Hawk 250 Mikuni 30mm Carb Jetting Land Elevation of 600 Feet

[Megadan]

^Your spark plug in that picture is in the range I like to call "factory good", which is how my Goldwings factory tuning is, along with many other bikes. It's in a safe range, but leaning very much toward the lean side of things. It's getting hot enough that the ceramic/electrode are getting cleaned almost back to white with just the slightest hint of tan. You could actually go a little richer and be ok. I have my Hawk tuned on the opposite end of the "good" spectrum, just a little rich but still stays clean. Closer to a light brown than a light tan covering the ceramic and electrode.

The real question is, at what throttle position is the plug getting in that condition? Was that after a WOT plug chop, or at low speed/idle?

[Musictrek]

Originally Posted by Megadan

^Your spark plug in that picture is in the range I like to call "factory good", which is how my Goldwings factory tuning is, along with many other bikes. It's in a safe range, but leaning very much toward the lean side of things. It's getting hot enough that the ceramic/electrode are getting cleaned almost back to white with just the slightest hint of tan. You could actually go a little richer and be ok. I have my Hawk tuned on the opposite end of the "good" spectrum, just a little rich but still stays clean. Closer to a light brown than a light tan covering the ceramic and electrode.

The real question is, at what throttle position is the plug getting in that condition? Was that after a WOT plug chop, or at low speed/idle?

I pretty much ran her up on the speed right before I got home. I think I should get her a little more rich. At least go to the 115 again.

[JerryHawk250]

Looks like it's running good but on the leaner side. Maybe the 115 main and 22.5 pilot but drop the needle one notch at a time to try to tune that 3/4 throttle transition.

[pete]

Something I read years ago in a bike mag before the internet...
bikes are jetted from the factory for 1000 foot elevation the bike
could run ok from sea level to something like 4000???? feet before any major jetting
changes were required...
wheather they still do this is anyone guess.....
artical also had a elevation jetting chart...

some more useless info.......



.

[Musictrek]

Originally Posted by JerryHawk250

Looks like it's running good but on the leaner side. Maybe the 115 main and 22.5 pilot but drop the needle one notch at a time to try to tune that 3/4 throttle transition.

I'll try that this weekend. I have the 115 and 22.5. Needle position I start in the middle. That's what so nice about the bike its easy to pull that carb out.

[Musictrek]

Originally Posted by Musictrek

I'll try that this weekend. I have the 115 and 22.5. Needle position I start in the middle. That's what so nice about the bike its easy to pull that carb out.

I didn't wait. its so easy that I went ahead this Friday night and put the 115 and 22.5 with middle needle position. She starts ok. But, 0-1/2 throttle she was a dog. Was not an even smooth idle when I twisted the throttle 1/4 or more. If I went from 0 twist to quick full throttle she would just die. I pulled the carb and changed the 22.5 to 20 and left the 115. I put back together and started her up. Not any better with that. I adjusted the mix screw underneath and made the full on throttle twist not so much die like it did. I took for a test ride. The throttle 0 to 1/2 or more was not a smooth rpm. On deceleration it would back fire quite a bit. For now, I put my 110 back in with the 20 and needle on full rich. It runs way better with that than what I tried tonight. On the 110 I can twist that throttle as fast as I can and that engine will ramp up smooth and fast and not the backfire on deceleration. not that the 110 has any effect on the 1/4 throttle setting.

[Musictrek]

Originally Posted by Musictrek

I didn't wait. its so easy that I went ahead this Friday night and put the 115 and 22.5 with middle needle position. She starts ok. But, 0-1/2 throttle she was a dog. Was not an even smooth idle when I twisted the throttle 1/4 or more. If I went from 0 twist to quick full throttle she would just die. I pulled the carb and changed the 22.5 to 20 and left the 115. I put back together and started her up. Not any better with that. I adjusted the mix screw underneath and made the full on throttle twist not so much die like it did. I took for a test ride. The throttle 0 to 1/2 or more was not a smooth rpm. On deceleration it would back fire quite a bit. For now, I put my 110 back in with the 20 and needle on full rich. It runs way better with that than what I tried tonight. On the 110 I can twist that throttle as fast as I can and that engine will ramp up smooth and fast and not the backfire on deceleration.

After reading up on tuning carbs, I learned that the main jet works on 3/4 to full throttle. I should be able to leave that 115 in there.

[Megadan]

Keep in mind that while certain jets have the greatest effect at certain throttle positions, that they still contribute to the total fueling as well. The simplest way I have seen it explained that the pilot jet is 90% of the fuel circuit up to 1/4, with the main/needle jet adding in 10%, and from 3/4 to WOT, it's backwards of that 10% pilot, 90% main. This may not be accurate, but it does outline a rule of thumb that any jet change can affect other jets operation.

Also, any time you change the main jet, you will also affect the needle fueling. The main jet screws into the bottom of the emulsion tube that the needle regulates. The bigger the main, the more fuel flow through said tube, and the more fuel at the same needle position.

The biggest issue with this style of carburetor is that even with the best tuning, they will still tend to go lean on the top end and rolling on the throttle unless you go fuel heavy on the other circuits. Then you run into the issue of being too rich on the top end and that is why Mikuni and Keihin introduced power jets/pumpers on their carbs.

These types of carbs allow you to lean out the middle and lower throttle positions while also running a larger main jet to help with the top end fueling. They help combat the overly lean condition when rolling on throttle and transitioning between jets, while also ensuring proper fueling on the top end of things.

[Ariel Red Hunter]

Originally Posted by Megadan

Keep in mind that while certain jets have the greatest effect at certain throttle positions, that they still contribute to the total fueling as well. The simplest way I have seen it explained that the pilot jet is 90% of the fuel circuit up to 1/4, with the main/needle jet adding in 10%, and from 3/4 to WOT, it's backwards of that 10% pilot, 90% main. This may not be accurate, but it does outline a rule of thumb that any jet change can affect other jets operation.

Also, any time you change the main jet, you will also affect the needle fueling. The main jet screws into the bottom of the emulsion tube that the needle regulates. The bigger the main, the more fuel flow through said tube, and the more fuel at the same needle position.

The biggest issue with this style of carburetor is that even with the best tuning, they will still tend to go lean on the top end and rolling on the throttle unless you go fuel heavy on the other circuits. Then you run into the issue of being too rich on the top end and that is why Mikuni and Keihin introduced power jets/pumpers on their carbs.

These types of carbs allow you to lean out the middle and lower throttle positions while also running a larger main jet to help with the top end fueling. They help combat the overly lean condition when rolling on throttle and transitioning between jets, while also ensuring proper fueling on the top end of things.

Dan, when you go richer on the main jet, you lower the needle to lean out the mid-range. I had the advantage of working mostly with Amal carburetors, which offered a variety of needles available (in my carb tuning box) to massage the mid-range mixture even more than the five notches on the needle do. As I've said before, on race night the temperature and humidity kept changing as the night went on. So we changed main jets often, but usually the other three worked all night long. The Harleys and Indians had adjustable main jets, which was a mixed blessing for their mechs, as riders tended to play with them. Especially the guys behind the leaders, to the detriment of their pistons...ARH

[Musictrek]

Originally Posted by Megadan

Keep in mind that while certain jets have the greatest effect at certain throttle positions, that they still contribute to the total fueling as well. The simplest way I have seen it explained that the pilot jet is 90% of the fuel circuit up to 1/4, with the main/needle jet adding in 10%, and from 3/4 to WOT, it's backwards of that 10% pilot, 90% main. This may not be accurate, but it does outline a rule of thumb that any jet change can affect other jets operation.

Also, any time you change the main jet, you will also affect the needle fueling. The main jet screws into the bottom of the emulsion tube that the needle regulates. The bigger the main, the more fuel flow through said tube, and the more fuel at the same needle position.

The biggest issue with this style of carburetor is that even with the best tuning, they will still tend to go lean on the top end and rolling on the throttle unless you go fuel heavy on the other circuits. Then you run into the issue of being too rich on the top end and that is why Mikuni and Keihin introduced power jets/pumpers on their carbs.

These types of carbs allow you to lean out the middle and lower throttle positions while also running a larger main jet to help with the top end fueling. They help combat the overly lean condition when rolling on throttle and transitioning between jets, while also ensuring proper fueling on the top end of things.

That does make sense on the main jet adding to the fuel mix overall. I even put the needle clip one slot down from the top for lean with the 115 and 20 to see if I could lean out the low end. On the mix screw on the bottom of the carb. If its turned in is that leaning the mix or making it rich. I read where if the screw is in the front of the carb vs. the back will determine rich or lean with screw direction.

[Megadan]

Originally Posted by Ariel Red Hunter

Dan, when you go richer on the main jet, you lower the needle to lean out the mid-range. I had the advantage of working mostly with Amal carburetors, which offered a variety of needles available (in my carb tuning box) to massage the mid-range mixture even more than the five notches on the needle do. As I've said before, on race night the temperature and humidity kept changing as the night went on. So we changed main jets often, but usually the other three worked all night long. The Harleys and Indians had adjustable main jets, which was a mixed blessing for their mechs, as riders tended to play with them. Especially the guys behind the leaders, to the detriment of their pistons...ARH

I am aware of all of this, but I have also found it helps people understand how these carbs work if you explain it the way I just did. Unless you are making a huge leap in jet sizes, then swapping around one jet shouldn't be enough to throw off the other circuits too much.

As far as the needles, I have found the Mikuni to have a slightly more aggressive taper to it than the stock carb, which lends itself better to the CG250's desire for a more hungry mid range. On my bike I noticed a small hesitation between the transition from needle to main when rolling off and on the throttle when I had the 115 main, so I put it 1 notch down. This often resulted in a slightly too rich half throttle. When I went from the 115 to the 117.5 main, I tried the needle back in the center position, and that hesitation was gone, and the mixture was much happier part throttle.

Originally Posted by Musictrek

That does make sense on the main jet adding to the fuel mix overall. I even put the needle clip one slot down from the top for lean with the 115 and 20 to see if I could lean out the low end. On the mix screw on the bottom of the carb. If its turned in is that leaning the mix or making it rich. I read where if the screw is in the front of the carb vs. the back will determine rich or lean with screw direction.

On the idle mixture screw. A basic rule of thumb is, if the screw is on the back side of the bowl (closer to the engine) then it controls fuel. if it is on the front side of the bowl, it controls air. On the stock carb and Mikuni clones, it controls fuel, so if you turn it in (righty tighty) it leans out the idle fuel mixture. Changing the pilot jet directly affects the idle mixture. If you go larger on the pilot you also increase fueling on the idle mixture.

As far as the needle setting goes, ideally you should strive to find the main jet size that allows the needle to best operate in the center position. Obviously, we rarely live in the ideal world, so don't focus too much on that needle position for the time being.

If your bike is happy on the 20 pilot, and you get no hesitation at light constant throttle, then I would say leave it alone. I tried to run a 25 pilot on my bike, and found it to be just about right, but a tad lean at light constant throttle/ The 27.5 pilot I currently use is honestly a tad rich on warmer days, but if I have to choose between the two, I choose a tad rich every time.

If you are unsure about the pilot jet, then I have found the easiest way to identify if it is lean or rich is to try and maintain a very light constant throttle at around 20 - 25mph in 2nd or 3rd gear. This puts your throttle position smack dead center in the pilot circuits primary range. If you have the fine motor control skills, you can even use plug chops to tune this range fairly accurately.

[HawkTwoFifty]

Originally Posted by Megadan

No. You want a vm28/486 pilot jet.
https://www.jetsrus.com/a_jets_by_ca...-486_pilot.htm

Megadan, I received the new jets for my VM26 carb but havent yet had the time to install them. But I am confused about something. With my carb being a VM26 carb, arent these jets for a VM28 carb? Will they work on my VM26 when they say VM28? Thanks!!!!

[Megadan]

Originally Posted by HawkTwoFifty

Megadan, I received the new jets for my VM26 carb but havent yet had the time to install them. But I am confused about something. With my carb being a VM26 carb, arent these jets for a VM28 carb? Will they work on my VM26 when they say VM28? Thanks!!!!

These are clone carbs, not genuine mikuni carbs, so they dont follow any normal patterns.

[Alexander M]

I got the Mikuni Vm26 30mm carb for my Vitacci Pentora 250 and I live about 150ft above sea level so I'm pretty close to sea level. So far all I have added was just a Nibbi high performance intake and took off the exhaust silencer. It runs pretty nice but I do have to start it with the choke for a sec and then take it off. Do you guys feel I should change any of the Jets with the upgrades I have so far and my elevation? I don't know much about carb jetting so I want to know what setting would make it run as perfect as possible. It seems like it runs a tad on the lean side as well.

[Megadan]

Originally Posted by Alexander M

I got the Mikuni Vm26 30mm carb for my Vitacci Pentora 250 and I live about 150ft above sea level so I'm pretty close to sea level. So far all I have added was just a Nibbi high performance intake and took off the exhaust silencer. It runs pretty nice but I do have to start it with the choke for a sec and then take it off. Do you guys feel I should change any of the Jets with the upgrades I have so far and my elevation? I don't know much about carb jetting so I want to know what setting would make it run as perfect as possible. It seems like it runs a tad on the lean side as well.

If it needs full choke to start and the air temps are over 50F, then it idles fine once warmed up it may just need the idle mixture adjust out a little bit. If it has any stumble or hesitation on very light throttle at low speeds as well, then it might need a step up on the pilot jet.

Do you know what jets are currently in the carb?