So I finally have a spark in this bike...it cranks fairly easily ,but really won't take any gas. If you turn it up slowly it will rev ,but if you try to turn it up even moderately fast it just bogs down and it won't pull you. Carburetor is spotless...all circuits are patent. Float level is correct. It calls for a #150 main jet and a #30 pilot jet.
I have the #150 in there, but have a #35 pilot jet. It bogs so bad I thought that flow was obstructed somehow, so I removed the air cleaner( no help), removed the exhaust pipe(no help) put a #165 main jet in (just because I had one) and that was no help. adjusted the airscrew many times.
Odd thing is it runs best with the pilot screw all the way in...even with a pilot jet that is bigger than standard. Needle jet in middle position and I have adjusted that up and down with no help..I've check the timing a couple of times straight by the manual and feel pretty good about that.
compression is good enough to blow your thumb off the sparkplug hole( has new rings).
normally I would think pilot jet...but it won't rev past half throttle. Left crank seal have never been changed and I would hate to take the armature off if I didn't have to.
So I need some ideas.

     Thanks,

          mp
 

OH OK..My turn.
ahhhh.....what bike and year is that?

I'm guessing we're talking an AT1 looking back on you posts. So still definitely lean. Not much left if you're sure main jet isn't hiding something--stuff behind it in the jet holder getting drawn up against it. Left crank seal is sounding like you'd better replace it to at least eliminate the possibility. Right seal not been forced out--unlikely if new & tight in the case. Bit of a mystery. Base gasket not sucked in. Try it without the main jet & see if it drowns to prove a point. Have you had the needle jet completely out of the carb as grow stuff up around the outside. There's an airway leading to it from mouth of the carb although symptoms aren't really pointing that way. I guess its an AT1 & not AT2 or 3 as could have a broken reed valve petal. AT2 etc run AT1 engine prefix.

My bad...1971 Yamaha AT1

Here is your 30.

www.mfgsupply.com/07-530.html

Thanks...it is the 1971 AT 1

My first guess is crank seal if it's lean.

Second guess is o-ring that seals float bowl to needle jet. (would make it rich)

Third guess is timing is off (wouldn't be the first that didn't understand the sometimes confusing manual) or maybe bad condenser or low battery.

Thanks

I'm an amateur for sure but i just went through both seals of my 1971 AT1C.  I had to take both sides apart twice, lol  Clutch side seal was blown and it provided so much smoke from sucking transmission oil to the engine my wife called it mosquito killer bike.  Left side hissed air as well.  you can look back at new guy from muscle shoals video to see my issues.  Thanks to Mark T and everyone here it runs really good now and i get to work on electricals and oil pump.  I have experieriened the bad rusty leaking tank, carb rebuild, stopped up exhaust, blown seals on both sides, clutch, and now auto lube.  Im really enjoying my project though.   i had one like it growing up.  im looking to buy another one lmao.

I replaced the right crank seal ... Just didn't want to pull the Left side apart.
apart. Guess I will.
. Thanks