[ckau]

We finally got the chance to play with a keihin cvk 30 mm pumper carb. A Dazon single seat 150 ,stock motor except for a bugle style header, straight through muffler and a uni with velocity tube was used for this trial.
First challenge was to get this carb mounted properly. There are spacers and intake extensions available but we didn’t like the idea of having the carb up above the motor mount bracket plus extending the intake runner length by using these extensions was not the best way to gain advantage out of this carb. We felt this setup would create tuning nightmares base on the complaints we have heard from other who have tried using the extensions Erratic idle, bogging and inability to get jetting correct. The optimum intake tract length for the gy6 is approximately 14.3 inches measured from the end of the velocity tube, through the carb, down the intake ito the head valve seat. It stands to reason when the tract gets too far out of the optimum range of length there’s going to be some issues.
Rich1, [buggy owner]\ came up with the brilliant idea of torching the plastic covering off a stock 24mm gy6 intake then fabricating an adapter conversion tube to mate 30mm carb to the stock intake. This “ conversion tube” gave the ability to fit the carb in the original location plus the ability to adjust the intake length if needed. The throttle cable had to rerouted and the starter housing was rotated to move the electric connection away from under the carb. It’s tight but fits!
I had suspected for quite some time these 150’s were under powered with a 24 mm carb . I felt the 24 was used as a matter of convenience and economy rather than from a performance stand point. The 30 proved this opinion to be correct!
We were astounded by the performance gains! Throttle response is crisp and instantaneous. Zero throttle lag at any speed! Motor turned almost a even 1,000 rpm more than ever before. Top speed increase by approximately 7 mph. Acceleration is smooth and strong. Top end speed is now only limited by the gearing. This buggy is now a real handfull to drive. We had to learn the characteristic of this buggy all over again. Whereas we used to anticipate throttle lag in turns , response is now instant . It’s too easy to overpower when accelerating out of turns.. It proved difficult to prevent wheelspin at times. Good brakes and proper braking techniques now comes into play. Rich1 commented “Oh great! Now we got to figure a shock setup that’s gonna keep the rear end on the ground!”
Jetting the 30mm really fooled us. Figured on something like a 135 or 40 would be sufficient but we ran out of jet sizes at 150 and it is still a touch lean. Didn’t worry too much about the pilot/slow jet at this time cause the pumper circuit would compensate for any critical adjustment temporarily. A set of main and pilots in the larger jet ranges are ordered so the fine tuning can be completed. It will be exciting to see exactly what can be had from this carb. I’ll give an update after the jets arrive.