I like to run plenty of end gap on my piston rings.  I used Wiseco’s recommended gap for “Street-Moderate Turbo/Nitrous”.  For a 3.820” bore you end up with .019” for the top ring and .021” for the second ring.  Better safe than sorry, it’s an air-cooled engine.

Installing a cam that closes the intake valves later will help reduce cylinder pressure.  The cam holds the valves open as the piston starts up on the compression stroke.  At lower speeds, this allows some of the pressure to bleed off through the open valves.  As engine speed increases, the inertia of the incoming charge prevents this bleed-off.  Since detonation is more prevalent at lower engine speeds, a cam that allows some bleed-off helps to mitigate detonation.

The OEM DR650 cam I was running closes the intake valves at about 44° ABDC.  I have a Web 402 grind that is supposed to close the intake valves at 55° ABDC.  It also has significantly more lift.  

Web doesn’t specify performance springs for the 402, but with .395” lift it takes the stock valve springs to the point where coils are touching (the early stages of coil bind).  Web installation instructions specify a minimum of .010” clearance between coils with the valve at max lift.  Stock springs aren’t gonna work with the Web 402.

Then install the degree wheel and get it adjusted correctly at TDC.

I don’t have factory specifications for the OEM DR cam.  All I have is data from checks I did many years ago.  The DR cam agreed with my previous data.  Intake opened at 10° BTDC and closed at 44° ABDC.  Max lift was .380”.

The Web 402 opened the intake valve at 10° BTDC and closed the intake at 50° ABDC.  Max lift was .395”.  The cam was well shy of the advertised 248 duration (240 vs 248).  Oh well, I want that intake valve closing significantly later.  Solution, retard cam timing about 5°.  Armen figured out a long time ago that there are 22 splines in the cam drive sprocket but there are only 19 teeth for the chain.  So, if you move the sprocket one-spline clockwise and the chain one tooth counterclockwise you should retard cam timing by 2.6°.  If you move the sprocket two-splines clockwise and the chain 2 teeth counterclockwise you retard the cam timing by 5.2°.   It gets a little more complicated if you want any more than a 5° change.  This simple table might prove helpful to some of you.  I’m sure there are more possible combinations, I’m just too lazy to figure them all out.

Now the intake valve opened at 4° BTDC.

While the checking springs were still installed, I did clay checks to verify that the valves weren’t getting too close to the piston.  It’s a bigger cam with more lift & overlap, and I have retarded the timing.  Best make sure that the valves have plenty of room.  No problem here, the valves didn’t even touch the clay.