Originally Posted by mattcfish
Actually the timing difference is where the key slot is positioned for the cam sprocket. It's just a hair difference. The slot is 3 degrees retarded which advances the valve timing by 6 degrees (cam spins half as fast as crank). There are several different opinions as to why this was done. Emissions regulations, to increase mid-range, or to improve idle. Your slotted oil pump, points style cam probably has strait 0 degree valve timing. Not sure, but I think the advanced cams came out with the bean cans.
I'm using the later bean can 336 on my /6. I've got light 9.5:1 1000cc pistons in Nikasil cylinders, titanium valve retainers, dual plugs, crank ignition, 1983 tranny with ultra light clutch/non-existent flywheel , 32/11 rear end , 2 into 1, and 38mm Dellortos (with spot on jetting). Gotta say I love this combo with the 336.
Cam specs are all at the crank. A cam that is advanced three degrees is advanced three degrees at the crank (and valves). Six degrees at the crank would adversely effect the exhaust timing too much by most tuners' opinions. Most tuners consider five degrees the absolute max to advance or retard a single cam. Most don't go past three degrees (at the crank). That's 1.5 degrees at the cam. Advanced or retarded at the cam? That all depends if it is gear or chain drive but most all tuners call what ever it is doing at the crank how they are modifying the setup at the cam.
Advancing the sport cam had nothing to do with emissions since the cam was never in a regulated bike. Advancing the 308 had to do with emissions but . . . . I don't know anyone that has changed a cam timing for idle. The timing was changed to lower peak torque around 500 rpm. Most single cams can be retarded and advanced three degrees for a total of six degrees to play with in order to move peak torque around about 1000 rpm.
As I understand it, they advanced the 336's timing before they changed the oil pump drive so this one could be either straight up or advanced.
I have a 336 in my '92 R100 which includes the later, lighter clutch and single row timing chain. 9.5:1 pistons. .025 off the heads. Titanium valve spring retainers. Custom made 4130 pushrods. 44mm intakes with modified valve head/seat ID ratio. Heavily re-shaped ports. Dual plugged with stock bean can and re-mapped ignition curve. Completely re-jetted 38mm Dellorto carbs. Heavily modified airbox. Staintune Sport mufflers. 33/11 FD. The thing makes WAY more power EVERYWHERE in the rev range including at least 1000rpm on top that it never had to start with! I love it!
I ran mine for over ten years without raising my stock 8.5:1 CR or much port work at all with great improvements from around 2500 rpm on up but now I wish I had raised the CR and ported my bike like I did just recently a long time ago. The thing is now running a LOT better than it was and it was already running a lot better than stock!