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A couple years ago I put the Mach 1 aside owing to a loss of compression. I thought I started a thread back then but now I can't find it and so I will restart one now.

So a couple of years ago I figured the M1 had lost compression as evidenced by the kick start level spinning the engine freely as I went to start it. Oddly, the compression returned for a brief while (perhaps a week) before it again disappeared.

That was enough as the second time I was eight miles from home. I transported the bike home and set it aside.

Two years on now I want to resolve this issue and get the M1 back into running condition and I find the engine has compression again. I know this is certainly temporary until I find the problem, so my plan is to take the valve covers off for a start just to see that the springs are where they should be.

Of course, as if to spite me the engine has regained its compression which is not going to help me as I try to look for the source of the problem.

Could this be a valve guide issue? Maybe a guide that is loose?

I don't suspect a broken ring as yet, but if you guys tell me that's what it could be I will keep that in mind.

Thanks,

Jim

[quote= JimF ...
" A couple years ago I put the Mach 1 aside owing to a loss of compression. I thought I started a thread back then but now I can't find it "

____ You mean this-one: viewtopic.php?f=3&t=658 , from three-years ago ?



" the M1 had lost compression
the compression returned for a brief while (perhaps a week) before it again disappeared.
and I find the engine has compression again.
the engine has regained its compression which is not going to help me as I try to look for the source of the problem. "

____ So-then does the engine once-again start-up & run now ?
__ My first suspicion is a valve-cap/shim that has been hastily ground-down unevenly,, and during the extra-low RPM kick-starting engine turn-over speed, the v.cap must then get time to be somewhat side-pressed into rotating-around between it's low & high* spots thus*then causing insufficient clearance for keeping compression build-up.
So I'd check-out the intake v.cap first.
__ I've found such poorly ground & unflat shim-caps to cause varying-amounts of clearance, but have never found any to have been ground-down so badly that their resulted high-spot (when relocated from 6 or 12 o'clock to 3 or 9 o'clock) could possibly take-up ALL the normal set-clearance !
So this plausible-answer may likely not be the actual case, (unless a really poor-mechanic had gotten his hands on your cyl.head).
____ Another (equally unlikely) possibility is that one of the valve-seats has worked-loose.
__ While neither of these conceivable suspicions is commonly suspected, I-myself (currently) can't think of any other answer for such a STRANGE*occurrence as you describe.
(* Where this problem/circumstance occurs REPEATEDLY.)
Otherwise I'd first suspect stuck compression-rings.
__ Perhaps maybe the four head-bolts aren't all torqued-down equally as they should be properly torqued to ?



" Could this be a valve guide issue? Maybe a guide that is loose? "

____ Well I of-course suppose that somehow that's certainly a considerably greater likelihood than a loose & temporarily-dislodged valve-seat ! _ As maybe a v.guide sometimes falls-loose from place & temporarily stays dislodged, and-thusly keeps it's then misguided valve-head from flatly seating.
However I-myself have never encountered such a condition, which could self-correct (as you've indicated). _ (But I think I've heard-of such happening before, in some Brit.bike-engine.)




____ For anyone caring to take another look back at the old posted pictures of Jim with his Mach-I (and Mark-III), then check-out this link... viewtopic.php?f=14&t=445&p=3115#p3115


Hopeful-Cheers,
DCT-Bob

Would the shim be on the adjustment screw?

[quote= JimF ...
" Would the shim be on the adjustment screw? "

____ Stock Mach-I heads don't employ adjustment-screws. _ The valve-shim/cap is found rather alone, between the valve-stem & rocker.
It's seen depicted as part-# 500 thru 511, (on the right-side of your posted pic.drawing).


Hopeful-Cheers,
-Bob

I am guessing: I should rotate the engine until the valve closes, then rotate the stack and use a feeler gauge to try and detect a change in the height?

I have the valve cover off now, and simply spinning the stack offers no visual clue that the rocker arm is moving.

Should I check the exhaust in similar fashion?

[quote= JimF ...
" I should rotate the engine until the valve closes, then rotate the stack and use a feeler gauge to try and detect a change in the height? "

____ Correct... Turn-over the engine until it reaches power-stroke TDC, then try inserting a rather thin* feeler-gauge/strip and slightly rotate the shim-cap to several alternate o'clock positions (no more than 60-degrees apart), to see if the rocker-clearance changes any at all.
(* You may likely have-to have your chosen f.gauge-strip be as thick as the recommended running-clearance for the M1-valves.)



" simply spinning the stack offers no visual clue that the rocker arm is moving. "

____ So then be sure to at-least check that the set rocker-clearance always remains consistent regardless of where-ever you positionally rotate the shim-cap to.



" Should I check the exhaust in similar fashion? "

____ The ex.valve is a lesser suspect since it's supposed set clearance is fairly larger (than that of the intake),, but if the intake side passes your clearance check, then the ex.clearance consistency is indeed the next suspect.


Hopeful-Cheers,
-Bob

Is there any scenario wherein the clutch, with no tension on the cable, could remain engaged?

____ Please note that I've finally update-edited my very-first initial-response post altered-to my finalized 'DCT-Bob' version.



[quote= JimF ...
" Is there any scenario wherein the clutch, with no tension on the cable, could remain engaged? "

____ You must mean temporarily left 'disengaged', right ? - (Or-else you possibly had rather meant the clutch 'line-tension' left "engaged", instead of the actual clutch-mechanism itself.) _ Cuz NORMALLY, the 'clutch' itself is left engaged (by default !) while it's line-tension is 'disengaged', (and vice-versa).
__ Now we're barking-up another tree...
We had started down this line of consideration, back three-years ago (in June-2011),, but I had assumed that you could certainly competently feel/perceive the certain difference between a slipping/disengaged clutch and the actual turning-over of a compression-less engine (whilst you kicked the stater-lever).
But even if-not then, you no-doubt would've certainly tried a push-start*attempt before choosing to abandon your prized Mach-I anywhere so far from home,, wouldn't you ? - (* At which point you most certainly would've more readily noticed the complete lack of the associated noises from the engine-mechanics turning-over.)
__ Anyhow, to reply to your specific question,, there's no common-issue that could cause the clutch to temporarily stay disengaged and-then also later become self-corrected.
And besides, you certainly would've noticed a floppy-loose clutch hand-lever (since the lever/cable-system doesn't include a separate clutch-line tension-spring [as some other bikes employ]),, so-thus if the hand-lever was returned back into normal-place, then so would have to have been the clutch-plates as well ! _ But if not, then it's fairly certain that you would've noticed & remembered that the hand-lever had no tension.
__ So bottom-line with this conceived consideration, is that it's got to be the wrong tree to bark at, (even as unlikely as the other scrutinized tree is the actual right-one !).
(As this-route is about as likely as the crankshaft/drive-gear key repeatedly failing & self-repairing.)


Dukaddy-DUKEs,
DCT-Bob

Jim I was going to say to check your clutch because of the way you described the kickstart "spinning the engine freely"
Like Bob said you should be able to tell the difference between the feel and sound of the clutch spinning and of a valve stuck open. Imagine holding in a compression release and kicking a bike over. Go to your other Duc and hold the clutch in and kick it over. Then try to decide what your Mach1 is actually doing.
Good luck

Well it's been a couple years... But I can't think of any scenario regarding the clutch that could create a disengaged condition without the cable being pulled.

I am still playing with engine rotation so as to check 'valve cap' clearances and I have been using the kick start lever but I may need to use the rear wheel with the bike in gear to get a finer resolution.

So far though I see nothing odd.

Jim