If the valve was even more than slightly bent, I would expect the cam to be stiff to turn or even not turn at all due to the valve not being able to close, jamming the camshaft (unless of course something has snapped off!) and if the piston isn't damaged then the head of the valve isn't floating around in there. Check your valve clearances, if they're still as they were, that would be good news. As for compression being zero, if the valves aren't working you won't be sucking any air in to compress, can you try filling the cylinder with compressed air as per a leakdown test?
That was exactly my thought, and why I was really pleased when the cam turned OK. It's obviously different with a Desmo than a normal valvetrain, and as you say you'd expect the closing lever to jam hard against the shim if a bent valve wouldn't let it close completely. The quick and dirty test for there being some clearance, spinning the shims with your fingers when there is no load on them, appears to show all is well. I can't get my Endoscope to see the very bottom of the piston top, because the head is too large, but I'd have thought that if the valve head had snapped off, even partially, and was rattling around in the cylinder there would be some damage to the piston top. That is a very good point, but I'd fitted an old belt to do the test and the valves were operational. The above really doesn't make sense, and I need to whip the head off for a look.
I've just thought, I'll pull the belt off the Vertical cylinder as soon as I get a chance and spin the Cam over by hand. That way I'll get a comparison of the feel of a known good valvetrain against the suspect one.
When you still have the belts connected - why not pull all 4 plugs and use the borescope to look at the valves from the plug opening - while turning engine over by hand??
Leakdown test? Fill cylinder with compressed air and see how quickly/from where the air escapes (compared to the known, good cylinder). You'll be able to see if air passes through the valves - if it doesn't then they are sealing properly, in which case they're not bent. Of course air/pressure will escape via the rings etc. but you should be able to see where from and how much, hence the reason to test the vertical as a comparison. If everything is free, valve clearances good, no visible dings in the piston and air isn't passing through the closed valves then you may not need to remove the head? though it maybe a wise precaution.
Unfortunately both plug holes point downwards into the cylinder, and I can't turn my endoscope head right back on itself to see the valves. It's head is a little large really for this type of thing. Thanks Dd I've got an old Spark plug somewhere with a tyre valve in it to use my tyre inflator and gauge as a basic leak down tester, but I think it's a bigger plug. I should knock one up with an old M12 plug to give it a go.
With the piston at BDC can u take the collets off the valve stem and drop the valve down into the bore for a better look ?
I saw on wheeler dealers .....Ant pressurising cylinder .....then listening in the exhaust ..... Intake ....and oil filler to figure if it was exhaust valve ....intake valve or cylinder bore leakdown .....
Still thinking aloud ......which is unsafe with my brain ......check inside valve covers for a foreign object....a missing clip from a rocker spindle .....etc ..... If a cam was momentarily jammed ....that would obvs be the start of this train of events .....
Worth a go, but I need to get the bike on my lift so I'm not scrabbling around on my knees whilst doing it. That will be at the weekend at the earliest. Yep, basically what Dukedesmo has suggested.
I'm not familiar with the DS engine (valve angle & plug location etc.) but on the 900 2v, you can see the valves through the plug hole**. You would also get a good view of the valves through the ports, especially the exhaust (most likely to have hit the piston IMO) and with the shims/retainers off you should be able to rotate them by hand to see any bend? That said, if a correctly timed belt was fitted when doing the comp. test so that the valves were operating and no pressure was measured then something's definitely not right so it's a head-off job to investigate. Edited to add *whilst they are opening*
A good description of leakdown test here; https://www.mobil.com/en/lubricants...vehicle-maintenance/how-to-do-a-leakdown-test
My 5p's worth is that if I understood correctly from earlier posts you have done a compression test with a spare belt fitted ? The rear, vertical cyclinder had around 110psi pressure, (assuming on a cold engine ?) & the forward horizontal cylinder had zero pressure. I would say that if the belt failed whilst the engine was running, even at tickover speed, at least one valve will be bent 99.9%. If you've had a look through the spark plug holes, at the tops of the pistons & there are no valves embedded in the top of either piston, only a slight mark where there was some slight contact that valve will be bent & not fully closing. It only needs to be very, very slightly bent so stop any compression building up. I would maybe try removing the belts & with the pistons positioned mid way up the bores you should be able to turn the two cylinder head cams & you should be able to feel some tightness in the suspect cylinder compared to the "good" cylinder. Ultimately though I think you are going to have to remove the suspect cylinder head to see exactly what you are dealing with. Hopefully you might get away with just a new valve or two, but you should also carefully check the two valve guides for any damage that might be caused by the valve stem. If you're going to do a leak-down test by pressurising the cylinders through the spark plug holes you need to be careful what position the piston is in on the cylinder you are testing because the compressed air can easily drive the piston down with some speed depending upon how much pressure you apply, which cause cause more damage ! Good luck & fingers crossed for a cheapish fix !!
Why not just take that head off? It’s four nuts as the belt is already gone anyway. Each of these longish thoughts about various tests take up more time than just removing the head. And you will have to do it anyway when you need to repair your unknown damage.
He can't look at it till the weekend. . We're all waiting ..... So.....until then. .... We're all twittering on.....
Sure, I learned it the hard way many moons ago: Don’t speak it, do it brings a lot of relief and saves tons of wasted time
I pulled the head today to see what the damage is. There is a definite mark across the straight edge of the Exhaust valve cut-out in the top of the piston: (Ignore the sideways scratches, that was me with a fibre rod down the plug hole the other day to show when the piston was going up and down) And a corresponding mark across the Exhaust valve head. Note the valve has spun round: The easiest way to see if there was any gaps between the valve head and seat was to turn the lights out and shove a small pen torch up the Exhaust port to see if any light escapes: Then spin the valve round 90 degrees to see if the light leakage moves, which it did: And then the light can't be seen with the mark towards the camera.
The stem of the valve is perfectly straight, but the head is canted over slightly. I put it in my lathe with a dial gauge and spun the chuck by hand. The result was a 0.65mm runout. Note that's on the bottom of the valve not the seat area, but it's good enough to show it's bent: The guide feels fine, with very little slack. Anyone got a spare Exhaust valve knocking about? It's a 7mm stem, 40mm head jobbie, @95.5mm long.
