I was just reading through the threads and saw a good thought for this problem... Check the key between the crank and flywheel.. That would definitely do it..

And the compression you have should be enough to get it going.. Engines will start and run down to about 80 lbs.. 60.. probably not or only with a real fast spin or tow. If you don't grind valves it's always a good idea to tip the head upside down and fill the combustion chamber with petrol and it should stay in there.. if petrol leaks out the valves need a grind.. Simple check for next time..

Well you see the thing is, if it fires every time the crank turns, it's going to be sparking at tdc on compression, and also at tdc when the exhaust valve is just closing and the inlet is just starting to open.. right ? And that would/could cause backfiring out the inlet.. So that thing is probably set up in it's electronics to only fire every second turn, and perhaps that's your problem, and what I was trying to ascertain with all that turning back and forwards.. Try and check somehow that the spark really is happening at tdc(or a bit before), on the compression stroke... Snap !

On a 4 stroke engine the timing marks can line up on the top of the compression stroke or the exhaust stroke. As Mech says , it's possible you have them lined up on the exhaust stroke. Everything will line up but 180 degrees out of time .

Ok, so I think I understand what you are saying, so let me give you an update. Pic below is my basic timing drawing. -IF its all lined up (timing correct) as it is...shouldn't the sensor/magnet on the flywheel match up exactly with the pickup coil for spark? Looks exactly that way in the manual. - I DO have spark. Have checked. My suspicion is: -the valves not sealing properly -not making enough rotations on the pull start alone to get it enough compression. I'm gonna have to find a way (hookup the wiring harness mess to the starter & battery) or do the ol power drill/turn the crank method. Keep you updated. Thanks.

That valve timing looks right. But perhaps it's not timed to match the ignition. Have you tried turning the crank back from firing by half a turn, with the sparkplug out and attached to it's lead, and then turning the crank forwards that half turn as fast as you can with a socket or spanner? Lots of cdi will make a spark with a half turn.. If there is no spark near tdc then turn the crank till both valves are open and it's at topdeadcentre, then turn it a half turn backwards again, then forwards as fast as you can with the socket again and look for spark again.. It might be firing when the pistons at top but with both valves open, the exhaust just closing and the inlet just opening.. It's a long shot because I'm presuming the ignition trigger is on the crank, and mostly those sorts fire every rotation..but some only fire after two turns.. That should be enough compression to get it spinning, and as you say, the compression will come up once it's had a run, but if it isn't spinning even once when it pops then I'd suspect it's firing on the wrong stroke of the piston.. Also the way you describe it, as a pop, that doesn't sound like a proper firing with compression huh.. it should really make a good hard bang..

@Dave Randolph Thanks for the response. Pics attached. The timing/cam is lined up as perfectly as I could get it. On the compression stroke. To be clear the ONLY time it "fired" was when I poured a bit of gas directly into the cylinder from the spark plug hole. Will not fire through carb. And when I say "fired" it made only a quick pop for a half second. Nothing actually started. Put on a new carb. Still nothing. Checked the compression. Ranged from 90-100psi. Very low, yes,. But as I understand it, the motor hasn't really even started, or moved more than a couple revolutions, so the rings aren't seated and it can't really build compression. What are the chances its the valves? Not completely sealing. I have a hard time believing that, as I would think something would run at this point...but I'm new to this so you never know. @Mech "Fires" just meant it did make a pop when I poured some gas directly into the spark plug hole, and pulled the starter rope. As I attached, the timing is lined up as perfectly as can be on the compression stroke. Wasn't apart for long, I checked every hose for blockage. I highly suspect the valves at this point. Just running out of options. It was running (badly) before I rebuilt the top end...now nothing.

When you say it fires for a second, do you mean it starts for a second or so, or that it makes smoke and a noise but not spin the motor ? If it spins the motor when it fires then the compression and ignition timing will be correct. If it burns the fuel and makes smoke but doesn't spin the motor up then you might have the ignition timing not matching with the cam timing. I'm not familiar with that motor but some motors you can time the cam and have comression but have the ignition firing on the wrong position, by which I mean it might be firing when the pistons at the top but the exhaust valve is just closing and the inlet just opening.. not on the compression stroke. You can probably check that by taking the sparkplug out and turning the crankshaft by hand and checking the spark occurs when the piston's near top on the stroke that has both valves closed. If that's correct then I'd suspect no fuel or a vacuum leak on the inlet. How long was it apart ? Any chance a mason bee could have made a nest in the fuel line or carby inlet ? Any chance you have a split in the rubber inlet manifold? Or a blocked exhaust stopping air being drawn in...

What's it sound like when it fires? Does it backfire from the exhaust? PO through the carb? The pin or dimple on the cam locates the cam shaft and the timing mark should look like the pic. With the timing lined up mark at the "T" Mark. Did you check the compression with a compression tester? What was the reading?