Advice/oppinions on intake lobe centerline location

xsjohn wrote;
From my experience with the 447 XS650 cam timing ....if the cam is advanced even a degree or especially 2 or even worse more than 2 beyond the factory specs the engine will be forced into artificial leanness where the cylinders don't fill completely because of the early closing event of the intake valves and the problem is especially noticeable at lower rpms...(these cams are already advanced to the "absolute" maximum and if advanced it becomes totally """un tunable""" although it will run but will appear lean even if the fuel is dumped to it...and especially at the lower rpms as I mentioned before.....this one I now from experience....).....a bit retarded moves the power band upward and can result in some loss of lower rpm torque.......never felt the need to experiment with retarded cams in these because I am not a rpm freak with these old pressed crank engines....... as the cam chain stretches these become slightly more retarded anyway.........



search 650 garage for the fine details on decreeing ....and maybe some other sites to

and as close to the wall that I have found the the advance on the stock cams to be it may be of some benefit to some to move the stock cams in the direction of retard instead of just dumping the fuel to them....or buying super expensive aftermarket cams with higher lift and adding heavy springs that just beat the engine to death and sound like marbles in a trash can .. Question .......

and on top of that I would bet that these are advanced just a degree or so too much anyway.......and even stokers could benefit from a tiny tad of retard especially at lower rpm's to negate artificial leanness which makes them run hot and harder to tune...and lean down there..instead of just adding fuel to wash out the rings and cylinders.

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TwoManyXS1Bs said:

Hey, Josh, this could be a maddening era in your build. I often obsess over precision, like Monk separating/organizing the peas and carrots, but cam timing on an XS could cause a meltdown.

There is a limit to the precision of the grind, and duplicating open/close events while adjusted to the entry/exit ramps can be frustrating. When rocker faces wear-in, these entry/exit ramp timing values change. Then there's the camchain, which can stretch over time to give you as much as 20° retard. Even the initial break-in stretch, and torquing of the head bolts/nuts will retard you a couple degrees.

It's amazing, but these engines will often still run with the cam off by one tooth.

There's some good info on MMM's site, and some buried here. A lot of good info is out there, but even more isn't, racer secrets, and the 'net didn't exist back then. You may come across dyno charts, identifying some of the components involved, but not the precise cam timing.

Then there's the build combo, intake/exhaust/head/fuel. Is it the best for that cam and the intended usage? But that's old song stuff for you.

In a way, it kinda boils down to 'good `nuff', with maybe a couple degrees advanced to anticipate initial break-in, or planning teardowns every month to get it precisely.

Wouldn't it be cool to be able to quickly and easily adjust the cam timing, not just centers but all events, from an external adjuster, and have a dyno sittin' there waiting to be used?

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