Seized Again, Almost Lost It | Yamaha YSR Engine Rebuild | Oregon Motorcycle 2021  

I found rebuild kits with oversized pistons❗️😍. This video, by the way, is NOT monetized, and is currently blocked in 4 countries 🤣

Those marks on the piston are from thrust, power thrust and compression thrust on the exhaust side and intake side respectively. Get your crank reworked and all new bearings! Have a good one!

That looks like it tried to heat seize where the piston got so hot that it outgrew the cylinder and would stick until it cooled down. When you get it back running you NEED to do spark plug readings and re-jet the bike. Personally I would keep upping the jetting until the engine "4 strokes" or kinda of bobbles at high rpm and then trim the jetting back until it just starts to peak out at high rpm. You gotta remember that this poor engine struggles and gets rode everywhere at wide open throttle so if you jet it for peak performance which is ok for a race bike that only gets rode for a few hours and rebuilt constantly, it will create alot of heat and seize. For longevity,unfortunately you need to jet it a bit rich on the main jet and give up a bit of performance up top but it will lower the combustion temperatures significantly and will greatly improve engine life. And remember that if you do eliminate the oil injection and run premix, your currently jetting will be extremely lean because the premix fuel is of a thicker viscosity than just regular gasoline. There are alot of variables to tuning but it is required to get it right if you want the engine to last. Also I personally have had the best luck using the OEM yamaha cast pistons. They seem to be alot more forgiving about the cylinder clearance and dont rattle in the bore like the forged wiescos do. I know that they make 3 different size ones. A standard, a .25 over sized and a .50 oversized piston in the yamaha oem cast type.

Also note to check piston size along the piston pin axis, a cold piston is oval and gets round when reaching working temperature!

Like the way you can kinda deduce your way through it!! Who has 1 of them micro-measuring devices like you used on the piston, on the shelf?!! That is way outta my league!! Nice one, Michael. Hey from Wales.

Hey man nice bike , enjoyed the video. Id suggest honing your jug and getting another 40.5 piston preferably wiseco. Alternatively you can bore it even bigger and run an even bigger piston, 44mm is the max bore for these motors. I have two running 43mm pistons with ported cylinders and they have been reliable for me.

3 thousands of an inch is a bit on the tight side. The good news that it is tight and not loose. I bet you can hone the cylinder and polish the piston and come out just right.

Furthest I have been on my YSR50 is about 35 miles each way, and that was sore enough! Mine is a Japanese market one. When I first bought it I looked at all the tuning bits. Then decided as it would be flat out almost everywhere I wanted it to be reliable, and if I wanted to go faster I would just take a different bike. It made 5.9hp on a Dynojet dyno. And managed a 23.57 second standing 1/4 @50mph!

Ohhhhhh NO!!! Thoughts and prayers for her.

Might be your 2 stroke oil, cheap oils can flash off quick and create a lack of lubrication

i had excessive piston slap on a kx80 and it really ate away the bottom the cylinder before the crankshaft bearings went. never had it cease up once on me though so honestly cant tell ya

Try and use Polytron oil treatment always, it's good stuff!

Digging the intro

Did you ever get it back to working condition?

It got too lean, and yes, too hot

That's pretty scary stuff, when your bike suddenly stops 😬 I've had several breakdowns on my bikes, but never seizing 😅 best of luck sorting it out 👍

Oil injection is normally not very trustworthy i prefer to run premix