Generic motorcycle carb question

Wigwam Jones · 07-10-2017, 12:53 PM

While I am collecting funds and trying to decide on what Chinese bike to get, I am also trying to resurrect my ancient 82 Yahama SR250 Exciter. It has some serious issues, not least of which is the carburetor.

I bought the bike recently because I owned a new one back in the day and I had a case of nostalgia. Expensive nostalgia apparently.

The factory carb is a CV unit - a Mikuni BS34-III. I don't know the jetting. It's in a bad way. I suspect it not only needs to be deep-cleaned, it also needs a new diaphragm, and who knows what else. It works, badly, at idle, but nothing as soon as I put a load on it by trying to ride; chugs, stalls, dies, etc. At random times (usually in the middle of the night), it decides to puke gasoline out the intake and all over the garage floor. It's currently disconnected for safety's sake.

https://www.facebook.com/thebillmatt...2680193094726/

So I am not a carb mechanic and I doubt my own abilities to fix this. It's on the edge of what I'd feel comfortable attempting on my own. Not that I really mind learning, but I can't afford a lot more expensive mistakes, and diaphragms are not cheap on this unit. I can send it out and have a pro do it, but that won't be cheap either.

So I am looking at replacement carbs. My first thought was to get a round slide carb of the same size - 34mm. However, I am surprised to find how few 250cc bikes have a carb that big on them. It appears that most are in the 28mm to 30mm size range when we're talking about flat or round slides instead of CV designs.

I should add that the SR250 is a single cylinder OHV motor.

So I am looking at carbs. Not sure how to go about buying the right size. Not sure if I really should be looking at cheap flat-slide OKO / KOSO / etc or the more expensive Mikuni round-slide units? Not sure what size to consider.

The one I *think* might be about right would be the Mikuni VM32-33. Found a guy with the same bike as mine who says that's what he used:

https://terrydean.wordpress.com/2013...e-sr250-parts/

But I also saw a comment on an Amazon carb that said she used a VM34-168 with stock jets and it works great on the same bike.

https://www.amazon.com/gp/product/B0...3C7LO58P&psc=1

Top customer reviews

5.0 out of 5 starsworks perfect. Elevation 300
ByNatalie R Smithon August 10, 2015
Verified Purchase
Put on my 1980 Yamaha SR250, works perfect. Elevation 300. So far stock main and pilot work fine, haven't gotten up 40+mph yet. Using Emgo 60mm pod filter and Spectre Breather filter for the crankcase.

So as you can probably guess, I'm confused and also I don't know what I am doing.

I am aware that if I change from the CV to either a round or flat slide, I'll have to redo the throttle cable, but that looks simple enough (famous last words). I am aware that throttle response will be different as well, and many say that CV carbs are to be preferred for street use. Yamaha certainly used the BS34 carbs in many of its motorcycles, either alone or in braces of four.

So I am open to suggestions here. Not in a tearing hurry, but I'd like to kind of pick a direction and move towards it.

Thanks!

**cheesy** · 07-10-2017, 01:20 PM

This doesn't seem unreasonable, though the shipping is nuts
http://jbmindustries.com/yamaha650.html

If it were me, I'd stick with the CV. Sounds like a thorough cleaning with an ultra sonic cleaner is what's needed. Peeing on the floor is a sign of either crud on the needle and seat or a worn needle. I'm leaning towards crud.

Carbs aren't hard(God knows how many times I was in the CVs on my Honda chasing an electrical problem

), and parts, for the most part, are cheap. Less of a headache to rebuild your carb than to adapt a slider unit to your bike, IMO.

Wigwam Jones · 07-10-2017, 01:28 PM

Quote:

Originally Posted by cheesy

This doesn't seem unreasonable, though the shipping is nuts
http://jbmindustries.com/yamaha650.html

If it were me, I'd stick with the CV. Sounds like a thorough cleaning with an ultra sonic cleaner is what's needed. Peeing on the floor is a sign of either crud on the needle and seat or a worn needle. I'm leaning towards crud.

Carbs aren't hard(God knows how many times I was in the CVs on my Honda chasing an electrical problem

), and parts, for the most part, are cheap. Less of a headache to rebuild your carb than to adapt a slider unit to your bike, IMO.

Thank you for the link! I can appreciate that rebuilding the carb myself is cheaper and perhaps easier than swapping carbs. I just tend to doubt my abilities in this area and although I agree that it would be good to gain the experience, at the moment I am pouring money down this rathole. The bike cost me quite a bit, then half that again for a 'tuneup' at the local dealer who didn't apparently test ride the bike and refuses to stand good for it, now the timing chain rattles but the adjuster is all the way in and the carb pees gasoline on the floor. If I rebuild this carb and mess it up, I'm going to take it into the back yard and set it on fire. That's where I am with that. So that's why I am considering a new carb. At least, hey, new.

EDIT: OK, so I was looking around and found this unit on eBay:
http://www.ebay.com/itm/80-82-YAMAHA...hZVfC0&vxp=mtr

Don't know if it would work, but that's a little less 'ouch' if I ruin it. Hmmm. Now I'm back to pondering my ability to rebuild a carburetor.

I rebuild an auto transmission once in high school, but I had a bunch of parts left over and it smelled funny after that.

humanbeing · 07-10-2017, 09:37 PM

Quote:

Originally Posted by Wigwam Jones

...I am looking at replacement carbs...
I should add that the SR250 is a single cylinder OHV motor...

Chinese loves replacing that BS34 w/ a TK model (as XF250 & tons GN250 clone)
Typo !!! https://www.cmsnl.com/yamaha-sr250-e...list/A-05.html

**cheesy** · 07-10-2017, 09:38 PM

Give it a shot, mistakes are how we learn.

Get a manual and read it.

Take your time and take pics of each step if you're unsure.

Use an old cup cake pan to place the parts in. Put some carb cleaner in the pots with the brass parts and let them soak.

Use the right size screw driver to remove the jets. The wrong size WILL eff things up.

Clean all the passages.

Clean all the passages. Again.

And one more time.

Triple check all the tiny holes in the emulsion tube.

Set float to specs.

Replace the Philips head bowl screws with Allen head screws.

Figure on about 2 hours from start to finish by being methodical.

Carbs are easy compared to automatic transmissions. And a hell of a lot less parts.

Swampy · 07-11-2017, 10:17 AM

Quote:

Originally Posted by cheesy

Give it a shot, mistakes are how we learn.

Use the right size screw driver to remove the jets. The wrong size WILL eff things up.

Clean all the passages.

Clean all the passages. Again.

And one more time.

I would clean the passages once more to be sure....... Great advice. I learned the screw driver size the hard way for sure.

Wigwam Jones · 07-11-2017, 11:21 AM

Thanks for the advice guys. I am going to buy a carb. I think a VM-32. I understand your advice and appreciate it very much. You have no idea the extent to which I do not want to wrench on my carb now that I know what's involved. That's just not me. I'll get mad and frustrated and pitch it in the trash. I'm going to buy a carb and try it out, and send my existing carb out to have a pro work on it. I'm just not that guy, it's not in me.

EDIT: On second thought, maybe I will tackle working on the original carb myself - but only after I have a functioning carb installed and running. Otherwise the risk and pressure of messing the only carb I have up, that's not something I can deal with. I'm weird, I know, but a man's got to know his limitations. Thanks!

goat67 · 07-11-2017, 08:45 PM

Quote:

Originally Posted by Wigwam Jones

Thanks for the advice guys. I am going to buy a carb. I think a VM-32. I understand your advice and appreciate it very much. You have no idea the extent to which I do not want to wrench on my carb now that I know what's involved. That's just not me. I'll get mad and frustrated and pitch it in the trash. I'm going to buy a carb and try it out, and send my existing carb out to have a pro work on it. I'm just not that guy, it's not in me.

EDIT: On second thought, maybe I will tackle working on the original carb myself - but only after I have a functioning carb installed and running. Otherwise the risk and pressure of messing the only carb I have up, that's not something I can deal with. I'm weird, I know, but a man's got to know his limitations. Thanks!

That sounds like a good plan.

If you can build a tube amp you can rebuild a carb. Really it is pretty simple.

Lots of experience here to help ya out.

goat67 · 07-11-2017, 08:57 PM

Found a guide to rebuilding your carb. https://drive.google.com/file/d/0B72...o1M1ZaRU0/edit

Wigwam Jones · 07-12-2017, 08:25 AM

Quote:

Originally Posted by goat67

Found a guide to rebuilding your carb. https://drive.google.com/file/d/0B72...o1M1ZaRU0/edit

Very cool guide, thank you!

**JerryHawk250** · 07-12-2017, 09:10 AM

Quote:

Originally Posted by Wigwam Jones

Very cool guide, thank you!

This will give you a chance to learn more about the carb.

There not as scary as they look lol. Since you have a new one coming just take your time and go through the old one and rebuild. We all started somewhere. It will be a good learning experience.

Wigwam Jones · 07-12-2017, 09:51 AM

Quote:

Originally Posted by JerryHawk250

Since you have a new one coming

I haven't ordered it yet. I was still hoping to get some advice as to size/type. I mean other than "rebuild your old one," LOL.

I was looking at this one:

https://www.amazon.com/gp/product/B0...V15NLPCQ&psc=1

Alvey 32 mm Carburetor (PD32J) for the Honda ATC250, TRX300, TRX350, & TRX400 ATVs

It's a 32mm that is about the right size for my bike. I note that the manifold side outer diameter is 40mm, same as my existing carb. The airfilter/airbox side is 55mm, again, same as my existing carb. It has a manual choke, same as mine, and it seems to have the same kind of CV-based throttle cable to a butterfly valve that my BS34-III has. The only thing is it's 32mm and not 34mm, which I am guessing is not really a big problem.

I also can't seem to find out how long it is from end-to-end. I found how 'high' it is, but that's not what I need to know. My existing carb is 115mm in length from the manifold side intake to the airbox connection.

And I see nothing about what kind of jets it uses. Can't find the manufacturer's website, only a lot of links to Chinese importers. No manual that I can find.

Alternatively, I can buy something like the Mikuni clone 28mm or 30mm carbs you guys buy for the Hawks, etc. I will have to come up with an adapter to mount it to my manifold side, and perhaps use a pod filter instead of my airbox or come up with some kind of adapter to lengthen my carb overall. And of course the throttle cable end is different with round and flat slide carbs over the CV style 'lawnmower' kind of throttle cables.

So I'm still pondering. Any help would be appreciated!

humanbeing · 07-12-2017, 10:42 AM

Half-moon (PWK/ Koso / OKO...) http://www.chinariders.net/showthread.php?t=16198 works fine & reasonable in price

goat67 · 07-13-2017, 07:54 AM

Go to this website http://kosonorthamerica.com

here is an image

Not cheap but looks to be high quality and jets are available you may not need to jet it anyway

Wigwam Jones · 07-16-2017, 01:28 PM

I bought a carb and fitted it on the bike today.

Everything appears to line up and mount OK, but my existing throttle cable is too short. I have ordered a new one from Amazon, should be here in a couple days, so I can try it out next weekend. Actually, I ordered a couple - the one that looks like it might work, and a 'universal' cable that you solder the ends on. That way I should be covered no matter what.

The side of the carb on the engine side is loose in the rubber mount to the engine; I don't think it is the 40mm outside diameter they claim; just a tiny bit smaller OD than the original carb. I guess I can make up the difference with electricians tape or something, and shop for a 39mm engine-side mount to do it the right way.

Now I feel better about ordering a rebuild kit for the original Mikuni carb and trying to rebuild the carb myself.

Thanks for everyone's advice and suggestions. I will keep this thread updated with results.

07-10-2017, 12:53 PM	#1
Wigwam Jones Join Date: Jul 2017 Location: SE Michigan Posts: 86	Generic motorcycle carb question While I am collecting funds and trying to decide on what Chinese bike to get, I am also trying to resurrect my ancient 82 Yahama SR250 Exciter. It has some serious issues, not least of which is the carburetor. I bought the bike recently because I owned a new one back in the day and I had a case of nostalgia. Expensive nostalgia apparently. The factory carb is a CV unit - a Mikuni BS34-III. I don't know the jetting. It's in a bad way. I suspect it not only needs to be deep-cleaned, it also needs a new diaphragm, and who knows what else. It works, badly, at idle, but nothing as soon as I put a load on it by trying to ride; chugs, stalls, dies, etc. At random times (usually in the middle of the night), it decides to puke gasoline out the intake and all over the garage floor. It's currently disconnected for safety's sake. https://www.facebook.com/thebillmatt...2680193094726/ So I am not a carb mechanic and I doubt my own abilities to fix this. It's on the edge of what I'd feel comfortable attempting on my own. Not that I really mind learning, but I can't afford a lot more expensive mistakes, and diaphragms are not cheap on this unit. I can send it out and have a pro do it, but that won't be cheap either. So I am looking at replacement carbs. My first thought was to get a round slide carb of the same size - 34mm. However, I am surprised to find how few 250cc bikes have a carb that big on them. It appears that most are in the 28mm to 30mm size range when we're talking about flat or round slides instead of CV designs. I should add that the SR250 is a single cylinder OHV motor. So I am looking at carbs. Not sure how to go about buying the right size. Not sure if I really should be looking at cheap flat-slide OKO / KOSO / etc or the more expensive Mikuni round-slide units? Not sure what size to consider. The one I think might be about right would be the Mikuni VM32-33. Found a guy with the same bike as mine who says that's what he used: https://terrydean.wordpress.com/2013...e-sr250-parts/ But I also saw a comment on an Amazon carb that said she used a VM34-168 with stock jets and it works great on the same bike. https://www.amazon.com/gp/product/B0...3C7LO58P&psc=1 Top customer reviews 5.0 out of 5 starsworks perfect. Elevation 300 ByNatalie R Smithon August 10, 2015 Verified Purchase Put on my 1980 Yamaha SR250, works perfect. Elevation 300. So far stock main and pilot work fine, haven't gotten up 40+mph yet. Using Emgo 60mm pod filter and Spectre Breather filter for the crankcase. So as you can probably guess, I'm confused and also I don't know what I am doing. I am aware that if I change from the CV to either a round or flat slide, I'll have to redo the throttle cable, but that looks simple enough (famous last words). I am aware that throttle response will be different as well, and many say that CV carbs are to be preferred for street use. Yamaha certainly used the BS34 carbs in many of its motorcycles, either alone or in braces of four. So I am open to suggestions here. Not in a tearing hurry, but I'd like to kind of pick a direction and move towards it. Thanks!
	0

07-10-2017, 01:20 PM	#2
cheesy Moderator Join Date: Jul 2010 Location: Elburn, IL Posts: 5,817	This doesn't seem unreasonable, though the shipping is nuts http://jbmindustries.com/yamaha650.html If it were me, I'd stick with the CV. Sounds like a thorough cleaning with an ultra sonic cleaner is what's needed. Peeing on the floor is a sign of either crud on the needle and seat or a worn needle. I'm leaning towards crud. Carbs aren't hard(God knows how many times I was in the CVs on my Honda chasing an electrical problem ), and parts, for the most part, are cheap. Less of a headache to rebuild your carb than to adapt a slider unit to your bike, IMO. __________________ Cheesy ______________________________________ 07 Hi Bird Enduro-Flown the Coop 07 Ural Gear Up 79 Honda CX500 77 VeloSolex 4600 V3 73 VeloSolex 3800 I'm making this up as I go.-Indiana Jones Telling me something 'is for my own good', is a fast way to a fat lip.-Me I don't even care about my own problems, why should I care about yours?-Quote on one of my favorite t-shirts
	0

07-10-2017, 09:38 PM	#5
cheesy Moderator Join Date: Jul 2010 Location: Elburn, IL Posts: 5,817	Give it a shot, mistakes are how we learn. Get a manual and read it. Take your time and take pics of each step if you're unsure. Use an old cup cake pan to place the parts in. Put some carb cleaner in the pots with the brass parts and let them soak. Use the right size screw driver to remove the jets. The wrong size WILL eff things up. Clean all the passages. Clean all the passages. Again. And one more time. Triple check all the tiny holes in the emulsion tube. Set float to specs. Replace the Philips head bowl screws with Allen head screws. Figure on about 2 hours from start to finish by being methodical. Carbs are easy compared to automatic transmissions. And a hell of a lot less parts. __________________ Cheesy ______________________________________ 07 Hi Bird Enduro-Flown the Coop 07 Ural Gear Up 79 Honda CX500 77 VeloSolex 4600 V3 73 VeloSolex 3800 I'm making this up as I go.-Indiana Jones Telling me something 'is for my own good', is a fast way to a fat lip.-Me I don't even care about my own problems, why should I care about yours?-Quote on one of my favorite t-shirts
	3

07-11-2017, 11:21 AM	#7
Wigwam Jones Join Date: Jul 2017 Location: SE Michigan Posts: 86	Thanks for the advice guys. I am going to buy a carb. I think a VM-32. I understand your advice and appreciate it very much. You have no idea the extent to which I do not want to wrench on my carb now that I know what's involved. That's just not me. I'll get mad and frustrated and pitch it in the trash. I'm going to buy a carb and try it out, and send my existing carb out to have a pro work on it. I'm just not that guy, it's not in me. EDIT: On second thought, maybe I will tackle working on the original carb myself - but only after I have a functioning carb installed and running. Otherwise the risk and pressure of messing the only carb I have up, that's not something I can deal with. I'm weird, I know, but a man's got to know his limitations. Thanks! Last edited by Wigwam Jones; 07-11-2017 at 12:02 PM.
	1

07-11-2017, 08:57 PM	#9
goat67 Join Date: Mar 2017 Location: Twin Cities MN Posts: 525	Found a guide to rebuilding your carb. https://drive.google.com/file/d/0B72...o1M1ZaRU0/edit __________________ 2016 Brozz
	3

07-12-2017, 10:42 AM	#13
humanbeing Join Date: Mar 2008 Posts: 2,436	Half-moon (PWK/ Koso / OKO...) http://www.chinariders.net/showthread.php?t=16198 works fine & reasonable in price __________________
	1

07-13-2017, 07:54 AM	#14
goat67 Join Date: Mar 2017 Location: Twin Cities MN Posts: 525	Go to this website http://kosonorthamerica.com here is an image Not cheap but looks to be high quality and jets are available you may not need to jet it anyway __________________ 2016 Brozz
	1

07-16-2017, 01:28 PM	#15
Wigwam Jones Join Date: Jul 2017 Location: SE Michigan Posts: 86	I bought a carb and fitted it on the bike today. Everything appears to line up and mount OK, but my existing throttle cable is too short. I have ordered a new one from Amazon, should be here in a couple days, so I can try it out next weekend. Actually, I ordered a couple - the one that looks like it might work, and a 'universal' cable that you solder the ends on. That way I should be covered no matter what. The side of the carb on the engine side is loose in the rubber mount to the engine; I don't think it is the 40mm outside diameter they claim; just a tiny bit smaller OD than the original carb. I guess I can make up the difference with electricians tape or something, and shop for a 39mm engine-side mount to do it the right way. Now I feel better about ordering a rebuild kit for the original Mikuni carb and trying to rebuild the carb myself. Thanks for everyone's advice and suggestions. I will keep this thread updated with results.
	0