I also had problems with the DLE20 throttle arm linkage hitting the crank case. I ended up using a z-bend in some 256 rod, and was able to bend it so that it would clear the crank case. Another option might be to use a different throttle arm (one that tightens from the side instead of the overhead screw). Another possible cause for the high idle is that the throttle plate is not exactly centered, so when you think it's closed there's actually still a gap, letting air in. This is common in these DLE's. The plate might just be off center enough so that you can't see it (specially with the internal positioning of that plate. Will have to take the carb off to do this). Try loosening the throttle plate screw and reseating the plate, then retighten. (this can happen with the choke plate, too.)

A high idle in general is another common problem with the DLE 20's (or used to be - I haven't bought a new one in a few years). If you richen the low end to get the idle down it bogs the engine down. Some have said that the fix is to buy an EI for a different model DLE, like for the DLE 55. Downside - that's $75. Or get a generic RCExel EI for $50. Another option - use your "throttle down" function, where you fly with a higher idle, but cut the idle down another 2-3% just before landing. This very-low idle might kill the engine in a minute, but it's just enough to land the plane and start taxiing, then you can flip the toggle switch back to higher idle in time to keep the engine running while it taxis back to you.

btw - you should also have a remote EI kill switch on all of your gassers, so you don't need to kill the engine with the throttle. And \ or use a choke servo. Many fields require a remote kill for gassers, for safety. With all of the vibration of a gas engine it's not unheard of to lose a throttle linkage in air, and flying around at 1/3 throttle for 25 mins waiting for the engine to die is not fun.