Valve Clearence changing in one week!

[lamoun]

So, here is the deal..

My GS was running like crap, backfiring into the carbs when cold.
Checked and my left cylinder valve clearence was less than 0.03mm, so I change the shim and the bike run perfectly.

...Until after about a week... Then it started backfiring again, and generaly not working as good as before.

I checked again the clearence and again I found it to be less than lower limit.


The valve seated more than 0.05mm in just a week..    So, what gives?

[GeeP]

Possibly a burned valve, carbon or crud under the valve when measuring the clearance the first time giving a faulty reading, error on measurement. 

Those are the most common things.

.05mm is about 1/3rd the diameter of the hair on your head.  It doesn't take much...

[bobthebiker]

I am going to suggest a compression and leakdown test,   you could well have a burnt valve, or carbon buildup under the seat causing this kind of thing.   

one of the best ways to figure out what it is, would be to pull the head and check, but I dont think you want to do that until you've gotten everything else ruled out.

[Ry_Guy]

Possibly a burned valve, carbon or crud under the valve when measuring the clearance the first time giving a faulty reading, error on measurement. 

Those are the most common things.

Good advice. I recently had the same problem...had to change shims every 2000 miles. Upon replacement I found a chunk of the exhuast valve to be, well, gone. Does your bike have problems starting up....backfire when it's cold out? Losing power? Stalling out when you pull the clutch in?
The OEM valves are made out of a cheaper composite metal that doesn't fare well with a big temperature changes.

[lamoun]

Thank you guys for your input! 

The number one problem was backfiring into the left carb when cold.

I have done a compression test (but with the motor cold, carbs out) and found both cylinders in spec, 130PSI.

Does that mean carbon buildup? If that is the case, changing shims will eventually remove the carbon, or I still need to remove the cylinder head to clean it up?

[GeeP]

Lamoun,

It would tend to indicate carbon buildup, yes.  If it was a measurement error that caused the problem, there's no need to do anything other than change the shim.

Also, if the engine was run for any length of time with tight valves, the seats will probably have eroded slightly.  Usually, the valves will bed themselves back into the seats if the damage is minimal, causing a loss of valve clearance.  Once they bed back in, all is fine.

Unless you have access to a borescope to have a look at the valves, or you're planning a long trip very soon, I would go down one shim size and re-check it in a few hundred miles or if symtoms return.

If symptoms return or the valve tightens again, I would remove the exhaust, stuff rope into the cylinder, remove the valve springs, and drop the valve.  Then you can inspect all 360 degrees with a dental mirror though the exhaust port by spinning the valve with your fingers.