Carburettor Tuning

[martinh]

Anyone know of any decent books on carb tuning? I twist the odd screw here and there, to get mine running how it should, but I would actually like to know what I am doing!

[richywrecka]

I got one, all you had to do is ask!

[panomphaean]

Anyone know of any decent books on carb tuning? I twist the odd screw here and there, to get mine running how it should, but I would actually like to know what I am doing!

Assuming a conventional slide carb. This is for a 4t but a 2T is similar save for oil/fuel mix and the alteration of that which affects mixture. i.e more oil is less fuel for same amount of air so hence a wee bit leaner and vice versa.

How do I adjust my carb and correct for changes in altitude/temperature and my riding style?

Make sure: Carb clean, vent tubes OK and not trapped, plug set, Vvs set Setting the Vvs at the max tolerance marginally helps the bottom/mid-range torque too. Make sure timing is OK, air filter clean/oiled, fresh clean fuel. No air leaks on carb/exhaust.

Order of fettling carb:

Float height/downtravel

Main jet

Idle system. Pilot jet and Pilot screw

Needle taper

Check Main jet

Air slide

Idle system. Pilot jet and Pilot screw

Needle groove

Repeat and repeat till happy.....

Here is what bits do what:

Idle to 1/4 throttle The top (usually a parallel diameter portion) of the needle goes from little to full effect

Idle to nearly 1/2 throttle The pilot screw goes from maximum to minimal effect

Idle to 1/4 throttle The pilot jet gives maximum effect, although it continues to flow throughout the throttle range reducing to minimal effect at WOT (Wide Open Throttle)

Just before 1/8th throttle The throttle valve cutaway goes from minimal and then to maximum effect at 1/4 and then back to minimal effect just after 1/2

The needle clip step It gives minimal effect at 1/4 raising to maximum effect just before 1/2 and tailing off toward 3/4

The needle jet taper or tapers Commence a minimum effect at 1/4 raising to maximum effect just before 1/2 and then tailing of at 7/8

The main jet Takes minimum effect after 1/2 crossing the tailing off point of the needle taper at 7/8 and then rising to maximum effect at WOT

To work out correction factors for Altitude and Temperature you can use this formula in lieu of my spreadsheet:

CF = The sum of (1.0778-0.00111 times T = 0.000000000238554 times A times A - 0.000010777 times A) Where T is temperature in Farenheit and A is Altitude in feet.

Multiply the standard jet size by the CF (Correction factor) to get the new main jet size. Round the resulting number up or down to the nearest jet size. (Honda main and slow jet sizes are numbered in increments of 2 or 3).

If CF is 0.95 or below, turn the pilot screw in 1/4 turn.

[martinh]

I got one, all you had to do is ask!

I'll borrow that then!

[gasgas]

An other tuning advice.

Look at the carb and intake traject and picture the air flow. See where there are any obstructions. Look for ridges and fabrication errors creating low pressure spots etc that will affect the flow. In this area getting rid of all the obstructions will generate more air into the motor hence more power. Will cost you nothing other then that dremel you already got and it is a nice feeling knowing you did some tuning other then just fiddling with the jets.

Be aware that more air will mean more fuel normally so keep that in mind.

Tip: where the airboot goes on the carb is a big error spot. It is easy to dremel and create more flow. You can even use a round file to do this.