In about May-June 2004 the bike started blowing more and more smoke on startup. So I did a compression test and arrrggghh, 3 out of 4 cylinders weren't much over 100psi, rising by about 15 or 20 psi 'wet'. So leaky rings were definitely part of the problem. Nothing for it but to tell the wife to forget buying food for a while, and then I set to pulling apart the top end (July 2004). And what did I find? Here goes...
Well, after 2 hours' leisurely work the top end of the engine was in pieces. After carting the head and cylinder block off to a renowned Perth motorcycle engineering crew, and having things analysed by folks more qualified in these matters than myself, the verdict was as follows:
- The exhaust valve guides were a bit worn, but well within tolerances and certainly not worth replacing.
- Pistons were fine.
- Dimension-wise, the bores were also fine.
- The rings looked a bit worn, especially the '2nd' rings, which had lost most of their bevelled edges.
- The real culprit in the oil-consumption and compression department, however, appears to have been that the bores had been honed too coarsely in the past, and it is this coarse hone that had chopped out the rings and led to the low compression figures. So the motorcycle engineer gurus performed a fine hone to rectify things.
- Naturally a new set of piston rings was needed. Click here to view Wiseco's instructions for piston ring installation.
- The head was bead-blasted to clean it all up -- including valve dressing and lapping-in. This revealed some small cracks developing between the spark plug holes and the valve seat inserts in the inner two cylinders, ie. cylinders 2 and 3 -- not good news. It indicates that the motor had been running hot. Of course I know the cause: the previous owner(s) had been running a bored-out cammed-up free-breathing engine on stock-standard jetting (see the dyno chart here for the way-lean air-fuel mixture over 5000 rpm when the bike had the standard jets installed).
- The motorcycle engineer chap said that there is a small chance (perhaps 5%) that these cracks could eventually cause a valve seat to drop out, causing... well, it doesn't take too much to imagine the consequences. But it would have cost me a further $400 to have the cracks welded up and a new lot of valve seats inserted and machined. So financial constraints mean that (gulp) I have taken a punt and decided to leave the cracks unfixed, especially since I think the overheating is now taken care of. Time alone will tell if I have been wise...
- Of course a new Wiseco head-gasket for the oversize pistons was required.
- And last -- but by no means least -- a fresh set of valve-stem seals was also popped in.
So, about two weeks after the tear-down, it was The Moment Of Truth. I went over things about 40 times in my head before I finally hit the starter button (naturally I had turned the crank over by hand to make sure that valves weren't going to crash into pistons, etc). Oil? Check. Cam timing? Check. All bolts tightened? Check. Cam-chain tensioner correctly installed? Check. Any bits left over? Nope. Etcetera.
Well after a fair bit of cranking the bike finally started. I think it just took some time for the fuel to get in. And once it was holding its own at idle, I took the bike for a quick strop around the block, up through the gears and back down again, and then let her cool down for a while. Now that things have settled in a little more, I must say that it is a different machine. It starts much more easily. And it sounds crisper and smoother. And no blue smoke when it starts, either -- which is a joyous thing indeed.
I might add that working on these engines is A PLEASURE. They are straightforward, well-engineered, sensible lumps. I mean, you try pulling apart the top-end of your average water-cooled machine in less than 2 hours. If I'd been doing this job on one of my previous GPz's, it would have taken at least 2 days...
The GS1000ST's engine rebuild page is here.
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