800xl wrote:
The high idle in 3rd and 4th is most definitely from the coasting richer valve on the stock carb. There is a switch on the trans for 3rd/4th, a switch on the gas pedal for not pushed, and a switch on the clutch for not pressed. If all three of those conditions are met, the valve on the carb opens to allow a small amount of extra fuel in. Your clutch switch is probably not hooked up or not adjusted as mentioned above so you are noticing it. It was an attempt to get less emissions when the truck was coasting at speed.
Most weber carbs don't have the valve so your problem should have gone away with the hitachi carb.
What he said, I was incorrect about the switch on the clutch, it is on the gas pedal. That is why we keep 800 around, so he can fix our misstakes, thanks for fixing that 800.