Yamaha SR250 Brat

I was speaking to a fella today about my bike, and he said because I have a different exhaust and air filter fitted I would have to adjust my carbs and jets or something. I didn't know what he was talking about, so to avoid looking like a dick I just nodded and went "Yeh, go to do those jets"
Does any one know what I should do/ adjust/ check when exhaust and air filter are changed.

Any help would be great and avoid future bruised ego.

DISCLAIMER: This is my understanding and should not be taken as the be all and end all. I have a low knowledge of carbs and am more than happy to be corrected.

When you change the exhaust to flow better you are going to have to change the jets as there will be less exhaust gas left in the chamber when the mixture enters.
When you change the air filter to flow better you are going to have more air in the mixture.

These two combined make for a very lean (low fuel to air ratio) mixture which will in turn run hotter.
The jets are located inside the carby and control the amount of fuel in the mixture.

The standard SR250 carb is a Mikuni BS34 which has 3 jets. One pilot (idle to 1/4 throttle) a secondary main (1/2 to 3/4) and a primary main (3/4 to full). These are easy to change and need to be done because of the mods.

1/4 to 1/2 is controlled by the needle. This is more difficult to change on the BS series (I didn't change it on my SR)

The hard part will be finding the right combination of jets for your bike. Ohh and the primary main is located in the top, do yourself a favor and change to Allen key bolts because the current ones WILL be an absolute pain inthe arse to remove.

Hope that helps out some

Thanks mate, that's already a 1000% more than I new haha. Ill do some looking into it this weekend and post how I go!

hey got a few questions on your paint of the motor.

did you bake that color?if yes how long?
did it show any chipping after letting the motor run several times?

how did you make these rad polished motor fins?
did you use a dremel after letting the paint cure?


thanks in advance buddy

Sure did a lot with this little bike.

the SR250 was my first bike owned as well and I enjoyed that little sucker. If I knew back then what I know now it could have been a great little bike.

The only issue I had with it was it vibrated like hell at around 100km/h and I lost 2 license plates because of this LOL.

PocketsVIC said:

I was speaking to a fella today about my bike, and he said because I have a different exhaust and air filter fitted I would have to adjust my carbs and jets or something. I didn't know what he was talking about, so to avoid looking like a dick I just nodded and went "Yeh, go to do those jets"
Does any one know what I should do/ adjust/ check when exhaust and air filter are changed.

Any help would be great and avoid future bruised ego.

Click to expand...

Hey mate, do you have the stock jet sizes? I rejetted for the exact same set up as you pretty successfully. I think it was a 130 main, 52.5 pilot and raised the needle 1 or 2 notches, then final adjustment of the mixture screw.

It was a while ago so I'm not 100%. It worked well for the Melbourne climate and altitude. Did find that the main jet needed to be changed for warmer or cooler months

Wondering which mechanic you went to for the RWC inspection? I was finding it hard to suss a mechanic to RWC custom bikes

Ryan Stecken said:

hey got a few questions on your paint of the motor.

did you bake that color?if yes how long?
did it show any chipping after letting the motor run several times?

how did you make these rad polished motor fins?
did you use a dremel after letting the paint cure?


thanks in advance buddy

Click to expand...

Hey mate. I didn't bake it. I took the head off and appart as well as the side cover to paint . I used primer first when I had to and then used matt black spray paint from a can. It was engine manifold paint so I knew it would take the heat. I only did 2 coats on the head because I didn't want to have it over heat. As for the fins and stuff, any thing on the side covers that is still chrome I taped up before I painted and the fins on the head I sanded back after I painted. It's a long boring job but I think the result is worth it.
I'm not sure if this is the best way to go about it, but being my 1st bike it's the best way I know. Hope that helps mate.

Learned a little about my battery and my bike today here. As much as I'd like to go no battery and a kick start, this thread makes me think I'll just try to hide my battery a bit better.

I see more SR250 brats than café builds. This one looks great. You made the stock tank work, not easy.

cosworth said:

Learned a little about my battery and my bike today here. As much as I'd like to go no battery and a kick start, this thread makes me think I'll just try to hide my battery a bit better.

I see more SR250 brats than café builds. This one looks great. You made the stock tank work, not easy.

Click to expand...

Thanks mate. Battery has gone flat a few times while I was working on the electrical and I couldn't start it as a result. If you want to hide the battery look at getting a Li-ion, I got a 4 cell Antigravity battery and it was a 1/4 the size and weight. Cost me $170 though. I'm not sure if you could get away with a capacitor.... might be worth looking into. good luck with it all.
Do you have a build thread?

Rmitch said:

Hey mate, do you have the stock jet sizes? I rejetted for the exact same set up as you pretty successfully. I think it was a 130 main, 52.5 pilot and raised the needle 1 or 2 notches, then final adjustment of the mixture screw.

It was a while ago so I'm not 100%. It worked well for the Melbourne climate and altitude. Did find that the main jet needed to be changed for warmer or cooler months

Click to expand...

Mate Ill have to read the manual. I'm sure it's a stock carby but not sure how to check the jet sizes, nor how to change them. I have been meaning to do some research ( hopefully find a youtube tutorial) but have been flat out. Life gets in the way hay! haha

PocketsVIC said:

Mate Ill have to read the manual. I'm sure it's a stock carby but not sure how to check the jet sizes, nor how to change them. I have been meaning to do some research ( hopefully find a youtube tutorial) but have been flat out. Life gets in the way hay! haha

Click to expand...

No worries, it's really easy to do- just be careful around the diaphragm when changing the needle height, and the posts that hold the float.

The sizes will be stamped on the actual jet. The smaller number in this carb is the pilot and the larger the main.

PocketsVIC said:

Thanks mate. Battery has gone flat a few times while I was working on the electrical and I couldn't start it as a result. If you want to hide the battery look at getting a Li-ion, I got a 4 cell Antigravity battery and it was a 1/4 the size and weight. Cost me $170 though. I'm not sure if you could get away with a capacitor.... might be worth looking into. good luck with it all.
Do you have a build thread?

Click to expand...

I don't have a build thread going yet. I've had some issues finding welding gasses on the island I live on, mostly due to them being closed when I'm off work and it's all exchange, no one fills tanks here. And if you don't have their tanks, they turn you away. So some of my progress has been slowed down. I digress. Lol.

So yeah after building my hybrid merkur/cosworth "XR500" I've gotten better at project management and controlling project creep. Barkingmadspeedshop has been really helpful finding some controls and other things for me.

This is what I have so far:

I am about to take my bike back in for a rego inspection and have been searching VIC ROADS web site for some sort of motorcycle roadworthiness requirements for victoria but can't for the life of me find anything. I have found one for cars but that don't help me. Surely there must be a generic checklist and set of rules, like "rear mudguard must come down to a 45 degree on the tyre", checklist that mechanics have to go off. Does any one know where to get one of these.
I have read the ADR's that apply for my bike but that's not what I'm after.

Also, does any one know if it's legal in VICTORIA AUS to mount your licence plate next to your back wheel behind your chain?
Also I got knocked back on my exhaust and want to know what the decible rating for a 81 bike is. or is the rule you cant exceed the original stock exhaust decible limit?

Thanks in advance.

Also I made a new seat last weekend. Thought this colour leather would turn out good, but over the past week it has done the opposite of grow on me haha. I rekon it pops out too much, so I'm keeping an eye out for old dark dark brown/ merone leather couches on the side of the road. That's were I got the material for this one.
One mans trash.....

Side view

PocketsVIC said:

I am about to take my bike back in for a rego inspection and have been searching VIC ROADS web site for some sort of motorcycle roadworthiness requirements for victoria but can't for the life of me find anything. I have found one for cars but that don't help me. Surely there must be a generic checklist and set of rules, like "rear mudguard must come down to a 45 degree on the tyre", checklist that mechanics have to go off. Does any one know where to get one of these.
I have read the ADR's that apply for my bike but that's not what I'm after.

Also, does any one know if it's legal in VICTORIA AUS to mount your licence plate next to your back wheel behind your chain?
Also I got knocked back on my exhaust and want to know what the decible rating for a 81 bike is. or is the rule you cant exceed the original stock exhaust decible limit?

Thanks in advance.

Click to expand...

http://www.vsis.ca/logon/logon.aspx?vid=QPLEGALEZE:cvim_view

You are either going to have to find someone who will let you go through the inspection manual, find the manual somewhere or pay for it.

In Alberta we can download our inspection manual and see what the bike would need to pass and let me tell you its every little stupid finiky thing you can think of.

My bike hasn't been insured since well the late 70s early 80s and it doesn't come up in the registry offices so I have to essentially get a inspection done the bike where they will even make sure signal lights are DOT satisfactory and wheel bearings are fine etc.

firebane said:

http://www.vsis.ca/logon/logon.aspx?vid=QPLEGALEZE:cvim_view

You are either going to have to find someone who will let you go through the inspection manual, find the manual somewhere or pay for it.

In Alberta we can download our inspection manual and see what the bike would need to pass and let me tell you its every little stupid finiky thing you can think of.

My bike hasn't been insured since well the late 70s early 80s and it doesn't come up in the registry offices so I have to essentially get a inspection done the bike where they will even make sure signal lights are DOT satisfactory and wheel bearings are fine etc.

Click to expand...

man that is rough. I thought I had it bad! I'll keep searching

number plate has to be centered, which is good, because chopper style off to the side looks awful anyway.

http://www.vicroads.vic.gov.au/NR/rdonlyres/2F4D64FE-B1A5-48D1-B848-19CD5F8176D7/0/VSI4.pdf

http://www.vicroads.vic.gov.au/NR/rdonlyres/1EDB44D4-0729-457B-BC74-4AAD84A6F8EC/0/VSI26_1212_WEB.pdf

. the 45 degree angle is correct, but it should be with someone sitting on the bike, so that gives you a few cm's you might need to pass, you can also screw a bit of rubber sheet one the bottom or similar. you're goign to need mirrors obviously, other then that, your bike looks like it could pass fine.
the rwc list is just the sensible things for safety, tyre tread, indicators, kill switch, bearings good, good chain, lights all work, lights all 'e' marked, no holes in seat, nothing is loose, brakes work well, etc etc.

Hi all.
i just jumped on JETSRUS.COM to order some new jets for my carb, but have no idea what the make and model the stock carb i have is. I couldn't see a model number stamped any were. Am i blind? Have i made a massive rouky error? Any help would be great.
thanks in advance.