C90 carb issue
Discussion
I doubt many of you will know much about them.
But have a parental C90 very dear to me for numerous reasons, a group of us have used it off road, (it is setup accordingly) for charity events.
Recently fitted a new carb, with a slightly louder can and am having a lot of carburation issues. (I would put it back standard but am voted down lol)
Runs fine, starts fine, goes fine until about 50% gas, then it starts missing a bit and top end it sort of chugs, runs, then feels like no fuel, the runs, etc etc. Like someone is momentarily pinching the fuel line almost!
Plug top is black, rich I presume, have tried mixture up and down, no real difference, I am presuming maybe jetting? Fuel is also fine, float bowl was immaculate upon inspection
But have a parental C90 very dear to me for numerous reasons, a group of us have used it off road, (it is setup accordingly) for charity events.
Recently fitted a new carb, with a slightly louder can and am having a lot of carburation issues. (I would put it back standard but am voted down lol)
Runs fine, starts fine, goes fine until about 50% gas, then it starts missing a bit and top end it sort of chugs, runs, then feels like no fuel, the runs, etc etc. Like someone is momentarily pinching the fuel line almost!
Plug top is black, rich I presume, have tried mixture up and down, no real difference, I am presuming maybe jetting? Fuel is also fine, float bowl was immaculate upon inspection
Mixture screw will only adjust the idle circuit so it'll only be effective at idle or part throttle. Certainly sounds like the main jet is too big if it's acting up at over 50% throttle. Can you re-jet it? Or at least, remove the main jet and see if the values stamped on it are right for the bike?
A quick Google suggests they have points/condenser, have a look here for adjustments : https://www.manualslib.com/manual/989554/Honda-S90...
Would also be worth checking the float and making sure there are no signs of gasket failure which could leak fuel. I had trouble with a Vmax carburettor running too rich, seemed to idle ok but cut out when any throttle was applied, turned out to be a leaking choke housing meaning the choke was essentially on all the time.
Would also be worth checking the float and making sure there are no signs of gasket failure which could leak fuel. I had trouble with a Vmax carburettor running too rich, seemed to idle ok but cut out when any throttle was applied, turned out to be a leaking choke housing meaning the choke was essentially on all the time.
Is the C90 a 6V or 12V variant ?
6V can be spotted by a wire running to the left cyl head cover, the points are under that cover and can be adjusted.
12V has electronic ignition ( as far as I know) and should be maintenance-free,
If you write about a free-flowing exhaust I would expect the mixture to be too lean, especially at more than 50% throttle.
First check out in which groove the carb slider needle sits, and move needle to the top, bigger main jet next.
6V can be spotted by a wire running to the left cyl head cover, the points are under that cover and can be adjusted.
12V has electronic ignition ( as far as I know) and should be maintenance-free,
If you write about a free-flowing exhaust I would expect the mixture to be too lean, especially at more than 50% throttle.
First check out in which groove the carb slider needle sits, and move needle to the top, bigger main jet next.
Benni said:
Is the C90 a 6V or 12V variant ?
6V can be spotted by a wire running to the left cyl head cover, the points are under that cover and can be adjusted.
12V has electronic ignition ( as far as I know) and should be maintenance-free,
If you write about a free-flowing exhaust I would expect the mixture to be too lean, especially at more than 50% throttle.
First check out in which groove the carb slider needle sits, and move needle to the top, bigger main jet next.
My first thought was a free flow exhaust would lean out the mixture but the blackened spark plug suggested rich, and the carb seems to be of unknown origin! Figured getting the jet stamp info would help, but sounds like you know your C90s and my advice was more general so I'll bow out...6V can be spotted by a wire running to the left cyl head cover, the points are under that cover and can be adjusted.
12V has electronic ignition ( as far as I know) and should be maintenance-free,
If you write about a free-flowing exhaust I would expect the mixture to be too lean, especially at more than 50% throttle.
First check out in which groove the carb slider needle sits, and move needle to the top, bigger main jet next.
I had similar with a BSA Starfire running massively rich to the point of 8-stroking. If your problem sounds like it's regularly firing every other time (rather than an irregular misfire) it could be that.
Warning - googling that results in very few results that don't also include the word "his" and "inch".
Try a leaner main jet/needle position. As others have said, the mixture screw will only adjust the idle circuit, so won't have an effect above 50% throttle.
Though also bear in mind that going richer is always safer than going too lean, so I'd try all the fixes that involve going rich, or making sure you have enough fuel (pinched fuel line maybe?) before going lean.
Warning - googling that results in very few results that don't also include the word "his" and "inch".
Try a leaner main jet/needle position. As others have said, the mixture screw will only adjust the idle circuit, so won't have an effect above 50% throttle.
Though also bear in mind that going richer is always safer than going too lean, so I'd try all the fixes that involve going rich, or making sure you have enough fuel (pinched fuel line maybe?) before going lean.
Edited by crofty1984 on Monday 9th August 13:36
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