2007 AE Has a low piston 4 pressure

[Jman]

Well I went and bought myself an FJR
It only had 14k miles and the price was low. It is a clean looking bike.
When riding it there is a bit of bogging at low speed and a little vibration in the handle bars at speed.
I really just thought it needed a throttle sync, and clutch engagement adjustment.
When I tested the piston pressure I got 200+ psi in pistons 1-3.
Piston 4 was around 140 psi. Threw some oil in the piston and I got 200+ psi.

Looks likes I (hopefully) need a piston ring.
Has anybody else had this problem in a low mileage FJR?
I have some experience working on engines and the service manual but any advice would be appreciated.
Thigs / quirks particular to this engine for example.

 

[Whooshka]

One easy fairly cheap thing to try is pick up a bottle of Yamaha Ring Free. You might have a ring that's "stuck" and not sealing against the cylinder wall correctly. This might help.

https://www.amazon.com/CYCLEMAX-Yam...7743&sprefix=yamaha+ring+free,aps,107&sr=8-16

 

[RossKean]

One easy fairly cheap thing to try is pick up a bottle of Yamaha Ring Free. You might have a ring that's "stuck" and not sealing against the cylinder wall correctly. This might help.

Agree! Certainly can't hurt.

 

[Diablo1]

Ring free is a very good suggestion, along with an Italian tuneup - put some miles on it at speed. That bike has been sitting around in storage and hasn’t been ridden much. Issues to check for are ignition switch and ground spiders.

 

[Jman]

Upon further investigation I found that the spark plug threads are ruined.

My testing tool got stuck and when I ran a thread chaser down there it had quite a bit of wobble even when bottomed out. Some gorilla screwed a spark plug in there sideways. That stripped out metal went into my piston...

So it looks like I am going to have to remove the piston head and use a time-sert tap on it.
While I am at it I am going to replace the piston ring.
Going to replace the cam chain tensioner with a green dot one.
Then I have to make sure the timing is good, check valves.
Exhaust is not sealing properly so I have to put new gaskets on there.

This is why I buy bikes in the fall/winter months. I do enjoy this kind of stuff but I wish the bike just needed a tune up.

Have to get time-sert kit, gaskets, cylinder head bolts and lots of little doodads.
Pretty absurd what this stuff costs. Almost the cost of buying a used engine.
All because someone was careless with a spark plug.

Always hand tighten bolts at first and use a socket that firmly fits your spark plug.

Anyhow Ill keep you all informed on my progress.