

crank looks good. everyhing turns super nice. connecting rod has no play... piston needle bearing is nice too.

only damage is the top of piston and the very front of the piston .. very top portion.

compression test showed the damaged cyl as 30psi.. would not think that high....
 
Could be a couple of isssues. It could be that the seal on the pto side went, When/if you put it back together and want to verify spray carb cleaner behing the clutch towards the seal and ifthe motor changes tune you will know that the seal is bad. Also too this sled is 20+ yrs old.
It could also be that there is dirt in the carb that the piston went. Also check the intake boot for cracks. also double check the jetting in both carbs to make sure that they are the same. Good luck with your findings.
 
ok, will be putting my other pistons/cyl's and head off my spare engine and buying new top end gaskets on Friday.

will be cleaning the carbs and checking the size of the jets.
what size should the stock jet's be?
 
decided to buy a complete gasket set. crank seals too. will be removing the engine from the sled and stripping her down completely.

will inspect the carb boots for any cracks.
carbs will be cleaned and will check to make sure the correct jets are in BOTH carbs. needle clip positions are the same.

wanted to do a complete overhaul this summer, just too bad the engine couldn't wait.
 
Did the ring turn? Is the end of the ring still at the locating pin?
 
i'll snap a pic of the piston tomorrow. i have it taken out of the engine already.
the ring did not pass the pin locator on the piston, the piece that broke allowed the 2 halfs of the ring to move towards the broken side of the piston, will post a pic tomorrow and show what i mean.
 
finished stripping the engie down today in my brothers shed.. ya.... no nice garage to work in but it has a small box stove which gives great heat!

i did not find anything wrong. crank seals had some miles on them but i don't believe they were leaking. crank looked real good, all bearings are in good shape. connecting rods are nice too.
looked over the cyl's off the spare engine, carb boots are not cracked.

heres some pic's of the work i did today.
engine is now sitting in my garage (no heat...) and i will try and put it in the sled tomorrow.

here she is, all dirty. head pistons and cyl's are off.
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stripped all apart.
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cleaned up and putting new PTO seal on.
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almost done. just the head left.
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all done. very easy motor to work on. well designed. had no problems, all bolts came out easily.
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also cleaned both carbs while i was at it. all jets were clean but blew everything out just to make sure.
both had 320 for mains.
will check the clip position on the needle tomorrow.

so, my best guess is the engine had some sort of ring failure...
never encountered this before in my life.

if anyone else has some idea's please post them up.
hate to have anything happen to this motor. LOL!!
 
this was not ring failure, that thing got hot as the sun! lol it either leaned out, wich you pretty much eliminated or it was overheating, the exhaust side of the piston is the hottest, and therefore burns first, if it was a ring problem, there really wouldnt be much melted metal in that pattern, how long did you have the throttle at the grip before she blew? if i had to guess it blew up at full throttle or close, i would think maybe you got too small jets, water in the gas (ice), plugged tank vent, broken pick up tubes in the tank, you might have nailed it with the seal, they always look good, but seldom are, also make sure is cooling correctly
 
i will check the carbs that came with my spare motor, see what jets are in it as i'm using the ones that came with the engine i bought (the one that blew.....)

i had it close to the bar for 20 seconds then let off to half throttle, about 5 seconds later the problem started then bang! major loss in power.

engine is still sitting on the garage floor, was planning on installing her today.
 
installed the engine a few days ago.
she's running but while installing the primary clutch i noticed the spring is broke... the very end of the spring you can see a break in it. fresh too as there is no rust where the break is.

will need to replace the spring before i can do any testing as the clutch engages around 1500rpm as it is........
 
From My Memory...LOL!

This is going back a ways but ...I remember the 570 Exciters having a real problem with part throttle melt downs. There were a few theories out there, engine vibrations, power jet carb issues ect.[they both had merit]

People were jetting their 570s with huge main jets and they would still go down!

Dynotech as well as PSI found that the stock needles had a real lean spot in and around 1/4 throttle. Initially PSI built and sold replacement needles that elliminated the lean spot and if you ran these needles, pretty well ended any melt downs that usually occurred after coming off wide open throttle and then running at part throttle.

In 1990/91 Yamaha updated the needles and all the new Exciter's came with a virtual copy of the PSI needles and this virtually elliminated the issue.

Number stamped on the newer richer needles: 6FL 82.

The 90/91 Exciters also came with an updated carb boot to help elliminate vibration issues.

So the moral of this story is...

If you have the 89 or older carbs on your Exciter, order in Needles from an Exciter 1991 , as well as the newer carb boots and that should end the problem. Also NEVER come off wide open to part throttle without pumping the throttle a bit to avoid a lean condition melt down.

JM.02c
 
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