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Auto-Decomp Sticking Open?

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2005 YZ450F.  

 

Was out doing a few laps at the track today, bike was running great.  After about 45 minutes of ride time, went to go restart and to run to the bathroom quick and bike had no compression.  Kicked several times and nothing.  It was obvious the decomp was sticking open for whatever reason.  Finally put the bike in gear and rocked it backwards. Didn't really help.  Held the throttle half open and kicked and it popped hard enough that the valve finally closed and then it fired right up.  Never gave me any more trouble and rode for another 30 minutes or so.  Anything I should be looking out for or needing to service?

 

Also, probably getting close to 10 hours on my oil as well (Maxima full synthetic) and probably headed to Moab this weekend.  I've heard it's ok doing the filter every other oil change, is this true?  All my old 2 strokes didn't have engine oil filters.  Also I've read up on that oil hole for the filter cover as well, so am aware of that issue.

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It isn't the auto decompression mechanism.  In the first place, the compression relief is supposed to take place during starting, so the device would be expected to be active at less than 750 RPM.  In the second place, it doesn't regulate compression such that it can be "out of adjustment" or "stuck open".

The setup consists of a pin embedded in the camshaft that is extended by spring force on an eccentric shaft.  The spring is too weak to hold it extended by itself, and depends on the eccentric going over center to keep it extended.  When the bike starts and exceeds 750 RPM, a flyweight on the cam sprocket that is linked to the eccentric shaft overcomes the spring and retracts the pin so the cam operates normally.

When extended, the decomp pin lifts one exhaust valve off its seat slightly just before the intake valves close, then lets the exhaust valve seat again about 20-25 degrees before top dead center (TDC).  This shortens the compression stroke to a manageable amount so human beings can turn the engine over.  If it were to stick in the "active" mode, the engine would make all kinds of clattering racket when it started, but the compression you feel while kicking would be normal.

So, then, what's actually happening?  The two main possibilities are:

The engine has skipped time. If the exhaust cam becomes retarded (late) by only one tooth, the exhaust valve lifted by the decomp pin will not seat until after TDC, and you'll loose the entire compression stroke.  This would be true whether the engine were hot or cold.

The valve are worn to at or beyond zero clearance. The unusual thing about these engines is that the head expands with heat to a greater degree than the valves do, and full temperature operating clearance is .001 - .002" tighter that what the cold clearance is.  If the valve clearance is tight enough, the valves may actually be slightly off their seat when hot.  If this is the case, "normal" starting compression usually returns once the engine cools.

 

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Weird, I'm not sure what happened then.  It had very little compression, then a few kicks later, it was fine.  Bike is running great, no misfire or stalling.

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I would check the valve clearance, if you haven't done so recently.

Two other things that can cause a temporary loss of compression are weak rings, or much more commonly, a sticking valve.  Carbon build up on the stems and around near the faces can prevent them from seating one time, then not the next.

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