SOHC.co.uk Forums > CB350/400
Carb Synchronising with a gauge.
scifi:
Hi All, I have several Hondas, including a cb400-4 that runs really well. I recently bought a 4 gauge meter for it and can see that the inlet manifolds have removable screws to connect the pipes. What should I adjust (at tickover..?) It probably need no adjustment, but I just want the practice. The only problem I see, is that the number 8 plugs ran sooty, but when I used number 7 plugs, the central electrode was a perfect colour, after a 2 x 30 mile run. The sootyness must be a high revs jetting, but the 4 gauges won't find this out, will it..?
My CB250 however sometime fails to start on two cylinders, and is more in need of synchronising, but I can't find any removable screws on the manifolds.
Any tips for anyone that has used these gauges, will be warmly welcomed.
Bryanj:
All you are doing with gsuges is evening out the 4 seperate pulses as much as you can at the tickover so you adjust the slides to give an even vacuum on each one
exvalvesetdabbler:
My goto phrase is if it's not broken don't b****r about with it.
To sort out the rich running, the carbs need a look first before you break out the vac guages.
Float heights,
Float valves
Emulsion tubes/ needles worn at the top
Bunged up air filter
Fuel getting passed main jet o ring
Static balance on the bench
I'm sure others will chime in with something I have missed.
Dave
K2-K6:
--- Quote from: exvalvesetdabbler on December 08, 2025, 09:19:01 PM ---My goto phrase is if it's not broken don't b****r about with it.
To sort out the rich running, the carbs need a look first before you break out the vac guages.
Float heights,
Float valves
Emulsion tubes/ needles worn at the top
Bunged up air filter
Fuel getting passed main jet o ring
Static balance on the bench
I'm sure others will chime in with something I have missed.
Dave
--- End quote ---
I agree with all points, particularly those I've highlighted.
All a synchronisation is supposed to achieve is parity of air slides on the carburettors. Importantly, it doesn't compensate for any other fault that may be present when you carry that out with vac gauges. By extension, if a fault existed on one cylinder, then the gauge method will, inappropriately, install a "wrong" correction onto the slides.
One of the most important influences is the ignition, particularly on the fours, as that will definitely affect the balance outcome if it's set inaccurately.
Mikep328:
Agree with the comments re sorting out other possible faults. Re sync gauges - the individual gauges quite likely will not agree out of the box. IOW, you might adjust all carbs to the same setting using the gauges but the carbs may not actually be set the same. So with a multiple gauge set, attach each gauge in turn to the same cylinder and adjust each gauge calibration so each reads the same. THEN you can attach all four gauges to the cylinders and get accurate indication of any difference between them.
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