I have a 1970 HT1 90 I'm working on. It was a basket case and a money pit, and this is my first 2-stroke project. It has extremely Low compression and of course won't even fire. I've honed the cylinder and installed a new piston & rings (2 rings). It's already been bored 2 over. I'm missing something here and am not sure I can even bore it out again as it's already 2nd over. It does get a good spark .    Any suggestions? Thank you!

Hey, I'm not sure I understand your post. Your old cylinder had low compression? I can understand that, but are you saying you bored the cylinder and with a new sized piston you still have low compression - something not right there. You could just get a better used or new head and piston and be good to go. Nowadays with lazer welders and so on everything is almost rebuildable if you want to spend the time.
Dude's like lowbuckgarage are always finding a route that gets the thing back on the road - and remember 'if you aint having fun you aint doing it right'

Yamaha have four oversizes , so you still have options .

Thank you for the info. Yes, it was obviously bored out before I got it, the piston says "50" on it. I just honed it and replaced the piston. I wasn't aware it could be bored 4 over. I'll see if I can find the next size piston and rings and try it again. Thanx for you help!

Checking the piston and rings at 28 minutes in, the piston is good on his bike, it's the rings that are gone, cheap easy fix, how much slap does your piston have when you push it side to side in the jug?

Love HT1's & had a lot of fun on mine.
If you turn it over by hand on the kickstart how much resistance can you feel.
Even with low compression it'd still run even if you had to push it down the road.
I spin my smaller motors with a reverse drill to save energy kicking it & getting nowhere fast.
Makes it easy providing your magneto flywheel nut is good & tight.
Here's how i start mine except this one kept locking up after about 2 seconds as the piston had carbon way down the skirt & never seen one so bad. So carbon meant it had zero clearance & expanded to drag it top a halt haha. Cleaned the piston up & she was good--& fast. Model LS3 100 singe that video.

Son even raced our HT1 against school kids for a laugh on their school built bikes with honda copy engines so i'm keen to hear an update & better news from yours.
Never had the cylinder off mine, just freed up a VERY stuck piston & went racing-as you do--or as i do lol. Here's mine as found.
photos.app.goo.gl/Ry1ouytudps8mgqu8
Found the video my son did for a laugh the day he raced it.
Bike never missed a beat all day pretty much.
Tank was rotted right through from the inside at the level the water or stale fuel water mix was sitting in the tank about halfway up.
Hence the plastic tank i used.
Goes on for a bit & gets boring so drag it along.