Superdunc Posted August 7, 2013 Report Share Posted August 7, 2013 Not my bike thankfully, but one I've been asked to look at. 748r with a low speed misfire, leak down a little high @ 15% on front cylinder, inlet valves leaking slightly, compression test 50psi down. Problem. Bore scoped front cylinder, hmmm, definite signs of exhaust valves hitting piston, time for a strip down. Valve clearances on the baggy side, but most in spec a couple slightly out. Cam timing correct, all marks line up correctly. Head and barrel removed no damage to rings or bore, big ends ok we think but will need looking at. The top of the piston though is very strange, no detonation, or signs of contact on the squish areas, but the exhaust valve pockets, appear to have been hit repeatedly by the exhaust valves, to the extent that they have score marks from where the valves are rotating and machining deeper pockets in the piston, around 5thou. I have never seen anything like it. Any ideas what is going on here. Link to comment Share on other sites More sharing options...
theo Posted August 7, 2013 Report Share Posted August 7, 2013 Its not helpful but Ill start the ball rolling with the exhaust valves have been hitting the piston. Thats where your problems lie. Link to comment Share on other sites More sharing options...
l3ddy Posted August 7, 2013 Report Share Posted August 7, 2013 Some sort of problem caused by over revving the engine? Link to comment Share on other sites More sharing options...
Superdunc Posted August 7, 2013 Author Report Share Posted August 7, 2013 Some sort of problem caused by over revving the engine? Desmodromic, positive closing valves. My first thought when we bore scoped it was that the closing rockers would be completely knackered, which isn't unheard of on these engines. @Theo, I can see why they gave you that fancy job now. Link to comment Share on other sites More sharing options...
Chrisball Posted August 7, 2013 Report Share Posted August 7, 2013 Not sure of the cause, doesit run the standard R cams?? But somthing that may help, i have a brand new set of Valves for 748R/RS. (2inlet 2 Exhaust) Sat in there ducati bags that i bought as spare when i had my 853R racebike. I may also have a pair of pistons, but would have to check at weekend! Let me know if they are any good to your customer. Link to comment Share on other sites More sharing options...
Superdunc Posted August 7, 2013 Author Report Share Posted August 7, 2013 Not sure of the cause, doesit run the standard R cams?? But somthing that may help, i have a brand new set of Valves for 748R/RS. (2inlet 2 Exhaust) Sat in there ducati bags that i bought as spare when i had my 853R racebike. I may also have a pair of pistons, but would have to check at weekend! Let me know if they are any good to your customer. Will do ta. Pm me a price, and let me know if you have any other bits for the 748-996 bikes up for grabs. Link to comment Share on other sites More sharing options...
2moto Posted August 7, 2013 Report Share Posted August 7, 2013 My guess, and this from my experience with 996 race motors, is that the exhaust valves will have a upturned lip on the valve head. This is from hammering the valves onto the hardened seat at high RPM. This would explain all the effects you mention. You will most likely need 4 new exhaust valves and a valve seat regrind. Pistons should be OK. Link to comment Share on other sites More sharing options...
Superdunc Posted August 7, 2013 Author Report Share Posted August 7, 2013 Had another look at the bike tonight on the way home, it's at a mates shop, and a few more things have come to light. The bike has never run very well in the time it's been with its current owner, previous owner had some work done on the front pot...... My mate has spoken to Rich at Lougimoto, who reckons that the wrong combination of head and base gasket are fitted, 0.4mm base gasket and thin head gasket. I'm amazed that a 0.6mm thinner base gasket is enough to cause the valves to hit the piston. I've told my mate to refit the piston, base gasket, and barrel, then measure the deck to piston top height. You can see in the picture that the valve has actually machining away the top of the piston, using bits of carbon as a grinding paste. The valve seats and the seat on the valve seem ok, no lip. I've advised that the big end bearing should be check also. Link to comment Share on other sites More sharing options...
2moto Posted August 7, 2013 Report Share Posted August 7, 2013 So what the cause for low compression and leak down? Looks to me that the valve to piston clearance was never checked. Link to comment Share on other sites More sharing options...
Superdunc Posted August 7, 2013 Author Report Share Posted August 7, 2013 I'm sure the valves are no longer seating correctly, which is causing the low compression reading, when I stated the seats looked ok, I meant no obvious wear or lip. The engine is supposedly standard so you wouldn't normally expect to check valve to piston clearance, when replacing like for like. If it was a standard 748 which someone had fitted 748r cams to I could understand. I think it is probably a result of a number of problems. Link to comment Share on other sites More sharing options...
V6Jon Posted August 7, 2013 Report Share Posted August 7, 2013 I know naf all about engines in the grand scheme of things having never got more into one then clutches and clearances, however I was reading an article on the net today about small block Guzzi's and their inherent problems as I have one that I plan on cafe-racer-ing at some point. The article mentioned an issue small Guzzi's suffered with where the valve stems stretched and ultimately dropped the valve head lunching the motor, signs were a loss of power as compression was lost as the seal deteriorated. Not sure if this could be what is happening here, but with it being fresh in my mind I thought it worth mentioning. Jon Link to comment Share on other sites More sharing options...
Superdunc Posted August 7, 2013 Author Report Share Posted August 7, 2013 I know naf all about engines in the grand scheme of things having never got more into one then clutches and clearances, however I was reading an article on the net today about small block Guzzi's and their inherent problems as I have one that I plan on cafe-racer-ing at some point. The article mentioned an issue small Guzzi's suffered with where the valve stems stretched and ultimately dropped the valve head lunching the motor, signs were a loss of power as compression was lost as the seal deteriorated. Not sure if this could be what is happening here, but with it being fresh in my mind I thought it worth mentioning. Jon It's a good call, but stretched valve stems show up as very tight/no valve clearances. The opening clearances were on the loose side. Link to comment Share on other sites More sharing options...
CRM Posted August 7, 2013 Report Share Posted August 7, 2013 twisted crank perhaps ? i know a few early 749s did this party trick and while would run, could bounce around the workshop on their sidestands like a Buell. Link to comment Share on other sites More sharing options...
2moto Posted August 7, 2013 Report Share Posted August 7, 2013 Sounds like someone tried to raise the compression by installing thinner head and base gaskets, which caused an ever so slight interference at high RPM. Doesn't look too bad. It may even be that all parts are salvagable, just careful checking and setting up required. A bit of machining of valve seats, etc. Link to comment Share on other sites More sharing options...
bigkev Posted August 7, 2013 Report Share Posted August 7, 2013 Are the cams machined from solid or are the lobes pressed on? I've seen an engine do something similar and the timing looked spot on on the cam wheels but the lobes had been knocked ever so slightly due to previous piston/valve interface. Link to comment Share on other sites More sharing options...
lorenzo Posted August 8, 2013 Report Share Posted August 8, 2013 I know it's fundamentally Italian but it's been assembled in Hinckley, has it? Link to comment Share on other sites More sharing options...
franky Posted August 8, 2013 Report Share Posted August 8, 2013 Not my bike thankfully, but one I've been asked to look at. 748r with a low speed misfire, leak down a little high @ 15% on front cylinder, inlet valves leaking slightly, compression test 50psi down. Problem. Bore scoped front cylinder, hmmm, definite signs of exhaust valves hitting piston, time for a strip down. Valve clearances on the baggy side, but most in spec a couple slightly out. Cam timing correct, all marks line up correctly. Head and barrel removed no damage to rings or bore, big ends ok we think but will need looking at. The top of the piston though is very strange, no detonation, or signs of contact on the squish areas, but the exhaust valve pockets, appear to have been hit repeatedly by the exhaust valves, to the extent that they have score marks from where the valves are rotating and machining deeper pockets in the piston, around 5thou. I have never seen anything like it. Any ideas what is going on here. Rod stretch? Does it get revved properly? Link to comment Share on other sites More sharing options...
Chrisball Posted August 8, 2013 Report Share Posted August 8, 2013 Rod stretch? Does it get revved properly? Cant see it with a road bike and Pankl Ti rods. Link to comment Share on other sites More sharing options...
hotballs Posted August 8, 2013 Report Share Posted August 8, 2013 it could be possible that the cam timing is out. Ignore the timing marks and check it with a dial gauge. maybe someone has replaced the cams at some stage and installed a different cam or simply the cam pulley , in which case it will be out. I for sure reckon that a few thou wont make the difference between the valve hitting the piston or not, plus its taken 5 thou out of the piston, which makes up the gasket thickness, and there is no way the valves run that close to the piston. Even with high comp pistons there should still be about 2 to 3 mm clearance at the closest. Is it possible that the pistons have been replaced with the wrong type in the past, perhaps with a lower gudgeon pin hole? i would measure the other cylinder and compare Link to comment Share on other sites More sharing options...
hotballs Posted August 10, 2013 Report Share Posted August 10, 2013 also a thought, these sometimes have offset woodruf keys on the cam pulleys, which can be installed the wrong way around. Link to comment Share on other sites More sharing options...
Superdunc Posted August 10, 2013 Author Report Share Posted August 10, 2013 also a thought, these sometimes have offset woodruf keys on the cam pulleys, which can be installed the wrong way around. This is something that I've already suggest that should be checked. Good call. Link to comment Share on other sites More sharing options...
Superdunc Posted August 12, 2013 Author Report Share Posted August 12, 2013 Right the culprit has been found. The bottom cam belt pulley is has some backlash on the intermediate shaft, causing the cam timing to float. The customer has decided that he doesn't want to spend any more money at the moment to investigate further.... Thanks for the advice and offers of parts. Link to comment Share on other sites More sharing options...
franky Posted August 12, 2013 Report Share Posted August 12, 2013 Does he want to sell it as is? Link to comment Share on other sites More sharing options...
Superdunc Posted August 12, 2013 Author Report Share Posted August 12, 2013 Does he want to sell it as is? Back off, I've already made an insulting offer for it. The owner still has 2 other 748Rs so I suspect this one will get broken for spares. Link to comment Share on other sites More sharing options...
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