Ton up SR250: 100mph, 100kg, 30hp

JerryAssburger

Active Member
Thanks for the support mate.

I stuck with the CV carb for a couple of reasons. One being they are very easy to tune and offer a good compromise for low down power and high rpm power. They also self adjust pretty well to different air conditions (altitude etc) and compensate for minor tuning errors. A slide operated carb may offer better throttle response but at the cost of bog if not controlled well (rider control) and if it does not have a pumper circuit. It may even flow better at high rpm, but no one knows because no one has done back to back testing (on the SR at least), including myself. Another reason is purely business - improving on what is already a good system is often better and more reliable that starting from scratch with an entirely new system and trying to make it work. There is also the cost factor - a new carb can be pretty expensive and is not always in the budget for an SR build. I have had the SR on a few different dynos for several days at a time and think that this would be required to get the VM to run well and be able to test it with different air filters and exhaust set ups. But thats just me, I like to do things thoroughly rather than just stabbing in the dark. I have also spent countless riding hours with a home built O2 sensor with and air-fuel ratio gauge set up doing countless jets swaps to test different set ups with the CV carb, so I am quite invested in it.

The only reason CV carbs can be finicky with air filters is if they cover the vacuum intake port (think cheap EMGO pod filter), otherwise any type of filter will work.

In the past I have somewhat successfully tuned a VM34 carb for a DT400 and thought they were pretty easy to work on. But beware, there are different needle jets (emulsion tubes) for the VMs for two-strokes and four-strokes - the two-strokes have somewhat of a shroud at the base of the throat of the carb - this is to do with the different type of vacuum two-strokes pull.

The XT250 came with a 28mm roundslide pumper carb. Already there you have two huge differences with the VM34 - the throat diameter and the pumper circuit. Without going into all the details there, these two factors affect a lot.

If anything, I would want to test a TM33 flatslide pumper carb. 33mm being a good compromise for low end and high end power and the pumper circuit giving good low end response and resistance to bog. The flatslide also flows much better than the roundslide. Alternatively, I would be very keen to test a LECTRON carb at some point! They look like the business! But again, would probably require a lot of testing.

I have had little time to work on my bikes recently and this long winded response shows that I am spending too much time working at the computer and too little time in the workshop! Time to get on the wrenches again :) Best of luck with your build and carb!
Hah! Thank you for the response! Two reasons why many of us are here! Fun project motorcycles and great reads! I guess the best way for us to find out stuff is to try and fail and document, so I'll try my goofball ideas and post the *ahem* amazing results, warts and all! Love your threads!
 

Flatcap Funk

New Member
Firstly, would just like to say, love this thread. Looking forward to the first top speed runs!

Jerry, I think a 34mm Mikuni is overly large for these motors. I have experimented with Mikuni round slide carbs from 28-32 mm on my XT250 and think 30 mm is ideal for mostly stock motors. I currently run a 32 mm Mikuni with no airbox, ported head and a 30 mm internal diameter header pipe. It runs well and pulls hard in the mid to upper rpm range, and will just creep over 80 mph, but throttle response is less than ideal at lower rpm's especially coming off the the idle circuit. When switching between CV and round slide carbs, a good rule of thumb is non-CV carbs should be 4-6 mm smaller, depending on desired peak output and other performance modifications. Generally speaking, it is always better to to err on the side of caution and go down a size down on the carb.

The XT250 was fitted with a 28mm carb and its dirt cousin, the TT250 came with a 30 mm carb; both pumper carbs. I agree with Jake, the TM33 could be a good choice for these motors but would require a lot of set-up time. Mikuni make a 28mm flat-slide pumper carb, the TM28-01, which could also be a good choice for largely stock motors. It is also possible to bore this carb 2 mm, which would give you a modern 30 mm flat slide pumper carb relatively cheaply. Depending on how deep your pockets are, CT Racing offer a bored 30 mm FCR carb that they use on their YFM250 builds that make ~30 hp. There are many options but whatever route you choose, expect lots of time fiddling with jets!

If your looking for extra power, why not just install Jakes intake, carb, and header package retaining the stock carb? A SR250 motor making 20 hp without messing with cams and pistons etc is impressive!

Below are my current carb specs for a 32 mm round slide Mikuni fitted to a XT250. Admittedly it's not fully dialled-in, and possibly running a bit rich, but should get you in the ball park if you decide to swap carbs. The mounting spigot on the carb was also machined down a couple mm to fit the stock XT250 intake manifold.

Main jet: 175
Needle jet: 159-P2
Pilot jet: 20
Jet needle: 6DH3
Clip position (from top): 2
Pilot air jet position (from fully seated): 1.75 turns
Main air jet: 2.0
Slide cutaway: 2.0

Hope this helps, have fun!
 

JerryAssburger

Active Member
Firstly, would just like to say, love this thread. Looking forward to the first top speed runs!

Jerry, I think a 34mm Mikuni is overly large for these motors. I have experimented with Mikuni round slide carbs from 28-32 mm on my XT250 and think 30 mm is ideal for mostly stock motors. I currently run a 32 mm Mikuni with no airbox, ported head and a 30 mm internal diameter header pipe. It runs well and pulls hard in the mid to upper rpm range, and will just creep over 80 mph, but throttle response is less than ideal at lower rpm's especially coming off the the idle circuit. When switching between CV and round slide carbs, a good rule of thumb is non-CV carbs should be 4-6 mm smaller, depending on desired peak output and other performance modifications. Generally speaking, it is always better to to err on the side of caution and go down a size down on the carb.

The XT250 was fitted with a 28mm carb and its dirt cousin, the TT250 came with a 30 mm carb; both pumper carbs. I agree with Jake, the TM33 could be a good choice for these motors but would require a lot of set-up time. Mikuni make a 28mm flat-slide pumper carb, the TM28-01, which could also be a good choice for largely stock motors. It is also possible to bore this carb 2 mm, which would give you a modern 30 mm flat slide pumper carb relatively cheaply. Depending on how deep your pockets are, CT Racing offer a bored 30 mm FCR carb that they use on their YFM250 builds that make ~30 hp. There are many options but whatever route you choose, expect lots of time fiddling with jets!

If your looking for extra power, why not just install Jakes intake, carb, and header package retaining the stock carb? A SR250 motor making 20 hp without messing with cams and pistons etc is impressive!

Below are my current carb specs for a 32 mm round slide Mikuni fitted to a XT250. Admittedly it's not fully dialled-in, and possibly running a bit rich, but should get you in the ball park if you decide to swap carbs. The mounting spigot on the carb was also machined down a couple mm to fit the stock XT250 intake manifold.

Main jet: 175
Needle jet: 159-P2
Pilot jet: 20
Jet needle: 6DH3
Clip position (from top): 2
Pilot air jet position (from fully seated): 1.75 turns
Main air jet: 2.0
Slide cutaway: 2.0

Hope this helps, have fun!
Thank you for the advice! (Especially the Cheat-Sheet on the jets!) To be entirely honest, you guys are going to find that I tend to reach for the cheapest, lowest-hanging fruit when I try to get projects going. The original carb works and seems to be in decent shape, but the airbox is missing. So my next "logical" (not really) step was to try and find something on Ebay that would fit the SR250's intake boot. (cheap, Cheap CHEAP!) and that ended up being any carb with the 40mm snout on it. (40mm is right, isn't it?) They had some PK Flat-Slides and the Mikuni VM34 Round Slides for decent prices. From what I might have erroneously gathered is that the round-slides are supposed to be more user-friendly than the flat-slides. I have been known to make VERY ugly mistakes in the past, so no surprises, here. I'm beginning to see the light as far as 28-30mm being the most ideal for a stock 2-valve 250 cc engine, so the 34 will be over-zealous unless I uncork the breathing in other areas.
*EDIT* Well now, I can publicly kick myself... they DO make a VM30 with the 40mm snout. Live and learn.
EBmikiVM34168.jpg

Also, I'm still in the "Gathering of Junk" Stage, which means, I haven't so much had time to TOUCH the bike. I have a 1st Priority of getting my Wife's No-name 50cc scooter running first, and THEN I get to "play".

As a side note, this is a question for Jadus and all Projecteers.... How loud are the $40-ish Ebay mufflers that fit these bikes? I ordered a 21" side-turn-out looking thing, and it seems well built, but I sure hope it isn't too loud.
EBglaspak.jpg
 
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JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Firstly, would just like to say, love this thread. Looking forward to the first top speed runs!
Thanks! I have actually signed up to participate in a half mile (804m) event in May at an old airport. This way, any recorded speeds will be official and also legal :p But also, very much looking forward to wringing it out on an open runway. I am also already nervous how it will go - I haven't even had the bike running this year yet and I would like to do some dyno runs before the even to get engine tuned right. Also the pressure (only from myself) having built this bike for the past 5 years coming to a climax haha. I will need all the luck I can get.
When switching between CV and round slide carbs, a good rule of thumb is non-CV carbs should be 4-6 mm smaller, depending on desired peak output and other performance modifications. Generally speaking, it is always better to to err on the side of caution and go down a size down on the carb.

The XT250 was fitted with a 28mm carb and its dirt cousin, the TT250 came with a 30 mm carb; both pumper carbs. I agree with Jake, the TM33 could be a good choice for these motors but would require a lot of set-up time. Mikuni make a 28mm flat-slide pumper carb, the TM28-01, which could also be a good choice for largely stock motors. It is also possible to bore this carb 2 mm, which would give you a modern 30 mm flat slide pumper carb relatively cheaply. Depending on how deep your pockets are, CT Racing offer a bored 30 mm FCR carb that they use on their YFM250 builds that make ~30 hp. There are many options but whatever route you choose, expect lots of time fiddling with jets!

Below are my current carb specs for a 32 mm round slide Mikuni fitted to a XT250. Admittedly it's not fully dialled-in, and possibly running a bit rich, but should get you in the ball park if you decide to swap carbs. The mounting spigot on the carb was also machined down a couple mm to fit the stock XT250 intake manifold.

Main jet: 175
Needle jet: 159-P2
Pilot jet: 20
Jet needle: 6DH3
Clip position (from top): 2
Pilot air jet position (from fully seated): 1.75 turns
Main air jet: 2.0
Slide cutaway: 2.0

Hope this helps, have fun!
That would be a great starting point for jetting. I also didn't know about the possibility of boring up the TM28, that would be perfect. And a good rule of thumb from the CV to slide carbs needing to be smaller, very true.
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Thank you for the advice! (Especially the Cheat-Sheet on the jets!) To be entirely honest, you guys are going to find that I tend to reach for the cheapest, lowest-hanging fruit when I try to get projects going. The original carb works and seems to be in decent shape, but the airbox is missing. So my next "logical" (not really) step was to try and find something on Ebay that would fit the SR250's intake boot. (cheap, Cheap CHEAP!) and that ended up being any carb with the 40mm snout on it. (40mm is right, isn't it?) They had some PK Flat-Slides and the Mikuni VM34 Round Slides for decent prices. From what I might have erroneously gathered is that the round-slides are supposed to be more user-friendly than the flat-slides. I have been known to make VERY ugly mistakes in the past, so no surprises, here. I'm beginning to see the light as far as 28-30mm being the most ideal for a stock 2-valve 250 cc engine, so the 34 will be over-zealous unless I uncork the breathing in other areas.
*EDIT* Well now, I can publicly kick myself... they DO make a VM30 with the 40mm snout. Live and learn.

Also, I'm still in the "Gathering of Junk" Stage, which means, I haven't so much had time to TOUCH the bike. I have a 1st Priority of getting my Wife's No-name 50cc scooter running first, and THEN I get to "play".

As a side note, this is a question for Jadus and all Projecteers.... How loud are the $40-ish Ebay mufflers that fit these bikes? I ordered a 21" side-turn-out looking thing, and it seems well built, but I sure hope it isn't too loud.
'I tend to reach for the cheapest, lowest-hanging fruit when I try to get projects going.' Perhaps cheapest in money but most likely not in time, but you may have more of that than I do!

I admire your ambition! Personally I have had bad experiences with Chinese carb copies as have many others, and I would not go that route. Who knows what kind of baseline they even have? Are they even comparable to the Mikuni? So that those settings will be the same settings in the Chinese carb? Tolerances, casting quality etc etc. I sound a bit negative here, but I can say that if you have a lot of time and are prepared to fiddle with jet settings a lot, you'll most likely get an ok running bike with your carb :)

When referring to carbs, it is the bore size or 'throat' that is usually referred to. That is the size of the carb after the venturi and the size that enters the carb boot/manifold.

To answer your question about the silencer; That will most likely be very loud, unless it has some form of internal baffling. There is a guy (Spencer) on the SR250 facebook group that installed one of those silencers on his bike and it looks pretty cool. So you could join the group and ask him how loud it is. He built himself a really cool bike on a pretty low budget so he may have some other tips too!

The restriction in the SRs exhaust is not in the silencer though so this would not really be 'low hanging fruit' or add anything other than more noise. The restriction is in the tiny internally piped header ;)
 

JerryAssburger

Active Member
'I tend to reach for the cheapest, lowest-hanging fruit when I try to get projects going.' Perhaps cheapest in money but most likely not in time, but you may have more of that than I do!

I admire your ambition! Personally I have had bad experiences with Chinese carb copies as have many others, and I would not go that route. Who knows what kind of baseline they even have? Are they even comparable to the Mikuni? So that those settings will be the same settings in the Chinese carb? Tolerances, casting quality etc etc. I sound a bit negative here, but I can say that if you have a lot of time and are prepared to fiddle with jet settings a lot, you'll most likely get an ok running bike with your carb :)

When referring to carbs, it is the bore size or 'throat' that is usually referred to. That is the size of the carb after the venturi and the size that enters the carb boot/manifold.

To answer your question about the silencer; That will most likely be very loud, unless it has some form of internal baffling. There is a guy (Spencer) on the SR250 facebook group that installed one of those silencers on his bike and it looks pretty cool. So you could join the group and ask him how loud it is. He built himself a really cool bike on a pretty low budget so he may have some other tips too!

The restriction in the SRs exhaust is not in the silencer though so this would not really be 'low hanging fruit' or add anything other than more noise. The restriction is in the tiny internally piped header ;)
That's why I gots me some learnin' to do... I thought the stock silencer, with it's dime-size outlet would be the overall issue. The muffler I got looks to be a glass-pack design, maintaining a minimum of 30mm throughout. I'm hoping it isn't too loud- don't wanna anger the neighbors. The header would be nice to improve on also, but for now, I'm just gathering the missing essential parts. Luckilly, my basket-case came with what's left of the header. :) As aor the carb, I might be lucky and the stock CV will work fine with the correct air-filter routing. Then I wouldn't have to mess with the round-slide Fake-Kuni!
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
The muffler I got looks to be a glass-pack design, maintaining a minimum of 30mm throughout. I'm hoping it isn't too loud- don't wanna anger the neighbors.
If it has a glasspack core it'll probably be fine. It won't be quiet haha but neither will it be straight pipe loud.
As aor the carb, I might be lucky and the stock CV will work fine with the correct air-filter routing. Then I wouldn't have to mess with the round-slide Fake-Kuni!
That might be a good place to start! Then maybe save some money for either the real VM34 or a TM like Flatcap Funk suggested.
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
Here is the location of the speed run tests - half mile track at an airport, perfect. I just hope that the bike has enough acceleration to come up to speed in 804m!

Looking forward to the first start and first ride of the year/season. I will continue the engine break in, change oil, start running 98RON and install my O2 sensor to start getting some air/fuel ratio values to start getting the jetting right. I built the engine to run on 95 and was hoping to continue with that. But just recently in Sweden and most European countries, 95RON is required to have a 10% mix (E10) of bio-fuel, whereas 98 with be E5, having a 5% mix. I am just not sure about how this fuel will affect the engine and the carb so I figure running 98 will be a safer bet. Plus with the added bonus of having better knock/detonation resistance. Any thoughts/personal experiences on this?
 

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JerryAssburger

Active Member
Here is the location of the speed run tests - half mile track at an airport, perfect. I just hope that the bike has enough acceleration to come up to speed in 804m!

Looking forward to the first start and first ride of the year/season. I will continue the engine break in, change oil, start running 98RON and install my O2 sensor to start getting some air/fuel ratio values to start getting the jetting right. I built the engine to run on 95 and was hoping to continue with that. But just recently in Sweden and most European countries, 95RON is required to have a 10% mix (E10) of bio-fuel, whereas 98 with be E5, having a 5% mix. I am just not sure about how this fuel will affect the engine and the carb so I figure running 98 will be a safer bet. Plus with the added bonus of having better knock/detonation resistance. Any thoughts/personal experiences on this?
This is going to be SO COOL! Full Racer's Tuck! Air Resistance is Speed's BIGGEST enemy! I know this thing is going to ZIP!
 

JerryAssburger

Active Member
If it has a glasspack core it'll probably be fine. It won't be quiet haha but neither will it be straight pipe loud.

That might be a good place to start! Then maybe save some money for either the real VM34 or a TM like Flatcap Funk suggested.
Thanks! I have a neighbor across the street who is the spiritual equivalent of Mr. Wilson from Dennis The Menace. I reeeeeeaaallly wish I had researched the 40mm spigot thing a bit further and bought the VM30, but at this point, IF it converts raw fuel to exhaust fumes on it's own, that'll be a Little Victory for me to start from! Then, I can focus on Doing The Ton... (of inches... in a minute... or two...)
 

der_nanno

Faster!
Personally I think those Chinese carbs aren't too bad. But a wideband AFR and a dyno (or at least which you can drive up and down 100 times to check against the changes you just made), will come in very handy.

Also there's unbranded Chinese carbs, which I would stay away from and then there's those, which have a name, e.g. OKO - those are lot hit and miss than the really cheap "fell off the truck and no one asked"-kind...
 

Tanshanomi

New Member
Personally I think those Chinese carbs aren't too bad. But a wideband AFR and a dyno (or at least which you can drive up and down 100 times to check against the changes you just made), will come in very handy.

Also there's unbranded Chinese carbs, which I would stay away from and then there's those, which have a name, e.g. OKO - those are lot hit and miss than the really cheap "fell off the truck and no one asked"-kind...
Nibbi is another good, affordable, upper-tier Chinese brand. (And unlike OKO, I am not aware of any shady factories knocking them off.) I have purchased two Nibbi Keihin PE24 copies (a PE24 and a PE24FL), and a PE30. No quality issues with any of them, good performance, and excellent tune-ability all the way around.

I know they make sizes from 19mm up to PWK32 and PWK34.

IMG_7720.jpg


Screen Shot copy.jpg
 

JadusMotorcycleParts

Over 1,000 Posts
DTT SUPPORTER
In the closing stages of this project and after such an extensive weight reduction program for the bike, it is only fair that I put myself on a weight reduction program too :D The goal: from 76kg to 73-4. I have already started and gotten to 75, so it seems achievable to get to the goal in 6 weeks!
 

porcelanowy

Active Member
Hello
Well, it is the cheapest way to drop some " racing weight" ;)
But 76kg is my curent "winter weight" and I am 175cm. I will drop to 74 kg - my "bathing suit weight" :)
You seems taller than that and pretty slim already. Don't get anorectic ;) :D

Any bike pics?
 

JerryAssburger

Active Member
Aerodynamics are gonna play a BIG part in your success. If the bike can be lowered temporarily, skinny tires at high pressure, etc.... I'd employ all of the tricks.
 

JadusMotorcycleParts

Over 1,000 Posts
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Hello
Well, it is the cheapest way to drop some " racing weight" ;)
But 76kg is my curent "winter weight" and I am 175cm. I will drop to 74 kg - my "bathing suit weight" :)
You seems taller than that and pretty slim already. Don't get anorectic ;) :D

Any bike pics?
Haha it's proving more difficult than thought but I'll think I'll get to the 74 mark - your bathing suit weight :D

A couple of pics... One to show the riding position. Lets hope this position is enough to reduce drag :p
 

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JerryAssburger

Active Member
Haha it's proving more difficult than thought but I'll think I'll get to the 74 mark - your bathing suit weight :D

A couple of pics... One to show the riding position. Lets hope this position is enough to reduce drag :p
It's fun to look at pix of the old board-trackers and the positions they would assume to eek out those last few MPH on the straights. One trick I can verify from testing on my 883 was the left hand off of the grip and just hanging on with the right, and then using the passenger pegs for the feet.
God Bless Arizona! NO WAY could I pull a stunt like that here in Oregon.
 

Rider52

Over 1,000 Posts
A friend weight 350 pounds and recently spent several thousand dollar son upgrading his Harley Road Glide engine. He posted on Face Book that he could really feel the difference in power! A mutual female friend replied "I bet if you lost 100 pounds it would be a rocket ship!". I laughed so hard I was crying.
 

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