checking valves, different readings, which is correct?

Hey! I'm checking the valves on the bike, but I'm getting inconsistent readings when the cam is at different positions. I'm guessing this is normal, but the question is... exactly what position should the cam be in when I check it for an accurate reading? All the tutorials and procedures I've read are very vague in this regard.

To describe exactly what I mean, if I kinda jam the feeler gauge in there WHILE I'm rotating the engine, when the cam reaches about 90 degrees, the gap opens immediately to .127mm (.05in), but then as I rotate the cam around to 180 degrees, or pointing directly away from the valve, the gap gets tighter by .01 or .02 inches, down to .105mm or .0800 or whatever.

Since I'm going to be buying new shims based of this precise reading, what's the most accurate number? I'm thinking the initial number, i.e. the initial "peak," but I want to make sure that the procedure is not specific to the backside of the cam and it really needs to be checked at 180 degrees.

 

The lobe of the cam needs to be 180 degrees, or pointing directly away from the valve when you take the readings

 

I'm curious what your source is of this information, because I've heard both sides from different places.

I'm hesitant to do it that way as it seems like the profiles of the backs of the cams are not totally consistent; some of them get way tight around 180 degrees and some of them stay open, or even open up a bit more as it comes around. If I just measure slowly as it opens up, the "peak" is generall always about at the same place, which is somewhere around 120 degrees.

 

It says in the manual that the clearances should be measured/adjusted when each cylinder is at Top Dead Center. You have to actually use the timing mark to know when the cylinders are at TDC, since based on the exact valve opening profile chosen, this may mean the top of the lobe is not exactly 180 degrees away (but it'll be pretty darn close).

That's from the 1985 xj700 manual. And it tells how many degrees between cylinders so you can advance the engine. This bike has:

1 - 0 to 180 degrees
2 - 180 to 360 degrees
3 - 360 to 540 degrees
4 - 540 to 720 degrees

So you rotate the engine correspondingly. Also, you're not supposed to "jam" the feeler gauges. It should be a very light sticky feeling (a little bit of drag) when you move it. Otherwise, you're just compressing the spring a teeny bit, which goes against the whole idea here.

 

Based on years and years of actual experience working on motorcycles,planes, trains and automobiles. Okay I have not really worked on trains. But I use the ( pause for effect) Still pausing!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!. Wait for it.....The Genuine Yamaha factory Service Manual

 

Sorry couldnt help myself now I am on my way to turn some wrenches on a Volvo Penta engine so add boats to the above. 3 carbs ya baby