2007 SMR 510 hard/no start when cold

I'm pretty frustrated. The 510 started struggling to turn over the engine. It'll turn over with the decompressor pulled in, but when you drop it, it stops. I put a car booster pack on it and it fired up immediately. Replaced the battery, same situation. Replaced the starter. Same situation. Check valve timing, valve clearances. Drained coolant and blew air in the cylinder. Felt it coming out the side of the block. Blown head gaskets can act like this. Replaced the head gasket, piston and rings. Bottom end turns over very smoothly. Moved smoothly with just the piston and cylinder on. Once I put the head on, it was much harder to turn over. Got it back together. Had to use a car and jumper leads to start it the first time. This morning I tried to start it cold and needed only a small battery booster. It turned over a few times on it's own before it couldn't anymore.

Pre-engine rebuild it had gotten so bad that even a car and jumper leads wouldn't start it. Now it's 90% there, but still off. I've order a solenoid and battery cable. They could be off a bit? I've done a million voltage checks with my meter. What am I missing? I've thrown 1000 dollars USD at this and I'm pretty pissed the problem still exists.

Any other information on the bike before this started?

look at your grounds and your starter relay/wires (frayed, broken or limp inside?), maybe. clean and tighten the connections (use vasolilne or dielectric grease before tightening them)

whatever the problem ends up being- let us know.

good luck.

any news? let us know.