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  #1  
Old 12 Jun 2003
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NX650 Valve clearances

Hi,

I'm having a bit of trouble understanding how to do the right hand exhaust valve clearance on the NX650 (is it the same as the XR650L??).
I understand that the valve rises due to the automatic decompression system. I want to know when i should be checking the clearance. Is it just before TDC, or before the valve starts moving?

The manual doesn't really explain it that clearly so if anyone has experience with this, please help me out!!

Cheers
Rob
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  #2  
Old 13 Jun 2003
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you should do this at TDC on the compression stroke.

The piston reaches TDC twice in a cycle - once when expelling the exhaust (exhaust valve wide open) and once when igniting the fuel (Exhaust valve closed and not touching the cam). You want the latter. You'll know you're at the right one becuase when you see the TDC mark on the flywheel on the compression stroke, it'll be very hard to turn the motor. You'll also know it becasue you can eactually get the feeler gauges into the gap

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  #3  
Old 13 Jun 2003
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oh, and DO NOT turn the motor backwards if you miss TDC, because it messes up the decompresion system. Keep going again until you get to the right spot.
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Old 13 Jun 2003
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Thanks for the reply,

I understand exactly what you are saying - and this has worked fine for 3 of the valves. The problem is with the right exhaust valve. Quote from the manual:

"Special care must be taken when measuring valve clearance at the right-hand exhaust valve. THe Half-decompressor slightly lifts the right hand exhaust valve when the crankshaft position is before compression TDC.
Therefore it is necessary to position the crankshaft where there is no valve lift in order to obtain the correct clearance."

Now this has confused me! When spinning the crank to the correct TDC, the right hand valve continues to rise after the left one has stopped. Once that has stopped you are not far from the "T mark" (TDC)on the flywheel.
Do i measure the valve clearance before the right continues to rise? This seems very far away from the TDC mark.

Am i making any sense?!! This damn decompressor thing has got me confused.

cheers
Rob



[This message has been edited by rob46 (edited 12 June 2003).]
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  #5  
Old 13 Jun 2003
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Hmmm.

On second thoughts i think my valves may all be adjusted at the wrong TDC. I'll have to have a look asap. The bike runs fine though and there is only a small amount of tappet noise.

Still any advice on the right exhaust clearance would be gratefully recieved, as that's another issue.
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  #6  
Old 13 Jun 2003
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Well, admittedly I'm no expert on what's going on with the decompressor - I don't know how it works, but I do know how the whole valvetrain works in general.

All I can tell you is that if the valves are moving, you are probably at the wrong TDC! Everything should be closed prior to reaching TDC on compression stroke until about the point the piston reaches BDC, when the exhaust valves open, so during that time the valves shouldn't be being pushed by anything.

Anyone else have ideas?
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Old 13 Jun 2003
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a "lazy" trick that many mechanics use (and I learnt it from a really good one) is to crank the engine using the starter and watch the valves going up and down. then, once you know the pattern of operation, blip the starter (I also used the kick on my XR4) to find the point where the're all closed and even that fourth rocker arm has some "play" in it. TDC or not, it gives good results so far - I've used this trick on sports bikes and singles for years with no problems. if there's no "play", its too tight already!

by the way, I'm not sure about this auto-decompressor lark as I found no trace of it on my 2002 XR650L when I stripped the engine to put the kick-start on. might the manual be misleading you into believing its there as it also covers earlier kick-start engines?
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Old 13 Jun 2003
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Thanks for the tips - i'll get into the garage and try it out.

As to what Rich was saying: Interesting point. The funny thing is, is that the small rider's manual (that comes with the bike) makes no mention of this in the valve clearance section.

My model is a 91, so the kickstart had just been discontinued. However, i have two honda service manuals (88-90, and 91) and they both have this note about the right exhaust valve. This valve also does move in a slightly different way to the left hand one which implies that something different is going on.

I might take the tappet cover off and take a look inside. That might explain it more.

Thanks again for your help.
Rob
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Old 13 Jun 2003
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Well having found the correct TDC this time it started to make sense, sort of!

A little before TDC the right exhaust tappet lowers and then rises. It only moves a small amount, and stays put until TDC. I adjusted the clearance just before the movement started but it was obviously wrong. When TDC was reached, the clearance was big enough to put a plank through! I skipped that and just set all the clearances at TDC. The bike sounds, and runs great.

Really not sure what the manual was after with this one!

Rob
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  #10  
Old 14 Jun 2003
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The reason manuals usually refer to setting the engine at TDC is that ALL the valves are supposed to be able to be checked with out moving the crank again. Meaning that none of the valves are being engaged by the cam lobe and is completely closed.

Now, since we know what we want is the clearance between the intake valve or which ever valve and the cam. Just turn the engine until that lobe is standing straight up and you KNOW its not engaging the valve at all. Measure the clearnance. Obviously, you will probably again have to rotate the crank to measure the opposite side of valves. This is similar to the thread above with "bumping " the starter to posistion the cam properly over the valves that are being measured.

Point of that overly long rant. IF your having the auto Decomp. messing with you close to TDC just rotate it past TDC and measure there, without interference from the auto decomp.

NOW, when removing cams from the head IT IS important to put the crank at TDC . Other wise the force of the valve will push the cam up as your trying to loosen the caps.

Thanks,
Todd Bellew
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  #11  
Old 23 Aug 2007
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auto decomp

hello everybody!

just wondering, my buddy (a mechanic) reckon that my auto decomp has worn out, and causing some noise. my dommie has done 60,0000km (more or less) just fixed my speedo gears! he's suggesting that we should just remove the auto decomp & things should be okay! I've read this on the forum before, but this bloke's bike has done 20,000miles, mine has a new sleeves. will it work?

please help me out
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  #12  
Old 29 Aug 2007
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grand mother..... eggs but.....

spin engine round until TDC - find this by watching the valves (inlet) go in then out, the next TDC will be the TDC on compression. Not much turning after this.

This is where you are going wrong as I have in the past, you think you have the TDC correct and even have a screw driver in there to check but the valve going in then out is the best way.

Get a good feeler guage, adjust the valves to when pushing the feeler guage in it bends but pulls out OK (bit of a bite).

hope this helps
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