wish it had obvious causes like yours. going to be putting in new seal thanks

just looking at an MR175 as near as CR125 i can find on partzilla & RH seal is inboard of the main bearing which is a total dismantle to change so i'd be thinking hard before doing--like is it really 'that' problem. Left side looks normal to change in the drawing so hopefully 'is' & not trapped from the inside like my KX125. Video is good.

'75 CR125 Clutch Side Crankcase

about 6 hours and very carefully replacing right and left seals and new gaskets of course. Exact same bogg. won't even pull itself. idles fine. harder to start now. it runs perfect for about 10 seconds or three revs and then the bogg or loading up starts. Its got me stumped....

I'd try a brand new spark plug.

Bad CDI would be my next guess... and it's just a guess.

video please with good audio & please make video as long as feasible timewise to get a good listen.

new spark plug wires too. All electronics have an infant mortality rate, anything you replaced since it was last running well would be highly suspect in my book, I've seen condensers and similar components fail in ~1h of initial use.

its gonna be a bit, I stripped one of the cylinder studs taking it apart so many times....

I have an extra coil and cdi. will give it a try. Ive been thru 4 new plugs, all still white and good.

The symptoms line up with fuel starvation due to the float bowl not refilling quickly enough.

Thin flakey crud in fuel tanks of older bikes can cause major restrictions in the fuel delivery from the tank to the carb.

The float bowl can fill while not running, but once started the fuel demand can exceeded the refill rate of the float bowl. There may be enough fuel delivered for idling conditions but larger throttle openings may exceed what the restricted delivery rate of fuel can support.

Also, verifying that the second carb was a known good one would help eliminate carburation as a strong possibility.