Cam timing on a 750 GT

J

Jotabene

New member
I suspect I have got the cam chain one tooth wrong when reassembling the engine. The engine runs but the sparks are totally black and it seems to "spit black sludge" from the exhaust. When measuring the top lift position it seems to be 110 degrees instead of 103.5. how much can one tooth wrong give in degrees?
Anyone having an experience of this?


 
From memory it is about 15 deg per tooth.
Out one way it'll run like a diesel, out the other it'll be really
weak down low. I have seen both.
 
It's not hard to work out. I'm not sure how many teeth on a 750 cam chain sprocket, but if it's 30 (same as the triples) then a shift of 1 tooth changes the cam timing by 12? at the camshaft or 24? at the crankshaft. An error of 6.5? (on a crankshaft degree wheel) is about 1/4 of a tooth. So your problem is not the chain on the wrong tooth. Have you had the sprocket off the shaft? If so, did it go back in exactly the same spot? Maybe the sprocket was flipped over or there's enough clearance in the bolt holes and shaft key to allow 3? movement in the cam sprocket.
 
Quite common to have one of the 2 camshafts out by as much as 10? compared to the other... check a lobe on the other cam to confirm your first readings.  13? at the cam was the largest fault I've found to date.  I suspect this to be down to differing tolerances across production batches.

"Easy" remedy is to make a stepped key to get both shafts back in line.  As the key has no part in transmitting any torque, it can be fitted in halves, makes it easier to adust just one shaft.  Also eases installation in the head.

Have also found phasing faults in 750 cranks, not so easy to repair.

piet
 
You must log in or register to reply here.
Back
Top