After 3+ months of problems, I am returning the 45mm carb for a new one. I have come to the conclusion that there is something wrong with it. It has a drift problem.

 

So Scoob,

What did you say is likely the problem if you rev your 45mm and it bogs when you do it quick?

I have a friend who got his new stage 1 kit and he said he was able to ajust the idle and it was fine but when you rev'ed it, it would bog for a moment and then go to high.

I told him I thought it was the needle valve but since you have experienced this first hand, thought I would ask.

Thanks

 

AZDS650, There are really two related problems here. First of all, in my opinion, the large motor with big, bulky, heavy components has trouble instantly getting that mass cranking from very low RPM, regardless of the type of carb used. One solution would be to lighten the crank, balance shaft, flywheel, etc. The Raptor engine was designed with this in mind, so it revs fast. Alternatively, this problem can be masked by adding basic mods like a pipe, K&N filter, better carburetion, ...

As far as the carbs go, the stock carb is a CV type that uses a vacuum to open the throttle valve to let air flow in. So when you instantly nail the throttle, you won't cause a momentary lean condition that can occur before the motor starts to rev up and suck air through the carb. The throttle valve will only open when the motor is ready for it. With the stock carb, there can be some bogging from very low RPM when you instantly nail the throttle with the engine under load.

In contrast, the Ron Wood carb is a flatside type carb that will raise the throttle valve instantly when you nail the throttle, whether the motor is ready for it or not! Because of this, Mikuni includes an accelerator pump with the carb. What happens with this carb without the accelerator pump connected is as follows. Lets say the engine is at idle with the throttle valve closed, running strictly off of the slow speed fuel system. If you instantly nail the throttle with the motor under load, there is a chicken and egg senario. For a brief moment, there is no airflow through the main bore and the main fuel system is still shut down. In order to draw the correct amount of fuel into the main bore and turn on the main fuel system, air flow is needed. Some fuel will trickle out of the needle jet and start to get the crank moving, but the air/fuel ratio at this particular moment will be too lean as you have let in a bunch of air but very little fuel and the air will be traveling faster than the trickle of fuel. Without the accelerator pump connected, it is possible for the motor to skip or hesitate during this period or not produce peak power due to a lean air/fuel ratio. As the motor starts to rev up, it sucks more air in through the main bore which draws more fuel into the main bore through the needle jet. This continues briefly and then the air/fuel ratio will be stable. What the accelerator pump does is act as a catalysis. It jump starts the main fuel system by squirting fuel down the main bore ahead of the air, into the intake. This eliminates the momentary lean condition and should prevent any skipping. However, there can still be some amount of hesitation or bog in getting the crank moving due to it's heavy weight.