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This is a carburation issue (I believe).
I'm just getting this sonex-a flying (after lengthy rebuild and mgl avionics issues). Now that I have a few flights (3) under my belt I'm seeing a pattern.

1. If I taxi out and give full throttle, the engine will stumble, once. If the throttle is then brought back to idle and again advanced, this second time, it does not happen. It's less of an issue if I advance the throttle (rather) slowly.

2. During takeoff roll, the engine will sometimes stumble but recover, but once solidly in the air this does not happen. Very disconcerting on the most critical phase of flight.

I suppose the Remaster and Aerovee engines to be similar enough for experiences to cross. Remaster's version of the slide carb is being used. Induction is direct ram air into a cone filter (pic). I'm wondering if air might be getting disturbed at the cowl intake or some other odd phenomena.

This sounds a lot like the "burps" that you've probably read about on this forum.

I had the exact same symptoms and resolved them by adding a fuel line directed upwards between the Aeroinjector input and the fuel tank vent. Be careful that your tee is as high as possible or you could end up like me with an unintentional syphon.

I think we have the problem because there's nothing like a float chamber to separate bubbles from the fuel.

Thanx Chris,
I hadn't considered that this could possibly be the burps since it happens when things are still cool under the cowl. But considering how we shut off the fuel at the tank, perhaps that's wen some air gets into the lines (sitting over night). Just thinking out loud, and doubting the burps idea but not discounting it either.

Yeah, unfortunately I don't think anyone has installed transparent hoses and fittings to figure out exactly what is happening, but I do think you're probably right thinking that air gets into the lines between flights and some of it gets forced out when you go full throttle.

I had the exact symptoms you describe even when I'd expect the under-cowl temperatures weren't very high. My fuel lines are insulated and about as short as possible. I kind-of got used to it and trained myself to be gentle with the throttle. I certainly don't believe I've had it as bad as what others have described. I flew it like that for several years. Like you, it would primarily stumble when applying throttle during takeoff or runup. It did seem that I could clear the stumble during runup and it would be fine for takeoff.

Once I finally decided to try the burp tube, all symptoms went away and the throttle response was incredible. I left it like that for over a year. Unfortunately, I installed my burp tube tee low enough that one time I completely filled the tank and when I raised the tail on takeoff, fuel was able to syphon through the burp tube and out the vent starving the engine of fuel. This was confusing until I thought about it and reproduced the exact scenario on the ground by lifting the tail with the cowl off. Because of this, I removed the burp tube until I think of a better way to install it on my airplane. The tee should probably be at least as high as the vent connection to the tank, mine was about an inch lower. Now that I've removed the burp tube my stumbles are back. It's possible I'm the only moron who would install it wrong, but be careful :-)

I have a suspicion that in addition to giving somewhere for bubbles to go, the "burp tube" also provides a reservoir of fuel that can respond to transients faster than the rest of the fuel system.

In my mind, tuning the carburetor richer could also help and might be a good first step if there's any chance you can comfortably go richer.

Chris,

I understand you are connecting one end of the burp tube to the fuel vent as high up as possible. Where does the other end connect? To the AeroVee Injector?

Thanks.

John

Onex 107 is a tri gear, Aerovee with Aeroinjector, engine. I had the stumble also when starting a takeoff. I discovered that to shut the fuel off I had to use the main shutoff because the mixture shutoff did not completely do the job. It still dripped. So I shut off both when shutting down the engine. I think the slow drip from the Aeroinjector mixture shutoff allowed an air bubble to get in the fuel line. The burbs are caused by heating the fuel lines until vapor bubbles are formed and they will continue until the engine cools. Very frightening. Your engine stumbles/stops every 15 seconds. Even at cruise speeds. Insulation will stop it.

My ONEX with AErovee and Aerocarb does exactly what Brock says......stumbles ONCE on takeoff only if the throttle is pushed in fairly rapidly. Does not do it with slow throttle advance and the stumble is very brief. So I just push the throttle in slowly. End of problem.

pilotyoung wrote:Chris,
I understand you are connecting one end of the burp tube to the fuel vent as high up as possible. Where does the other end connect? To the AeroVee Injector?

Yes, I added a tee fitting to the elbow I already had coming out of the Aeroinjector. I used -4AN stainless steel braided teflon hose (smaller than the rest of the fuel system). Keep in mind that shutting down the engine will take a little longer because the burp tube will keep supplying fuel after the fuel valve is shut off.

Also, I'm not necessarily evangelizing for adding a burp tube. I like the simplicity of not having one, but in my experience, I can't say I resolved all the little hiccups and stumbles until I added one, then it was extremely satisfying. After I removed it I had someone comment about a stumble they heard during a touch-and-go. It's a little embarrassing, but something I can mostly manage by being slow on the throttle applications.

flyingbear wrote:My ONEX with AErovee and Aerocarb does exactly what Brock says......stumbles ONCE on takeoff only if the throttle is pushed in fairly rapidly. Does not do it with slow throttle advance and the stumble is very brief. So I just push the throttle in slowly. End of problem.

Only happens to me on very, very hot days, and pulling the mixture out to lean it solves it instantly. If you saw how much insulation I have on my fuel lines you'd be amazed! I never, ever have burps on startup, even right after refueling on a 100+ deg day, so it's definitely the mixture (for me).

I've personally never put much credence in the bubble theory. 540 hours on the AeroVee and AeroInjector with never a hiccup, except on the rare occasion noted above. But note that I don't have a gas collator, either. By super insulating the fuel lines, and using the mixture knob appropriately, I have never reset the mixture in 7 years (I fly in 20F deg in the winter, and 100F deg in the summer). On take-off, mixture knob "in" for the winter, and partly "out" for the summer (if it burps on throttle up, pull it out a little more).

Your mileage may vary!

The three Aerovee's I have owned and the two others I have flown with Aerocarbs all do/did this. They are simply a bit rich at idle. I lean for taxi and go full rich just before mag check. I richen again if I'm not immediately ready to go. No more burps, coughs, or stumbles. If I sit for a minute without leaning, it stumbles on throttle advance.