Valve Clearance Readings Mystery!

[Guest]

I’m in the middle of the valve clearance check on my ’83 GS650GL restoration project. 25K miles on the clock. Not ridden since 1993. Recently rebuilt the carbs, etc., and it runs just fine now.

OK, here’s the mystery. I have three different valve clearance readings depending on which position the cam lobe is in when it’s checked.

The factory service manual says that the clearance can be check with the cam lobes in two different positions, A or B, as shown in the illustrations below.





I have always believed that the best position for the cam lobe is half way between positions A and B, which places the exact bottom dead center (BDC) of the cam lobe facing the valve shim.

My clearance readings are listed in the table below:



I checked the clearances twice in each of the three cam lobe positions described above, and got three different clearance readings in each cam lobe position. That should not happen, since the bottom hemisphere (180 degrees) of the cam lobe should have the same diameter. So, that’s my mystery. How in the heck can there be three different clearance readings?

Any and all explanations are welcome.

My present thought is to leave the valves exactly as they are and not adjust any of them. That conclusion is based on the readings in the “midway between A & B” cam positions all being pretty much in spec (.003 mm to .008 mm) with the exception of two that are just a few thousands over. I currently have no valve noise at all now, and those slightly big readings should get smaller with wear.

What would you do?

???

[Nessism]

There is only one way to positions the cams, not optional methods. You need have them in the proper position so both adjacent valves are both on the base circle at the same time, otherwise the adjacent valve will skew the cam within the journal clearance and give false readings. Basscliff has a nice tutorial on his website you might want to check out.

[Steve]

What would I do? The first thing that I would do is to follow the manual and position the cams correctly.

Yes, it seems normal to assume that the 'correct' position would be to have the lobe pointing away from the valve being checked, but that likely puts the other lobe on that side of the cam pushing down on its valve. That will tend to push the cam off-center in its bearing, giving you some false clearance on the valve you are attempting to check.

Yes, the picture is rather confusing, so look at it this way: position the cam so that both of the lobes on that side of the cam are at about 45 degree angle away from the valves. That way, you can be sure that neither one of them is pushing on a valve. Now, as the manual states, CHECK BOTH VALVES, without moving the cam.

There are some items here that can be used as "shortcuts", but they are really just "following the correct sequence". Start with Exhaust valves 1 and 2. After checking them, rotate the crank 1/2 turn (180 degrees), you will find that IN 1&2 are now set. After checking them, rotate the crank another 1/2 turn, you will find EX 3&4 are set. A final 1/2 turn of the crank will set up IN 3&4.

Yep, set the crank in the proper postion just FOUR times, you can check all eight valves.

.

[tom203]

I had same confusion on my 650 based on my valve checking on cars, until I realized that adjacent cam lobe was giving false clearances . Repeat using Steve's advice and be absolutely sure. I run mine at .0015 to.004 inch.

[WingMan71]

Thanks all for the answer regarding the adjacent valve causing a false reading if its cam lobe is also not on it's base.

I'm just surprised that there can be that much slop in the cam journal bearings! If there is that much slop in those journal bearings you'd think they would need to be replaced.

I'll get back out in the shop and make sure the adjacent valves both have their cam lobes on their base before I take a reading.

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[Nessism]

Position 1E lobe facing forward - "B" position. Check 1E and 2E valves.

Position 4E lobe facing forward - "B" position. Check 3E and 4E valves.

Position 1I lobe facing upward - "A" position. Check 1I and 2I valves.

Position 4I lobe facing upwards - "A" position. Check 3I and 4I valves.

[Steve]

I'm just surprised that there can be that much slop in the cam journal bearings! If there is that much slop in those journal bearings you'd think they would need to be replaced.

There is no "slop", there is OIL CLEARANCE.

Take a look at the cam specs, you will see that the cam journal CLEARANCE is just about the same as the valve clearance spec.
In fact, the 'service limit' is almost TWICE the valve clearance limit.

This might help to point out the importance of checking your valve clearances.

Position 1E lobe facing forward - "B" position. Check 1E and 2E valves.

Position 4E lobe facing forward - "B" position. Check 3E and 4E valves.

Position 1I lobe facing upward - "A" position. Check 1I and 2I valves.

Position 4I lobe facing upwards - "A" position. Check 3I and 4I valves.

Pretty much what I said, but different order. Just more crank-turning involved.

.

[WingMan71]

There is no "slop", there is OIL CLEARANCE.

Take a look at the cam specs, you will see that the cam journal CLEARANCE is just about the same as the valve clearance spec. In fact, the 'service limit' is almost TWICE the valve clearance limit.

OK, now THAT makes sense!

Thank you!

That said, I'm surprised to see that much service limit on the cam journal clearance!

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