My 10 year old son rides a 95 stock blaster. He’s light and he does not ride super hard. Over the holidays, on the last day of our seven day trip at Dumont Dunes, the motor suddenly seized riding in moderate dunes. Upon disassembly I found that the rod bearing on the crank had seized. The cylinder and piston look fine, except for black and brown discoloration on the back side of the dome and along the side of the piston below the rings. I was told the discoloration on the backside of the piston was a sign of overheating. The crank spins free and bearings appear fine. No visible seal leaks. Crankcase, crank, and piston were wet with oil film. Clutch is good, no blue plates indicating a slipping clutch (causing engine to rev).

- Plug color – brownish
- Measured piston clearance, nearly .009 inches (I’ve been told .004 is max –sloppy)
- Oil injector has been removed and I run 32:1 mix (two years ago)
- Main Jet 240 (230 stock) – so it should be on the richer side, went up as suggested for pre-mix
- 60-70 hours since last top end
- New Vito Crank installed two years ago

I can usually diagnose these types of problems, but I’m at loss to explain this one. I’m planning to rebuild and install a new crank(obviously) and piston. I want to avoid doing this again. Please, can some maybe explain or give me some clues why this might have happened and what to look for

Thanks for any help

 

well rod bearing usually seize when you rip on the engine too hard, this same thing happened to my buddy's blaster, but he only needs a new rod kit, maybe that's all you need, you also said you were riding in dunes, sand possibly could've got in there and locked it up. the discoloration usually happens with all quads and bikes so i wouldn't necessarily say it was from overheating, it's carbon burning onto the piston. and also .009 is too much clearance, .003 is the average recommended clearance. i hope this little bit of info helps you

later on
NichelsonRacing87

 

Normally there shouldn't be any carbon build up on the piston skirts at all. Are the rings bad? Maybe the combustion gasses were blowing by the rings, just a thought.

Does the rod have alot of movment on the crank, up and down? It could have just thrown the bearing or somthing, what happened when it seized, how did it act? Did it just lock up, or make any funny noises first? These all could be clues as to what happened.

Also check out the crank seal under the flywheel. The blasters are natorious for popping these. They lead straight into the crankcase and if they are leaking then you can bet it was sucking in air and leaned out the mixture which seized up the rod. Pull the flywheel and check around the seal. If its wet then it was blowing gases out when the crankcase comes under pressure, then then when the piston goes to TDC it creats a vacuum in the crankcase which draws in air. Could be a possibility.

Hope this heps
Shav-

 

if the crank seal blew then ur trans oil was bein pumped in with ur gas into the piston. that would cause any excess carbon on ur piston. check ur trans oil and make sure theres still some in there

 

If that was to happen it would richen the mixture so much you would never be able to drive it. I've had that seal let go twice, and both times it was would suck in so much oil it wouldn't run.

 

I appreciate everyone's comments and feedback! I've read the posts and I will try to comment to most of the questions asked (good ones by the way)

1. I was not ridding the quad when it siezed, my son was and to get a detail of sounds and feels when it siezed is really not practical. The best I got was "Dad, it just quit running"

2. No build up of carbon on the piston skirts, but black carbon and some brown coloration directly below the second set of rings. With .009 measured clearance, that might explain it? I did check the back side of the piston (under the dome) and it was real black and had the brown discoloration. Some of my other piston trophies I've kept show none of these signs (ones that siezed). I was told by someone to check this for discoloration, as this is another tell tale sign of of overheating.

3. The rod right now does not move on the crank, it is very tight and you can feel that the bearing is gone! However, the crank seems to spin (the small amount it can with a siezed rod) freely in the cases. I can not check for any type of rod-to-crank clearances, just tight.

4. I did pull the Flywheel the other night and did note some wetness around the seal. I replaced it when I changed the crank a few years back. I should have done a leak down test before I tore the engine down, that might have confirmed things for sure. It is very realistic that I did lean it out and maybe the reason the piston did not sieze first is due the excessive clearance (.009)? Anyone think that is a realistic theory, rod bearing siezed before piston (which has excessive clearance)?

5. I don't think I'm sucking in any trans oil through that seal. No signs or usage of trans oil.

6. Someone has suggested to me that I should buy a Wiesco Hot rod/crank over the vito's. Any reasons one way or the other. Not sure it will cost me any less to replace just the rod vs. the entire crank and rod assembly.

7. I've been using the stock air box and filter treated with a real sticky foam oil by Maxim (I think). A few have suggested I might have sucked some sand in (always possible) but with a stock box (lid on) and a clean, well oiled filter?


I hope to here more feedback to my latest updates and responses. Unless others have a better suggestion or idea to move forward, my plan is to finish tearing down and replace the rod/crank, crank seals, crank bearings (if they need it), new bore and piston.