Yamaha YZ490 head modification

Cylinder head: The head is prone to warpage. Look for black gas trails across the copper gasket. You can lapp the top of the cylinder and head to remove the warpage. I like to countersink the stud holes to prevent “mushrooming” of the stud hole that can cause a leak. Regarding the head dimensions, if you have access to a lathe with a 3-jaw chuck, you can use a spark plug mandrel available for $25 from Goodson, a large aftermarket automotive tool supplier. Then narrow the width of the squishban by adding a blend angle to the combustion chamber. The bland angle should be 30 degrees and start 8mm from the edge of the squishband.
Car jetting: That carb was a mess. We used these baseline settings for super-unleaded premium (no ehtanhol!) at 80 degrees Fahrenheit, and 40:1 with Spectro. 40 pilet, 3.5 slide, R-0 needle jet, needle in the middle clip, and a 440 main. The pilet jet, slide, and main jet are leaner than stock but the needle jet is much richer. Use stock reeds because Boyesen dual stage make the bike surge at low rpm and mimic a lean condition. Make sure that the hole in the slide is uncovered by the cable retainer and that the rubber seal in the top of the cap is in good shape. Replace the needle if it has any ridges down the length, those are wear marks caused by vibration. The slide can be filed to a 3.5mm cutaway height so you’ll need to buy the R-0 needle jet, pilet and main. That will cost you under $25. DGY probably has the parts in stock Tel. (630) 971-2602
Air leaks: All the fancy head mods in the world won’t help it if the engine has air leaks. Check the intake manifold for dry rotting, the left crank seal for leaks (pump gas with ethanhol causes swelling of the seal and leaks), cylinder base gasket leaks, and seal the front of the exhaust pipe with RTV high temp silicone.