SOHC.co.uk Forums > CB750
750 K2 Strip & Rebuild - help please
K2-K6:
"Thanks Nige - all makes perfect sense. The only remaining question is ‘how much’ gap to finally leave wrt the “closed / running positionâ€â€¦. 01mm, 0.5mm? I do have the original ‘thread remaining’ figures, and so indeed could roughly set this."
This doesn't matter as you'll be setting the rpm as soon as you start it, so just eyeball it to nearly closed as it will mean you have to use the twist grip to start it running if it's just a little too closed. The whole setup routine (Bryan method) establishes the components in the correct orientation one to another to give potential for in use service adjustment in either direction needed. The final closed gap is always going to be dependent on rpm when running.
K2-K6:
'(To labour the perfect sense bit; I’m left bloody wondering why I bought a set of vacuum gauges 😂, though am indeed happy to leave ‘em in the cupboard!)"
It's certainly become folklaw that using vacuum gauges is the right route to correct any sort of unspecified maladies :) and very wrongly used in someone hoping that it's going to fix their particular problem.
It will of course only fix the slide balance from one to another IF it was wrong in the first place. Also conditional is that all other service items should be resolved first PRIOR to setting with gauges, else you'll establish a fault by wrong reading.
They are a convenient way of setting carb balance if used appropriately (and don't require a workshop to remove carbs to complete) but only with re-establish and accurate prior routine. Often given as a solution though from unreliable scources.
There's a different use though on one bike in these SOHC engine and that's the four cable carbs. They are used to set the individual slide position and balance at idle (there's no master lift parity on that system) after which the in service adjustment is done equally by the "clock face" grooves on the slide stop position screw to keep them that way.
Also at a higher rpm too when the parity of the cables is tested independently of their stop position, this to give full load parity (again because it's not controlled be the lift bar) and important for cylinder balance under maximum torque running. Certainly there's more value there from owning gauges, but most don't don't know why that detail exists in the Honda manual.
Bryanj:
Using the small drill bit ses the slides at the same height which gets you close but the gauges then take into account all the minor manufacturing imperfections in the carbs
Lobo:
Well Bryan, that’s thrown the cat amongst the pigeons in my wee brain…. I was ready to mothball the vacuum gauges.
Did Honda (ie dealerships) routinely vacuum gauge their bikes at the standard valve service intervals? I genuinely thought it was a ‘set & forget’ type procedure, which was never really necessary unless major works had been required.
I definitely like and respect the idea of mechanically matching the sliders once and for all, and ask whether valves set iaw the manual have a significant effect upon MPs and warrant de syncing those sliders? Would you not end up chasing your tail over the years?
Specifically, my 750 has been rebored, new pistons, rings, and a comprehensively serviced head… to the point that any (In/Out) valve could go into any (In/Out) seat (I never knew such tolerances existed). The carbs have been fully serviced. So, in my case here, is there a case for simply leaving alone once bench set?
An interesting topic - appreciated.
Bryanj:
I think carbs were supposed to be checked at first service and then every other major service or annualy and if everything else was adjusted first were frequently inside tolerance but rarely exact.
Always checked after major work and then at first service afterwards, or if removed/disturbed for any reason
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