Understanding carb jetting.

Hawk2016 · 10-07-2016, 03:49 PM

I have been fine tuning my Mikuni clone vm28 and trying to get the most
perfect jet setup based on my current air flow threw the Hawk engine I
found this youtube video and I like how this guy explains the jetting process and I was thinking this would be a good place to share it and maybe get some more input

'16 TT250 · 10-07-2016, 09:16 PM

Lots of information, but I think your idle air/fuel screw controls fuel and not air so it'll be backwards from his description.

pete · 10-07-2016, 10:41 PM

Quote:

Originally Posted by '16 TT250

Lots of information, but I think your idle air/fuel screw controls fuel and not air so it'll be backwards from his description.

it dose both..one leads to another ...
changes the vacume around the
emultion tube making it easyer or harder for the motor to draw fuel
at low rpms / vacume..

..

'16 TT250 · 10-08-2016, 12:37 AM

No, it does one or the other, it either meters air or fuel. If it meters air, as the carb in the video does, then in is rich and out is lean. If it meters fuel, as I think these carbs do, then it's the opposite.

pete · 10-08-2016, 12:53 AM

Quote:

Originally Posted by '16 TT250

No, it does one or the other, it either meters air or fuel. If it meters air, as the carb in the video does, then in is rich and out is lean. If it meters fuel, as I think these carbs do, then it's the opposite.

after this air has been metered .. what dose that air do ?

.....

Ariel Red Hunter · 10-08-2016, 10:20 AM

Quote:

Originally Posted by pete

it dose both..one leads to another ...
changes the vacume around the
emultion tube making it easyer or harder for the motor to draw fuel
at low rpms / vacume..

..

That is correct. It doesn't matter which way it is done. The emulsion tube is permitted to either draw more or less air, or more or less fuel depending on which way the designers preferred. Because of the limited range of adjustment available, it is important to have a pilot jet that allows the mixture screw to be somewhere near the middle of its adjustment range. The pilot jet controls the fuel mixture from idle to about 1/4 throttle, not 1/8th throttle. At 1/8th throttle, the main jet, through the needle jet, is starting to have an effect on mixture. But from 1/4 throttle to 3/4 throttle, the needle taper controls the mixture. From 3/4 throttle to wide open the main jet controls the quality of the mixture.
The best way to do your jetting, in my opinion, is to install a new spark plug on a warm engine, then take the bike out and run it wide open in high gear for a mile or so. Then, ALL AT THE SAME TIME, pull in the clutch, shut off the throttle, and shut off the ignition. This is called a plug chop. Keep the clutch pulled in, and coast to a stop. pull the plug and look at it. If it's white, too lean. If it's black, too rich. If it's brown, well, pretty close. When you have the right main jet, or are close, you will be able to run the needle in the middle slot. You should have clean running from 1/4 throttle on all the way to wide open. No flat spots, no hesitation, and no gurgles. Then put in a pilot jet that allows the idle screw adjustment to be in the middle of its range, and you are good to go. Until you change to a different type air filter, different exhaust system or open up the air box . Then you get to do it all over again!

'16 TT250 · 10-08-2016, 12:28 PM

Quote:

Originally Posted by pete

after this air has been metered .. what dose that air do ?

.....

If the air flow is restricted by turning an air screw in it makes the bike run richer.

Not trying to argue and I'll let it go after clarification. I was merely pointing out that if someone follows the instruction in the video and applies it to their carb with fuel screws the adjustments will be backwards. I've read numerous posts from people who got completely screwed up because they followed air screw instructions on a fuel screw carb or vice-versa and were pulling their hair out because they couldn't find a pilot jet/screw setting that would make their motor run right.

Generally speaking, if the screw is on the cylinder head side of the carb slide it's a fuel screw, if it's on the air filter side it's an air screw. Also generally, 4 strokes have a fuel screw where 2 strokes may have air or fuel. Some carbs have both and/or have adjustments for air/fuel adjustments beyond the idle circuit, but I've never seen those on a stock motorcycle that I can recall.

10-07-2016, 03:49 PM	#1
Hawk2016 Join Date: Sep 2016 Location: Spencer IN. Posts: 9	Understanding carb jetting. I have been fine tuning my Mikuni clone vm28 and trying to get the most perfect jet setup based on my current air flow threw the Hawk engine I found this youtube video and I like how this guy explains the jetting process and I was thinking this would be a good place to share it and maybe get some more input __________________ “Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety.”
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10-07-2016, 09:16 PM	#2
'16 TT250 Join Date: May 2016 Location: Hacienda Heights, Ca Posts: 281	Lots of information, but I think your idle air/fuel screw controls fuel and not air so it'll be backwards from his description.
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10-08-2016, 12:37 AM	#4
'16 TT250 Join Date: May 2016 Location: Hacienda Heights, Ca Posts: 281	No, it does one or the other, it either meters air or fuel. If it meters air, as the carb in the video does, then in is rich and out is lean. If it meters fuel, as I think these carbs do, then it's the opposite.
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