Assuming you do NOT have a compression gauge..... check your oil!!! If you did an oil change and for some reason ended up 'not' getting the drain plugs properly sealed, you may have lost all your oil on your little spin....and in turn ended up seizing a valve in a guide (no compression). If you DO have oil, then I'd check you plugs next to make sure they're properly tightened. If so, when you kick over the engine, you can possibly hear a 'compression' loss...either through the exhaust ports, a spark plug hole, or back through the intake. If you killed ALL the compression, then it seems most likely to me that the valves hung up. You may be able to check this visibly by pulling the 4 tappet covers and just looking at your clearances. If you've got valves 'stuck' open, you'll see it. If the valves ARE moving properly, there's one other possibility..... a long shot, but possibly worth checking. Sometimes 'no compression' can be a result of 'no intake air'. I've seen mice and rat nests in air boxes before....and when not removed before running, they can either 'clog' the intakes, or a jam a valve slightly open. If you DO have a compression gauge and you've confirmed ZERO compression on both cylinders......yet it ran fine the day before....then something stuck 'open'. You can pull the air box boots off the back of the carbs and see if you're getting any 'suction' on the intake stroke. Finally, you can put each cylinder at BDC on the power stroke and apply compressed air to the spark plug hole and see if you hear it coming out through either the exhaust or intake side. If so, that will tell you which valve is leaking. Seems pretty unlikely that you 'cracked' both pistons or blew the head gasket on BOTH cylinders....but still possible. NOTE, do NOT apply compressed air at TDC, as the piston WILL shoot downward and this can possibly result in injury or damage.....depending on 'stupid stuff' like having a bike in gear!
Good luck, WC
