I reassembled it using a genuine Suzuki DR650 cam with stock valve springs.  I was able to retard the cam timing 6° and ended up with the intake valve closing at 50° ABDC.  The cranking pressure was now 180 psi (that’s a mystery, possibly attributable to reusing the rings).

The odometer is currently reading 5331 miles.  I have put about 2500 miles on the new setup.  It is running excellent.  Power seems to be exactly as it was with the 402 cam.  It’s also very quiet.  No oil consumption.  Fuel consumption is averaging about 55 mpg.  This seems to be a very good combo.  Retarding the cam timing seems to work out well.  Guess I got lucky.

I will update this post periodically to fill you in on the project.

Hey Dave, do you have any photos of the combustion footprint on the top of your 94mm flat-top?

Thanks for the photo Dave.  I think that the combustion deposit pattern might be useful in analyzing detonation problems.  I'm curious why the deposits don't accumulate around the periphery of the piston top.  Looks like you have even less deposits than I do.  The periphery is the quench area and as such the charge is forced out, but I think there should still be some combustion taking place in that zone.  Increasing the deck height to lower my compression seems to have caused the deposits on my piston to fill in a lot of the areas that were previously clean as a whistle.

I've been snooping around on the web trying to see if there is any useful info on interpreting combustion deposits.  So far, all I have been able to find is stuff related to oil consumption or overly rich mixtures, nothing related to detonation.

I was able to find some photos of deposits on Milwaukee 8 engines.  They seem to accumulate in a pattern similar to ours.  Those engines have compression ratios upwards of 10:1. I included a photo of some M-8 pistons.

My engine is running very well, and I have no reason to open it up, but I want to learn as much as I can from this project.  

Mike:

I wonder if the piston skirt has anything to do with it - perhaps the piston top is cooler where it is right above the piston skirt.

Finn:

Adding a second spark plug is far beyond those of us with amateur machining skills.

Perhaps the igniter from a DR650 would work....they use 2 spark plugs on the later version.

Dave