I just changed my head gasket on my 1991 Honda XR250L. Now that I have it rebuild, the bike wont start. I have made sure the piston is at TDC, the cam shaft is placed right. I dont know what is wrong. any help would be great

chains are not repairable only replace with new ! remove camshaft etc then cylinder head , then barrel and with the side cover etc removed then replace the chain ! tdc the piston when barrel back on feeding the chain up , fit head then fit cam with all valves closed and should line up to a mark on cam cog , fit chain then bolt the cam down , rotate the engine several times back to tdc and check valve clearences and refit all parts and change the oil !

Definitely the head gasket mate water cant really get above piston any other way - the piston would be sucking water into it on the down stroke then on the up stroke water would be passing pass the piston rings into sump oil
the only test you can do with out the bike started is a compressed air leak down test I happy to elaborate on this test if you need but if you do all your own rebuilds then I sure you know all about it..
when you take head off to replace gasket I would highly recommend you get the face of the head machined as pretty much any head that age in my experience at least would have some warpage to the face which will become harder and harder to get good head gasket seal even with new gaskets also if budget allows this would be great time to consider new piston and rings kit even new cylinder as its all apart any how if you can afford it would be very worth while
when you get running again I would also change that oil completely and oil filter every ride for at least 2 or 3 times to make sure all that water is all gone - or it will bite you later with far worse problems than head gasket issues

If the valves "rock" at TDC, the piston is at TDC EXHAUST, not TDC COMPRESSION. Chances are the engine is flooded. Install a fresh spark plug and go easy on twisting the throttle when starting the bike. Be sure the battery is in good condition. Push the kick lever to just past TDC compression then give a strong kick. Don't try to kick start with the piston going upward on the compression stroke. Be sure the air cleaner is not dirty and you have good gas. Please rate my answer. Thanks!

TDC is when the #1 piston is at the top of the stroke. I.E. flush with the top of the block. It should also read 'zero' on the crank pulley in which there is usually a mark on the block and the pulley.

If it is a constant smoke and or alot, your head gasket is blown. stop driving it if you want to save this engine. also engines with really high miles don't like head jobs, remanned heads can blow out the rings on the pistons and that leads to an engine rebuild. If your car has really high miles , you would be better off replacing the engine

If you don't have compression on your bike's engine it could be either broken piston rings or faulty head gasket. Try changing resealing the head gasket first and see if it works other wise consider opening the cylinder block and check the piston rings and bore of the cylinder.

I assume you are talking about the cam timing. If so, then you first turn the engine so that the "T" mark on the flywheel lines up with the index mark in the timing hole cover. This places the piston at top dead center (TDC). Now position the camshaft so that the cam lobes are both facing downward. Align the two marks on the cam sprocket with the cylinder head cover gasket surface (making sure the piston is still at TDC). Install the two bolts through the sprocket and screw them into the camshaft. Done.

if its goit a lot of miles on it or burning or using oil then rebuild or replace it!
if your doing the head then you may aswell do the block as the new headwork can expose other issues like piston ring wear etc.
Cheers Rob

Oil change only...
XR250L XR250R................................. 1.3 liters (1.37 US qt)
XR400R.............................................. 1.7 liters (1.8 US qt)

Oil & filter change...
XR250L XR250R................................. 1.4 liters (1.5 US qt)
XR400R.............................................. 1.8 liters (1.9 US qt)

After engine rebuild...
XR250R, 1986 through 1995 XR250L..... 1.6 liters (1.7 US qt)
1996 and later XR250R........................ 1.7 liters (1.8 US qt)
XR400R.............................................. 2.2 liters (2.3 US qt)

Sounds like you have the distributor timing 180* out.
Remove the distributor and turn engine 360* and reinsert distributor.