Yamaha99MM700
New member
I'm a huge noob to anything mechanical and this is my first sled, but I have a shop manual and am trying to learn, so please forgive my lack of knowledge on these things. I just wanna get this thing fixed because there's still some winter left here in Alaska!
Last winter I went over a large bump and went flying pretty high and landed hard on flat hard surface on my 99 Mountain Max 700. Immediately sounded like it was running on two cylinders but I rode it home. It was even starting up after (I know I shouldn't have..) until it no longer started months later. I recently did a compression test and the center cylinder had 60 psi, pto 110, mag 120. I took the top off and two cylinders were blackish except the center which was clean. Took the pistons off and the skirts were broken off on both sides of the center piston, and on one side of the mag piston (other side was cracked). Pto piston looked fine. Also, all the piston rings were very loose. (are they supposed to be tight?)
I'm going to take the crankcase apart to see if there's debris from the skirts in there and if it looks good just buy a new wiseco piston kit I guess? The cylinders look fine but I read that I should hone them? Also, should the connecting rods have any movement to the side? If it means anything, I've been premixing my gas, and haven't piped it yet. Bought the sled used. Am very curious what could cause this and would appreciate any help. Thanks!
Last winter I went over a large bump and went flying pretty high and landed hard on flat hard surface on my 99 Mountain Max 700. Immediately sounded like it was running on two cylinders but I rode it home. It was even starting up after (I know I shouldn't have..) until it no longer started months later. I recently did a compression test and the center cylinder had 60 psi, pto 110, mag 120. I took the top off and two cylinders were blackish except the center which was clean. Took the pistons off and the skirts were broken off on both sides of the center piston, and on one side of the mag piston (other side was cracked). Pto piston looked fine. Also, all the piston rings were very loose. (are they supposed to be tight?)
I'm going to take the crankcase apart to see if there's debris from the skirts in there and if it looks good just buy a new wiseco piston kit I guess? The cylinders look fine but I read that I should hone them? Also, should the connecting rods have any movement to the side? If it means anything, I've been premixing my gas, and haven't piped it yet. Bought the sled used. Am very curious what could cause this and would appreciate any help. Thanks!
High mileage is the cause. Pistons are worn to the point of rattling. Unlike a cast iron bore, your cylinders maybe alright as is but I'd measure them. Scotch bright and wash with soap and water if they're okay. Unless you plan to make a major change to your warm up cycle, don't use Wiseco. They are not forgiving.
Huge noob would suggest you may want to have someone look at the crank when you have it apart. Pay close attention to the case halves for cracks or even holes from the broken piston skirts. Plan on replacing the crank seals while you have the case apart.
If your service manual isn't a genuine Yamaha, get one. Why are you not using the oil injection?
Huge noob would suggest you may want to have someone look at the crank when you have it apart. Pay close attention to the case halves for cracks or even holes from the broken piston skirts. Plan on replacing the crank seals while you have the case apart.
If your service manual isn't a genuine Yamaha, get one. Why are you not using the oil injection?