YZ Engine Rebuild in Progress

As anyone reading would note, YZ hasn’t been happy recently. Its been making noises I didn’t like the sound of so I decided to pull the barrel off and take a look, the first time I’ve done so. You can do this with the engine still in the bike. The first thing of note was the piston, the exhaust side was unremarkable but the inlet side had fairly deep scoring, above what I’d consider normal:

Its a widely accepted fact of life that two stroke engines do need maintenance from time to time and with two years of (ab)use, I can hardly complain and I’d expected it would need a piston. The barrel didn’t photograph well but I think its probably serviceable which is a relief as getting a new coating put in is a pain. I had removed the piston so I turned my attention to look at the bottom end of the engine and specifically the big end bearing:

The big end was fine however in checking it, I thought the little end felt like I’d filled it with sand. A closer look revealed:

Which is something I’d not expected, the little end is to put it technical engineering terms, knackered. I’ve damaged two stokes in a multitude of different ways but I’d never broken a little end bearing until now. First time for everything I guess. Sadly, with damage like that a new con-rod is needed and that means splitting the casings to get the crank out. That in turn means taking the whole engine out the bike. To do that there are a number of serious nuts that need undoing. It was one of these that I previously managed to snap ligaments in my hand whilst attempting to undo. This time around with having the barrel off, its possible to rather robustly brace things to stop them turning:

This may look abusive but the little end is already broken so it can’t really damage much else. The bike looks rather sorry for itself without the engine. You can see from the photo how spindly the frame is which makes more sense when you realise the engine is actually part of the frame (a stressed member):

So I’ve found it needs a new piston and a new con-rod but why was the bike rattling? My best theory so far is the power valves are also in need of attention. The push rod from the bottom to top end is loose and the free play in it could account for the rattle. The fact it doesn’t work directly as engine rpm changes also matches well with the symptoms. In looking at the valves themselves on the workbench, it was clear the left hand valve was not completely closing. Wear in the power valve cams and the cam following pins would appear to account for this. In an ideal world, I would just replace all these parts however to do so looks like adding hundreds of pounds to the repair bill. I think there is adjustment which would allow the free play in the cams to be taken up, the push rod may need to be replaced however.

A photo of the engine partially dismantled on the workbench. I can’t split the casings until I find a puller to take the ignition off. The cams are on the left and right of the square oily opening on the barrel. The followers are on the ends of the forks on the black thing on the workbench on the bottom left.

It could be a lot worse, something could have seized whilst it was running and caused considerably more damage. Finding reasonable priced spare parts seems to be a bit of a challenge and I haven’t figured out what to do with the power valves yet. The YZ has some life left in it yet and will live again!

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