Author Topic: Carb swap?  (Read 228 times)

Smithy

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Carb swap?
« on: June 23, 2020, 04:26:47 PM »
Hi all

I don't suppose anyone knows whether it is possible to swap a CV Mikuni BS34 carb to a round slide Mikuni VM carb? I have a BS34SS CV carb on my Suzuki GN250 which I'm not too impressed by. The plug is very sooty but I can't tune out the richness no matter how I try. It seems to be affecting the running just off idle but the mixture screw does nothing and trying to lower the needle just in case sees the bike running rubbish. I did buy a rebuild kit but this has made no difference either.

I would replace the diaphragm but a replacement would cost me almost as much as the bike is worth! Hence why I wonder if it would be better to change to a slide carb. There looks to be enough space to fit a throttle cable on a top fitting carb as opposed to the side fitting of the BS but I wonder if anyone knows of any reason why I shouldn't give it a go.

Just to illustrate why I want to try this a slide and diaphragm would cost anywhere from £66 to £199 whereas a Chinese copy VM is around £37. I know there may be quality issues with a Chinese carb but can it be any worse than a 33 year old original? I know you can buy replacement diaphragms for £25 but when I tried fitting one it was really hard and I put a pin prick in it so I don't want to throw any more money down that route.

Any help gratefully received.

Ian

johnr

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Re: Carb swap?
« Reply #1 on: June 23, 2020, 06:16:42 PM »
if the plug is sooty, and youve not got a restrictive exhaust or clogged air filter, then id guess that the main jet was too big rather than the air screw or needle position being wrong.
having said that, i cant see there being a problem going for a slide carb, but just dont go too big. remember that the cv carb will be a few mm bigger than the equivalent optimum slide carb. so if its currently running a 34mm cv then the slide carb you need would probably be 28-30mm. i bought a chinese mikuni copy for my bsa, it transformed the way it runs, so theres no probleem with the quality of them. the originals are probably made in china now anyway!

Smithy

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Re: Carb swap?
« Reply #2 on: June 24, 2020, 06:27:51 AM »
Hi John

Thanks for that. My exhaust is very non-restrictive and at the moment I have no air filter as the original crumbled and I've not got around to replacing it. The main jet was in the rebuild kit I bought so is presumably standard.

I didn't know about the size difference between CV and slide carbs so thanks for that info. Good to know that if I decide to change to a slide carb the Chinese ones are ok.

Cheers

Ian

spooky

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Re: Carb swap?
« Reply #3 on: June 24, 2020, 07:58:43 PM »
If I remember right, I drilled some holes in the airbox , I only ever revved it to about 8500, actually, I always revved it to 8500 !  Never had a sooty plug, might be worth a bit of duct tape over the holes...  carb wasn`t touched in the years I had colin. Just had the garage upside down looking for an air filter but must have used it on one of the other gn`s or lost it.
if it`s not broke fix it until it is

Schéinen Dag nach

Smithy

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Re: Carb swap?
« Reply #4 on: June 25, 2020, 08:41:06 AM »
Hi Rufus

No problem mate I seemed to have solved the issue now. I reset the float chamber height and all is well. I don't go past 6000rpm so maybe was experiencing symptoms you didn't get. Thanks anyway for looking. I saw another GN250 on ebay and in the write up it mentioned that the plug was black and sooty so perhaps it is a common thing.

Anyway Colin is flying along and now has 3000 extra miles on from when I picked him up and is destined for a lot more now more places are opening up.

Cheers

Ian

johnr

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Re: Carb swap?
« Reply #5 on: June 26, 2020, 08:24:11 AM »
Hi John

Thanks for that. My exhaust is very non-restrictive and at the moment I have no air filter as the original crumbled and I've not got around to replacing it. The main jet was in the rebuild kit I bought so is presumably standard.

I didn't know about the size difference between CV and slide carbs so thanks for that info. Good to know that if I decide to change to a slide carb the Chinese ones are ok.

Cheers

Ian

yeah, its a peculiarity of how cv carbs work, reliant as they are on the incoming air to create the pressure differential that sucks the slide up, slide carbs are completely manual and so work on the position of the riders wrist rather than the position of the slide in the carb. as an example, suzuki gs1000, first model, 26mm slide carbs, next model, exact same power output and torque curve, but power delivery smoothed out by fitting 32mm cv carbs. cv's aree also 'greener' in that they always give the optimum fuel air mix for a given engine speed/throttle setting.