Friday, 31 July 2009

Cut Me Some Slack

CB Special ownership frustration levels have reached bursting point. Local scrap dealer calls at Diplo HQ once a month and I've been contemplating loading the bike onto his truck ! Shocking mal-combustion issues at anything over 1/2 throttle have been causing grief. Compression ? Balance comp test is holding 90 psi (as much as I could hold against the kicker) for more than a minute with no drop off. Carburation ? Complete carb overhaul X2 including comprehensive clean, float checks, new jets, needles, etc etc etc including up and down main jet sizes. No fix. Ignition ? Timing set, new coil, plugs, condenser, points, leads etc. NO FIX !!!!. Valve adjustment or timing ? Comp test rules out adj but re-set just in case, slight miss-time could be enough to cause trouble at big revs without kissing a valve - Off With His Head then.
Upon initial inspection camshaft is timed perfectly - BUT - cam chain looks a bit slack !?%&@? Could this be enough (low tech variable valve timing) to cause grief at high rpm ? With the head off I'm beginning to suspect the chain tensioner wasn't engaged following some earlier re-build (it locks up out of harms way for assembly and is released post-build.
Very un-honda looking grooves machined in the front of the timing tunnel happen to fit the profile of the timing chain perfectly !!!!!!! WOW - to cause this wear (and so quietly) the cam chain has been centrifugally flung about 25mm from its intended route ! I'm working on the assumption that this "adjustment" is greater at higher revs and is sufficient to advance the valve timing a fraction and give me an open exhaust port when we're really not ready for it.
You can see from the state of the uncleaned piston we've acquired a bit of carbon en-route which can't help. Damage to head indicates earlier valve or piston issues but should clean up ok. Variable valve timing Indeed ! MORE ALUMINIUM SWARF !

1 comment:

Affer said...

I feel for you....I have a similar unidentfiable misfire on the Efelump. I remember spending some hours working on a Lancia Beta until, on a hunch, I took off the fuel filler cap....there was just a hint of a swoosh and I had fixed it!