81 SR250 Carb Help

JORDAN4

New Member
Thank you guys for your detailed info.. Of course, I am not buying the UFO..

I ordered the VM34 from kickstarters. Hope everything will be fine..

When its on, I wii inform about the perfomance and any help will be highly appreciated!
 

JerryAssburger

Been Around the Block
Sorry- another bump. My SR250 is missing it's airbox, so I'll be trying the VM34 Route. Promise to report the results- good or bad.
 

Ichiban Moto

Bad Ass Cafe' Style
I need to make an electronic fuel injection conversion video.
34mm throttle body with engine side of 40mm od to fit stock engine boots, right?
 

cxman

Over 1,000 Posts
DTT SUPPORTER
try this if you got a 34

vm 34 number 3 slide
no air correct take it out
35 pilot (get a 32.5 as well) 145 main needle in the second groove (get a 150 main as well)
leave the needle and jet needle it comes with alone
set the mixture screw to 1.5 if its been messed with

if you have not ordered a 34 pm me if you ride the bike around town a lot a 32 would suit you better
 

pidjones

Over 1,000 Posts
This> if you have not ordered a 34 pm me if you ride the bike around town a lot a 32 would suit you better
 

cxman

Over 1,000 Posts
DTT SUPPORTER
mikuni has a whole department that their job is to figure out what they can put in a stock carb that nobody can use
anything in it that gets replaced (all of it) is a value-added sale
 

JerryAssburger

Been Around the Block
HAH! I never thought of it that way..... So you're suggesting starting with a 145-150 main and a 35 pilot....? Looks like I'll be ordering a collection of jets! I will be running a pod filter and a glass-pack muffler- otherwise bone stock.
 

JerryAssburger

Been Around the Block
So a little more schlooping on the Net has shown some success with 135 main, 22.5 pilot, 159 Q-2, and 6DHA needle. I realized I boogered an opportunity to get a VM30 instead of the VM34, thinking I had to MATCH the size of the CV. Wish I had researched first. Even a VM26 or VM 28 is more appropriate, except for the 35mm spigot diameter.
 

JerryAssburger

Been Around the Block
CXMAN, if I can bug you on something?- as you know, I muffed up and ordered a VM34, (thinking I was supposed to match the CV34). For a low revving two-valve single, it's overkill. Should I get a VW30, or go down to a VM28 and adapt the spigot with a Gates PowerGrip Thermoplastic hose clamp 1 3/16-1 1/2" as a spacer? (Found that little gem on a different forum.) Both set-ups will cost about the same.
Or can I just "take it easy" and use the VM34? I'm guessing, "NO".
My bike is stock, except for a glass-pack muffler and sponge/pod filter.
Many thanks for your help.
 

JerryAssburger

Been Around the Block
i would go vm32 i just got one for my own xl250
Thanks. Lesson learned.
I remember on my old SL350, that it was cheaper and easier to swap the head and carbs from my CB350 (larger passages and carbs) than to find replacement SL carbs (pre-ebay era) and wow- I did NOT like how it robbed the bike of it's "grunt". I really don't want to repeat my mistake. I'll try the VM34, but I'm guessing I'm not going to like it.
 

teazer

Over 10,000 Posts
DTT BOTM WINNER
A CV carb doesn't flow as well as a simple slide carb because things like the butterfly valve and shaft disrupt flow. I read somewhere years ago that a rule of thumb when replacing a CV with slide carb is that the slide carb should be 85% of the size of the CV it replaces.

That's 85% of the area. Not the diameter, so that works out to 31mm more or less.

But if you want more performance, you can go larger than that "rule" which takes you to the 32 that CXman recommended.

An example of a poor flowing CV carb is the BS40 fitted to the GT750 Suzuki. The CV carb is 40mm outlet but has a lot of restrictions and it supposedly equates to a 32mm slide carb and not 36mm which the rule would suggest
 

JerryAssburger

Been Around the Block
A CV carb doesn't flow as well as a simple slide carb because things like the butterfly valve and shaft disrupt flow. I read somewhere years ago that a rule of thumb when replacing a CV with slide carb is that the slide carb should be 85% of the size of the CV it replaces.

That's 85% of the area. Not the diameter, so that works out to 31mm more or less.

But if you want more performance, you can go larger than that "rule" which takes you to the 32 that CXman recommended.

An example of a poor flowing CV carb is the BS40 fitted to the GT750 Suzuki. The CV carb is 40mm outlet but has a lot of restrictions and it supposedly equates to a 32mm slide carb and not 36mm which the rule would suggest
NICE INFO! Thank you!
Judging from Jadus' dyno runs, he was making little progress with trying to free up and/or increase intake flow, suggesting to me that the cam lift/timing and valve/ports are set up to top out at 7500-8000 rpm max. (It was a dirt-bike engine, and they're usually set up to pull stumps.) The former slide carb was a 28mm (in dirt-bike application).
I get the feeling that although I made a poor stupid choice with the VM34, (not my last, for sure) that if I press on with using it, low RPM pull will suffer unless I take care not to modulate the throttle properly. I still think I'm going to spring for the VM30 if it's intolerable. I'm a fan of a motor that pulls from low, over maximum "top-end", so I'll eventually get the VM30.

So HERE'S something interesting... if Yamaha wanted to maintain the VM28's flow in a CV carb, a "BS31-ish" carb would've been the equivalent. And like you pointed out, going from a BS34 CV carb to a VM slide is... VM31-ish!
...seems that for the SR250's stock breathing, the VM30-VM32 is right on. SO.... how does uncorking that horrific stock exhaust affect things?
This stuff is addicting.
 

ex119x

Been Around the Block
The exhaust is another completely different can of worms. There is so much misinformation and straight up BS about 4 stroke pipes that it makes my head hurt. The best answer I have come up with on my racebikes is to just try different pipes and see what works best. I have found a few items which seem to be consistent, but not yet quantified by me. Most exhaust header pipes are too big and too long to put the power where it is usable. Combine that with too big of a carb and you have a bike that is boggy everywhere and won't clean up on top end.
My suggestion based on just my gut feeling would be to go with a VM30-32 and a head pipe which is only slightly larger diameter than stock, stock length with a Supertrapp muffler that you can tune with the discs.
 

JerryAssburger

Been Around the Block
The exhaust is another completely different can of worms. There is so much misinformation and straight up BS about 4 stroke pipes that it makes my head hurt. The best answer I have come up with on my racebikes is to just try different pipes and see what works best. I have found a few items which seem to be consistent, but not yet quantified by me. Most exhaust header pipes are too big and too long to put the power where it is usable. Combine that with too big of a carb and you have a bike that is boggy everywhere and won't clean up on top end.
My suggestion based on just my gut feeling would be to go with a VM30-32 and a head pipe which is only slightly larger diameter than stock, stock length with a Supertrapp muffler that you can tune with the discs.
I will definitely be taking the path of Trial & Error, as evidenced by my purchase of the VM34. The whole header length/diameter/RPM thing is a different bag of magic beans, yes. I plan on running the stock header that I have, (the silencer has been already hacked off by the PO.) and a glasspack muffler. I don't plan on racing any GSXRs, etc. ! :D
 

Brodie

Gold Coast, Queensland
DTT SUPPORTER
The issue with the stock pipe is that it is double walled with the inner tube being just above an inch in size.

4 stroke exhaust gets complicated when you start looking at header length and pulse reverb timing.
I would go for a 1.25" header and have it run the way you want without worrying too much about length.
 

JerryAssburger

Been Around the Block
The issue with the stock pipe is that it is double walled with the inner tube being just above an inch in size.

4 stroke exhaust gets complicated when you start looking at header length and pulse reverb timing.
I would go for a 1.25" header and have it run the way you want without worrying too much about length.
Thank you! So much that can be done to these things. A new header would be sweet. That will go on my Wish List. I'd also love to go 18" rims front and rear, but that is WAY above my budget. I don't know how the folks in Australia can afford this; I've read the forums for a bit now, and it seems they pay a lot more for stuff... yet some of the coolest rides come from there! My hat is off to you.
 

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