rough mid-range and carb questions
Greetings,
I've got a 912UL that has been running well, except for roughness mainly from about 2300-3800 rpm. Idle and higher power is smooth. I've been trying to get rid of this for some time now, but it wasn't severe so it wasn't a big priority. Recently, I upgraded the cylinders and pistons to 100 HP units modified by South MS Light Aircraft. They machine the top of the cylinders to fit the 80 HP heads, and they dish the pistons to lower the compression to 9:1. They feel like this is as far as you should go for the older 80 HP engines, and it seemed reasonable. It's quite a noticeable increase in power. After the upgrade, my old vibration got much worse. I'm certain this isn't anything mechanical, or any issue with the new cylinders. I just think the larger cylinders are causing the original problem to be more pronounced. Of course it also got considerably colder during the upgrade period, so maybe that's part of it as well. I only started flying the engine in March this year, so winter will be a new experience for it.
At one point, I thought it was ignition related, but after spending a lot of time and money at the bottom of that rabbit hole, I'm 99% sure it's not ignition.
It's been mentioned that the gear box can cause this, but the previous owner of the engine sent it to Brian Harrington to have it inspected just under 4 years and 60 hrs ago. The log entry shows that he completed the 600 hr inspection, replaced "bad upper step collar" and both disc springs, then reshimmed. Those parts are in a bag of returned old parts. The engine has 730 hrs on it, and if all that was done 60 hrs ago it's hard to imagine this would be gear box related.
That brings us to the most likely cause, carbs. At the inspection 4 years and 60 hrs ago, the carbs were rebuilt using the LEAF level 3 kits https://www.leadingedgeairfoils.com/rotax-engines-parts/genuine-rotax-parts/4-stroke-parts-912-914-series/912-914-series-parts/carburetors-fuel-system/912uls-912ul-standard-carburetor-kit.html I have to believe 5 years is a pretty conservative time for a full rebuild, so at just under 4, it's not likely needed yet.
I do think I need to take a look for anything obvious in the carbs that might be causing this. Main jets are 158 according to the inspection notes, and EGTs for high cruise power are 1400-1450. I'm planning to pull the float bowls to check the float type, condition, and weights. I'll look at the float arms, and level setting, and make sure there's nothing in the bowls that shouldn't be there.
There's no note about the needle setting, but it's recommended to be in the 3rd notch I believe. I'm not sure how much effect the needle would really have at high cruise powers, but there would be some. I do believe it's the prime suspect for my 2300-3800 roughness though. We're only at 900 ft elevation, so not really a high altitude situation. I need to confirm what notch the needles are in, then possibly try changing them. I've never read about anyone using any notch other than 2 or 3 though.
Am I missing something obvious to check?
Thanks,
Rusty
PS- I initially forgot to mention that I've checked the carb sync a dozen times, and at various RPMs, which never made any difference.
