SOHC.co.uk Forums > CB750
750 K2 Strip & Rebuild - help please
Sesman:
I have a set of mercury vacuum gauges from the 70’s. I can honestly state that all the multi cylinder bikes I owned in the 70s and very early 80s benefitted from regular carb vacuum balancing. The improvements were very notable on the 400/4 and Suzuki 550 fours I owned. The mech sync will provide a reasonably accurate physical/dimensional sync, but the vacuum gauges iron out any differences in fluid dynamics created by manufacturing tolerances and inlet flow characteristics. I guess it depends on how fickle and fussy you are with the set up.
K2-K6:
Original manual, 1970, for 750 calls for carb checking firstly at 3000 miles in service schedule, then check at 3000 mile interval. And that's for the four cable system.
Certainly by setting them mechanically but then putting the gauges on will confirm or dispel something of this topic just from the readings that are present.
I feel that review of this element is significantly skewed by not fully appreciating the role the airscrew adjustment takes in setting these up, which most contemporary workshops didn't even have significant understanding of when working with them. The adjustment of most carb rack slide position has insufficient resolution to fully trim the cylinder parity at idle which by design is completed by the idle mixture circuit. That strategy is a sound one and transferred to fuel injection system on virtually all current engine, in that the flywheel pulse monitoring is used to trim fuel quantity for each cylinder to bring smooth idle irrespective of intake tract variances. This is the same as these carburettor (they are particularly well designed) in working with cylinder energy output to give idle parity which then is ignored as throttle gets wider open.
Carb slide position (at idle) is not the right tool to compensate for cylinder to cylinder casting/flow variation as you'd need the maximum flow at full open to control that condition. It would likely give unavoidable poor refinement at idle though, if needed.
K2-K6:
Essentially the vacuum measurement is only controlled (at idle) by the geometry of the space under the throttle slide, that's a small part of the venturi arc in combination with the underside of the slide itself. It cannot take into account most intake tract variability as they would lay outside the scope of influence of the above named parts.
Likely to be more affected by piston ring sealing (pulling the vacuum to be measured) more than anything else, and probably why Honda don't call for monitoring until running in is well out of the way.
Sesman:
Interestingly the Suzuki 550 models of the era had a higher vacuum setting on the two outer cylinders than the inner two.
The rules for best results are to mech sync then vacuum sync with instruments. Then check at regular intervals….it works.
Bryanj:
Now im awake--ish.
When at Meads i had to do a full sevice job on one of the 500 V4's and was trying to get the carbs closer, after 4 hours of peeing about cos we didnt have the correct double bent screwdriver tool i gave up and told the customer i was having problems and it would not be finished till next morning.
Came in fresh, pulled the carb bank and set them up with a 1/16 drill shank on the butterflies.
After reinstalling vacuum was well within book spec but not spot on so buttoned it up and reported as within service spec, customer went away and came back saying he was very happy and thought it was running best it ever had plus thanking me for being honest,
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