Hello William,
I want to share with you my last odysee with bad vibes between the 2000 and 4500 RPM range. It started with slight vibes in this so called mid-range and became slightly worse with time. Idle was good, WOT was good. So started to concentrate on the mid range band.
First thing I have to note is, that on this specific aircraft (Shark UL/LSA) you need to swap the carbs at engine installation, due to limited space under the cowl, so the throttles/"chokes" sit inside. So, when I installed the engine (was installed after "IRAN" done by another mechanic) I swapped the carbs, without thinking, that the idle mix and the jet needle position could be way out of basic values. So the RH carb, now sitting on the LH side had an idle mix setting very rich and the LH carb (now RH carb...) had a leaner (#2) needle clip position. By realizing that, I set both back to "basic" but this did not dramatically change/worsen the vibes (with good carb balance at 3500/idle). So to make it a bit shorter, I pulled both carbs, cleaned them, installed new needles/needle jets and seals, set both to basic settings. The only remarkable thing were the "closing balls", they were all uncovered (cracked or missing epoxy), so maybe some "bad air" may had changed one or both carbs. Therefore I closed them with new epoxy.
Installed the carbs and had to note, that the LH throttle "cable" was suddenly sticky, not allowing a smooth throttle movement.....! During closer inspection I have found broken strands at the throttle attachment underneath the throttle/"choke" quadrant. Wooow, I have thought, that makes really sense to that issue and changed it. That cable was really bad and did not allow soft movements, which would explain the "unbalanced" range, and due to normal vibes, and pushing to WOT the spring might have pulled the cable across the damaged area. So with new cables, fresh cleaned carbs I was positive minded to adjust the carbs and all will be gone. Bad thought.... balancing carbs, played a bit with idle mixture made the whole run a bit better but not good enough as it is possible, normally.
AND now the real shocker - I could not get static RPM anymore. WOT, RPM 4700-4800. Low EGT. What the h.... is going on there. So removed plugs, looked inside the cylinders with a borescope - nothing real bad, checked plugs - mixture looked ok/even....???
So removed valve covers and checked the valve train and had to realize that pushing in the exh. valves did not re-seat them properly and there was no compression for 10-15 revs by hand and then they "came back". So I did that now on all 4 valves, reassembled all 4 covers and ran the engine again..... ALL good, NO vibes at all, WOT/Static RPM exactly there, where I had set them months ago, when I have installed that engine to the aircraft....
My explanation would be sticking vlaves due to the extremely rich carb (LH) before and that many ground runs during this troubleshooting session.
Note: During my troubleshooting I was also comparing the flame color between #3 and #4, where #4 was pretty rich and #3 was on the lean side. When doing the choke test during that "troubleshooting session" at ~2200 RPM the RPM raised about 200 RPM and then the engine almost died.
So, this might not directly help to solve your problem, but sometimes it is a "crazy" combo of issues and sometimes stay back a bit and start to think outside the box. My experience is, that two standard adjusted carbs, freshly cleaned an serviced just work, no vibes etc. even, if there is no airbox.
And by the way, I know and run a e-props on my Bristell, it runs like a sewing machine ( that applies for both, the fixed pitch and the constant speed) at all speeds.
So may be that helps a bit, good luck!
Gerhard
