No compression

[MotoMike]

Jim
sounds like a leak down tester would be usefull for you. easy enough to make or borrow if you don't have one. plus with it in the plug hole and running, hearing anything amiss from the intake or exhaust would be easier and a one man job. I've used them to check the condition of multi cylinder engines where checking that all cyclinders are about the same is usually good, but if memory serves, 5% leak down on a fresh engine and no more than 10% on a healthy engine of any miles. But even if you didn't use a regulated gauged tester, just an air fitting on a spark plug body would let you run air through it while testing for leaks. a piece of thin tubing inserted in the exhauste, carb and even the crankcase dip stick hole or breather tube while you listen at the other end might give you some data.

[Bevel bob]

Check the head bolts for slack.

[JimF]

I have both valve covers off - a spin of the engine reveals nothing out of place, that is to say that the rockers operate both valves. Nothing is obviously wrong.

I have the intake manifold off, the central oil union nut is off, the tack drive cover is off, the exhuast header is off. I will likely take the coil off to give myself some head room (no pun intended) between the head and the frame.

Before I take out the head bolts I have a few questions:

What do I do about the bevel tower? Presumably the tower comes off with the head, yes?

Is there anything at the base of the bevel tower I have to disconnect or concern myslelf with?

Is there any better engine cycle I should be on (piston up or down or don't care?) or should I align the bevel drive marks for easier reassembly?

[DewCatTea-Bob]

" I have both valve covers off - a spin of the engine reveals nothing out of place, that is to say that the rockers operate both valves. Nothing is obviously wrong. "

____ Have you also checked both valve-clearances? _ If so, what are they at ?


" I have the intake manifold off, the central oil union nut is off, the tack drive cover is off, the exhuast header is off. I will likely take the coil off to give myself some head room (no pun intended) between the head and the frame. "

____ With a 250, you need only unscrew the plastic coil-lead cap.


" Before I take out the head bolts I have a few questions: "

____ Have you first made-sure that all four head-bolts were previously torqued-town tightly enough ?


" What do I do about the bevel tower? Presumably the tower comes off with the head, yes? "

____ Right, just leave the tower-shaft housing-tube (and all), as part of the cyl.head,, as it's best removed as a complete-assy.
BUT, before you lift-off the head,, rotate the engine until the timing-dots on both bevel-gears are aligned, (which will take less than 10-revs to accomplish !).


" Is there anything at the base of the bevel tower I have to disconnect or concern myslelf with? "

____ No,, just lift-up about a half-inch, then turn/twist the cyl.head forward & rightward & outward (clockwise, fulcrum-ed at the tower-tube), then lift-off cyl.head/tower-tube up & away from the tower-tube flange-base... and you're then completely removed !


" Is there any better engine cycle I should be on (piston up or down or don't care?) or should I align the bevel drive marks for easier reassembly? "

____ The piston will be at TDC when the bevel-gear timing-marks are aligned.
By having the bevel-gear timing-marks aligned when the cyl.head is removed, you'll then leave all the other remaining internal gear-timing, 'in time' !

____ After you get the cyl.head off, commence with checking for valve & seat leakage...
Prop/set the head with one of the ports facing upward, and pour-in an ounce or so of gas/fuel or penetrating-oil, and note any leakage past the valve-head, (while timing-dot/marks are kept aligned),, then likewise for opposite-port.
__ Looking-forward to your report-back.


Hopeful-Cheers,
-Bob

[JimF]

The engine was rebuilt by Syd Tunstall.

I have both a beam torque wrench and a clicker - would you prefer that I loosen the bolts and see and what torque they break free? Or should I tighten them to see at what torque they are at? Clicker or beam?

[DewCatTea-Bob]

" would you prefer that I loosen the bolts and see and what torque they break free? Or should I tighten them to see at what torque they are at? Clicker or beam? "

____ Use your beam-type torque-indicator to either tighten (if you're optimistic), or loosen (if you're expecting worse causation),, to get your torque-reading (with turning of bolt, twist-point).
__ The reading to note, is while twisting (as opposed to the 'break-loose' point).
The torque should be near 28ft/lbs. _ If you find it to be under 20 for any bolt, then it could be the source of your compression-issue.


DUCATIly,
-Bob

[MotoMike]

This is definately DCTBob's wheel house.

An air hose regulated at 100 lbs or less, connected to a fitting from a compression tester screwed into the plug hole with the engine at tdc on compression would let you listen around and tell where you are leaking. I guess if it was not from slack head bolts, you have to take it down anyway.
Nigel recommended I have a friend holding the cylinder in place while removing the head to keep the cylinder base gasket intact. Unless you expect that you'll need to go further than the head.
I am anxious to hear the cause.

[DewCatTea-Bob]

" Nigel recommended I have a friend holding the cylinder in place while removing the head to keep the cylinder base gasket intact. "

____ The cyl.head very rarely ever sticks to the cyl.jug, so gravity adequately holds-down the jug well enough. _ But if it does seem that the jug wants to lift-up along-with the cyl.head, then merely give the cyl.head just a little wobble-shake as you lift upward on it.
Far more often (than having the head sticking onto the jug's oil-passage nipple), is finding that the tower-shaft's lower-tip is stuck into the shaft-sleeve (which in turn is also stuck onto the lower drive-shaft. _ But that issue almost never happens with an engine that's been running in the past year ! _ That stuck-shaft issue is either due to rust build-up or from an undersized shaft-connector sleeve (having been used to replace the original connector-sleeve.)
THESE possibilities are the main instances for actually needing to really shake-hard on the cyl.head while lifting it ! _ But otherwise, only a slight wobbling-pressure (if any at all, while lifting the cyl.head), is quite sufficient.
__ Also, it's fairly rare that the paper cyl.base-gasket will be harmed in any way, if the cyl.jug happens to get lifted-up/off it's place against the motor-case. _ If that base-gasket happens to be adhered to both surfaces, (which is doubtful !),, then it's adherence alone ought be sufficient to keep the cyl.jug held-down, while the cyl.head is lifted off.
(Note: In my first-post, I got a bit ahead & carried-on as if the cyl.jug were also being removed... I've gone-back & edited-out that remark about rags on the case.
If you want details about pulling the jug, I'll add them in a related-post.)


DUCATIly,
-Bob

[JimF]

____ Have you also checked both valve-clearances? _ If so, what are they at ?

Not to dismiss the importance of valve clearances, but so long as I see clearance appearing between the rockers and the valves I would think that indicates the springs are free to retract the valves to their fullest extent. Given that, valve clearances can't be part of the compression problem, can they? Or am I taking too simplistic an overview?

[DewCatTea-Bob]

' but so long as I see clearance appearing between the rockers and the valves I would think that indicates the springs are free to retract the valves to their fullest extent. Given that, valve clearances can't be part of the compression problem, can they? "

____ Well if you can actually "see" clearance, it's then likely that's not relevant to your issue,
but because of how you stated your engine ran (just before giving-up), it then seems possible that a valve heated-up enough to more than absorb available (possibly insufficient) clearance.
__ Clearances for a stock cyl.head of your Duke-model, should be at LEAST .004" on intake, and .006" on exhaust, (although the factory-recommended figures may be up to twice that).


Hopeful-Cheers,
-Bob