heck of a difference in milling a 2-stroke and a four-stroke, milling changes the installed height of the camshaft with respect to the cam drive, which in turn retards valve timing and ignition timing (if the cams also drive a distributor or crankshaft/camshaft position sensor).I had the head milled down 15 thousandths on my razz and it made a big improvement. Don't think I would go that far with the 400 but with all the engines I got why not have 5 or 10 taken off
Unless you can get some good gas,more compression won't give much return from the milling and resultant compression boost.Chain probs would be something to consider as well. lha
In these with a chain drive cam, assuming valve clearances were reset (obviously) I don't see how timing would be effected....but I'm willing to admit I have not studied the issue in great detail. The only issue i see is making sure the cam chain tensioner has enough take-up for the chain.
I think Jayel is right. Your valve timing AND your ignition timing would be slightly affected unless you re-degree the sprocket on the cam. Valve clearances shouldn't be affected at all.