First, Build a Shed

[Meyer]

Inspection soon?

Congrats!!

[Cherrybark]

First engine start on a Rotax 912iS. On a beautiful Fall morning the plane was rolled out of the hanger and parked in a clear spot in front of the hanger. Three or four of the regulars came by for the event - two are very familiar with the 912 carbureted but this injected engine, with all of the electronics, was new to them. Knowing it would be an exciting morning, with plenty of distractions, I made a pretty detailed checklist for Start, Warmup, Lane Checks, and shutdown. Put a 1/4 tank of fuel in each wing the day before and was pleased there wasn't a hint of gas leaks. Master On, Lane Lamps lite and extinguished, one fuel pump on, push the starter and the engine rumbled to life. "Rumble" is the key word here. The engine ran, barely, at a very rough idle and changes to the throttle made absolutely no difference. Oil pressure came up but the shaking was terrible so it was shut down after a few seconds. Switches off and we started poking around. Butterfly valve in the throttle body moves with the throttle control. No leaks. No detached hoses. Nothing looks out of the ordinary. Tried a second start with the same results. Terrible shaking and no response to throttle changes.

Pushed the plane back into the hanger and the head scratching started.

I called Kitfox and John McBean responded later in the day. He suggested it was fuel starvation from something plumbed incorrectly in the fuel lines. Fuel flow tests showed plenty of flow at the output of the fuel pumps but measures nothing beyond that point. John suggested I plumb a mechanical fuel pressure gauge where the fuel line enters the engine and, with years of experience, rattled off the list of fitting sizes that would be needed to make the connections. If I'm not getting 40-43 PSI at that point, something is wrong with the way plumbing of the filter assembly between the pumps and the engine. There are a couple of "directional" components in the fuel line - one located under the center console in the return line to the header tank and a second in the fine filter assembly on the engine side of the firewall. Hopefully one of these is installed backwards and, even better, has clear markings showing flow direction.

One of the locals suggested this rough idle can do real damage to the gear box. Thinks the "knocking" sound is being made when a lobe slams into place in the gear box. At higher RPM there isn't such a mechanical "slap" but at this extremely low RPM there can be significant damage. I would appreciate any comments on this thinking.

This will require some quiet time in a closed hanger working through the problem. I know the engine ran successfully before it left the Rotax factory. John says the Lane lights turning on then extinguishing after three seconds confirms the wiring is correct. I do need to spend some time setting up the Garmin <> Rotax 912is configuration and figuring out if the G3X can show a full page of engine information for the next start. This is a problem where too many helpers chase in too many different directions.

Happily, Kitfox will supply all of the support I need to solve the problem. I'm very thankful to have their help with this problem and at all stages along the way.

[Shadowrider]

I bet it’s something simple. Do you have a fuel pressure readout of what the 912is is seeing. Assuming you don’t if John is telling you to get one. I also highly recommend fuel pressure gauge. I would put a sender in that feeds info to the g3x if the 912IS can’t.

[JMH]

Carl, I had the same thing happening on my first start. Turns out I needed to crack the throttle a bit more when cranking. Mine would run rough, shaking violently, like it was only running on 2 cylinders. These engines need to run at a much higher rpm than we are comfortable with. Before you go through too much effort, try again with the throttle open more before you crank it. Hope this helps.

John

[tracstarr]

I'm planning a first start on my 912is today or tomorrow. I believe we have a similar setup as I'm using the G3X as well. I already have everything setup in the G3X and I've tested that my fuel pressure sensor is reading and showing on the G3X. I've tested with each fuel pump individually and together - reading around 43psi if I remember correctly. This is the sensor in the line manifold where it goes through a pressure valve (which if your pressure is too high it runs through the bypass at this point). Also keep in mind that if you have to crack a fuel line after the pumps it will still be under pressure - probably very low - but enough to squirt into your eyes if you're not careful.

That said, a friend of mine will be here for the first start - he's heavy maintenance trained on the 912is - so I can run it by him too for you. I can also help with G3X setup if you need. You won't find a full page display of engine parameters, just the side strip and a split page.

[efwd]

Mine ran rough for a spell as the engine had been pickled. Took a short while to blow that stuff out. The throttle response you report is confusing. You say no response to throttle but later state that if the RPM is higher the "slapping" stops. When I started mine I believe the book suggested a throttle setting that I used but it was far more throttle than idle. Mine roared to life and I had to quickley retard the throttle. I think you should be looking for 2000 rpm after startup. Your oil system has been purged through and you have verified all the valve lifters are tight? I think that I once started the engine after an improperly conducted oil change and a friend thought the engine sounded "clacky" on start up. It went away after a moment or two. Your "slapping" description brings this to mind but now that it has run the system is likely all purged now. None of that would explain the lack of throttle response I suppose. I hope this proves to be an easy fix. Your so close.

[Cherrybark]

Nice collection helpful comments.

My first mistake with the engine start was not configuring the Garmin to display useful information beforehand. Oil pressure is displayed by default and I didn't think beyond that critical piece of information. I did discover the two strips of engine display. Would be nice to have a full screen but I suppose they consider a bit of inflight info is always useful. I don't know that it's possible to load configuration data from the smart card. I emailed g3xpert@garmin.com to ask if they had a pre-configured G3X - Rotax 912iS that could be loaded. They are usually very quick to reply to support questions but nothing back on this question.

I do have a fuel pressure gauge installed in the the aluminum block of the "Fine Filter Sub-Assembly". Honestly, while the engine was banging I was to distracted by playing with the throttle and worrying about what was going on to notice the pressure reading. I ran the throttle from the initial "cracked" to almost full power with no change in the engine. Back in the hanger, I double checked that the throttle body cover was not left in place and the butterfly valve does change with throttle changes.

One of the kibitzers comment that fuel was coming out of the exhaust pipe. Obviously this is an important clue and means fuel is getting to the engine under pressure. I'm beginning to think the check valve in the return line to the header tank is installed backwards. But I don't know enough about injected engine to guess what happens when you basically block a return line and they are overwhelmed with fuel. An answer to how raw fuel would get to the exhaust system would be very helpful.

I understand the plumbing of the fuel system up to the Fine Filter Assembly. Can someone explain how fuel flows through that circled assembly on Figure G-3 Fuel Filter and Fuel Pump Connections?

The engine only ran at a low, knocking RPM. The comment about gear box lobes "slapping" was a warning from one of the guys that the engine / gearbox could be quickly damaged unless RMP is increased to smooth the gearbox operation. Not being able to increase RPM meant the slapping never quit. These were to very short runs and, as JMH said, it seemed like it was running on only a couple of cylinders.

Keith, I would appreciate any help with the G3X setup.

I'm sure this is something simple. The major headache will be controlling the mess of fuel when disconnecting lines and fiddling with wrenches in tight places.

[Eric Page]

No specific experience with the Rotax 912, but thinking generally, it seems like an ignition problem might explain what you're seeing. If only two or three cylinders are firing, it could explain the roughness, and any dead cylinder might pump fuel into the exhaust.

[efwd]

If it is an ignition problem then I have lost all confidence in the redundancy of the computers on the 912iS. His lane lights are checking out OK. I honestly can't say I know what happens if you don't have a fuel return line that is functioning but the fuel injection requires it if I recollect. I understand that the UL versions run without a return but the return line was added in order to prevent vapor lock on those engines. Certainly someone like Larry will know more.

[Eric Page]

Plug wire not seated on a spark plug? Like I said, I have no experience with Rotaxes.