Mikuni Carburetor Jetting INFO

[NickThrash]

I put the Mikuni Carb on my 250 Hawk and love it. It's winter and very cold up here and have tested it out a few times and it feels great.

The stock carb seemed to run really well in the cold but once the bike started to get really hot, it would go back to being really boggy again just like it did in the summer. Over the summer that's all it ever was. (Again this is referring to the old stock carb)

I put the 110 jet on the mikuni because that's what i was recommended. But i see A LOT of people using the 115. I was wondering if i should go up to the 115 or if i will likely need to once it starts to get hotter out? or if i would have to go to the 100. I ride at low altitude. (ive seen people mention high altitudes when referring to the jetting)

Also if anyone can give me some info on what a higher or lower number jet does exactly and how i should know which way to go if i start to have problems. I want to have my bike set up for warmer weather since thats when i will be using it the most. and if i run into problems with the jetting i would like to know which direction i should go

[joeboyk]

What mods have you done?Also where exactly do you live ,for above sea level elevation.

[NickThrash]

Originally Posted by joeboyk

What mods have you done?Also where exactly do you live ,for above sea level elevation.

Not many performance mods. I put on the 17t F sprocket. Opened up the air box. And the Mikuni Carb. Thats about it. I live in OHIO and i think we are about 600ft above sea level. The bike runs great so far in the colder weather, but summers here get into the 80-90+ degrees. and was wondering if i should go with a different jetting.

[Weldangrind]

You need a fatter jet in colder weather, since the air is more dense. Jet sizes are in millimetres, so a 110 is 1.1mm, a 115 is 1.15mm etc. Before changing jets, evaluate how the engine runs and go from there. Does it fail to idle? Does it bog when you first open the throttle? Does it have a hiccup at mid-throttle? Does it feel like it's running out of gas at WOT? All of these questions need answers before changing jets. A plug chop is also wise.

[Ariel Red Hunter]

Originally Posted by NickThrash

I put the Mikuni Carb on my 250 Hawk and love it. It's winter and very cold up here and have tested it out a few times and it feels great.

The stock carb seemed to run really well in the cold but once the bike started to get really hot, it would go back to being really boggy again just like it did in the summer. Over the summer that's all it ever was. (Again this is referring to the old stock carb)

I put the 110 jet on the mikuni because that's what i was recommended. But i see A LOT of people using the 115. I was wondering if i should go up to the 115 or if i will likely need to once it starts to get hotter out? or if i would have to go to the 100. I ride at low altitude. (ive seen people mention high altitudes when referring to the jetting)

Also if anyone can give me some info on what a higher or lower number jet does exactly and how i should know which way to go if i start to have problems. I want to have my bike set up for warmer weather since thats when i will be using it the most. and if i run into problems with the jetting i would like to know which direction i should go

Ok, the higher the number, the more fuel flow per second, minute, whatever. Personally, I do this by putting the needle in the middle slot, and the idle mixture at 1 1/2 turns out. Then, I ride the bike around for 20 minutes, or so, to get it well warmed up. Then run it a mile or so, and do a plug chop. A real plug chop. At full throttle, hot and winding, whip out the clutch and kill the ignition with the red button on the right hand grip. Release throttle. Make sure the engine does not turn over all the way to a dead stop. If it has any black on the plug, when you pull it out, it's too rich. If it is almost white to ivory color it is too lean. If it is tan, it is just right. Now check the throttle response from between 1/4 and 3/4 throttle, and then on to full throttle. Should be seamless. Now for a dual purpose motorcycle, the most important part of all - idle to 1/4 throttle. What you are looking for is a smooth transition from idle all the way up to wide open throttle. The pilot jet controls the fuel mixture from idle to 1/4 throttle. So, leave the idle mixture screw alone until you have a pilot jet that allows you to roll on the throttle at low rpm, clutch fully engaged, and wind out to wide open, without a hiccup or hesitation anywhere. For the last part, turn the idle speed down to a speed that the engine can run at, and adjust the idle mixture control screw to the highest speed idle obtainable. I can't really get you any closer than this from here, because some of these carbs have an idle air control, and some have an idle fuel control. Richer and leaner is obtained by going 180 degree different with the idle mixture adjusting screw. Ideally, you want it just rich enough to pick up (a healthy first "thump") from the lowest idle speed the engine can maintain. Usually 1350-1400 rpm. When you have your carburation set up like this, your clutch will thank you by not getting hot........ARH