I modified my US 41 in the following ways:

o-A walbro wa144 carb-see tuning notes below.
o-A Bennett muffler.
o-A lite-ply reinforcement on the crap 90deg crank.

I was turning a TF powerpoint 18/12 at 7100 before and 7800 after. Not bad.

The carb tuning was done as follows:

Bench the engine.

Install carb.

Open both needles 2 turns. The engine will idle at these settings at about 3500 and have a top end of 4000.

Set the high first, then keep closing the low about 1/4 turn at a time until there is no transition..Open it up until the transition is good..Go back and re set the high... then the low...then the high...

This takes a lot of time, but the job is made easier by the new placement of the needles which allows easy adjusting with a screwdriver from a distance with the engine running.

TAKE YOUR TIME! This is where you get the power. You don't need to modify the phenolic spacer. You shouldn't even need to change gaskets if you're careful. Pump venting slots in the gaskets matched up. But the key is listening to the engine and making the adjustments over and over. Wear foamies so you can listen to the pitch. Leave the engine running at a setting AT LEAST 30 seconds after each tweak before deciding if it's right. The more I did it, the better I got at hearing subtle frequency changes, and my adjustments got smaller and tighter. I ran three tanks through doing this.

Here's a photo of the rig:

Comments welcome, but I think we have a pretty good mod here.