81 SR250 Carb Help

510antihero

New Member
Hello I just purchased a 1981 Yamaha sr250. The bike is in great shape, but needs a new carburetor. Past owner lost it. I know Mikuni makes great carbs. But I'm not sure which one will be best for the bike. I was thinking vm32 or vm34 but not sure if either would work. So any help would be great.

Btw found these two carbs on Craigs
http://phoenix.craigslist.org/nph/mcy/3138863381.html
http://phoenix.craigslist.org/nph/mcy/3138806674.html
Would this work? Though I'd probably be better off buying new right?

Thanks,
Justin
 

510antihero

New Member
Bump

So I guess no one here knows carbs? Or maybe you don't understand what bike I have? If unclear this is the bike I have http://www.bikez.com/motorcycles/yamaha_sr_250_special_1981.php Not much of a bike I know. Definetly won't do the don lol. Oh wait in order to be on this forum does your bike have to do the ton? Though I've seen many bikes on here that I doubt will do the ton? Any ways I just got this bike for cheap so I can ride while I fix up my 71 Honda sl350. Any ways I tried to do some further research and I almost sure the Mikuni vm32 will work. Just will need a universal throttle and cable. But I'm not a mechanic so anybody here with more knowledge than me feel free to speak up and give me a hand please.

Thanks,
Justin
 

MILKY

cafe con leche
dude..

i got the same bike.. sr250... i too need a new carb so i will "bump2 this post..

any research i come across i'll pass on too..

good luck!

i only just starting stripping back the bike... looks like shit! lol

[/img]
 

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hmenzies

Been Around the Block
I don't know anything about the sr250 but I had to do a lot of carb research when replacing a carb on my CB550. I was able to find some charts with the carb number listed for each CB in that era. Maybe try finding one of those? I would also suggest trying to find the stock carburetor. If you get one different you might have trouble tuning it.
 

MILKY

cafe con leche
hmenzies said:
I don't know anything about the sr250 but I had to do a lot of carb research when replacing a carb on my CB550. I was able to find some charts with the carb number listed for each CB in that era. Maybe try finding one of those? I would also suggest trying to find the stock carburetor. If you get one different you might have trouble tuning it.

i got a meesage from "swivel" he says a slide carb from a same year xt will fit.. gonna do some fleabaying!
 

PocketsVIC

Active Member
I also have a 1981 sr250. Im wanting to re do the jets as i have fitted a new air filter and exhaust, but i don't know the make and model number of the stock carb. Any help would be greatly appreciated.
thanks in advance.
 

nofacemonster

New Member
I am sorry to evoke an old thread but i am also in a simillar situation. my SR 250 needs a carburetor. on the ebay i came across few good slide carbs which is 34mm

http://www.ebay.com/itm/Performance-PWK-34mm-Carburetor-Scooter-ATV-Go-Kart-PIT-DIRT-BIKE-QUAD-CARB-/161914660903?hash=item25b2dda827:g:H~AAAOSwu4BVxTcm&vxp=mtr

http://www.ebay.com/itm/Mikuni-Geniune-VM-34mm-34-mm-Round-Slide-Carb-Carburetor-VM34-389-/191793803045?hash=item2ca7cd2f25:g:tLoAAOSwv0tVHsoL&vxp=mtr

will one of these work? if the PWK 34mm works that's perfect. Any idea people?

and also the stock carburetor of SR250 is 34mm or 32mm? i am kind of confused... will both fit? sorry for sounding stupid....
 

crazypj

Split personality, I fake being smart
Contact member 'cosworth' he's done a load of work on the SR250 motor
 

nofacemonster

New Member
510antihero said:
Hello I just purchased a 1981 Yamaha sr250. The bike is in great shape, but needs a new carburetor. Past owner lost it. I know Mikuni makes great carbs. But I'm not sure which one will be best for the bike. I was thinking vm32 or vm34 but not sure if either would work. So any help would be great.

Btw found these two carbs on Craigs
http://phoenix.craigslist.org/nph/mcy/3138863381.html
http://phoenix.craigslist.org/nph/mcy/3138806674.html
Would this work? Though I'd probably be better off buying new right?

Thanks,
Justin

Check this out... i think VM34's work

https://youtu.be/ZLPDJzqAOac

i am in the same bandwagon as you guys are...
 

cosworth

Coast to Coast
Ok if you are going to swap carbs it will be a bit fussy. You'll have to buy a vm34 carb, then a whole jet setup including needle. And you'll have to mod the throttle cable.

The jets and needle needed to swap a vm34 on there are here:

http://www.braigasen.com/Mikuni_jetting_chart_four_stroke.htm

I recommend getting a stock carb. I run a bit of a hopped up motor with a header and a carb trumpet. All stock jets except main jet, which is 137.5. Idle screw 2 turns out. I live at sea level and it's cold here.

You might only need a 132.5 with a stock air box and a pipe.
 

JORDAN4

New Member
cosworth said:
Ok if you are going to swap carbs it will be a bit fussy. You'll have to buy a vm34 carb, then a whole jet setup including needle. And you'll have to mod the throttle cable.

The jets and needle needed to swap a vm34 on there are here:

http://www.braigasen.com/Mikuni_jetting_chart_four_stroke.htm

I recommend getting a stock carb. I run a bit of a hopped up motor with a header and a carb trumpet. All stock jets except main jet, which is 137.5. Idle screw 2 turns out. I live at sea level and it's cold here.

You might only need a 132.5 with a stock air box and a pipe.
Hello to everyone,
I know its an old topic but I have the same problem on my 81 SR250. My stock carb is in bad shape and if I buy the VM34, I don't know what jets and needle will be needed( unfortunately cosworth's link not working).
Thanks in advance. Hope someone can help me.
Jordan
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
For the sake of simplicity, it might be easier to buy a good used stock carb - there is tons of info on jetting them. Where as if you go down the VM34 route, you will be down to trial and era a bit. Not saying it can't be done, I have done it to a DT400. But it cost a lot of money in jets and plugs and took a long time to get right.

One more thing, the SR250's carb is a CV carb with a 34mm throat. The fact that it is a CV carb allows that throat to be so large and flow a lot at high rpm, while still maintaining enough air velocity at low rpms. If you go to a 34mm round slide carb, you are not going to have the same low rpm velocity and it might be fussy to jet right. There is a reason the trail versions of this bike - the TT250 and the XT250 came stock with 28mm and 30mm carbs!

You'll find help here either route you go ;D
 

JORDAN4

New Member
Thank you very much for your answer. I understand that it will be tricky to set the VM34 right.
But once jetted, will it be an improvement for the bike?
If not, then there is no point on going the "hard"way..

P.S. i can find some online stores selling prejetted VM34 for SR250. Can I trust them or not worth the risk?
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
I highly doubt anywhere will sell a pre jetted carb for this bike.

If you want to know about performance/flow of different carbs there is a lot of technical stuff to wade through. Check out this comparison here for a Harley CV carb vs a flat slide carb - for a stock engine, performance increase in minimal. So with the headaches and cost counted in... http://www.nrhsperformance.com/tech_mikunivscv.shtml

If you do get a VM round slide, you could install one of these: http://www.dcvmx.com/xcart/product.php?productid=20385

I have seen some flow bench tests and these bring the flow numbers up to almost the same as a flat slide carb!

Also, read though this: http://www.ducatimeccanica.com/dellorto_guide/dellorto_3_1.html

According to the chart, say the stock SR puts out 13-15rwp or maybe even 18hp at the crank, you would only need a 30mm carb throat to support that kind of power and you would have much better air velocity for the rest of the rpm range than just up top ;)

This makes me feel like testing this at some point in the future! ;D
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Just realised I'm trying to point you away from something you might want to try.... Soz!

The thing is with VM carbs is you can pick em up brand new for real cheap on ebay, so there really would be no harm in giving it a go. I'll guide you through the jetting if you do decide to give it a crack. Then you can report on the performance ;D
 

JORDAN4

New Member
No no! Don't even think about it :) I read carefully your opinion but I am thinking that a try for the VM worth the time!

I also made a lot of research and found that these guys can do the jetting for me.. http://www.kickstartershop.de/en/mikuni-vm34-vergaser-fuer-yamaha-sr-xt-tt-400-500-rennvergaser-rundschieber-vergaser-carburator-carb

I asked them and they told me the jets they use for the SR250.. And then I found out that they are the same "cosworth" suggests.. https://m.reddit.com/r/motorcycles/comments/445il6/question_about_1981_xt250_yamaha/

So I am almost ready to choose the VM way.. 8)

Hope you can also help me :)

P.S. What do you think about the UFO (Ultimate Flow Optimizer )? You suggested it to me,it seems a very cheap way to gain power but I read some things saying about poor build quality..
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Awesome man. I'm sure it'll run sweet with it. And amazing that kickstarted will pre-jet it for you!

I found this discussion somewhere else about the UFO. Someone has spent a lot of time on the flow bench comparing with/without and then vs a flat slide. Check it. The results are pretty impressive I think...
 

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JORDAN4

New Member
I hope she will run ok my friend! Because with the stock I had soo many problems..

I think that the UFO results are more than interesting.. I am one click away from ordering it!

The only thing is that they say **When installing UFOs, the pilot jet must be reduced substantially (usually about 50%, sometimes even more).** With the #25 pilot jet on the prejetted VM-34, will I have to buy a #12.5?

Also, I read that on many occasions, there were serious problems getting the idle mixture right – running way too rich http://suzukisavage.com/cgi-bin/YaBB.pl?num=1149364740

What do you think?

P.S. Is it possible to fit a TM34 on the SR250? Its a thought because the cost to get the UFO to Greece + the VM34 is almost the same..
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Well I think I have changed my mind after reading that forum thread! It's not worth it if you cannot get the idle mixture correct.

Aren't the TM carbs more expensive? If not, go for that one!
 

cxman

Over 1,000 Posts
DTT SUPPORTER
flow optomizers wind up needing hand fitting per carb to work right and have even close to a drive able transition to midrange

they really reduce the slide cut away to almost nothing then when you reduce the pilot to compensate the bike falls on its nose over 1/8 throttle

till it hits about 3/8-1/2

and i dont even want to talk about the one that simply fell off the throttle slide during testing got stuck in a valve and the valve hit the piston

if you need the extra hp get a flat slide
 

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