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Discussion Starter · #1 · (Edited)
Hi All -

This February I bought my first bike - a 1980 Yamaha SR250 modified by the previous owner to loosely fit the "cafe racer" style. And certainly in need of a fair bit of work, mostly cosmetic.

The seller mentioned the carb he installed, a Mikuni 34mm VM with stock jets on all circuits, would probably need tuned or rejetted, as the last adjustments were made when he lived in California (now having moved to Nebraska). Naive me figured this was no big issue. The bike made it the 2 hour ride home, with minimal issues. I only noticed it start running poorly once I was in town and it finally got a chance to properly warm up after going full tilt down the highway at 40 degrees F.

Fast forward a few months, I finally have the bike nice and purty, and fixed a slow oil leak. It was then that I cleaned the carbs and began the tuning process. It ran like a dog and sputtered through all throttle positions, at first leading me to believe I had really screwed up putting the carb back together. After about 3 weeks of tinkering, it turns out it was just running rich on all circuits. I have dropped a size on the pilot, main, and jet needle. I did one at a time of course. Each circuit I dropped a size gave a significant improvement to the throttle range it controlled. Last night I was finally comfortable enough to ride her around town for about 45 minutes without fear of stalling. I am getting great throttle response nearly everywhere - EXCEPT around 1/8 to 3/8 throttle in all gears. It sputters and accelerates roughly before eventually catching smooth. To the best of my knowledge, this is where the exchange between pilot and jet needle/needle jet circuits takes place. The size of the throttle slide cut out also has an effect here as well. I have performed plug checks in all throttle positions, and it continues to foul at this position, and this position only. I am hesitant to drop another size on the jet needle for fear of leaning out the upper mid range too much. The e clip is already in the leanest position. Leaning out the air screw more than another 1/8 of a turn gives me an awful hanging idle. I am running with fresh gas, brand new fuel lines, and new air filter. :confused:

Does someone with more practical experience than me have any suggestions? Should I be looking for a throttle valve with a larger cutaway, which, according to the wisdom of random people on the internet, seems unlikely? I also haven't messed with the needle jet (emulsion tube). Go easy on me, as of 6 months ago I had no idea how a carburetor even worked. :eek: I'm learning a little more every day I continue with these adjustments.

Hope this makes a shred of sense!
 

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Still sounds like you're a little rich on your pilot jets.

Have you done a red line plug chop yet? How do the plugs look after casual riding as well?
 

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Discussion Starter · #5 ·
Still sounds like you're a little rich on your pilot jets.

Have you done a red line plug chop yet? How do the plugs look after casual riding as well?
Have not done a red line plug chop yet. After a 45 minute casual ride last night, the plugs were slightly fouled. Not as severely as before the rejet, but certainly not a nice tan color. I don't know if this matters, but I was riding in town last night so I never really opened the throttle past 3/4, let alone had it wide open. Most of the time spent at 0 - 1/2.

I tried adjusting the air screw this afternoon to lean out the pilot jets a bit more, and any more than another 1/8 of a turn out made the idle rpm shoot up. It's currently seated at 1 1/8 turns out. Perhaps I just need to adjust the idle mixture screw as I'm leaning out the pilot to prevent this?
 

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Discussion Starter · #7 ·
BTW - that's a pretty big carb for a 250 single.
That's what I thought after a bit of research. The previous owner seems to have thrown it on there without much thought. Jets seem to be cheaper than a new carb however, so I thought I would go that route first and learn a little something along the way :D
 

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to give you some perspective, I'm currently tuning some CR carbs on my Z1. The bike sputters under acceleration and has horrible fuel mileage. Redline chops leave the seat fouled but the tip is a good tan color. Pulling the plugs after regular riding and idling have the whole plug black. I adjusted the needle one "notch" and there wasn't an appreciable change. I then did some research and went from a 68 pilot down to a 52... a huge change, but it should get me far more into the ballpark than I already was.
 

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Discussion Starter · #9 ·
to give you some perspective, I'm currently tuning some CR carbs on my Z1. The bike sputters under acceleration and has horrible fuel mileage. Redline chops leave the seat fouled but the tip is a good tan color. Pulling the plugs after regular riding and idling have the whole plug black. I adjusted the needle one "notch" and there wasn't an appreciable change. I then did some research and went from a 68 pilot down to a 52... a huge change, but it should get me far more into the ballpark than I already was.
Sounds like I'll being going after the pilots - good thing I ordered the next size down as well. 35 was the stock on this, and I've dropped to a 30. Got a 25 on hand if I can't get it evened out with the screw. I also dropped the needle from 6DH4 to a 6DP17 (stock for Mikuni's 32 mm VM), and the main from a 260 to a 250.
 

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Sounds like I'll being going after the pilots - good thing I ordered the next size down as well. 35 was the stock on this, and I've dropped to a 30. Got a 25 on hand if I can't get it evened out with the screw. I also dropped the needle from 6DH4 to a 6DP17 (stock for Mikuni's 32 mm VM), and the main from a 260 to a 250.
That's the other thing to look at as well as changing the pilot - find a needle with a different taper.
 

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how long is the taper on the needle? you might find that it's still on the root diameter. 6FL14 has a larger root. the mikuni needle system is nuts.

n-0 needle jet, assuming it has an n-2 now.

what sort of air filter does it have? tried removing it?
 

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250 main?! Hory clap! Are you running methanol?

you need to do some WOT throttle chops and get your main jet circuit dialed in first. THEN, work on the needle and transition circuits. The main affects the mid range as well.
 

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Discussion Starter · #13 ·
how long is the taper on the needle? you might find that it's still on the root diameter. 6FL14 has a larger root. the mikuni needle system is nuts.

n-0 needle jet, assuming it has an n-2 now.

what sort of air filter does it have? tried removing it?
I've spent a lot of time with this page lately - http://www.sudco.com/images/Tuning%20Jetting%20pg%202.jpg . I agree, for a first timer it's a lot to learn and take in.

Taper on the current needle is 32.1. Comparing it to the stock needle, which I should have done before ordering, it only leans out the center. Diameter is increased in the middle, and decreased at the root and tip. Doh.

The needle jet is the stock Q-2, 159 series. Ordered 3 smaller sizes this morning, though I won't get to try them out for another week.

Air filter is one of those supposedly washable/reusable "K&N Universal Chrome" filters. Haven't tried removing it yet.
 

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Discussion Starter · #14 ·
250 main?! Hory clap! Are you running methanol?

you need to do some WOT throttle chops and get your main jet circuit dialed in first. THEN, work on the needle and transition circuits. The main affects the mid range as well.
Hahaha. I'm running on 0 experience and a busy residential area that doesn't lend itself well to running WOT plug chops. Now that I have it in a semi reliable running condition I can actually get out of the middle of town and do some better work on the top end. Also waiting for 200-240 mains in the mail. The first drop in main jet size made a noticeable difference at least.
 

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a dyno and a professional tuner guy or a test track, a professional rider and a professional tuner plus a slew of parts would do it. Do you ride with any decent rider or tuner types or do you have bags of money and time to throw at it?

Carburetor tuning is acquired knowledge but it all starts with understanding how each little assembly works, it's not rocket science, you're on the right track by acquiring the knowledge. Then get out there and get all gasoline smelly.
 

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As mentioned before, a VM34 is HUGE for that bike. The PO probably saw the original 34mm CV carb and figured, "If 34mm works for the CV, it must be the same for the VM type." Nope. As an example, my 79GS550 came with a rack of VM22's. In 1980 they went to CV type carbs and the bore went up to 32mm. The fueling ratio didn't magically change. 14.7:1 was still stoichiometric efficiency. It's just that VM's and CV's work differently. I have a feeling that you may have trouble getting this carb to run lean enough, and you will most likely have to be smaller than that 200 main you ordered.

Also for reference, the Keihin mechanical slide carb on my XR400, is around 32mm, if I recall. So 34mm bore on a similar type carb for a 250? Might be a bit of a challenge. But who knows. I've never dealt with that particular set up, so Maybe you can get it straight.

You have to work from the top down when tuning. Typically, I start wiith getting the idle close, just so it will run smooth (that's after setting valve clearance, and doing some other static checks). Then get the main jet dialed in because the main affects the mid range (needle), then dial in the transition off the pilot, because the needle position affects the transition, which puts you back to optimizing the idle circuit.

Also, don't dick around with cheap ass pod filters. http://www.caferacer.net/forum/tips-tricks/19639-pod-filter-thread-geeto.html
 

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not sure how much you're paying for jets, but a vm28 is $115. how big is the inlet port at the head?

a carb that's too big can be harder to tune at low throttle ime, as it's just not getting enough signal for consistant operation as designed. similarly, it'll need bigger than expected mains due to the lower vacuum not pulling so much fuel through.

dyno time is a great idea. you can download dynojet run viewers so you can look at it all at home too, but it's a great way to get a heap of info quick. you've only got one jet to change too, with i'm guessing prety easy access.
 
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