Other 1978 YZ400 Piston Seized

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aoyue2702

So... I was riding down the road breaking in the new piston and freshly bored cylinder today, was going about 55 mph on the pavement in 5th gear. All the sudden the engine dies and the rear wheel locks up causing me to go sliding left and right until I thought to pull the clutch in. Pulled over to the side of the of the road, put it in netrual, and it started first kick.


I went back to the house and I tore the top end apart again.

Here's what I found. Two corners appear to have seized on the piston, and the other two corners appear to look nearly brand new??? What could have caused a 2-corner seizure? The cylinder looks to be in better shape than the piston, however two corners of the cylinder does have some vertical scratches and scuffs in those spots.

Is there any way I could possibly sand this or something along those lines and run it for the summer? (I know, crappy way to go about things, but I really, really dont have another $200 to drop on a fresh top end again.) Also, I noticed the top of the piston had little bitty specks all over it, and when I looked a bit closer, it appears to be micro chips? What could have caused this?

The jetting in the manual calls for a 370 jet, however, I put a new carb on it and it's got a much smaller jet (I think a 320 or 330), would that have caused such a seizure? The bike also has a header leak and is missing the muffler.

There was almost no vertical play in the connecting rod, The bike was almost warmed up fully, probably been on for maybe 3-5 minutes


Heres some pictures

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Mihylo33

Mihylo33

Yes you need a new piston
Yes the new carb needs to be jetted properly so that means increasing your jet size.
I hope you are running a 20:1 mixture in that awesome monster as well.
Before reassembly check your ring end gap and your piston clearance.
It sucks and I'm sorry to report this but this is how we all learned so you aren't the 1st one to do this....
Good luck and welcome to DBA!
 
A

aoyue2702

Ok. I already changed the jet size to 370, but of course I haven't got to ride it.... The damn piston for these things cost $150 so it may be a while before I try and get this running again :( . Sure does suck throwing $200 down the drain (piston and bore job cost). A local shop had done the bore job, but they have recently become known for doing crappy work ever since they hired in drug addicts to do the mechanical work . I was worried they could have possibly screwed up the bore job on the cylinder.

Do you think I also need to re-bore or what should I do about the cylinder?
 
Mojavekid18

Mojavekid18

From the pictures it looks like the cylinder is run-able. I would definitely get a new piston though. Also make sure you take it easy on that bike until you get it all warmed up. Sounds like a possible cold seizure. With the new jetting, piston, and proper warm up time i don't think you should have any issues
 
Mihylo33

Mihylo33

There's a possibility that the cylinder is out of round but just take it to a local shop and have them hone the cylinder and check it for out of round.
 
A

aoyue2702

I will probably take it to a different local shop, I really regret doing business with the one who done the bore job, they are absolute trash, I should have known better than going to them in the first place. I'm not saying its the shops fault, (probably mine for the lean jetting issue) but they have been doing some really, really crappy work for several of my riding buddies.
 
josmoloco

josmoloco

aoyue2702:
Ok. I already changed the jet size to 370, but of course I haven't got to ride it.... The damn piston for these things cost $150 so it may be a while before I try and get this running again :( . Sure does suck throwing $200 down the drain (piston and bore job cost). A local shop had done the bore job, but they have recently become known for doing crappy work ever since they hired in drug addicts to do the mechanical work . I was worried they could have possibly screwed up the bore job on the cylinder.

Do you think I also need to re-bore or what should I do about the cylinder?
Click to expand...
http://www.ebay.com/sch/i.html?_trk...TR0.TRC0&_nkw=yz400+piston&_sacat=0&_from=R40
 
cleonard

cleonard

Replace the crank seals. They are likely from 1977 and leak some serious air.

How deep are the scratches on the cylinder? Do they catch your fingernail? If they are not too deep you might get by with some aggressive honing. I'd break out some sandpaper on that piston if it's not too bad. You will lost the oil holding surface machining on that spot, but I bet it would run.

One thing that is critical is knowing the piston to bore clearance. If it's too small it will just seize again.

If the damage is too great on the bore then it's new piston time.

No shop will do this though. I've assembled and ran engines that were a lot worse. Is it the "right" thing to do? Of course not. Remember it's a $500 bike tops. It's a bit of a different approach than I'd use with a $10k new bike.
 
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