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

[abobber91]

1978 Yamaha XS400-2E (2 cylinder engine)

The bike runs and feels to have good power, (it will speed up to 40mph from a stop light pretty quick if i make it) but feels like it could be better.

Engine does not have any bad sounding vibrations for as far as my noob ears can tell.

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. About 100-115 psi for each.

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!!

 

[Tombo]

Why did you start a new thread!? Did you do the wet compression test like I mentioned? That is relatively low compression, I should be about 150. No adjustment can make up for a lack of compression - and it's hard to predict what this low compression will limit you to as far as interstate riding (which btw - it's doable on these bikes, but my personal preference for longer distances at 70+ mph is to ride my bigger bike. The 400 will go 85ish MAX, and it will struggle to hold 75mph up hills.)

They are great little bikes, lots of fun. But if you do a wet compression test and your number goes up, then you need rings and a re-bore (just like I told you in the other thread )

 

[xschris]

I would first give it a good oil change. Have you adjusted the valves yet? About 120psi is the low end for these bikes new is about 155psi. Does is smoke at all? If you put a little oil in the cylinders and your compression goes up then its rings if it stays about the same then valves.

 

[abobber91]

Why did you start a new thread!?

Bah! I dont know.. thought I'd see if I could get more input, not saying that your advice was over looked!

Where might I find oversize pistons and rings for this machine?

 

[Tombo]

No worries - I understood why you'd re-post, and I took no offense! Did you do the wet test? What was the result?

I agree with chris that a valve adjustment could improve compression. If any valve is so tight that there is a valve not closing completely, you could lose some compression. Valve djustment takes like 20 minutes once you have the tank and cover off, so do that, retest, then if result is the same, then wet test.

 

[xschris]

Make sure your gauge is accurate also a guy one here had the same issue but it was a bad gauge There is a guy on ebay that sells standard size rings but over sized pistons and rings are hard to come by. Yamaha no longer has them and on ebay finding a matched set is not easy. The prices for them are most often very pricey.

 

[Drewcifer]

Abobber, do the wet test please, we need answers to help you.

Then do what I had to do. Inspect the threading of your spark plug holes. My bike had torn ones with only 2 threads left on each side. I had to helicoil them but afterwards the bike ran much better.
Good luck

Drewcifer