Main jet is dependent on throttle position, not RPM.  So if it's rich at less than 3/4 throttle... even at high RPM... you don't want to be changing the main jet.

MarkT great write up on throttle position not Rpm’s hopefully this will help others too

Main jet was Mikuni 150 it ran Rich at half and 3/4 throttle. All Mikuni parts. I put a Mikuni 140 and 135. It ran really well. 135 it was pulling at 7000 to 8000 rpm’s.

The only problem it has when coming to stop it stays at 2500 to 3000 rpm’s and does not go down to 1800 for a idle. I need to play with idle screw to get it down.
What do you think?


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I'm not sure what your experience level is with jetting, what it's doing that makes it seem rich, or if it has another issue like a clogged muffler or who knows what.  I also have a 1980 MX175 and it feels like it has pretty much identical power to the DT175.

I do know that I've never found the stock jetting in that model to be rich if all is in good shape.

Pretty much any two stroke will get a neat "boost" when you lean it out, even if it has an issue...  so maybe something is wrong and when you lean it out it seems to have more power?  That doesn't necessarily mean it was rich.  Unfortunately sometimes just when you think you have it "leaned out" just perfect, the next sound you hear is "BWAHHHH..." as it coasts to the side of the road with no compression... or it might flat lock up.  Unfortunately I know this from experience.  Actually "experiences". 

It also concerns me a little that you said 1/2 to full throttle...  main just barely starts to work at 1/2 throttle and isn't truly effective until 3/4 throttle. 

All I can say is "be cautious". 

Also the "hanging idle" is a sign of being lean...  ignition side crank seal on that model often is leaking.  Best to use 77-79 IT175 double lip seal.