Dirt Bike Repair & Maintenance Forum

This is by no means a conclusive test, but it's interesting nonetheless.

I ran my CRF150RB for the 1st 2 hours on the original oil, then drained it.  Exhaust valves had tightened by .002" and .0015"

I then refilled with Mobil (petroleum, not synthetic) and ran it to 7.5 hours.  Valves tightened by .002" each.  I then replaced the oil and filter, again with the Mobil.  Valves tightened by .0015" each.  I then drained the oil and filled it with Motul 300V Competition 15w50 and ran it 7.5 hours.  The valves didn't move.

Hmmm...  
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Here we go again!

lol. thats actually really interesting. good post. now if you were to drain the synthetic out and replace it again with petroleum based would the move again? how do we know that they just decided to stop moving haha. anyway. now you got me thinking. im not good with fourstrokes. do you have a crasher course on the valves i could read? a previous post i missed in the search perhaps?  
PEACE GUYS ITS BEEN A BLAST!!!!! ~patrick

Very good questions!  They very well could have stopped moving, but my experience is that the motor and valve train are broken in by 2 hours.  This bike stopped tightening valves at 17-24.5 hours - during the period I ran Motul and long after this engine should have broken in.

I plan on running petrolium the next oil change and see what happens.  If it moves, I'll go back to Motul.  If it stops moving with the Motul, then I think we're nearing 'conclusive'.

Valves.  All engine designs have them!  On a 2-stroke the piston moves down and exposes holes in the cylinder (ports) - these regulate the expulsion of exhaust gasses and the intake of fuel.  On a 4-stroke, the cylinder is solid.  Valves at the top of the head are operated by the camshafts, which are connected via a chain to the crankshaft.  After the piston starts moving down on the 1st stroke, the intake valves open allowing fuel in.  When the piston reaches the bottom, the valves close. When the piston reaches TDC on the second stroke, the mixture is compressed and all valves are closed. BOOM - spark fires!  The downstroke is the power stroke.  When the piston starts coming back to TDC on the final stroke, the exhaust valves open releasing the burned gasses. Then it all starts all over.

The cams press on lifters that operate the valves (opening them).  You need clearance inbetween the cam and lifter, so that when the engine heats and the metal expands, you don't get a situation where the cam is always pressing on the valve and keeping it slightly open. Over time, the exhaust valve seats wear, an this reduces the clearance (which is why we have to adjust the valve clearances). On the intake side, the valves either won't move at all, or the clearances will grow over time due to carbon build up.  The exhaust valves take the brunt of the absue because the exhaust side is much hotter than the intake side.  
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Here we go again!

ahh thank you for clearing that up skip. i knew what the valves were but not why you would have to adjust them. i've had a revelation! now if only i could get jetting... hahaha jk ~pat  
PEACE GUYS ITS BEEN A BLAST!!!!! ~patrick

interesting skipper post back after the next oil change i want too see what happens
also i want to see what you think about this thread??

http://forums.dirtrider.com/70/7047348/kawasaki-dirt-bikes/04-kx250f/index.html  
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ridin the dirt and the pow