I run those Ngk plugs but 1 size colder -8, no fouling problems. A vacuum leak that big would be instantly noticable. You wouldn't be able to idle below 1200 rpm's. It sounds like the engine runs well in the upper rpm's, just not at idle. Probably too much fuel at idle and then the opening power valve just making it worse.

Not sure on the head gaskets, I did not put them it. I am a 2 -stroke guy.
All I know is they are Cometic, the block I belive is a Gen 6. I have a Mallory 130 gph fuel pump at 4.5 psi.
The motor idles at 800 as solid as a rock, start easily. It just eats plugs.
Interestingly, the intake never fit right, and had to be machined to make it seal.........

RT

99% sure but LOOK FOR VAC LEAK AT FRONT OF INTAKE.....

and dang it listen to Mike 280 and panther.... they know yer problem.. stop bein ignorant about 4 strokes just cause ya play with a lapptop and 2 strokes... tunin a carb is highschool mechaniics class basics... even a retarded Special ed student can do it....

Does the motor smoke or use oil? Possible intake leak sucking oil from lifter vally.

nope, that is good. Just burns your eyes at idle.

any chance the valves are adjusted to tight

what does that do?

Makes it run like crap.

I don't think this your problem though since it seems to run good except at idle/low rpm.

I would suspect you would have more trouble at higher rpm if the valves were tight. More heat, and less time to close.

It would be sluggish and way down on power at higher rpms.