Hey guys.
I ran my MC16 up yesterday, got it up to temperature. I had a bit of an exhaust leak on the front cylinder, so I took the pipe off to give it a clean and I decided to have a look up the exhaust port. I spotted these 4 marks on the piston skirt. (see pic)
I did a bit of research and it says that it's the start of a 4 point seizure, due to lean mixture. Would that be due to the exhaust leak?
If anyone has any ideas or comments, I'd like to know.
Cheers

Right, first things first, a 4 point seizure is NOT caused by lean mixture.

It may well be avoided by an overly rich mixture (causing cool running) but the root cause is insufficient clearance, either due to; thrashing from cold, running in condtions so cold the barrel never gets warm (I did this at Brands last year) or bad matching of components at assemby.

Those marks do look quite clear (good pic by the way) but I've seen plenty of barrels come off perfectly working engines and have similar marks on the piston.
I'm not saying it's ideal as it's clearly not, but if it's been running for years in this setup.....
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Cheers for that fontyy,
This is a new set up i.e carbs, cylinder and pipes are all MC18.
I'll pop the other pipe off today and check the rear cylinder.
I might have to go up another jet size or two.

I had another look at the bike today. I found the offending cylinder's air screw was over 2 1/2 turns out and the balancing was way out. So I think the piston got hot, expanded and scraped on the way up.
I've still got some richer jets coming.
Should I need to move the needles?

In my opinion you should read my post, take the heads off and check the clearance.

If the bike runs well, isn't detonating and the plugs come out a nice coour there isn't a problem with the air/fuel ratio.
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Last edited by fontyyy on Mon Mar 30, 2009 7:40 am; edited 1 time in total

Yep, you were right. I've just discovered on the other side of the cylinder that it's an "A" cylinder. The other cylinder is a "B" when I was ordering the parts I assumed they were the same as I could not find a mark on it.
Nothing like assumptions, it makes an ass outta me.
So. Can I just order a new piston for an "A" cylinder or do I have to get new rings too?
Guttered.