Carburators

[guyom]

Hi all, perhaps the topic was already started and done.. but i do not find informations about carburator tuning. i have simple question :

1- tuning should be done with a hot motor right (so after a 15 minutes ride) ?

2- the mixture screw : when the screw is closed (i mean when the screw is tightened to the maximum) is it lean or rich ? (i do not know if the screw lets go the air or the fuel...)

3- i have some "pops" when i decelerate, or on neutral position... meaning i think it is too lean... so i need to enrich the mixture right ?

4- is there a preference to start with the left or right carb ?

5- thanks for time !!  :-)

Guillaume

[Barry]

1.  Yes, engine and oil should be up to temperature so min 15 mins or 10 miles and more if tuning is done in cool weather.

2. The mixture screw controls fuel so turning in leans the mixture.  It's usual to start from the factory baseline setting of 1/2 turn out from gently closed.

3. Popping could be air leaks but if not then in general it does indicate a lean mixture.

4. No preference for which carb to tune first.

5.  You are very welcome.  I have a fuller description of the carb tuning procedure if you need it.

[Bob_Roller]

Make sure the valves are adjusted before attempting a carb synch .

[guyom]

Thanks a lot for answer. I have made some tuning... too lean and open the screw helps a lot but not perfect.

[marcmiller001]

I don't understand why recommended settings are so low at 1/2 turn. I am at at least a turn and a half to get my carbs tuned and I still think it is too lean.... could be the removed emissions system, header wrap and cone exhausts... but how much is too much?

[Justin B.]

The change in exhaust can cause mixture changes.  On most carbs I turn in until RPM drops, then out until RPM drops, then put my initial setting in the middle of these two points.

[Barry]

Mixture settings are influenced by float levels.  When I had heavy floats 1/4 turn was the optimum setting on one carb. It would also work the other way with low float levels needing more turns out than the base line figure.  Even so only 40mm carbs had a base setting of more than 1 turn out.

[guyom]

I have a last question : can i unplug by removing the ignition cable... so to have the left or right piston and do the good setup of the mixture or it is not recommended ?

my idea was to focus on one carb and just listen one at the time and after set up the synchronization.

[Barry]

You can do it but not by pulling a plug lead which could damage the ignition system on a points bike and almost guarantee damage to an electronic ignition bike. What happens when you open circuit the HT  is the coil tries to generate a much higher voltage than normal until ultimately it breaks down insulation somewhere.

It's safer if you short the spark plug on one cylinder to earth. I have some metal extensions made up that separate the plug cap from the plug. I use a long  screw driver to short one plug at a time to the engine fins.

Assuming you already have perfect vacuum balance and idle mixture in the ball park, what will usually happen when the engine is run on one cylinder is that the engine dies after a number of revolutions. The length of time it takes to die on alternate cylinders is an indication of mixture balance between the two.  I don't use the shorting method that often but have found it useful.   

[Semper Gumby]

I set the Idle Mixture with a Colourtune plug. 

It's always right.

[guyom]

hi thanks for answer.. so just what is a Colourtune plug ??

[Bob_Roller]

It's a tool you use in place of the spark plug , it looks like a spark plug, but it has clear glass instead of white ceramic insulator .

You can see the color of the combustion in the  cylinder and adjust the mixture to the correct color .

[guyom]

ok ! Thanks for time !
Guillaume