When setting the valve timing you have to take care because the pressure of the valve springs on the cam will move the camshaft.
You think that you have them both in the correct position and then as you remove the tool that turns/sets the cam, the cam moves and you don't always notice it ......
I got round this by slackening all the tappets as far as they would go (ie effectively out of the rocker), repositioning the cam, fitting the peg VERY carefully and tightening everything
Once done, set the gaps correctly and then turn the engine over checking the sequence, ie firing on 1 - both valves closed, rotate engine until No.3 is at TDC and both valves closed etc etc
If you have followed the gapping in workshop manual , all should be well. Whilst you are at it , check the position of the rotor arm at each firing point
Sorry grandmother, I thought you'd put your teeth in