Nitro Dew...Another thing I was thinking about if I was building a UCD for the first time. Air flow. Especially with a 4 stroke engine. You need a great amount of air flow. The first couple I built I didn't get enough air flow through the cowl and they would run fine for 5 minutes or so and then start to load up and run rich. You have to really open the cowl up. On the front and especilly underneath. Here's a picture to kinda help.

Thanks
Barry