Cummins 400 big cam

Is there a ratio adapter at the pump or the end of the cable at the tach? If so, try removing it and connect the cable direct.

 

maybe the engine IS idling at 300 rpm.

 

if you don't have it idled up a bigcam can idle 300 to 400 rpm.

there is an adjustment screw that can be used to set it higher but I forget where it is now. I've tried to scrub every memory of them POS motors from my brain

 

On the old style pump it was inside the plug at the rear of the pump. The screw was in the removeable cover where you put the shims in at. Just remove the plug and use a straight blade screwdriver.

See arrow in picture.

 

Give the guy some credit...... he wants to use a simple "formula" to figure his rpm because he bought the wrong tach...... don't really know why anyone would want to do that but....... don't expect any "formula" can be figured without knowing what ratio the correct tach should be though..

But to suggest that he adjust the low idle screw on his motor to correct a wrong ratio tach is loco.....

 

Why do you think his tach has the wrong ratio? It could be right or wrong. I merely stated how to set the idle IF it turns out the engine is really idling at 300. When idling that low, there is usually a lot of transmission rattle and truck vibration.

 

It should flat foot, no load, at around 2150-2200, unless it is a formula engine, then 1900.

 

IIRC most of the one's i drove were set at around 450 rpm's...