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Hey guys, been tuning up my new Jabiru 3300. Im running an Aeroinjector with no.3 needle. I got it running very nice from idle to 2300 RPM. Im running a Sensenich 54-64 prop. Past 2300 rpm it breaks up horrible and begins to spark knock. Ive been leaning for best power, which is about 4 turns and I can achieve 2700 rpm static. once in flight I need to further lean or else cylinder 4 and 6 start creeping up past 1,400 degrees. I have no problem leaning but it just runs like crap below 2200 rpm.

I tried turning the needle in, and i can run it up to 2700 rpm no issues with good EGT but it will not idle even with the idle screw all the way in. If I back it off half turn the idle is fine again but still rich at WOT. ive found the "sweet spot" but its just not where it needs to be. I tried a 2.5 needle and just made things worse. I've tried tuning with cowl off, on, full fuel, cold days, hot days. Engine is brand new and has no induction leaks and I confirmed with smoke check. If I lean the engine for best power on takeoff it goes great but like I said it's super touchy and if I put it back to full rich in cruise or landing, once I give it full throttle it balks and the EGTS sky rocket after 2300 rpm. I pull the mixture to lean and it clears up.

What can I do?

Hi Mr Frustrated!
Are you trying to tune the motor including when the throttle slide is wide open? Don't get caught out as it is a trap. Advance the throttle until it starts to run rough and then shut it down. If you take a look at the throttle slide you will see that it still has maybe 1/4" - 5/32" to go until full throttle. If you adjust the length of your throttle cable so that full throttle in the cockpit equals that much throttle slide opening. I think that you are trying to make it run smooth with the slide open past the sweet spot which is then stuffing up the tuning down low. I personally think that when they go to wide open throttle the lack of a venturi causes the fuel to not atomize properly.

If you do a search for what Jeff Schulz has written he explains it pretty well. I do like the system.....now. But there were times I felt like kicking the blasted thing into the weeds.

Good luck, remember to take deep breath, count to 10 and then REALLY let fly with that cursing!

Sounds like you need to set everything back to factory default position, and restart your tuning for WOT. Once WOT is running well, then you can start adjusting the idle screw.

Also, make sure your system can flow enough fuel. If you’re flow testing isn’t giving you a number in the mid to high teens, you should address that first. No amount of carb tweaking is going to help if the rest of the fuel system is not flowing well.

I don’t restrict my throttle range. You’re EGTs will be lower, but you’ll also lose power. My last flight I had a peak EGT of 1335F. It’s been more at times, but you can always reduce throttle/power if you need to. I’m usually at 800AGL before my EGTs peak anyway, but the extra 50rpm is significant.

I did the fuel flow test and it exceeded the mimium rate needed. Wouldn't it be running lean if the fuel flow was too little? Like I said it seems way too rich at WOT and leaning the mixture clears it up. Plugs also indicate rich.

I didn't think of the throttle slide. I am using the push open style and can't really go to the other style because the builder of the airplane integrated it into the riveted panel. Ill take picture of the slide open and closed and post it here. But I did go back to factory settings. Thats where i can get it halfway decent.

Have you tried a 2.5 needle? I'm not sure how many 3300's are using it, but the profile would give you a leaner mixture at open throttle than the 3.0 needle (assuming mixture at idle is the same.)

I agree that it appears you are way too rich.

Are you sure the needle hasn’t been altered by the original builder?

I think I typically have to screw the idle screw in a little if I richen WOT. Your result makes me suspect the needle or the carb, assuming there aren’t other factors such as a fuel pump, incorrect needle orientation, etc...

I had a similar issue not long ago. (General Discussion: Normal for an Aeroinjector?) Group "tunnel vision" would have you believe the injector/carb is never rich at WOT and lean at idle.

I purchased a replacement needle to compare it to what I had. They were for all intents and purposes, identical. What I did discover about the needle, however, was when it was leaned in the slide enough to make the engine run good at WOT, there was no taper showing in the orifice with the slide closed at idle. Armed with that tid-bit of information, I extended the taper with a few file strokes. My engine has been running fat-dumb-and-happy ever since. (Except for that Burp-thing).

The only explanation I have is an oversized orifice, which would require the needle to be set deeper than normal to facilitate WOT, thus starving the engine at idle.

When you pulled the screw/ needle slide was there a lot of slop between the needle holder and the set screw? I had around 20 thous in mine and had to jerry rig a washer with a piece of aluminum. That really made it much easier to tune it by taking up the slack. It kept the needle from jumping around. Doesn’t take much to add or reduce a lot of fuel

sonex1566 wrote:If you take a look at the throttle slide you will see that it still has maybe 1/4" - 5/32" to go until full throttle. If you adjust the length of your throttle cable so that full throttle in the cockpit equals that much throttle slide opening. I think that you are trying to make it run smooth with the slide open past the sweet spot which is then stuffing up the tuning down low.

This is specifically mentioned in the AeroInjector manual. I was tweaking last week for best static RPM and went a touch too "open" on the slide and replicated this issue.

Engine "balking" or "over carburation" at full throttle? Yes it's possible, but the carburetor is sized for the Jabiru 3300.

My understanding is that over carburation occurs when an engine isn't pulling enough airflow to deliver the fuel adequately. This is mostly due to the Venturi effect in standard carbuators, but I guess that manifold pressure and air velocity could play a part in the AeroInjector.

If you're only getting 2700 RPM at WOT, then it may be true that you're over carburated, but it seems like that would mean the propeller was oversized, or there is an induction restriction, that is the root problem of the lower RPM, and thus airflow. In that case, limiting the throttle is patching the problem.

Perhaps I'm just lucky, but my 3300 can spin 2900+RPM static; on hot days 2950 RPM. On climb out approx 2850 RPM. She likes to breath, even which additional ducting for cold air.

Whatever the case, I hope you resolve the issue quickly.