Hello BigDog302,
I am building a very similar bike- 75 frame with and 76 IT400 motor, Vape ignition, pod air filter, and I rigged a IT 400 exhaust with the dt250 silencer.
I was wondering if you could tell me your carb jets?
My pilot is a 60
and the main is 300, is that too small?

The bike doesn't seem to want to rev out like it should,

It seemed to run better with the DT exhaust- 

I do have one of those "frisby size" rear sprockets which I am sure doesn't help

Maybe you’ve already moved the clip on the needle up or down? That’s what I try first as much quicker and easier than dismantling the carb multiple times for jets. May have to do it anyway but start with the clip up or down one notch.

I had similar problem with my dt400 i had to replace the needle jet and it cleared up the problem up. I had ported it, run the stock pipe, and oval bored the carb, but although the old needle jet worked for the stock carb size it was too worn for the new carb size.  I dont know how this could be but it cleared up the problem. I just went to the stock size but it worked. But maybe it won't work for you as I went to 36mm on the carb.

Update-
I found a 340 main in my garage swapped it out- still ran crappy, so I decided take your advice and adjust the needle.
When I pulled it, the clip was at the top, so one down still crappy, one more down (middle) still crappy, one more down- big difference, front wheel came flying up.
While blasting around the neighborhood I encountered another issue which I hope to tackle in another post. But I was wondering should I keep going maybe try a richer needle?
thanks again for your help

I sold the bike.    I regret it as I miss that bike. a guy offered me decent money for it and I needed the money at the time.

I am sorry to hear that, I am sure one day you will build another super DT 400!
I had one more question for you-Did you think the V-force was worth it or noticable improvement for the money?
Thanks again


 

I never ran it with the V-force. it was too restrictive for a 400. I found out it was for a Honda CR125. it made so much power I did not feel the need to experiment with it. as it was with Boyesen reeds in the stock reed cage that the bike really ran 90 flat out and it had more left. the wind as very stiff at 90 mph.

I do have another DT400 frame however and a complete DT360 engine.

I'm not a huge fan of the V-force.  They seem to work a little better on high revving motors like a modern 125cc MX bike if you want to rev it to 14000 RPM. 

My 360 responded very well to a modern 250cc reed block, four petals and much wider than the stock block.  You have to widen and enlarge the reed cavity significantly to fit.  Anything that fits into the stock casting without modification is going to provide minimal benefit.

I have a V-force 3 on my 1978 DT175 and on that bike it does very well.