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Last season, had a dramatic power loss and then could not keep sled running ('91 Yam SRV540). Finally, 7 months later, I am trying to fix it. So, I found that one of the head gaskets was burned through. Got new ones ordered, easy to replace. But, I have some questions that I hope someone would not mind helping me out with:

1. What was the cause of this?
2. Is there any possible underlying cause I should be concerned with?
3. The top of the piston has a pit burned in it, not all the way through. I know it should be replaced, but will it run OK or am I risking some other huge problem?
4. Should I, Can I remove the carbon build up on the inside of the head? How?

Sorry for all the probable dumb questions, but I just am not that knowledgeable on 2-cycle engines!

Thanks,
-dave
 

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the head bolts may have needed retorqing, and that may cause it, i know a lot of machine call to retorque the head bolt after so many hours, so that could be the cause, or it could be detination, and since the pistons look funny that is probably what happened, have you cleaned the carbs? and is your machine stock?
 

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I agree with Matt that it could be detonation. What type of plugs were in the sled? They might have been too hot. Did you check the head for cracks?

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Thanks guys for the replies!

Bought this used and as far as I know the only mods were a PSI (I think) Pipe, some different flapper things in the carb. I do not see any cracks in the head and no I have not cleaned carbs. I will pull the carb today and see what there is to see! Plugs that came with the mach were not as speced. Hadn't noticed that one :-( What does detonation mean?
 

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I think that we will need some more info on what exactly happened when your sled lost power. The more detailed the better. It is hard to tell anything from just "lost power"
1. What did you plugs look like, did they both look nice and light brown at the tip? If one was all black and wet right after, 1 cylinder was not firing.
2. Did it start to backfire or make any unusual noises at all?
3. Did your temp light go on before hand.
4. Would it run at all, idle, wide-open?
5. Any missing peices from the reeds, (flapper things)?
6. Is the fuel pump working OK?

I would change that piston for sure.
Power loss could have just be from the loss of compresion from the burnt gasket, caused misfiring in that pot.


When hell freezes over, we'll ride there too.
 

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If this is a problem on only one cylinder on a multiple cylinder engine, im guessing that you either need to re-jet your carb or look for air leaks. The pot or hole that in burned into your piston is either caused by detonation or bye a lean mixture most likely caused by an air leak or improper jetting (possibly due to the pipe). Were the carbs re-jetted when the pipe was installed? Are the rubber boots (carb flanges) in good shape or are they all cracked and weather checked? Going back to OEM specified plugs would also be a good idea.

Detonation - simply put, the air/fuel mix is being ignited at the wrong time, usually too early and trying to reverse the rotation of the crankshaft. This causes major damage to any engine.

Now how lucky do you feel about that piston...if you should decide to leave it in there, here are the risks you are taking.

1. That pit is now a weak spot in that piston. So if it is weak enough to break under the tremendous rpm or combustion pressures, you run the chance of spewing chunks of piston down into the crankcase which may end up screwing up your crank bearings.

2. That pit may cause further detonation by the fact that it could now hold more heat than the rest of the piston head and igniting the mixture before it is supposed to be. (more detonation)

3. Your performance will be down due to the fact that you will have lower compression in that cylinder. thus making the other cylinder(s) work harder.

Hope this helps some

Polaris is my way out - Other people just use a door.
 

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Discussion Starter #7
Sorry it took me so long to get back. It was one of *those* weekends! Well I pulled the carb and it does not look crudy on the inside. Rubber boots look good, etc. I have no idea what jet this thing is running, PSI pipe came with it from prev. owner. Guy at the parts joint sez the reeds (flapper things) are "power reeds".

500fasEnuf: Started to backfire about an hour before quitting. Ran OK at idle. Coming off idle, had to feather it to keep it from killing. At WOT, weak power and backfiring. No temp problem, no other unusual noises, no flapper chunks.


So, I am now convinced that since I want to run this thing for a few more years, here is what I should do:

1. Rebuild carb and go back to stock reeds and stock jet (#150 or #145).
2. Replace both pistons and rings (Never done anything like this. words of encouragement and advice welcomed, I am mechanical, just never did this on anything but a lawn more 4-stroker).
3. Go back to stock pipe, which I got with the sled.

I'll take it from there and see how she runs for me!


I guess I should look into the fuel pump too. I am not sure how to tell if it is working OK, but I think I am about to learn more about this than I wanted to know! Wish I had $7K to buy I new one!

Thanks again for advice.
-Dave
 

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dude if this thing hot enough to start to burn a piston theres something wrong, check to see what plugs this sled should be running make sure your plugs arnt too hot, ive seen people run a too hot plug to compansate for a too rich carb problem, "not a good idea" also something else nobody touched on is check and make sure that it this machine has oil injection it is working as it should, i beleive that yams have a inline filter in the oil injector line make sure it not blocked, that would cause your meltdown, also as others said here check your jetting get the stock baseline, then go from there, remember sometimes a bit rich will make your sled last longer, also before i put the heads back on that sled if it got that hot they may be warped so get a peice of glass and lay it flat put some ultra fine emery paper on the glass, the put the head down sealing surface down and in slow circular motions this will "level the head and assure that things are flat for a good seal then just follow the gasket touque instructions, after all thats done do a good compression check too good luck with you sled

if its not a CAT, then it must be a dog!
 

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Check to see if you have any oil deposits at the crank seals, if the seal is leaking it could cause a lean condition, and burn a piston. It happened to me on the mag side [pull cord side] on my old Phazer.
 

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If you burnt the head gasket due to loose head bolts the it is quite possible the the piston melt marks are due to continuing to run the motor. It would suck air in by the bad gasket causing the fuel mixture to go lean?
 

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Well if it was backfiring out the carbs, you had a lean condition, backfiring out the exause i'd say your timing could be messed up....but i'd bet on the lean condition....If this is the case i wouldnt worry about changing back to stock form, rather just jet the carbs 10 or 20 higher (larger jets) and watch the plugs for signs of a lean condition.

Polaris is my way out - Other people just use a door.
 
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