Duncan Avengers can be found pretty cheap at certain Walmarts, and are good targets for modding. There is a new revision of them, to be released shortly, so keep in mind that these mods all apply to the old style.
Adjustable Gap
The packaging claims that the Avenger has an adjustable gap... if it does, it's pretty lame. The only slightly adjustable thing about it is the nylon locknut, but it's not tight enough to be effective. To give it a better adjustable gap, pop out the locknut and flip it over. The easiest way to do this is to pop the bolt out of the other side: place the threads side of the bolt on a table, and press down on the yo-yo half; the bolt should pop out easily. Remove it from that yo-yo half and screw it into the locknut in the other half. Put the head of the bolt down on the table and press down on the yo-yo half. This should pop out the locknut fairly easily. Then comes the trick: flip it over and stick it back in the yo-yo half, so the flat side is facing out. Put the bolt back in its half and screw the yo-yo back together. Be careful that you screw it in straight, or you will mess up the threads. It should be a little tougher to screw and unscrew, and should give the Avenger a better adjustable gap. It will also widen the gap a bit, so be careful.
Performance Rings
With the wider gap made by flipping the locknut, putting in a Custom Performance Ring is recommended. There is now a description of how to make performance rings! Click here.
Replacing parts
Another easy way to quickly increase the gap size is to replace the Avenger's axle bolt with one from a TL/Variflex, and also use the TL/Variflex's o-rings under the spacers. This will double the gap size, if not more, so performance rings are necessary. Another way is to replace the Avenger's bearing and spacers with those of a TL/Yomega.
Extra Starbursts
The starbursts on the original Avenger are small and a little weak. To give them a little more grip, take an X-acto hobby knife and cut some more radiating lines into the plastic, as shown in this picture.
Thanks to YeTI, Greg Clarke (crash), Gabe Nichols, Fiend X, and Darryl "Killboy" Cannon for these mods.