modifying keihin carbs

M

muskrat

Active Member
howdy.
has anyone heard of any ways to make the stock cv carbs on a cb350 (twin) perform better.
i've had experience modifying cv carbs on harley's with coils cut from the diaphragm spring and drilling the slide for increased throttle response but i can't seem to find any info on the smaller carbs on the hondas.
i'll be rebuilding my carbs next week and figured i'd do whatever i could to give the little guy an edge.
thanks
muskrat
 
Re: modifying keihnin carbs

hey pj.
i've read so much wisdom that you've imparted on this site and appreciate it all.
that being said, i have seen the throttle response differences on a dyno making changes one at a time from the drilled slide first, to cut coils in one coil increments til we got diminishing returns then to a custom spring (by dynojet).
this was at a dyno seminar run by dynojet in the early 90's when they we're pushing modification of the new style cv carbs on harleys over switching out to s&s carbs in an attempt to show you could have a cv carb with close to as good throttle response as the ol' tin can style the harley guys seem to love.
i understand the physics of the spring but up til about 3 coils removed, we got better throttle response before it began to drop off at 4 coils cut. of course the custom spring brought the greatest improvement but dynojet doesn't deal in anything older than mid 80's (i believe)
i can't seem to find any info on the size of these carbs (or the model for that mater) so that i can dive in deeper.
i've found this mod talked about on allota other forums but there talking about bigger bikes than a 350.
maybe you can point me in the right direction for the carb specs on a 71 cb350 twin so i can do more research.
i appreciate any help you can throw my way.
thanks
muskrat
 
to tell ya the truth, i don't recall, it was ages ago.
the mod could have been designed that way for some reason but the dyno charts showed that it worked like gangbusters.
have you had experience drilling the slides (no coil cutting) and if so what size do the holes start at and what size do you finnish drill to.
alternately, are there any mods for these carbs or is it a switch to mikuni kinda deal for most people wanting improvements.
i'm not looking to race but anything to improve throttle response is aces in my book.
thanks
muskrat
 
so it goes something like this (emphasis on something).
theres a small hole in the bottom of the slide which allows air pressure changes between the venturi and the diaphragm allowing the slide to open as air pressure differences increase from giving the throttle a twist. the change in air pressure causes the the slide to raise as it overcomes the spring pressure holding it down. if you increase the diameter of the hole in the slide it allows quicker air pressure differences resulting in the slide raising quicker which translates to quicker throttle response. you can increase the effect by installing a weaker spring making it that much easier for the slide to overcome the spring pressure.
this is however something i would not recommend without some frame of reference to work with as far as how big to drill the hole or what mods (if any) you'd want to try on the spring.
theres lotsa info on the larger keihins the carburated harleys and newer triumphs run, drilling the hole out to an !/8". dynojet even makes a nice kit with jets, weaker spring and a drillbit.
however, without a dyno and a buncha slides to destroy, theres no way to know how big to drill on other models.
thats why i'm hoping to find someone with experience with this mod on the smaller cv's as i've no longer got free access to a dyno and i've only got 2 carbs and i need them both.
unfortunately i think most people just switch to mikunis as i can't seem to find any info yet but hopefully someone will chime in who's had success with this.
thanks
muskrat
 
well, so much for the modifications.
first i need to find the missing parts the PO forgot to replace when they supposedly rebuilt the carbs :mad:
so i opened up the top to find pristine diaphragms and (2) 1/8" vacuum holes in the slide ???
then i moved to the float bowls where the real fun begins.
popped them off expecting to find a float and a coupla jets and to my surprise...there's no jets, and no jet holder on either carb ???
the needle jet is there and the main jet holder but no jets and the jet holder that doubles as a limiter for the float (i think) are not there on either of them. also the needle jet an main jet holders are pretty banged up from what i'm assuming was a very over enthusiastic, under educated screwdriver wielding gazooni, yeah i said it, gazooni.
then to add insult to injury, the float bowl gaskets are wrong.
anyone know where to get the jet holder, i've found the correct gaskets and i'm gonna assume the jets aren't hard to find.
i'll get pix up for you later tonight crazypj.
thanks
muskrat
 
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