Airscrew and fuel screw tuning

Since I can't find any threads on this I'll make a new one! I'm trying to richen up my bike a bit, but don't know much about the screws. I know the screw on the top of the carb is the air screw and on bottom is the fuel screw. I don't know how much I should turn them. Is it screw out to richen? And screw in to lean? How do I know if it worked? My very left carb is running super lean and I don't like that. help? Thanks

You do not indicate what bike and or carbs you have. This makes it somewhat difficult to give advice. But I will assume that you have vm style carbs and that they have both the air and fuel screws. If that is the case then try this. The pilot fuel screws are located on the engine side underneath, near the float bowl. Turn these out 1 3/4 turns and the air screws which are on the side of the carb bodies about 1 3/8 turns from lightly seated.

Omaha Rider wrote:

Since I can't find any threads on this I'll make a new one! I'm trying to richen up my bike a bit, but don't know much about the screws. I know the screw on the top of the carb is the air screw and on bottom is the fuel screw. I don't know how much I should turn them. Is it screw out to richen? And screw in to lean? How do I know if it worked? My very left carb is running super lean and I don't like that. help? Thanks

air screw -- out to lean, in to richen.

mixture screw -- out to richen, in to lean.

Good Fortune!

So If I have an aftermarket 4-1 header installed and now my plugs are turning white which means my motor is running lean. Would replacing the MAIN JET with a larger one be the answer? Or do I have to replace the SLOW JET and JET NEEDLE aswell? Here it says there are multiple things that I could replace.

Quick Jetting Guide.
1. IDLE: Set idle speed to correct r.p.m. by adjusting the IDLE SPEED SCREW. Turn The AIR SCREW to achieve the highest idle speed and best response. After this adjustment has been made adjust the IDLE SPEED again back to the correct r.p.m.

2. OFF IDLE to 1/4 THROTTLE: The SLOW JET and the AIR SCREW are most effective in this range. When you want a richer mixture use a larger SLOW JET or turn the AIR SCREW in. The opposite holds true for a leaner Mixture.

3. 1/4 to 3/4 THROTTLE: The JET NEEDLE is the most effective component in this range. Raising the needle by lowering the clip position at the top of the needle will richen the mixture. Lowering the needle will lean the mixture.

4. WIDE OPEN THROTTLE (W.O.T.): Changing the MAIN JET effects this range. Select the size which offers the best W.O.T. performance, then install one size larger MAIN JET for ideal engine durability.

Anyone?

You have the needed info in your post. In general you should not have to replace the needle, but you may need to move it up via the clip. You may also need to replace the main and the pilot jets.
Having said all of that it is difficult to answer your question as you have not given information on what type of riding you have been doing before you checked the plugs color. Was it mid throttle or wide open. In general to richen mid throttle move the needle up by lowering the clip. If on the other hand you have been riding at 3/4 to wide open throttle then the main would need to be increased.

A general rule of thumb for jetting after you alter either or both intake and or exhaust is as follows.

1 jet size for pod (K&N, Uni, generic) filter (single inside the air box)
1 jet size for drilling out the bottom of the air box
2 jet sizes for both single K&N and drilled air box
2 jet sizes for individual pod filters
2 jet sizes for 4 into 1 exhaust
1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size).

Check plug color often, sync carbs after each jet change and make sure the floats are set correctly.

Example
Stock main jets: 102.5
Stock pilot jets: 15
2 sizes for 4 into 1 exhaust
2 size for pod filters
Main Jet Calculation = (2+2) - 1 = 3 size main jet increase or 102.5 + (2.5 * 3) = 110
Pilot Jet Calculation = 1 size pilot jet increase or 15 + (2.5 * 1) = 17

Amazing information! Thank you

If only one carb is runing lean, i would suspect a defficiency in the combustion - compression? valve clearances? an air/exhaust leak?

gd4now wrote:

You have the needed info in your post. In general you should not have to replace the needle, but you may need to move it up via the clip. You may also need to replace the main and the pilot jets.
Having said all of that it is difficult to answer your question as you have not given information on what type of riding you have been doing before you checked the plugs color. Was it mid throttle or wide open. In general to richen mid throttle move the needle up by lowering the clip. If on the other hand you have been riding at 3/4 to wide open throttle then the main would need to be increased.

A general rule of thumb for jetting after you alter either or both intake and or exhaust is as follows.

1 jet size for pod (K&N, Uni, generic) filter (single inside the air box)
1 jet size for drilling out the bottom of the air box
2 jet sizes for both single K&N and drilled air box
2 jet sizes for individual pod filters
2 jet sizes for 4 into 1 exhaust
1 pilot jet size for every 3 main jet size increase

Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size).

Check plug color often, sync carbs after each jet change and make sure the floats are set correctly.

Example
Stock main jets: 102.5
Stock pilot jets: 15
2 sizes for 4 into 1 exhaust
2 size for pod filters
Main Jet Calculation = (2+2) - 1 = 3 size main jet increase or 102.5 + (2.5 * 3) = 110
Pilot Jet Calculation = 1 size pilot jet increase or 15 + (2.5 * 1) = 17

Nice cut n' paste :laugh:

...been there, done that... :silly:

Yes it is cut and paste - and if I came accross as though it was my creation that was not my intention. I was just attempting to pass on info that I have learned here. But your point is a good one, I should have indicated that it was not my creation.

FirstLY, yes, if you have more airflow out the exhaust than your previous exhaust you will need more fuel.
2ndly, if the mixture screw is between the fuel bowl and head it controls fuel, so in is leaner and out richer, IF scew is on filter side of fuel bowl then it controls air, in is richer and out is leaner. Depending what exhaust you had and how it ran, versus what you now have you may have to go richer. What did you have? Did you run a baffle? What do you have now? Are you running a baffle? Follow your manual and set the air screws. Make sure you start with either new plugs or nice clean ones, ride for 20mins at constant throttle,top gear,2500rpm, stop and pull your plugs, have a look at colour, once its nice (refer to NGK Spark Plug site for pictures of good jetting colour)you can then do your main, same procedure BUT do it on a nice quiet road or the LAW will be annoyed! Once main is nice, try steady cruise at quarter, same procedure as before, then cruise at half, same procedure as before. OR, post up engine specs, exhaust specs,carb type and what jets your have NOW and someone on here is sure to be running what your need and may advise, either way works, the first way is longer and $$$ but you will learn a lot!

cheers from Emu Plains,
KT.

So for just a 4-1 exhaust, I should increase my main jet size by one size making it 105? If stock is 102.5 and mine are stock. (2-1=1) 2.5*1=2.5 102.5+2.5 = 105

So four of these?
www.z1enterprises.com/detail.aspx?ID=1921

Or these?
www.z1enterprises.com/detail.aspx?ID=53
Or these?
www.z1enterprises.com/detail.aspx?ID=54