SOHC.co.uk Forums > CB750
750 K2 Strip & Rebuild - help please
Lobo:
Bench carb set-up…
Know about the 3mm drill trick in initially setting the sliders - but which end of the carb to stick it in?!
If I use the engine manifold side (straight skirt, and from a previous SOHC post) I’m left with abouts 7mm of thread protruding above the adjuster lock-nut. This goes against my manual suggestions of 1-2mm, which was also what I had when I dismantled the carbs… (ie 1.5mm)
If, otoh, I go for the Airbox side (domed skirt) I do indeed get the 2mm free thread, but virtually closed throat on the opposite side.
Cheers in anticipation….
Oddjob:
It's meant to be a closed throat on the other side as this is how the carbs are at idle.
Lobo:
Cheers Ken. This fits with the 2mm (and my carbs @ dismantle) free thread spec, and suggests the drill goes in the domed skirt / airbox side. That however, seems to be in confliction with other posts I’ve read on this SOHC Forum.
Ho hum.. will go with your advice / the 2mm free thread spec.
Oddjob:
Actually I may have it the wrong way round. IIRC the drill bit goes into the engine side as that the gap the carbs need to idle properly. The idea being that they are all the same gap when you fit them and you then adjust the idle speed correctly using the large screw on the linkage bar.
In the bottom pic it's the left hand carb which is orientated correctly for the drill bit.
K2-K6:
--- Quote from: Oddjob on July 01, 2022, 02:16:09 AM ---Actually I may have it the wrong way round. IIRC the drill bit goes into the engine side as that the gap the carbs need to idle properly. The idea being that they are all the same gap when you fit them and you then adjust the idle speed correctly using the large screw on the linkage bar.
In the bottom pic it's the left hand carb which is orientated correctly for the drill bit.
--- End quote ---
Agree with this to set on bench.
Initially set the threads as you want to match original Simon, then turn the "master" tickover screw to lift all the slides such that you can get the drill of choice underneath the #2 throttle slide like a feeler gauge just sliding fit. Now set the other three carb with that same drill as feeler gauge with their own threads to get parity. Once it slides concisely into all four you've equalled them as a set.
Now bring that "master" control back down to their closed/running position to set them ready to start the engine.
Once it's up and running, don't reset the carb balance (it will be correct iif done accurately) and allow running in to be completed and then valve gaps etc to be confirmed/reset to specification (these gaps will affect vacuum reading and so shouldn't be relied upon during this period) then take a reading of vacuum if you need to quantify it.
It's an odd situation (often propogated by statement of "just needing a carb sync") but mechanical accuracy in setup will hold absolute technical priority over vac gauges if done with diligence. It's primary function is to set the slides at parity for full power balance (this is not measured with vacuum gauges) with conditional low speed running as a side benefit of it.
Fine cylinder "tickover" balance is completed with individual airscrew optimisation in finally setting that smooth running.
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