lawn tractor - stubborn reversing gear

The reversing gear on my elderly (pushing 25 years now) lawn tractor's getting a little stubborn - at least while the engine's running; it seems happy switching between forward and reverse when the engine's stopped. Google turned up a few incidents of this same problem, but unfortunately I didn't see any actual solutions.
There doesn't seem to be any significant wear on the gears / dog clutches within the diff. Everything stops turning with the clutch pedal pressed, but I *think* what's happening (given the way it works OK with the engine stopped) is that pushing the clutch isn't fully releasing tension on the transmission (and therefore the input shaft to the diff). With the engine running, if I really haul on the forward/reverse lever while "tickling" the clutch pedal, it'll usually work - but I suspect that I'll eventually break something doing that...
Question is, are there any "usual suspects" to look at? I can adjust the rod which connects the clutch pedal to the arm containing the dual-cone drive clutch (which serves as the transmission's gearbox) so that it travels further and therefore slackens the belts more - but I'm reluctant to mess with it if the problem's likely elsewhere, as I assume that messing with that will also result in belts potentially slipping during normal drive.
Another possibility: The gearbox side of things operates fine, which means that the conical clutch is able to slide up and down on its shaft as it's supposed to, but maybe this isn't sliding as well as it should (and so although tension is removed on the belt which links the engine to the clutch, it's not being removed adequately on the belt which links the clutch to the diff)?
(the mower's an old MTD/Craftsman with 38" deck and 11HP B+S vertical- shaft engine, not that it probably matters as I bet the drive arrangement's typical for all sorts of lawn tractors, even if the parts are slightly different)
cheers
Jules
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On Mon, 18 Oct 2010 17:22:11 +0000 (UTC), Jules Richardson

How about low fluid levels in the gearbox/transmission?
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On Mon, 18 Oct 2010 10:47:09 -0700, Oren wrote:

Yes, no problem in the diff (which is where the reversing gear is). I've actually got two diffs - on one, there was some visible wear on the reversing gear, so I swapped with the other one. That seemed better for a few weeks - until my main drive belt wore out, which makes me think it may have just been coincidence / wishful thinking, and it was the belt starting to give out which was making it better (again perhaps hinting at a drivetrain tension issue) cheers
Jules
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On Oct 18, 4:12 pm, Jules Richardson

i had the cant shift problem, traced to rusty shafts where they entered the transmission, so i flloded the area with oil and all was well.....
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On Oct 18, 12:22 pm, Jules Richardson

When this problem occurs in an automobile with manual transmission, it is usually the transmission input shaft bearing in the flywheel. With age, wear and no lubrication, the bearing begins to behave like a sprag clutch and when the input shaft is turning the gears simply don't want to behave. Something similar could be happening in this case.
Joe
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On Mon, 18 Oct 2010 12:23:36 -0700, Joe wrote:

Thing is, there's nothing actually turning at that point - the clutch pedal decouples drive from the engine, and also operates a small disc brake mounted on the rear axle.
My thinking on the tension though was that the "decoupling" happens by moving the conical clutch toward the engine, therefore making the main drive belt go slack - but the engine pulley's still going to rub against the belt, so there's always going to be *some* tension passing all through the drivetrain as far as the diff. The slacker the belt is, the smaller that tension gets, but it'll always be there, because there's no way of completely removing the belt from the drive pulley.
There must be a fine balance I think between the belt being slack enough not to cause the reversing mechanism to bind, but yet still tight enough when the clutch is released to allow drive without slipping. But I'm hoping people who have had this particular problem and overcome it can comment, as it'd be nice to know whether I'm barking up the wrong tree or not :-)
cheers
Jules
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On Mon, 18 Oct 2010 20:05:33 +0000, Jules Richardson wrote:

Follow-up thought - I think I'm wrong there. I think that the clutch adjustment must dictate *only* by how much the conical clutch moves toward the engine (and hence how slack the belt goes).
Traveling in the "other direction", the clutch rod doesn't play a part - at some point, the clutch pedal fully engages the clutch and the "currently selected gear" linkage takes over, and it's this which says how far the conical clutch is able to move away from the engine (the distance from the engine, with the belt "biting", being proportional to the gear ratio due to the way that the conical transmission works)
It's dark and cold and full o' bears outside now, but I'll have to have a look tomorrow and see if that seems sensible :-) In my head it does, though. The upshot of all that is that I should be able to adjust the clutch rod as much as I want as a test - the worst that can happen is that I make the belt too slack and it comes off one of the pulleys, but it shouldn't disturb the gearing or belt tension.
(Ingenious little transmissions, these - amazing they work at all given the harsh environment that they're subjected to)
cheers
Jules
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