Bores on MK II should be Nikasil or Nigasul or however you choose to spell it. So that should not be a problem (unlike the dreaded chromed bores) How do the valves look regarding tappet adjustment? If they are near the bottom then the valve has recessed in the seat so it may be time for an unleaded conversion. I have never had clear answer on the unleaded conversion debate and it appears that most engines don't suffer, only a few cases. Me if I was having to renew valves and guides than I'd certainly have it done, but tearing it apart for that alone, not me anyway. Borgo's idea sounds good. If you've had the head off at anytime you usually need to retorque the head bolts a few times after several hundred miles as the gaskets bed in too.
I suppose the OP could try re-torquing the head nuts, but if there was compression leak from the head gasket, it would be pretty noticeable / visual, bearing in mind it had a 10.2:1 compression ratio (as opposed to the MkIIIs measly 9.8:1)........ Hopefully, I'm wrong but, using unleaded petrol means the valves stems and guides aren't being lubricated by the lead in 4 / 5 star, so they are prone to wear and tightening up..........these are effing big valves and they run pretty hot, which means that instead of closing properly, they are being held tight in the guides.........I expect the OP to find a bent pushrod. If rings had broken, I would have expected to see blue smoke from the exhaust.
In the workshop now, got the rocker cover off and nothing looks bent or broken but the valve clearance on the exhaust valve has grown. I checked/reset the clearances less than 500 miles ago, can't remember which were OK and which I altered (I think 2) but they were only very minor adjustments. I checked/set to 0.22mm as per the Haynes manual and the intake is measuring 0.24mm but the exhaust measures 0.54mm. The adjuster screw isn't loose so something is amiss?...
Bent pushrod because valve seized in the guide.......I expected it to be the exhaust valve side BTW, 'cos it runs hotter than the inlet. These are big valves with big stems........so I wouldn't expect the valve stem to bend before a pushrod will. What happens when you turn the engine over by hand? Can you judge if the valve gets full travel?
Engine turns over by hand fine, valve gear off now but seemed OK before (albeit with 0.3mm less valve opening). Pushrods look fine and move up/down nice & free. Just wondering why a too big clearance would give a low compression, the valve is (unless something else adrift) still closing...
Actually although the pushrods look straight there is a very slight bend on the exhaust rod, noticeable when rolling on a true flat surface. Don't know if that's enough to be significant?
Don't forget I'm only going by gut feeling and a bit of Guzzi experience but I reckon it's enough to tell you that the valve became tight.......don't forget the rods are pretty tough / springy and will bend more under load, only return to nearly straight..... Unfortunately that slight kink is enough to flex more under load from those really tough valve springs, thus causing rattle from the valve gear. Then when the valve gets hot and tightens up, compression is lost, plus the pushrod and rocker are banging away at something that isn't moving properly. I anticipate you won't see any indication of oil burning from the exhaust, because there is no blow-by on the valves stems when they have got hot and tight in the guides. The valve should close OK now because the engine has cooled down....... You may get away with a new pushrod and some lead additive in the petrol...... ......but I would have the heads off and have a really good look...... The valve guides are also easy to change.....Pity I'm not nearer to you......I would love to get my hands dirty on a Guzzi engine again.
Thanks for the advice, these are new to me (despite them being older than the ark!). I don't think the slight bend is enough to set the opening wrong though? as I have tried leaving a feeler gauge in the gap so that it's tight with no clearance and the rod will still turn freely with no tightspots - although at speed and when hot I don't know what happens??? Also tightened it beyond clearance so that the valve will open more and it still turns over (by hand) just as easily so valve opens freely, at least when cold. Not sure where to go now, may do a compression test with the clearance set to spec again, obviously if still low (and I can't see how it would have rectified itself?) then further investigation required....
I think the bores are cast iron? because when I had the sump off I did a magnet test on the inside and it stuck. So unless there's some other steel lining under the Nikasil (thought it was put onto aluminium?) then they're cast iron sleeved. As for the tappet adjustment, the exhaust is further in than the intake although I have now just wound it in 0.3mm more, here's a pic as it stands now with 0.22mm clearance; I don't know if the head's been off, bike's only done 12k miles so maybe not? but the head nuts don't move when I put 30lbs/ft on them so probably OK and there's no sign of any leaks.
FWIW with the clearance reset to spec the compression is still way down, tested again (this time cold); RH - 145psi LH - 45psi
Al is offering a lot of very sound advice based on experience. I disagree a little on the steel gear (coz I'm running a set) but apart from that I'm in agreement with it all Did you check head bolt at the spark plug and also opposite the plug on the inside, access is covered by a hex plug Bore material, this might help http://www.thisoldtractor.com/guzzitech.dk/pdf/identifying-cylinder-bores-on-roundfin-guzzis.pdf Hard to say from that angle but it does look like your getting a bit of recession on the exhaust valve (it will always be the one) Taking the head to pieces is easy and a couple of hours will see it to bits and back together. Removing the head can be a bugger as that hex key mentioned earlier is usually corroded in and needs some heat plus a breaker bar to loosen. Heads info Cylinder Heads - Technical : The complex stuff - exciting but heavy reading, take your time. - guzzitech.dk archive - courtesy of Jens Lyck I'd replace the pushrods, I don't think they are that expensive and I was told the standard check is rolling them on a glass table or a surface plate, if they wobble then it replacement time. Engine strip info Engine strip - with pictures - Technical : The complex stuff - exciting but heavy reading, take your time. - guzzitech.dk archive - courtesy of Jens Lyck These were all written by a guy who is a bit of God in the Guzzi community so its all good info
FWIW did you do the check WOT (wide open throttle)? I assume you did judging by the results. Did you also check that both carb slides were up fully? If the left is down a bit, i.e. out of balance, then it will have an impact, not that much I guess but its worth checking all this before you tear it all to bits.
I think you will find that it is easy enough to set the clearance correctly with a slightly bent rod, but as soon as it is under pressure from the valve springs it will flex more.....then, as it gets hot it will expand in length slightly, but also bend more if the valve is getting tight. As yours is a 1980 model (probably made in 1979, therefore it will be the same as mine) it will almost certainly have Nikasil plating on the bores.......Hopefully nobody fitted chrome rings and it doesn't seem as though they have, otherwise I expect the engine to have seized a bit.......... .......but don't forget, these engines aren't run in until they have done about 10,000 miles. Maybe a burnt valve seat is the issue......again, unleaded can cause that sort of problem.......... Thinking about a worn valve guide, it would really only let blow-by/loss of compression occur if the valve wasn't seating properly. Whatver the issue is, it seems that 'heads off' is going to provide the answer. I'm currently looking for an in-depth write up on the MkII which covered the spec in detail...........I know I have it somewhere, but F knows where
This what you're after Al? The original "base" manual for the 1000 SP most of the others up to the Daytona were an addendum of this http://www.thisoldtractor.com/mg_manuals/workshop_manual_g5_sp.pdf The MK II additions http://www.thisoldtractor.com/mg_manuals/workshop_manual_850-le-mans-ii.pdf
Thanks for the info. Yes compression tested at WOT and both carbs are fully open. It is quite possible that the compressions were different on the cylinders before as I didn't actually test them, but I doubt that it was as much because it was running very well and would idle cleanly at under 1000rpm even when cold and without the choke after the first 30 seconds or so.
Valve looks OK from down the exhaust port. I didn't check the nut under the hex plug, 'cos the plug won't shift even with a 2ft long bar on the socket but I doubt that is the problem because the other 5 are all tight and I really don't think the head's ever been off - will be coming off now though. As for my frustrations I'm going to take them out on the Monster...
Yes and no.....these are the OEM ones more or less, but actually I had a genuine Italian OEM one and genuine Parts Book, specifically for the Le Mans II........ What I was thinking of was a thorough really in detail write up by Chris Dabbs.....but I still can't find it....... This might be usueful though....... Moto Guzzi 850 Le Mans an option for English as well. I don't recall ever having an issue with the hex plug thingy........the first time I had an engine apart was on the bench and although I must have used an extension bar, it certainly wasn't a breaker bar.....it came undone quite easily........Just lucky I guess. Thinking about the problem a bit more, I wonder if a valve stem got bent which has had the effect making the valve seat on the skew.......... After all, if a pushrod is bent, there is no reason why a valve stem couldn't have got bent as well... .......I hope I'm wrong, but IMO it has to be in the top end somewhere, rather than busted rings or scored bore otherwise I think the smoke would have been obvious.
