Cummins 400 big cam

mine would idle just below 400 if not idled up. if I remember correctly the idle adjuster worked anywhere from the 300 range up to 500. go take it for a drive and see if you think the tach is off.

if I remember correctly the ratios were 1:1 and .5:1 so if it's the wrong one you could be at 600rpm. either way it is easier to find out by driving it, if it tops out at 1000 rpm you know you have the wrong one.

personally I went with a mechanical to electronic pulse generator and used the elctronic tach.

 

Deadly oil pressure at 300 rpm too.....
I guess some folks really can't tell when a motor sounds like it's running backwards... or idling so low it's tryin to lock up... but a tach isn't going to help that either..... That's what made the techno-rigs so popular to start with... most drivers need training wheels.....

I don't recomend any kind of throttle lock for idling up a PT pump neither unless you like runaway engines..... If the governor spring is shimmed like 99% of them are I'd stay away from floorboarding in neutral too.....
And the PT pumps that "range" out of idle and won't stay adjusted = Wore Out.....

 

The Idle spec on a NTC400 is 575 rpm. This spec is on your engine data tag on left front of engine. If you need any tach drive ratio info, supply your cummins dealer or pump shop with CPL # from either engine data tag or PT pump data tag. The CPL # is on both tags.

 

Sounds like you need the .5 to 1 ratio tach.

 

If you want it to look right then the fancy thing to do is X2 all the numbers and write them on the glass with a sharpie...