Generic motorcycle carb question

[Wigwam Jones]

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.

https://scontent-ort2-1.xx.fbcdn.net/v/t1.0-9/19748662_10212680134733267_718935314849622759_n.jp g?oh=abbdc1156a9af1204d3b1e368b09d13f&oe=5A0CC963

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/thebillmattocks/videos/10212680193094726/

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.

https://scontent-ort2-1.xx.fbcdn.net/v/t1.0-9/19748642_10212680129493136_7141524625176961912_n.j pg?oh=8a3573ab27ae2dcebd2923a95988f130&oe=5A044D32

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/2013/01/18/more-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/B000GV8QAO/ref=ox_sc_act_title_1?smid=A1TFBK3C7LO58P&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]

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 :ohno:), 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]

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 :ohno:), 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-SR250-CARBURETOR-SLIDE-PISTON-RUBBER-DIAPHRAGM-22-3F7DP-/142430818555?hash=item212989d0fb:g:XuMAAOSwVJhZVfC 0&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]

...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-exciter1-1982-c-usa_model8871/partslist/A-05.html

[cheesy]

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]

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]

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]

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]

Found a guide to rebuilding your carb. https://drive.google.com/file/d/0B72SKWdDvNtmX016T0o1M1ZaRU0/edit

[Wigwam Jones]

Found a guide to rebuilding your carb. https://drive.google.com/file/d/0B72SKWdDvNtmX016T0o1M1ZaRU0/edit

Very cool guide, thank you!

[JerryHawk250]

Very cool guide, thank you!
This will give you a chance to learn more about the carb. :tup: 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]

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/B01LWZ55Y0/ref=ox_sc_act_title_1?smid=AE1GSV15NLPCQ&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]

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

[goat67]

Go to this website http://kosonorthamerica.com

here is an image
http://kosonorthamerica.com/wp-content/uploads/2016/11/KSR-Carburetor-02_3-2.jpg

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

[Wigwam Jones]

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

https://scontent-ort2-1.xx.fbcdn.net/v/t1.0-9/20106597_10212767310312602_5844958195649983892_n.j pg?oh=9478f1419d352fe1ec3fe537c3853903&oe=5A0EE699

https://scontent-ort2-1.xx.fbcdn.net/v/t31.0-8/20045470_10212767315192724_3808382274591851937_o.j pg?oh=88287d6f66031f7d7ee7104be4a3f038&oe=59C31F91

https://scontent-ort2-1.xx.fbcdn.net/v/t1.0-9/20108204_10212767314312702_5856520917898971485_n.j pg?oh=eff8bfb11662d19ea4fb42f3a39a1b72&oe=59C42054

https://scontent-ort2-1.xx.fbcdn.net/v/t1.0-9/20046652_10212767310472606_4282169704295489770_n.j pg?oh=138ceff2479a3a9cb43e09551734debf&oe=59FC445F

https://scontent-ort2-1.xx.fbcdn.net/v/t1.0-9/r180/20031892_10212767312952668_5678601829112169632_n.j pg?oh=43e684407c687c8077cb97426707c8ec&oe=59FAADE5

https://scontent-ort2-1.xx.fbcdn.net/v/t31.0-8/r90/20017597_10212767316232750_1332660281727315210_o.j pg?oh=2f314b94caf7668f181247e6c01e23ea&oe=59C3D818

https://scontent-ort2-1.xx.fbcdn.net/v/t31.0-8/20045294_10212767325872991_5318280041619152065_o.j pg?oh=62fffa454c5eaa9df43cdc720c09ff78&oe=5A0E394A

https://scontent-ort2-1.xx.fbcdn.net/v/t31.0-8/20017684_10212767322032895_5054173612560644961_o.j pg?oh=22e1d3a809b7c8bcf5c57078c313fe35&oe=5A0A30BA

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.

https://scontent-ort2-1.xx.fbcdn.net/v/t1.0-9/20046328_10212767318392804_6179669396394387407_n.j pg?oh=889270f24fe49be7d134e071e50d657b&oe=59F4EBEC

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.

[Wigwam Jones]

So I built the throttle cable today and installed it. Took the bike out for little spin. Surprise, all the issues it had been experiencing went away! Bike now runs great!

I do not like the automatic choke that it has on it. More suited for what the carb was intended for, ATV use, I think.

However, now that I have something that definitely works and definitely proves the problems were with the original carb and not with the motor, I just ordered a rebuild kit for the original OEM Mikuni BS34-III carburetor, and I'm going to have a go at rebuilding it myself.

More information as it happens. But at the moment, I'm well chuffed. Thanks, all!

[Azhule]

Do you have something that will screw into that threaded area by the cable? Or did you drill it out and make it smooth? Wouldn't want those threads to chew up your brand new throttle cable

[Wigwam Jones]

Do you have something that will screw into that threaded area by the cable? Or did you drill it out and make it smooth? Wouldn't want those threads to chew up your brand new throttle cable

The 'universal throttle cable' kit I bought on Amazon had a nice little bit that threaded into the carb mounting point very perfectly, and allowed me another place to adjust the cable in and out as well (beside the stock adjuster at the top of the cable by the throttle). I had to sacrifice my stock cable, because I needed the metal bent bit that comes out of the throttle - the universal kit had nothing that the throttle could 'hold in' to keep the cable end in.

The universal kit also came with nice little rubber boots to fit over the adjusters on the top and bottom, very nice, clearly with ATV use in mind, but lovely to have. The carb also came with a cover that goes over the entire throttle mechanism, but I haven't installed it yet, assuming I ever do.

So all in all, I used the inner wire from the universal kit, the threaded-in carb bit from the kit, the bent metal top bit from my old throttle cable, and the solder-on ends from the universal kit. I used a wire cutter to cut the throttle case and wire to length, and then soldered the ends on.

It all worked pretty well. I manage to make the cable a couple mm too long - beyond the adjuster's ability to take up slack. So if I was going to keep this setup, I would fix that by unsoldering one end, making it a tiny bit shorter and resoldering. But at least I can do that - hard to make a cable longer.

I have ordered another universal throttle kit - they are cheap and it's good to have, I figure. I also ordered a replacement 'stock' throttle cable to replace the one I had to cut up - when I put the stock carb back on, I'll want that.

[goat67]

Looks good. Did you figure out what to do with the engine side of the carb? You do not want any air gaps.

[Wigwam Jones]

Looks good. Did you figure out what to do with the engine side of the carb? You do not want any air gaps.

The fit was very close - like .1mm off. So I removed the carb, wrapped the engine side in two wraps of electricians tape, and slid it back in. Snug fit then. I checked for air gaps with the motor running, didn't notice anything amiss.

However, I only ran the bike a dozen times up and down my street; enough to confirm that the stalling, bucking, hesitating, etc, was completely gone and that the bike was no longer experiencing the problems it had previously.

It's parked now and will probably remain that way until I get the original carb cleaned and put back on again. If I decide to leave the new carb on for a bit, I'll order a different rubber boot intake manifold to get a firmer air-tight fight without the need for a shim made of electrician's tape.

[Wigwam Jones]

Rebuilt the stock Mikuni BS34-III today using a kit from Japan that I bought on eBay. So far, so good. No leaks (fingers crossed) and it idles well. If it would stop raining, I'd go give it a try. Thanks again for all your words of encouragement and your suggestions.

https://www.facebook.com/media/set/?set=a.10213018159583677.1073741918.1082932103&type=1&l=94a44eff1a

[goat67]

Let us know how it runs after you take it out for a spin