So I had my 1975 dt 175 motor completely rebuilt. And had to bore out 1 size up. I believe I installed a 67mn piston. Its idling fine starts up in one kick but it falls on its face as soon as I try to open it up in gear. In neutral I can wind it up but no/ under load. I dont know if the jets in it are same from factory won't know till tomorrow or sat. When I can get around d to it. I also live in Florida so basically sea level. 80 mile above at most. I did bring it from Pennsylvania so maybe the jets are set for there. But what would you recommend?

Jet sizes can be checked against the manual, either the online ones shown on Partzilla, CMSNL etc, or maybe an original one in the Tech Library on this forum (see toolbar above).

Might also be issues with a dodgy air filter or a gummed up exhaust.

By falling on its face does it go rich & like its being choke--in which case i'd be looking at the exhaust in the tail section or try it with complete tail muffler removed if its that type of removable tail muffler. But--what i started to say, was, if its lean--very lean or blocked main jet--& you whack the throttle fully open it'll stop the motor if you do it from idle. So question is--rich or lean. Rich being from a restricted blocked up exhaust, or lean from a piece of something stuck in the main jet. Could be other things but that's enough for a start.
Also--minor point but at 67mm you're 4 sized over from 66mm in Yamaha sizing. Doesn't matter so long as the piston fits the hole. Yamy go up in 0.25mm steps from 66 but if using Wiseco brand or possibly other brands they go in 0.50mm steps. Sometimes a .25 step may not be enough to fully clean up the wear so your machine shop will ask for the next size--then ya stuck with one to sell. Anyway--hope you get gets it sorted. Cheers from NZ.