Piston failure

1

1980SRX440

New member
Joined
Feb 25, 2011
Messages
10
Age
50
Location
PA
Hey guys,

Been looking around here the past few days for some answers for a piston problem.

Here's the back story:

Picked up a 1980 SRX 440 from a buddy, the guy he got it from had it garage stored but hadn't ridden it for a few years. My buddy took the carbs aparts, cleaned them (previous owner draind the gas from the tank and carbs), put them back together. He got it running, brought it over and we rode it for 2 hours roughly. On my trip back, I pulled up to talk and left the sled idle and it just shut off. We couldn't get it re-started. Brought it home did a compression test, and found 30 PSI in the one lung, the other was like 135 or something,I can't actually recall exactly, but well with in spec. So he gave the sled to me and went and bought a newer sled. I tore the top end down and found the PTO side piston had the exhaust side of the piston rounded off. He was running race gas (112 octane) and we thought it may have been too much for the sled, so we assumed that that was probably it. I bought 2 new pistons (cylinders actually looked good), ran a ball hone thru and put the top end back together. I replaced all the fuel/oil lines, bled the oil pump, mixed up some 50:1 put it in the tank and got the sled running, wide opened the oil pump to make sure the lines were full and proceeded to break the sled in. Rode the sled around for an hour or so varying the speed and such. Everything seemed good. Went back the next day and rode it around some more, while out in the field something didn't seem right so I went back to the shop and shut it down.I pulled the plugs and they were black, figured they were fouled out, put two new plugs in and she was a no start. Pulled the MAG side plug and pulled it over, no resistance. Replaced the MAG plug, removed the PTO plug, lots of resistance. Back onto the trailer, back to my shop, compression test on PTO side 30 psi. Pulled the exhaust looked in and sure enough, rounded the piston again. Fast forward I am here today!

I have pulled the motor, made block off plates, pressure tested with 7 PSI, holds for hours or until I release the pressure. So from that I assume the crank seals are good. Intake boots looked in great shape (nothing visible). Obviously the injection was working plus the gas was mixed at 50:1.

Checked the timing with a dial indicator, TDC at 0, rotate crank counterclockwise 0.055 and the timing mark is off a bit. If I rotate to get the timing lined up I'm actually at 0.046.

Took the carbs apart : Mains are a 230 Pilots are an 85. Float level seem low, should be 30mm +/-1mm and I measure 28mm. I may be a little too precise (measuring with verniers)

Altitude where I am is roughly 230 feet above sea level. By looking at the Yamaha chart at 32 degrees F, sea level should be a 250, and up to 3000 feet should be a 230.

I'm assuming that it is a lean condition, but could the jetting really be this specfic? Why then only would the one side burn down? Could the timing be causing this (comparing my readings)? I now want to rebuild this thing again, but I this time I want to send the cylinders out and have them nikasil coated and install wiescos since OEM are not available.

Any help or insight would be great, I really enjoyed the small amount of seat time I got on this thing. Before it went the first time it was wicked fast. Just alittle gun shy of putting the money into it just to have it gag again.
 
LET ME UNDERSTAND. YOUR STILL USEING THE OIL INJECTION AND 50:1 PRE-MIX AND THIS STILL HAPPENED. ALSO THAT 112 OCTANE IS NOT HURTING THAT SLED ONE BIT. MATTER OF FACT, ITS ACTUALLY GOOD TO RUN THIS. ONE THING YOU DID NOT MENTION IS HOW ABOUT A OVERHEATING ISSUE. POSSIBLEY STUCK THERMISTAT ? ALMOST SOUNDS LIKE COLD SIZEURE ISSUES. IF TOP OF THESE PISTONS THAT ARE FAILING LOOK GOOD, OCTANE IS NOT YOUR PROBLEM. ITS EITHER LEAN OR COLD SIZEING. 3:16 (yammie tony)
 
Make sure you coolanf liquid is circulating and did you measure your ringend cap when putting new piston?
 
YAMMIEGOD3:16 said:
LET ME UNDERSTAND. YOUR STILL USEING THE OIL INJECTION AND 50:1 PRE-MIX AND THIS STILL HAPPENED. ALSO THAT 112 OCTANE IS NOT HURTING THAT SLED ONE BIT. MATTER OF FACT, ITS ACTUALLY GOOD TO RUN THIS. ONE THING YOU DID NOT MENTION IS HOW ABOUT A OVERHEATING ISSUE. POSSIBLEY STUCK THERMISTAT ? ALMOST SOUNDS LIKE COLD SIZEURE ISSUES. IF TOP OF THESE PISTONS THAT ARE FAILING LOOK GOOD, OCTANE IS NOT YOUR PROBLEM. ITS EITHER LEAN OR COLD SIZEING. 3:16 (yammie tony)
Click to expand...


I was running the 50:1 in the first tank (break in procedure) as if it were a new sled just fresh from the crate. As far as the coolant, followed the service manual procedure on filling and bleeding. Temps seemed fine while running, never any where near the red zone. The top edge to the ring is where they're rounding off. It's actually melting the piston off. I haven't torn the top end down yet, just incase there was some other test I need to do other than the leak down. The MAG piston still looks brand new.
 
Reimond said:
Make sure you coolanf liquid is circulating and did you measure your ringend cap when putting new piston?
Click to expand...


Coolant was definitely circulating and no I didn't check the end gap.
 
'' MAG PISTON STILL LOOKS NEW ''. WELL THAN BELIEVE IT OF NOT, THAT SLED IS RICH IF YOUR WASH LOOKS NEW. I WOULD START WITH GOING THROUGH THAT CARB AGAIN YOUR HAVEING ISSUES WITH. 3:16 (yammie tony)
 
sounds like the intermediate carb circuit is the problem if its the same cylinder, the butterfly carbs dont have a needle so they are alot more fussy, go thru all the circuits in carb and squirt carb cleaner thru all of them followed with compressed air, you have junk in there.
 
mrviper700 said:
sounds like the intermediate carb circuit is the problem if its the same cylinder, the butterfly carbs dont have a needle so they are alot more fussy, go thru all the circuits in carb and squirt carb cleaner thru all of them followed with compressed air, you have junk in there.
Click to expand...


I'll check this out today or tomorrow, thanks for the suggestion.
 
Alright,


I re-checked the carbs, all passages are/were clear 100% with out a doubt. I'll post a pick of the original piston tomorrow. Pics are worth a thousand words!
 
yamaton said:
Sounds like crank seals to me.
Click to expand...

That's what I thought, but a pressure test will hold 7 PSI for as long as I want until I release the pressure. One question I do have is , are the cranks sealed in the middle like a Rotax liquid cooled sled. On my liquid cooled Skidoo's the cranks have seals sealing each "1/2" of the crank case rotary valve shaft. They seal that because the rotary valve shaft is constantly lubed. I've never had a yamaha bottom end apart, but from the microfisch I don't see any seals called out in the center of the crank.

Does any one have the procedure for a pressure test. I made block off plates for the imtake and exhaust ports and used my mity vac hand pump and pumped thru the pulse line and pressurized to 7 PSI. Like I said it will hold for as long as I want. Sprayed case halves, intake and exhaust ports, and cylinder bases with no bubbles forming. Is this the proper procedure for a yammie?
 
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its been a long time since i had a 80 butterfly carb in front of me, but if they are like the phazer carbs theres a certain place for those rubber plugs that fit in the bottom, you might want to research that. Tony could probally tell you with more certainty.

I think a couple posted pics are like you said worth a 1000 words. make sure to show pics of good piston as well.

you did exactly right on your assesment that its not a crank seal if it holds 7psi that long, its not the problem. I know the butterfly carbs have no needle to adjust so your midrange air/fuel is relied soley on a hole in the carb, this is why alot of guys changed the 80 carbs back to the slide carbs of the 79's, or went bigger 38 mikunis. Only thing was you lost the oil injection and had to premix.
 
Yes the original pistons were the original yamaha pistons, the second one was a parts unlimited. I went with that piston for the fact that it accepted the factory tolerance, unlike the tolerances of a Wiesco piston.

As far as the jetting the service manual I have called out 250 as the "standard" main. Thus havine a 230 main meant it was jetted up to 3000 ft altitude. I have no problems putting 270's in and trying that and then going to the 260's after a plug run. I'm just afraid to put about 600+ bucks down to burn another piston.

I do have a 79 parts sled with the slide carbs but they would need some major restoration. And as far as getting rid of the injection----I could care less, sort of peace of mind that atleast I know the oil is in there!

Piston pics tonight for sure! Thanks again for all your help guys.
 
PARTS UNLIMITED STUFF IN THESE MOTORS IS A REAL BAD IDEA. I KNOW IT HAD YAMAHA PISTON FIRST GO ROUND. TUFF HERE BECAUSE IT REALLY SOUNDS YOU HAVE GONE THROUGH THIS STUFF REALLY GOOD. I,LL WAIT FOR YOUR PICTURES. 3:16 (yammie tony)
 
Here are the pics

Original piston:
DSC_1284.jpg



Second piston, still together
DSC_1288.jpg

DSC_1286.jpg


Sorry for the bad pics, hard to focus in the jug, but the same as the original piston but looks to be burned down worse on the exhaust side. Probably due to the material difference from the OEM piston to the PU piston.
 
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Sorry to see you are having these problems. It is definately a lean condition from the intake. Too bad you didn't ask here before the second time. :( You either were just too lean for those air conditions (probably mid range) or the carb mount was left loose or leaking from the back. Good luck!
 
Hey guys, I have a few questions about this "Labyrinth" seal.

I assume this seals each half of the crank case from each other, right?

Reason I'm asking is how do you know if it is good or bad? Like in my skidoos I can pressurize the rotary valve shaft cavity and remove either the MAG or PTO sparkplug and that test will tell me if either the MAG or PTO side seal is either good (cavity holds pressure) or bad (leaks out thru the sparkplug hole on the corresponding side telling you that the seal is shot on the center of the crank on that specfic side).

So is there a procedure on this and if it is bad is it replaceable by me or does it require a full crank disassembly and realignment? And where are people finding suitable replacements?

I was talikng to my buddy who gave me the sled and he mentiond that the idle was sometime erratic (hang alittle high the drop down toa normal idle), I did not notice this, but then again I did not let the sled sit and idle since I just did the top end.

Sorry to throw in this last minute detail. I really do appreciate all the suggestions given so far. I'm going to drip som trans fluid thru the carb passages tomorrow to verify(in color) that all the circuits are open in the carbs, can't hurt to check the for a 7th time.
 
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