Hi Norman.

The tank position in this case will have no effect at all. Don't concern yourself with that.

The idle rpm indicates that either the engine is very rich, or the throttle idle stop screw is set too high and should be backed out a bit. The engine is now in a cowl, and will run warmer, effecting overall operations a little. You may just be ingesting warmer (less oxygen) air into the carb, casuing part of the problem. Also having a spinner will have a small effect, but usually this will add to the rpm.

Without a cowl, bring the engine up to full power while it's being well restrained and peak the high end needle, then back off of it about 1/8 turn or a little less. That should be very close to the high speed throttle setting.

With a new engine, setting the low end needle is not an exact science. It will require changes in settings as the engine breaks in. Go with the manufacturer setting and work back and forth from that 1/2 turn to see what you need to do to get a smooth transition from low to high throttle. Blubbering is too rich. Perform the low end settings below 3,000 rpm.

Cutting the ehhaust stacks on the Slimline will have little effect on the engine. The need for back pressure on a gas engine is very low. Since you have a rear mounted carb, are you certain the the available air intake is adaquate for the engine? And is there ample area (cooling air area squared) for hot air to exhaust from the engine? Both of theses could have an effect.

Good luck,
Pat