Random Question... (Compression test/valve clereance)



Jose J Ortega

Registered
Just a random question: I heard and see in videos from Motorcycle mechanics that they do a compression test first then they check the valve clearance. Correct me if I am wrong but, The compression test if the valve is not on spec can throw a false PSI #. Correct? Why not better check the valve clearance first, replace any shim if is necessary and then do a compression test? I not a mechanic just trying to find the logic.
 

RansomT

Registered
It's always better to do a compression and leak down test first to gauge the health of the engine. It takes minutes to raise the tank, remove the airbox, plugs, and crank access cover to do a leak down and compression test.

Valves have a tendency to "tighten", which will show up as a low compression reading because the valve doesn't seal. Leak down will pinpoint it, especially if you do a cold vs warm test.

If you do a valve adjustment first and then find out that the bike has a hurt piston or a burnt valve, you have wasted valuable shop time that you won't get paid for.
 

Jose J Ortega

Registered
It's always better to do a compression and leak down test first to gauge the health of the engine. It takes minutes to raise the tank, remove the airbox, plugs, and crank access cover to do a leak down and compression test.

Valves have a tendency to "tighten", which will show up as a low compression reading because the valve doesn't seal. Leak down will pinpoint it, especially if you do a cold vs warm test.

If you do a valve adjustment first and then find out that the bike has a hurt piston or a burnt valve, you have wasted valuable shop time that you won't get paid for.
Thank you for your fast reply RansomT... Just asking as I am planning myself to do a valve check next week and knowing the tendency like you said, that the valves tighten overtime, it will give me a false reading as if is a issue a issue with cylinder/rings, etc and maybe it was just the need to just adjust the valves.

Though that was better to check and fix anything that can affect the result of a low PSI in a Piston. I can just check it on the beginning do any Valve adjustment and check it again on the end.
 


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