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cb400f - carb sync

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blackeagles:
finally got the carbs set up something like it says in the book - bit difficult judging 1mm for the throttle slide even if you do it with the slide down - anyway its near enought but with the air pilot screw out 2 3/4 turns it runs (a bit rough) but its very rich - when i take the plugs out they are black.

when i get the sync guages and start resetting the hight of the slides to get the 16-24 cm hg +- 3cm hg reading it is going to move the slide away from the 1mm setting which i assume is just to allow you to get the bike started...which brings me onto the question

most of the youtube carb sync videos seem to say that the cm/hg reading is not important just get them all the same so how many of you folks set them between 16/24 or do you all just 'get them the same' and forget heo reading?.

i'm asking this 'cos since i'm already running very rich then the more i raise the slide (to reach the correct reading) the richer it will become (and the more the primary chain is going to 'rattle')

paul G:
Raising the slides will not make it richer that is to do with the float height and the air screw ;)

K2-K6:
The synchro routine has virtually nothing at all to do with jetting. It's just to get parity across the four carbs,  and hence cylinders, to make sure that during use the engine is running with all four pulling equally.

Mixture is controlled by the jets, needle position and trimming pilot circuit with screws to get each part of the range correct.

Yes to just getting all vac levels to match and don't worry about absolute reading. If absolute is really far out when running then it would bring up different questions about engine condition, possibly. But certainly wouldn't worry about it at this stage.

Rob62:
To answer the original question "do you set to a specific vac level of just get them all the same". You will find that as you adjust the individual slides the idle speed will go up and down and you will have to keep adjusting all the slides up or down until you reach a point where they are all showing equal vacuum and the motor is at the correct idle speed. At that point the level of vacuum will totally depend on the idle speed i.e. the lower the idle the higher the vacuum (as the motor is being throttled)… the higher the idle the lower the vacuum.. So the short answer is just balance them out and the level of vacuum takes care of itself.

billywingnut:
I bought a set of cheap gauges, before using them it's best to check there calibration. I rigged up some plastic tubing ot connect all four gauges together then connected them to one of the carb inlets. I was then able to check all four gauges gave the same readings.
I tweaked one to get them all the same before using them.
It's true that as long as all for carbs read the same the levels are not that important. Dont adjust all four though, you need one carb as a reference.
two and a half turns out of the airscrew seems to be a good starting point.
good luck

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