Motorcycle Repair: 1986 honda rebel 450, honda rebel 450, exhaust gasses

Question
Both pistons come TDC at the same time, does that mean i set the intake and exhaust valves at the
same time on both cylinders? How do i know which cylinder fires first? Please help i have never
adjusted valves when two pistons come TDC at the same time?    Thanks Ronnie

Answer
Ronnie,
 I think you may need to recheck your interpetation of TDC.  There are two TDC's at alternates of the four strokes however you are only concerned w/ the compression stroke, the one that happens right before the plug fires.  There is the intake stroke, when the piston drops to BDC (bottom dead center) and the intake valves are open ( NO space between the rocker and the valve stem, they are being pushed down).  The compression stroke, in which the piston is at TDC and all valves are closed (space between all the rockers and valves stems.  This is the TDC you want for measuring the valve clearance.)  The power stroke, the mixture ignites and the piston goes down.  Finally the exhaust stroke, the piston goes up to expel the exhaust gasses and the exhaust valves are only open.  This is not the TDC you want to measure valve clearance.  The only inline twins that fire at the same time are older British bikes like Norton, BSA and the like.  All Japanese twins fire 180 degrees opposed.  In other words while one piston is at TDC ready to ignite, the other piston is at TDC on the exhaust stroke.  
 Just remember, when the cylinder your adjusting is at TDC make sure all the valves are closed/there is space between the valve stem and rocker.  You will then be at the compression stroke, the right stroke for adjusting.  If your exhaust valves are being pushed down and there is no space you are at the wrong TDC, the top of the exhaust stroke.

Hope this helps,
Mike