Bike has about 13k miles. Low compression, 100ish psi for both cylinders. Bore...

[Fred]

...seem necessary? 1978 Yamaha XS400-2E (2 cylinder engine)

The bike runs... but feels like it could be better.

Engine does not have any bad sounding vibrations.

Have not looked at the pistons yet but plan to when i have time.

Did a compression test today and got a low reading for both cylinders, cold and immediately after being thoroughly warmed up, throttle wide open and with throttle closed.

The readings for both cylinders came out very close to each other which seems to be a good sign.

I want this bike to be good enough to ride interstate hopefully.

I just am seeking some kind of idea as to what the cylinders and pistons will need. Does low compression always mean you must get the cylinders bored??

Im kind of hoping that all they will need is new piston rings and maybe a light honing of the cylinders. The mileage does not seem to be that much to cause the cylinders to warp, but maybe I'm wrong??

I've seen bikes have their first bore oversize at like 100k miles...

Any input is much appreciated for my peace of mind!!

 

[Firestormer1]

100 does seem a bit low but I agree with you about both cylinders being the same being a good thing.

You must realise that these bikes had a reputation for being pretty gutless (the 250s were worse). If it starts and runs ok, then that is probably as good as it gets.

Personally i would just leave it alone and just ride. You could spend $500 or more on new rings etc but the bike will still be a slug. It would be far cheaper to just make sure the tuning a etc is spot on.

 

[AdamS]

it cold possibly be the valves seeing as people did not check the clearances often enough and they could be damaged,could also be a gasket possibly but clearances should be the next place to check if posible.