srinath wrote on 08/20/09 at 08:06:42:Oh yes this is a great test. The key though, you need to get that throttle to WFO and stay there for a few seconds till you see it really start pulling ...
And speed is almost irrelevant. You need load, so the slowest it can run in 4th or 5th ... And throttle position determines what jet you are using, not rpm or speed. In a way, I find it is easier and less confusing to tune a bike wihtout a tach. You mark on the grip 0 (where the free play ends and actual throttle action begins) 1/8, 1/4. 3/8th, 1/2, as it goes on till full in increments of an 8th. Then go by that when you make your observations.
Someone please correct me.
The piston valve (needle) isn't directly connected to the throttle. It's raised up by the vacuum in the venturi.
To really test the main jet, we need to get that valve as far open as possible. Otherwise the needle will obstruct some of the flow. That means we need the biggest vacuum in the venturi. Since the vacuum is caused by the movement of air, we need the maximum amount of air moving through the venturi.
Opening the throttle completely provides the opportunity for the maximum amount of airflow.
Wouldn't a high RPM also be required to actually pull the maximum amount air through?
The alternative is that the piston valve is so sensitive to the pressure drop in the venturi that even with low air velocity, the high volume will open it completely.
-D. Dwarf