Just had a routine service on the 900SS - oil, fuel filter, cam belts, hydraulic fluids. Air filter and spark plugs were fine and not replaced (they were done 2000 miles ago). Bike running nicely, particularly notice the clutch being smoother after the dust was blown out. During the belt change, the mechanic mentioned that the valve clearances required attention. I was a little surprised by the method - he spun the horizontal cam pulley and said it felt tight. I also had a feel and while it spun freely it felt a little notchy. While I don't doubt the mechanic, can anyone explain how this is the case? I elected to wait a few hundred miles (mostly as I'm skint, and it's apparently a 4-5 hour job). Wish I had the ability to adjust the valve clearances myself, but without a proper garage and tools (and skill) I'm a bit stuck. Surprised about the frequency - the bike last had the valves done (before my ownership, at Baines) at 10,400 miles, and the current mileage is 14,200. Guess I'm used to my Honda Bros, where you can ignore the valves for up to 10,000 miles.. Cheers, Humbug
Well, I don't know about anyone else, but I think what he said about the clearances was bollox. I'm still trying to work out how turning the cam pulley can measure the gap unless there is no gap.......even then, with no gap it will still push the valves up and down, so it will feel a bit 'notchy' anyway.....it's a cam and there is a spring on one of the rocker arms, plus the horiz one is driving the rev counter as well..... And if you are sure about the mileage and that it was done by Baines the last time, then 3800 miles is sod all. That's my opinion and I'm sticking to it...... AL
You can't measure the gap, but you can 'gauge' it....... Opener no, but you can feel if the closer has gone tight because the rocker is rubbing the cam. The only way to be sure is of course to open it up, but I hazard a guess from what's been said the closing clearance has gone tight.
If there is a spring holding the rocker against the cam, then it won't make any difference if the gap is 10 thou or 5 thou..... ....unless of course someone can measure an air gap with the feel of their fingers.....which I doubt..... Of course, I may have got it all wrong how the desmo system works; and as the shims and rockers wear, the gaps get tighter instead of wider...:wink: AL.
The spring is holding the rocker against the shim. If there's any clearance then you won't feel the drag. If the clearance is very tight, you can't get a full rotation on the cam as the assembly locks up as the valve seats tight. That's exaggerated, but you can feel the drag if the clearance is tight ie zero. If its left, then you get lobe wear and/or follower wear. you measure anything unless its opened up, but I don't think that's what the OP has been told. I'd guess the tech has felt resistance on the cam and that could be tight closing clearances
I had a good dig around here and .ms before posting, and couldn't find anything close to an answer, so thank you both for responding. Arquebus - I have all the MOT certs for the last few years, and I'm guessing the mileage is correct, though you never know if a speedo cable has snapped for a while. I bought the bike in April last year. Nelly - I've been struggling to think of a way of describing what I felt when I turned the cam..apart from a bit notchy (without feeling tight). I'm guessing that you know the previous owner (policeman, lives in Mansfield, has a 749?) though possibly not the bike, as he serviced it himself. My guess is that the tech has done enough valve clearances over the last 10 years to know when one needs doing..and the question I've got is whether I try and squeeze in the valve adjustment before I make a trip to Belgium on the bike at the end of the month (about 400-500 miles), or wait until I get back. I'm supposing though that nobody's going to be willing to say either way!
Sorry all...I still think it is bollox.......From someone who was commended by Moto Guzzi at the factory, for getting their engine to run better than they could.... I'll butt out now... AL
I had shims go out of whack in Portugal on my 907, It started running rough halfway through France getting more and more difficult to start and not ticking over. Ended up getting recovered from Lisbon. Carole Nash was great though, had us on a flight the next day, the Monday after the MGP with taxis too and from the airport all arranged, bike arrived home 2 weeks later. I thought I'd holed a piston or 2 as I was running cans without a chip or I'dve got it fixed.
I'm with Arq on this but wouldn't put it quite so strongly. Maybe I am missing something but wear on both opener and closer mechansim (Cam lobes/rockers/shims) will increase clearances not decrease them. Conversely, valves hammering into seats, as they all do eventually, will increase closer and decrease opener clearances. Therefore I don't see how the closer can become tight with use unless it was incorrectly shimmed in the first place. hope this helps
I'm like Steve R, helper springs removed, but I don't fit them back mine are true desmo. Raced for the last 4 years like that with no probs at all. I also just do it by drag feel.
Generally over time, closer clearances get bigger and openers get tighter. This is one reason you can set closers tight knowing they won't get any tighter.
And i found that a new set of half rings will bring the closing tolerance back. ...as said.. Good feel. They get flattened over time.
Sorry to possibly muddy the water further but why no increased drag on a closed down opener Neil? If it's closed down purely due to valve recession then the valve has 'somewhere to go to' as the closer clearance will be increased at the same time but it will still be 'fighting' the closer spring as it holds the valve off it's seat? I know you have built more units than I have or possibly ever will, but unless i'm going mad I think I used the very indication i.e. slight 'binding' to hazard a guess of closed down (i.e. zero (or even minus) clearance) on an opener in the past. Seems to be the first thing to happen on a 2 valve depending on mileage/neglect and usually the exhaust first. Sorry AL - not taking sides here, and I would not be confident enough to diagnose on 'possibly increased binding' alone but it would give me enough reason to pursue clearance checking.
I said I was butting out......because in my mind, wear takes place, therefore tolerances are greater.... ...but bearing in mind what Dukedesmo says above, if that is the case, maybe Humbug ask his mechanic which clearances have got tighter....openers or closers. I'm really a valve spring and collets type.... AL
You can get drag on the opener Chris. Its much less common though. Two scenarios........openers opening up and closers tightening.... Mainly due to carbon build up on the valve seat. As the valve is moved away from the seat, or down, however you want to visualise it, then the closing clearance is taken up and the opening clearance widens. Zero clearance on closing rocker means it will rub the cam lobe and you can feel it or sometimes can't fully rotate the cam pulley. This is the most common I see... Second scenario...... Half rings flatten and the closing clearance opens. As mentioned, most of the time simply replacing the half rings brings it back in spec. Openers tighten due to valve sinking in seat, extreme scenario burning of the seat/valve. If it tightens up enough, the follower rubs on the cam. There is reduced rotation of the pulley and again, you can feel it. Less pronounced than the closing scenario, but still detectable. older 2V carb bikes tend to build up on the seat. The newer stuff, especially with narrower seats tend to burn in. Everyone is welcome to their opinion and many will be different. the OP asked a question and i gave mine from my experience. It happens, and you can pick it up. It's an easy check if just doing belts and no valves as I've often been asked to. Next time you're checking them, drop a feeler gauge in between the the shim and the rocker to take out the gap plus a thou or so then turn the cam.....
as I said, I don't doubt your experience for a minute Neil. I have come across deformed/worn collet split rings of course and it's a blessing when that's all it is but have encountered more due to valve recession myself (2 valve).
I've just been sitting here for half and hour studying a working animation of the valve gear............... I still can't see that the gaps can get tighter..........the closer operates against the closing shim, therefore something wears (the shim most likely) and the collets can compress......it seems to me that in all those cases they have the effect of allowing the valve to drop lower and not shut fully. With regard to the opening shim and rocker, if the shim wears (or something does) the gap is greater and the valve doesn't open as far as it should. AL.
If if the closer shim or, more likely the collets, wear then the closer clearances open up but if the valve seat or valve wears then it will sit further into the head meaning the opener clearance tightens as the valve is closed that little bit more.
Yep, I follow that.....so what wears first....the shim or the valve seat...... What you are telling me is that it may not be shims and the gaps, but something more serious..........therefore piddling around with shim/valve adjustment won't really cure it. I'm glad I'm good with springs and collets....:wink: AL
No worries Chris. I follow what you're saying. The closing spring will act on the closing shim and pull the valve in to the seat as tight as it can get. Although the clearances are measured at the shims, what's actually being measured, albeit indirectly, is clearance at the cam lobes. If the valve, shims and half rings are viewed as a single assembly that's suspended between the bounds of the two followers with clearances on each lobe allowing 'float' then you will have no drag at either end. Ignoring any wear to cam lobe and/or follower, if the valve head moves into or away from the valve seat or the half rings deform, then the valve assembly will move within the confines of the followers which are effectively fixed by their contact with the cams. If it moves far enough then one of the followers will lose its gap and remain in contact with its relevant lobe. There's the drag... if you add in a fairly heavy spring, forcing the valve assembly hard in to the seat, then this movement is exaggerated and the drag is amplified as your turning the lobes against the pressure of the spring pushing the tight shim on to its relevant follower and hence lobe. There is of course mechanical wear also in the equation, but IMO most of the adjustment comes about from the valve seat and half rings.
