mag side piston problems

1987vmax · Post by **1987vmax** » Wed Feb 04, 2009 8:27 pm

Hi there I am looking for help with my 87 v max.When I bought the sled the piston on the mag side was melted down. I bought new cylinders & pistons had the carbs done at the local Yamaha shop. I installed new crankseals & did everything I could to make sure it was right. It burned down again then I found the main jets were to lean I thought the dealer would have put them to stock! since I have put #330 MAIN jets & 130 & 135 power jets put it back together & this time the piston looks like somebody took a cheisel to it. I did notice that the mag cylinder runs alot hotter when I put my hand on it. I was running premium fuel as well ANY IDEAS??????????? I have since removed the head & the exhaust side of the piston there are chunks missing from the crown & a peice of the ring is gone aswell Looks like pre detnation.?????

Joe · Post by **Joe** » Thu Feb 05, 2009 2:45 am

Are you running the stock air box?
Have you checked to make sure it is all there?
A reduction of the intake restriction will remove some vacuum signal making the engine run lean.

Post by **Vmax540** » Thu Feb 05, 2009 8:40 am

1987 Vmax

The 330 jets are plenty safe if your airbox has the baffle, there is no porting and there are no air leaks. My best guess is the crank case if not flushed throughly had a piece of old piston/ring or other debris and was sucked into the top of the piston thru the transfer ports and is in effect sandblasting the top of the piston.

1987vmax · Post by **1987vmax** » Sat Feb 07, 2009 1:58 pm

Vmax540 wrote:1987 Vmax

The 330 jets are plenty safe if your airbox has the baffle, there is no porting and there are no air leaks. My best guess is the crank case if not flushed throughly had a piece of old piston/ring or other debris and was sucked into the top of the piston thru the transfer ports and is in effect sandblasting the top of the piston.

chrisrider63 · Post by **chrisrider63** » Sat Feb 07, 2009 4:46 pm

That sounds like that motor blew hard damn!!

Joe · Post by **Joe** » Sun Feb 08, 2009 12:16 am

1987vmax:

Can you post a photo of the piston crown?

1987vmax · Post by **1987vmax** » Thu Feb 19, 2009 9:41 pm

jipcee · Post by **jipcee** » Thu Feb 19, 2009 11:15 pm

How did you set the timing?

Joe · Post by **Joe** » Fri Feb 20, 2009 4:45 am

What does the head look like?
Does it have the same texture as the piston?

1987vmax · Post by **1987vmax** » Fri Feb 20, 2009 8:56 am

the texture of the head is the same I fiqure it was either pre detnation or the timing or both. To answer your question about timing I just back to where it was when I got it. I have an 83 service manual & if the timing is the same as an 1987 than it is not correct. I am determined to fiqure this problem out.

vmax-540 · Post by **vmax-540** » Fri Feb 20, 2009 6:40 pm

The problem is above the rings not below them so it is not a oil or piston fit problem, The timing is the same on the 83 and 87 it should be (1.6 +0r- 0.05mm or 0.063 +or-0.002 inch)before TDC. You said that the carbs were done by a Yamaha dealer did they synchronize them on the sled? You said the mag cylinder feels warmer then the PTO cylinder does ,so it sounds like either that cylinder is working more then the other one or maybe the other one is not putting out like it should ,I have seen that happen before ,also this sled has the carbs jetted different on the mag then the pto could you have gotten them mixed up by mistake, they are numbered right next to the choke plunger . Also when in the power band does this happen ,does it happen at full or part throttle ,just because you richened the main up it could be a different jet that is cousing the problem, I would take the carbs back apart and check ALL the jets to make sure they are right.. Here is the standard jetting for the 87, also check you fuel line for restriction ,and what is the compression on both cylinders? . Good luck

PTO carb 80N00L Mag carb 80N00R
Mains ______________ 290's ________________
Pilots _______________ # 40's ________________
Air screw _____________ 1/2 turn out + or - 1/4 _____
Power jets #135 #130
Power air jet #1.5 #1.4
Needle jet _____________ 6DH1-4 ___________________

1987vmax · Post by **1987vmax** » Sat Feb 21, 2009 11:08 am

the carbs weren't syncronized but that is something that I will be checking now as for the jets they are correct & I made sure they went on the right side.The one thing I didn't mention was when I put the sled back together It wouldn't idle for some reason. I never had that problem before. I also checked the air box the baffles are still in it.The local sled shop says I should run OEM pistons as well if they are still available. It currently has kimpex pistons in it. But for now I am going to put a motor out of am 83 in

sxr1000 · Post by **sxr1000** » Sat Feb 21, 2009 12:29 pm

This could also be from a crankshaft bearing going out and the metal pieces are pounding the top of the piston,did you find any metal in the pipe or muffler?

vmax-540 · Post by **vmax-540** » Sat Feb 21, 2009 2:41 pm

Kimpex pistons are fine, I have used them before without any problems.If it wouldn't idle then maybe there is an air leak some where?

Post by **tyler440** » Sat Feb 21, 2009 4:36 pm

sxr1000 wrote:This could also be from a crankshaft bearing going out and the metal pieces are pounding the top of the piston,did you find any metal in the pipe or muffler?

i was kinda wondering the same thing myself but it looks like the outside of the piston is all beat up and not so much in the middle... i would think if it was crank bearing or part of a ring it would hit all of the piston equally

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