912 run rough on climb or heavy load

"Not sure what you meant about fuel pressure.. mine had never fluctuates really during power changes. Might just be the smoothing in my guage though.  But does not drop when issue arises."

Carburetted engines run on fuel volume/flow. In theory you could have good pressure and nil fuel flow.

As the engine increases speed, the demand for fuel increases and the pressure usually drops. On my Sonex/912 ULS/Dynon, fuel pressure drops from a high of around 5.5psi down to about 2 psi, when I go from taxi power (say 3000 rpm) to take-off (5800 rpm). The Dynon will alarm below 2 psi.

I do have a fuel flow reading  - it can show up to 27L/hr at take-off  and just a few L/hr at say 2500 rpm.(I don't think its very accurate). There is no alarm on the fuel flow.

So a good pressure reading alone does not guarantee that your carbs/engine are receiving adequate flow.

That you don't see much change in your pressure reading is odd but as you say may be due to gauge anomalies.

When you did your "open"  hose fuel flow check, did you also check your fuel pressure? - it should have been at or close to zero.

If not zero, you have either a dud gauge or a restriction/blockage in the fuel supply system😈

Hi Dave,

White residue of any amount on Rotax 912 spark plugs? Never have I ever seen this on my engine. This indicates you are running a bit too hot. I suspect you are running just a bit too lean. Just enough to be good below 5,000 rpm, but not enough fuel is available when you reach 5,000. That rpm should be just beyond the transition point from needle jet to main jet within the carburetors. Either one side of the engine is receiving more fuel than the other when you reach 5,000 rpm, or you have an air leak that can no longer be compensated for by the balance tube when you reach that rpm. Do both carbs come off the idle stops simultaneously and also reach their respective WOT stops? I know you looked at the diaphragms, but how long has the engine been in service? If applicable, when was the last five year rubber replacement service completed, including all carburetor rubber components, internal and external? 

@Jim it only has 200 hours over the past 4 years (almost 5).. Carbs are in sync for sure at the cable and run very smooth from idle to 5000rpm.. as my comments before it is when the engine is held at 5000 for approx 10 - 20 seconds that it begins to breakdown. I also kind of suspect it is lean however this did just fly into this airport perfectly and I flew the evening before for 1 hour with no issues. Always stored in a comfortable hangar as well. 

Still sounds like an intermittent fuel problem, which could be supply (blockage) or air entry (leak).

I guess there is a distant potential for an ignition issue however with two systems operating, both would need to partially fail, at the same time, This seems unlikly.😈

So I finally got back to the airport today to do more tests. Spoiler alert I was still not able to find anything specific but here is what I tried. Keep in mind this is at an airport that is approx 1.5hrs away so I am taking tools etc with me to try and resolve so not at my home base where I can pull all the cowlings carbs and engine apart to start checking internals etc. 

Fuel Related

Anyway so to hopefully eliminate the fuel delivery options I ran new fuel lines and as a test ran a new fuel line from the suction side of the fuel pump into a jerry can of fresh high octane fuel. no change. I ran a primer to the fittings on the bottom of the carb and tried that. really expected some sort of change but it did not seem to resolve anything either good or bad which was surprising to me. I did try to run with the air filters removed (rotax mechanic told me that was the problem) if anything and keep in mind it is a very random amount of time before it begins to breakdown but if anything with the filters removed it 'seemed' to make it worse. in other words it seemed to breakdown after about 5 seconds. I will revisit this test to see if that is repeatable. I also tried lifting the needles which were in position 2 as recommended but that made no change. 

Electrical Related.

Not much to test here without swaping parts but I did run the engine on 1 ignition then the other until it began to breakdown and it did not seem to make any noticeable difference. I am feeling more confident that it can't be electrical but never say never. As mentioned in earlier posts i have checked grounds and cleaned. very unlikely that both ignitions would fail the same or 2 coils would exhibit the exact same symptoms but difficult to test without spare parts to swap. 

I am very frustrated that this engine would put me in this position to start with and be so fragile that I can't easily at least find what is causing it. Not that it factors in so please don't take it the wrong way but I was a mechanic on performance snowmobiles and motorcycles for more than 12 years yet this seemingly solid engine that I fly approx 3 times a week would all of a sudden exhibit this behavior and I can't figure out the problem.

Sorry for that last bit just frustrated. The only other unique thing I can think of is that when my friend was sitting on a chair watching it run he could not easily notice any difference in sound but when I had him sit in the cab with me so he could see the dash shake he was very surprised explaining how radically different the experience is in the plane vs what it sounds like.