SOHC.co.uk Forums > CB750
Rewiring K2 with later high beam switch
the-chauffeur:
Pete - thanks so much for the kind words. I thought about using copper grease on the base gasket but decided against it for no very good reason. Should've gone with my first instincts, but again, I've got another spare if the one that's on there turns out to be dead.
Your post couldn't've come at a better time, because guess what? Bikewise, today got a lot worse.
After putting the 750 to one side, I decided to go get some food and thought I'd take the SL350. What could go wrong? Well, the top of the left shock absorber separating itself from the bottom four miles out wasn't something that would've immediately sprung to mind. But that's exactly what it did. Oddly, I didn't notice anything wrong until I got off the bike (although the spring had wedged itself against the frame, so I didn't lose much in the way of rebound) but I needed recovering 'cos there was no way I was riding it like that once I'd noticed it. So I don't think I was ever in any real danger.
However, what the separation did do was take a chunk out of the rare-as-hell-and-virtually-irreplaceable sidepanel which I spent a bundle on getting painted. Fortunately, it's not holed or cracked - I think it's just taken a layer out about the size of a 10p coin - but colour matching is going to be a pig of a job. On the upside, Silvers have got the collars that disappeared themselves when the shock disassembled itself are still, so at least that's not lost. Guess that's one of the benefits of such a rare bike - no-one else needs the parts.
It's not good, but I can only look at the whole sorry mess and say hey, it could've been so much worse. I'm still in one piece, and the remedial work on both bikes is more time than money (well, maybe not the sidepanel . . . ).
Right, let's tear down that 750.
the-chauffeur:
2 hours - that's how long it's taken me to get the engine down to the crankcases and tidy the garage. Not bad, huh?
Looking at the areas where the leak was emanating from, it seems to be attributable to one of a couple of things - the favourites are a poor seal around the thin strip of gasket covering the case near the front of the cam chain guide, or something due south of the up oilways (round the rear cylinder studs) from the crankcase. The surfaces are completely flat, so it's not warp or scarring, but I can't remember if I used new o-rings around the oilway studs. Having said that, if they weren't sealing, I would have thought any leakage would have taken the much shorter route to air - i.e. go out from the cylinders backwards and onto the top of the crankcases. There was also nothing particularly unusual about the o-rings around the cylinder bases. Whatever, it shouldn't be a particularly tricky fix.
I'll let the faces a while to dry out, and run them over with thinners before putting everything back. I'll also pull the valves, check the guides, clean up the seats, gently re-lap the faces and put on new seals. I already kinda live by Brian's earlier maxim of never throwing anything away, and numerous engine gasket kits have provided me with more than enough spares. 'll also clean the cylinder bases and reseat the larger o-rings before reassembly.
Suffice it to say I've got the bit between my teeth. The problems I've got are ones that I understand and can fix, so it's just a matter of getting on and doing it. I find it's easier to be motivated by this kind of situation than be down about it, 'cos I'm pretty sure it just needs a bit more twiddling to get it perfect. And by perfect I mean oil tight and running without smoke. Talking of which, pistons 1 and 2 had some carbon deposits on the tops, but 3 and 4 (remember 4 was smoking a bit) were pretty much clean as a whistle. Dunno what - if anything - that means.
Tired now. Busy day.
UK Pete:
Well done Neil, positive attitude gets you everywhere, these old bike can be a test of anyones patience, i have lots of oldish jap bikes each one has presented me with some sort of problem , but evrything comes good eventually,
I am now off to ride some of mine today i think first of all i will take the 750F2 up the oakdene cafe, then come back and take the K1 out for a 30 mile or so ride just to keep it healthy, then when back from that ride i will go ballistic on my FZ1, for an hour to pump up the adrenaline, it keeps you feeling young you know
pete
the-chauffeur:
Looks like the weather held for you today . . .
On further inspection, it looks to me like the leak around the cylinder base was caused by a casting flaw in the underside of the cylinder block. There's a small patch around one of the oil returns that isn't completely flat - it's very slightly rough. It doesn't run the width of the band from the return through to the engine exterior, but the pressure inside the crankcases now that the oil pump has been overhauled and works at pretty much maximum potential is enough to force tiny amounts of oil/air out.
So I've refitted the base gasket after coating most of it in copper grease and the bit that sits over the rough patch (and areas in front of the oil returns) with thin coats of sealant on both faces. Top went back on with pretty much the same ease as the last time I put it together, and there's just a few jobs left for the re-rebuild to be completed. One benefit of picking up he new head I hadn't foreseen is that it came with better fitting exhaust flanges - the bits that join the exhausts to the head - than I've got on mine. I never really thought about it until now, but mine were really sloppy, which could explain the with smoke from #4 (I always thought it was an air leak).
So that's cool.
the-chauffeur:
Well, I thought it was cool. I was wrong.
After stripping and rebuilding the top end, I got to run the bike last week. Top end was pretty much fine, and the smoke from #4 was residual oil (WD40 or similar) in the exhaust pipe. However, I couldn't help notice what seemed to be oil dripping from the front filter housing. Closer inspection revealed the housing was fine, so the next logical conclusion was that it was old oil that had leaked from the cylinder base, dried on and then liquefied when everything heated up. Sadly, that would just be too easy.
Seems the base gasket leak masked a smaller, but equally significant leak - from the joint between the crankcase halves. Never seen that before, but it's there. Like the other leak, I'm putting it down to a properly functioning oil pump increasing the internal pressure, but that's little consolation.
Looks like the only remedy is to pull the engine, flip it, unseal and reseal and put it back in again. At least that way I can be sure it's done right.
I'm sure every step I go forward is getting me closer to having a properly sorted bike, but it sure as hell is putting me through the ringer to get there.
:'(
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