MZ/MuZ Riders

OK,
you read my issues on my baggy, So can i get a Yamaha 660 head and internal parts and rebuild the top end? Or is the MZ yamaha, but not yamaha?

yes, top end is same as early 660 raptor

what's wrong with your head, your top end? how many miles?

2500 miles and I put the compression release valve right through the top of the motor. Haven't had it torn down, But hoping its just the top end and nothing went lower.

Theres a guy in Westerly, RI selling a few Raptor 660 heads on Ebay, oddly enough. Name is muffsmotorsports , he has a good rating and appears to do a lot of work on these...might give him a call.

so, has anyone else on the forum experienced a sudden decompression failure? Ever find a root cause?

Sam

[Yeah I saw that Sam, ill be reaching out soon

well it's now down at Bikeworx in MA. They kinda couldn't believe that I did that much damage at 25 mph. Then again niether can I. we will see just how bad it is in a few days...

OK,
Galin at Bikeworx called me yesterday. It appears that one of the lobes on the cam sheared off, and bounced around pretty much destroying the top end, and busted up one of the lobes on the case, which if it cant be refabricated, will also have to be replaced. He feels that either this was a huge failure, or someone pulled it apart and did not put it back together correctly. But he couldn't tell which it could be. OK, other than reaching out to MZ to see if failure might warrant pity and help in parts or a rebuilt engine, Any thoughts as to were I could find a rebuild motor for this thing? I am so ready to just sell it for parts, and I am bumming having only rode it 6 times.

(sigh) as the seller of this bike I'm more than a little sympathetic, but nothing it did in the ~200 mi I had it showed any indication of impending failure. When I bought it I was somewhat suspect of the ~2,500K on the odometer, as it appeared that the speedo had been removed/reinstalled at some point. But, riding it around, the trahcmission had that tight, low mileage feel to it, the fasteners seemed like they hadn't been disturbed for some time, and the rest of the bike - foot pegs, bearings, etc, seemed to indeed be very fresh with no signs of work done on them. There were some hints of previous work - the radiator clamps were non stock, and (in hindsight) the exhaust nuts were incorrect. But overall, it seemed like a low mileage motor with original looking sealant on the valve cover/head, case seals, etc...and it ran well. The guy *I* brought it from seemed relatively clueless; I wouldn't have expected him to know if the PO before him screwed him, but he claimed to have put several hundred miles on it. If he was bilking me he was damn good. But that is a weird, *big* failure.

Come on Sam, you dam well I'm not blaming you for this. As you get to know me, you'll realize it just those kind of things that make up my life. I think it would be boring if it didn't.

sdbct1 wrote:Come on Sam, you dam well I'm not blaming you for this. As you get to know me, you'll realize it just those kind of things that make up my life. I think it would be boring if it didn't.

Hey, don't worry; I know you're not pointing any fingers at me. I appreciate that, and I know I did my due dilligence on that bike. But that doesn't keep me from having a world of empathy, and an even deeper curiosity as to WTF went wrong!

So, I've ended up with the bad motor: I sold the good one out of my Skorpion to sdbct1 and am building up a new one. This was as much out of opportunity to put a better motor together for my bike as anything. I just went over the bits last night and here's my own forensics:

The cause of failure was a bolt coming out of the cam sprocket. The sprocket started to wobble, breaking the decompressor bits off; at least one of those bits got on top of the cam sprocket which pushed it right through the cam cover. This, in turn provided the leverage to break one lobe of the cam off with the remaining bolt fastened to the sprocket and allowing the timing chain to slip off the sprocket. The chain wedged between the inside of the head and the sprocket - pushing the sprocket outwards enough to crack the outer casing of both the head and cam cover before the chain could slip by into the case. So, head, valve cover, valves, cam, and sprocket are scrap.

At this point the engine was still spinning, and the cam was as well, but not necessarily at the same rate as the piston. All 5 valves hit and were bent, at least one guide is cracked from the bent valve. Looking at the piston I'd say they each only hit once, the piston looks perfectly reusable, as does the cylinder.

When the chain dropped into the lower case it wrapped around the lower tensioner mount cast in the cases, tearing it off. somewhat surprisingly, this is the only damage to the bottom end. I'll machine a new mount, mill a flat in the case where the old mount was, then bolt it in - this way I won't have to deal with potential distortions caused by welding heat on the case. So, I can save the cases.

Now *why* did that bolt come out?!? There *was* red Loctite on the bolt, and neither bolt nor cam thread are stripped. Perhaps there was oil on the bolt/thread and it just didn't stick; who knows? And - did it not stick when put together at Yamaha, or when someone else was in the cam cover for some reason? For me, though - case closed.

Still waiting on Bikeworx to get to it. I figure i still have a few days of good riding left....Oh wait its raining

....as I was splitting the cases I found the chunk of the case that formerly held the lower part of the timing chain. Not surprising, until I tried to match it up to the hacked up bit that was left on the case, only to discover that it has been already build up with weld! It was nowhere near as substantial as the original part which is why I hadn't noticed it, plus I just wasn't looking for welded up bits inside my engine. But now, looking at it, and the *lack* of damage on the front timing belt tensioner, I'm pretty confident that it was holding it's own when the cam went.

So - *someone*, for some reason had this lower mount crack and fail. They went in, made a functional fix by welding a little blob where the more substantial mount was. They then almost certainly tried to Loctite the cam bolts w/o cleaning the residual oil off them, one backed out, and here we are.

Sam

Bike shop STILL hasn't gotten to it. I am probaly going to pick up all the pieces and put it together myself. At this point it can't be any longer with me doing it than the bike shop.