I can hold my speed at 45-60 MPH and after about 2 minutes just holding the throttle there, my transmission will kick down a gear and back up, causing the truck to jerk. I have a 400 motor with a Holley 2 barrel carb. I have blocked off all unnecessary vacuum ports. I have the dizzy advance hooked up to the carb. My EGR is blocked off with a thick carb gasket. I'm just using the EGR plate as a spacer. I have the adjustable modulator valve, which I have turned the screw inside it to the left about 4 whole turns.
the kick down return and the spring for it is fine, I checked them the first time it was doing it. I have checked all vacuum hoses. I think what is happening is there will be a sudden vacuum pressure drop and it shifts the transmission down, and back up to boost the pressure back up.
well... those hoses were taken off and never replaced when my motor was rebuilt, but all the vacuum ports on the motor that are not being used is plugged up or blocked off. what does not off a source that is metered mean? thanks for helping though.
so where do I find a non ported vacuum? I have the existing hose coming off the giant T connector right beside the oil pressure sending unit. which also has 1 line going to the power brakes. the rest of the plugs on it have been capped off.
You are hooked up to the right vacuum source, so that’s not the problem. Check the vacuum lines very carefully. Look for cracks and excessive softness. The hose could be collapsing inside and cutting off the vacuum to the modulator. Replace the hoses with a good quality of vacuum line, also make sure the metal line running to the tranny is not kicked anywhere. Good luck.