Compression puzzle

[GLJ]

I'm a bit puzzled. As many of you know I have a 72 XS2. This is the start of the 2nd season on this engine since rebuilding the motor. Last year was 3000+ miles. This year I've already put all most 3000 on it. It has been running fine. Before leaving on a 1700 mile trip to the Ozarks a couple of weeks ago I took a quick compression test and got 120ish on each side. I thought there may be something wrong with the tester and didn't have time to go any further. Road it on the trip trip and it ran fine. No problem starting it, gas mileage was upper 40s, it pulled up hills just fine even with a 32 tooth sprocket on it and loaded down with extra weight from luggage. Since I got back I have been cleaning and doing some service on it. Well today I was finishing up a few thing on it and decided to check compression.
Now the puzzle. It would hardly get to 25 lbs on either cylinder. What the he!!. Checked the tester and could not find any problems. Definitely a compression problem, didn't even try to blow my finger off either plug hole. Head scratching time, all I've done to it since I road it into the shop is change oil and clean it. Hadn't even tried to start it. Off came the valve covers and checked valve clearance. I have been running 3 and 6. They may have been a little tight but I could move the rockers some with my fingers, definitely not zero lash. WTF. Loosened them up as much as my elephant foot adjuster allow. Rechecked compression 145. OK maybe they were tighter than I thought. Reset to 3 and 6. Still low, went to 4 and 7 better, 5 and 9 more better. 6 and 9 better but still low 125. Hmm. Earlier I had checked and adjusted the cam chain, it was a little loose. Decided to check cam timing. It's a little late. I don't think it's a tooth late. If it is it's been that way all along. One other thing the carbs are on a work bench waiting to go back on after being cleaned. Yes I took the paper towels out of the boots before checking compression.
I've been using the E-start to check the compression. With the valve adjusters backed off I got the 145 using the E-start. I think I'll get the battery back to full charge and think about it while it's charging.
Anyone have any thoughts

 

[Jim]

Dunno Greg.... the only wear I can think of that closes clearances is wear on the valve face and seat. Anything else in the valve train would cause bigger gaps. You positive you're on TDC when checking lash?

 

[Jim]

Try adjusting 1 valve to normal clearance and play with the other. At least that way you could narrow it down to either intake or exhaust causing it.

 

[GLJ]

Dunno Greg.... the only wear I can think of that closes clearances is wear on the valve face and seat. Anything else in the valve train would cause bigger gaps. You positive you're on TDC when checking lash?

Valve and valve seat wear will close up the gap. They were not at zero lash. Loosening them up as far as the adjusters allow gets me to normal compression. Adjusting them looser than I had been running them gets it better. I will try 6 and 10 and see what I get after the battery is charged back up. I like your thought on loosening just one and see. I'll try the intake side first.

 

[5twins]

That is odd. When you rebuilt the motor, did you lap the valves in and check for leaks afterward?

 

[GLJ]

That is odd. When you rebuilt the motor, did you lap the valves in and check for leaks afterward?

Had the head done at a automotive machine shop. 3 angles on the seats.
I've ruled out valve sealing in my mind because it will develop 145 psi with the adjusters backed off.
I'm curious to see what guys that have more experience with the 650 engines think of the late cam timing. It's not a lot but some. I used NOS Yamaha chain in it, 6000ish smiles ago.

 

[Jim]

Here's a pic I marked up showing what one tooth off looks like while you still have the rocker box off. As a guess, where does yours fall?

 

[GLJ]

I set the crank using the timing marks on the rotor/stator. I think I need to get a more accurate TDC setting.
I'm just puzzled by the fact I rode it into the shop and 10 days later compression was that low.
I know on car engines when the cam chain was worn out they jumped timing when you shut them off. Engine rolling backwards from compression and slack in the chain. My cam chain adjustment was maybe 1 mm out. Or in depending how you look at it.

 

[timbeck]

Things that make you say, hmmmmmm.........

tim

 

[grizld1]

Probably not all and maybe not any of the problem, but Yamaha's spec for the 256 cam in your motor was .006" and .012". The tighter settings were recommended for the later 447 cam. As the old saying goes, "It's better to hear your valves than to smell 'em." Doesn't explain why the grief showed up suddenly after many miles of good operation, though.

 

[GLJ]

"It's better to hear your valves than to smell 'em."

Heard that a lot growing up.

Doesn't explain why the grief showed up suddenly after many miles of good operation, though.

Ran fine to the Ozarks and back. Couldn't keep up in the corners. That had nothing to do with the motor.

 

[Mailman]

This is very weird Greg. I just set the valves on my bike at .003 and .006 and did a compression test and I’m getting just under 150 lbs for both cylinders. When you back your adjusters off all the way, you get good compression. Something has got to have your cam timing off just a little , it’s gotta be cracking your valves open just before TDC and letting your compression bleed off. Or is that the conclusion you guys have already reached?

 

[gggGary]

I saw Chaves over by your bike one night at the Ozarks but didn't think anything of it. Maybe he got tired of how hard your were pushing him in the twisties.

WJL I set valves at .003 and .006 when I got it in 2019, haven't checked them since, same as I set all my other 650 motors. Compression then was L152 R142
I changed oil and checked WJL when I got back also, L150 R is now 160
I was using a quart a thousand during the ride, very consistent adding 1/4 qt at a time UP to the lower mark on the stick. Bike would leave a couple drops after parking it. GLJ noticed a puff from the RH cylinder one time otherwise noticed no smoke. RH plug shows a bit of fluffy build up, pic is a bit blurry but this is prolly the smoking gun for oil use. there is some pitting, rust, evidence in RH cylinder only, LH is clean, smooth crosshatches.
Probly cuz I baby my motors.

Some period SB's RE valve setting in 256 motors

I think that 2 and 4 for the TX650A got revised to 3 and 6 later on.

 

[GLJ]

GLJ noticed a puff from the RH cylinder one time otherwise noticed no smoke.

It was coal black smoke, no blue at all.

I saw Chaves over by your bike one night at the Ozarks but didn't think anything of it. Maybe he got tired of how hard your were pushing him in the twisties.

Was probably wondering how anything could go that slow around a corner and not fall down.

 

[gggGary]

It was coal black smoke, no blue at all.
.

Rolling coal,
BACK OFF baby!

 

[gggGary]

Sorry for barging, a "better" pic from before the trip.

 

[TwoManyXS1Bs]

Ohboy, conjecture time.

A late-closing intake valve will reduce compression test values.

Consider the possibility of worn rocker pads.
Possibly worse than this, as we've seen rocker pads in much worse shape.


A pad with that wear line no longer follows the camshaft profile, and will lead to early opening and late closure.


There *is* a way to test for this.
Using the early XS1-XS2 valve timing chart,


Note that the intake valve should close at 113° BTDC (180° - 67°). And, those valve timing specs are done with 0.012" lash. So, with a tighter lash, the intake valve will close even later, maybe 100° BTDC. A retarded camshaft could bump that beyond 90°, and a worn rocker pad would push that even later. Not much piston travel left to get a good compression reading.

Set your valve lash at TDC compression stroke.
Rotate the engine 360°.
You're now at TDC, overlap, beginning of intake stroke.
Both valves should be tight, slightly opened.

Continue rotating the engine another 180°.
The exhaust valve should be closing, confirm by wiggling its rocker, checking for looseness.
The intake valve will fully open, then start to close.

You're now at BDC, beginning of compression stroke.
Continue rotating the engine another 90°.
The intake valve should be closed by now, confirm by wiggling its rocker, checking for looseness.
The rotor timing mark should be hidden behind the stator housing web.


If the timing mark is visible, then you're dealing with a late-closure valve, reducing compression value.

Try this with various lash values. Pencil lines on the rotor face for reference.
With a good camshaft, good timing chain so it's in proper time, and good rockers, variations of valve lash should alter the closure timing less than 10°. If greater variations occur, you may have found a worn rocker pad...

 

[GLJ]

Checked the TDC marks on the rotor/stator and they are pretty close. Backed off one intake valve all the way, 145 lbs. Set it to .012 and got 125. Then set the cam at TDC by the setting the pin hole to 12 o'clock and looked to see where the TDC marks were at on the crank. As expected crank is past TDC mark. Looks to me about 10.6 degrees or 1 cam tooth. I realize I am eyeballing setting the cam at TDC and the amount the crank mark is off. I do think I am accurate enough to say it's a tooth off. @Jim your pic was very helpful to put things in perspective. I think if set the intakes at 12 it would run. I think the motor is coming out and apart. Got a few more trips planned this year. I've got other toys to play with while this bike is down. Would like to thank everybody for their comments.
I still would like to know how I road it into the shop and then loose compression.

 

[grizld1]

Cam turns at 1/2 crank speed, so 10.6* (1 tooth on the 34 tooth sprocket)=21.2* at the crank (I know you know that, but some readers might not have thought it through). Congrats on trapping the gremlin! Cage the rascal and poke it now and again to make it squeal, that'll make its buddies clear out. Gary, Jim, and 2M, thanks for the education; I wasn't aware of the 1972 spec change for valve lash, and I might have been fooling with dial indicator and degree wheel instead of checking valve timing the quick and easy way.

 

[TwoManyXS1Bs]

GLJ, your cam is about as retarded as mine, and mine still runs. I have a few posts scattered around in here about retarded camshafts, that may shed more light.

Do a forum search on "retarded cam", posted by "TwoManyXS1Bs"...