Xs650 valve adjustment

Jacob Canada

Jacob Canada

XS650 Enthusiast
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Happy Friday everyone. I am finishing up a fresh top end rebuild on my 76 and I need to get my valves adjusted. I have read a ton on it and get the gist of what i'm doing for the most part. The problem I am having is when I adjust once cylinder and rotate the engine a couple rotations the rocker arms on the adjusted cylinder are will be loose as if they weren't adjusted then when it comes back around they are back in adjustment? Its like they get loose on when the cylinder is on a non compression stroke. Am I missing something? If I get the chance tonight I will up load a video of what is happening. Any help would be greatly appreciated.
 
Rotate the alternator rotor CCW and watch the rockers on the left cylinder. . Keep turning until you see the exhaust open.... keep rotatin' and it will close and the intake will open.... keep going..... intake will close.... keep going 'till you line up the timing mark with the TDC mark....... STOP. This is the point where you adjust both valves on the left cylinder. Since the XS is a 360 twin, all you gotta do is rotate the crank (still CCW) 360 deg.... one complete revolution, line up the marks and repeat on the right cyl.
 
Thanks Jim for the quick reply. That's exactly what I'm doing. Why are the rocker arms loose when it's on the cylinder that is not in a compression stroke? I will get a vid uploaded as soon as I get home today. Or is that just how its supposed to be? Thanks
 
When at TDC, the valves should only be loose on one of the cylinders. That is the one that will be at TDC on it's compression stroke, and the one you can set the valves on. The other cylinder will be at TDC on it's exhaust stroke and it's valves cannot be set. Both should be tight still with no clearance. The exhaust will just be finishing closing and the intake will just be starting to open.

If you had the top off your motor and could observe the cam, you could see this. The rockers ride on top of the cam. In the pic below, the right cylinder would be at TDC on it's compression stroke. The rockers would be riding on the base circles (lowest point) of the cam lobes on that side. The left cylinder's rockers would still be riding on the approach ramps to the lobe high points, which would make the valves tight .....

eCVfNLK.jpg
 
The other thing to remember when dealing with anything to do with timing (valves, ignition etc.) is that the camshaft turns exactly ONCE for every TWO revolutions of the crankshaft. That ratio is the same for all four-stroke cycle engines - ALWAYS.

The reason for the name is that each piston travels up and down in the cylinder bore TWICE per engine operating cycle. Thus, the piston travels the entire stroke distance four times and that is why our engines are called "four stroke" or "four cycle" engines - but really, the correct term is "four stroke cycle" engines.

That is why you need to turn the crankshaft TWO revolutions to get the camshaft back to the correct location for the timing marks and cam lobes shown in 5Twin's photo above to work properly.

I teach mechanical and automotive engineering and it is amazing how often students will forget that little factoid and muff a calculation.

Incidentally, that is also the reason why early XS650 cranks and cams cannot be mixed with later cranks and cams: MamaYama changed the number of teeth on the sprockets on the crankshaft and camshaft in <I think> 1972. If you try to mix an early crank with a later camshaft - the 2:1 ratio will not be correct and valves will hit pistons.
 
Last edited:
Jacob Canada said:
Happy Friday everyone. I am finishing up a fresh top end rebuild on my 76 and I need to get my valves adjusted.
Click to expand...
Your fresh top end will likely need valve adjustment again soon. I'd just get em close for now , maybe even on the loose side knowing that will be checked again soon ?
 
Actually, it seems you didn't quite give it enough thought. Or maybe you're just not fully aware of how your 650 engine operates. Both pistons rise and fall together but they are on different strokes. When one is on the compression stroke, the other is on the exhaust stroke. You can't set the valves on that exhaust stroke cylinder, and if you did so, they would be really, really loose when that cylinder did come up on it's compression stroke. Run through Jim's method of watching the exhaust then intake open and close to insure you are at TDC on the compression stroke for that cylinder. One thing I'll add - if the exhaust valve starts to open again after the intake opens and closes, you've gone too far, past the point you should have stopped at. If you do that, it's best to cycle the motor through again instead of turning it backwards.
 
Pete, the change you referred to occurred with the introduction of the 447 engine on the TX650A (1974). As 5twins says, even with adjusters backed out all the way it's impossible to open a gap between tappet and valve on the cylinder that's at TDC at the top of the intake/exhust cycle. Here's the old memory aid, often cited by Brother Kop, for the functions of the four strokes of the cylinder: "Suck, squeeze, bang, blow."
 
Jacob Canada said:
But rather be safe then sorry.
Click to expand...

ZACKLEY!!!

I’m afraid I can’t agree with the set ‘em at the loosest point” method. While that Could potentially give a result that will work, the fact is that is not really correct.

The timing marks are there for a reason and the correct way to time the various engine functions is to use them properly.
 
Jacob Canada said:
So basically when I see the intake valve open then close on the left cylinder that's when I want to make my adjustment?
Click to expand...
No. If your rotating counterclockwise... the intake closing just tells you you're coming up on the compression stroke. You need to keep going 'till you line up the TDC mark.
 

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