G16 has no pulling power

[dragracerz]

Hello,
Having issue with g16.
I've owned it since new, and use it to pull my race car at the track. Last year I put in a brand new Yamaha engine and aftermarket primary clutch, it pulled the car great, like it always did. The only modification was the zip tie trick to the governor.
This year, the cart was very slow pulling the car and top speed was down to 15 mph. I installed a governor speed spring and top end went to 22 mph but still struggled pulling the car. I put the stock Yamaha primary clutch and belt back on it and it pulled the car pretty good on the first attempt but slowed after that and is back to being a snail.
I do have full throttle at the carburetor.
Should I be looking at the secondary clutch ?
I'm lost here, any advise is greatly appreciated.
Thanks

[dragracerz]

I should add that the cart still has stock tires and wheels on it

[Dave in VA]

Raise the rear end, run it and watch the clutches. The belt should start at the bottom of the primary and rise to the top--it should be at the top of the secondary and go to the bottom. Belt width when new is 1 3/16".

[dragracerz]

hi Dave
I did that and it appears that the clutches are working properly. The newer aftermarket belt is 1-3/16 wide and the Yamaha belt is 1-1/8 wide. The aftermarket belt is a little longer so I tested with the yamaha belt
The throttle opens fully, then when the governor kicks in, it drops to 1/2-3/4 throttle
I have the governor turned all the way in and clocked it without pulling the car at 23 mph

[slonomo]

What about tune up?

[dragracerz]

New plug and air filter this spring