SOHC.co.uk Forums > CB350/400
Carb balancing
Ken4004:
Hi everyone just been trying to set up carbs with the Carbtune kit, not as easy as I thought and very time consuming.
I have managed to get the carbs synchronised but the revs at idle go up and down after tightening everything by this I mean they are a little unstable, also I expected that the engine should pick up fairly well after balancing but it seems to be a bit sluggish.
I am just wondering if it’s due to the air screw position , I have read that the factory set position is around 1 1/2 turns from fully in and also after this initial setting the position can be adjusted with the bike warmed up and the screws are adjusted to get the highest RPM .
If anyone can advise me as to what sort of vacuum values they are getting for all the carbs at 2000 revs and what their opinion is concerning air screw settings and adjustment.
Many thanks regards Ken
Nurse Julie:
The air screws should be set at 2 +/- ½. So start at 2, which is the datum, if running lean, screw in ½ a turn meaning more fuel, if running rich, screw out ½ turn, meaning more air.
Ken4004:
Thanks Julie
I’ll reset the air screws, also just to let you know I have fitted the new seals you sent me for the oil pump which seems to have fixed the oil leak many thanks.
Regards Ken
Nurse Julie:
--- Quote from: Ken4004 on October 21, 2025, 06:58:24 PM ---Thanks Julie
I’ll reset the air screws, also just to let you know I have fitted the new seals you sent me for the oil pump which seems to have fixed the oil leak many thanks.
Regards Ken
--- End quote ---
Great, nothing worse than an incontinent oil pump 😊😊😊
K2-K6:
--- Quote from: Ken4004 on October 21, 2025, 06:42:55 PM ---Hi everyone just been trying to set up carbs with the Carbtune kit, not as easy as I thought and very time consuming.
I have managed to get the carbs synchronised but the revs at idle go up and down after tightening everything by this I mean they are a little unstable, also I expected that the engine should pick up fairly well after balancing but it seems to be a bit sluggish.
I am just wondering if it’s due to the air screw position , I have read that the factory set position is around 1 1/2 turns from fully in and also after this initial setting the position can be adjusted with the bike warmed up and the screws are adjusted to get the highest RPM .
If anyone can advise me as to what sort of vacuum values they are getting for all the carbs at 2000 revs and what their opinion is concerning air screw settings and adjustment.
Many thanks regards Ken
--- End quote ---
There's a little piece missing, that's in Honda's officially detailed method, in addition to that statement.
It calls for this in turning the screw out to get highest rpm ... then to turn it back in again to get a 100rpm drop in revs .... that'll be its final position.
What that's doing is to bring it to it's leanest sustainable mixture point (this being too lean to give decent response to throttle) then to bring it back from that datum point by adding fuel to bring optimal setting for each cylinder.
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