How do you tune the power system on a motor that drops to almost 0 vacuum above 1/8th throttle? This is the 3.8L in the Cutlass below.

As it is right now, the motor is PIG rich at part throttle and gas mileage sucks, but the mixture looks good at WOT. Unfortunately since the power system is on at part throttle, leaning the mixture out there will also lean the WOT mixture.

Now, ordinarily one would correct this problem by changing to a softer spring to make the power system come in later. How on earth can I make it come in later than practically 0?

And of course, the other question... I assume vacuum dropping to 0 that quickly is a good indication of too little motor for the weight it's hauling around...

 

Low vacuum at WOT just tells you that there is very little restriction in the induction system..... big carb on a small engine.

I doubt anyone can offer any helpful advice until you tell us what carburetor you're using.

 

0 above 1/8 throttle?? Not possible. I've run HUGE cams with HUGE carbs and still pulled 7-8". You sure you're measuring intake manifold vacuum from a reliable source?

 

I didn't get that good a look at the vacuum gauge, apparently. It actually reads 4-5" at 1/8th throttle.

I fixed the problem by putting in a much softer power piston spring.