74' 1st gear problems

[wardog750]

Ok, so I rebuilt a 74 SOHC engine and when the bike was purchased it was a non-running project. I checked if it went through the gears and had a good transmission before purchasing. i had rocked the bike back and forth and all 5 gears locked in and out like they should when testing to see any skipping issues or gears that would have no teeth left. All that fun stuff. When I got the bike back together after the top-end rebuild I broke it in and did multiple heat cycles and one final oil change and filter with almost no shavings or concerning metal deposits. I went to put the bike into first gear and it clicked in perfectly with no issues. when I let out the clutch to move the motorcycle it locked in and moved. I gave it very little acceleration on the throttle and it popped out of gear pop back in very quickly. tried to hold the shifter down with my foot to see if the fork internally was the issue but still does the same thing. i then decided to stop and shift into second and it took off with no issues and so did all of the other gears with no issues or sounds. I then shifted from 2nd to 1st when going about 10mph. it would downshift and rev up the engine like any normal bike would and had no issues until I tried to accel the throttle to speed up then it pops in and out of gear quickly and made a horrible sound. Still does this even if i hold the shifter down in the first gear position with my foot!!!.

any ideas are welcome. just very confused and lost about what to do next.

thanks!!!