Aerocarb Alternatives

[builderflyer]

Thanks Noel,

Yes, I've read Busch's writings for years. I wish he had been around years ago when I owned and operated an IO-520. But I don't know if his huge amount of work and experience on Lycoming and Continental engines is directly transferable to a Jabiru engine. Maybe so.

In any event, the CHTS on cylinder nos. 5 and 6 do follow the EGTS indicating a true lean condition especially in the heat of our 100 F summers. A step climb is necessary to keep from frying the back cylinders and I don't believe that would be as important if the EGTS on those cylinders could be brought down to the level on the other cylinders when operating at WOT.

Any other ideas out there?

Art,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,Jab 3300 #261

[Arjay]

I solved the loose needle issue this way: found a thin washer that just took up the slop in the needle carrier, cut a notch out of it and wedged it tightly next to the ball. No more slop. Needle Now keeps position reliably.

My Aero carb, aerovee now runs well with no 2 needle. (tried no 2.5, but that was way too rich). Now Needs leaning at all throttle settings. Engine speeds up very slightly when cutting off with mixture to full lean at idle (1000 rpm). Needs aggressive leaning at cruise, but cht must be closely monitored and mixture adjusted to keep cht under 400 f. Goes too rich (stalls engine) at 4 g pull up. My mechanic had to set it down on highway after he couldn't restart.

Would love to hear about others experience. Does anyone else stall out during hi g's?

Ron, legacy sonex tail dragger aerovee with aero carb.

[kmacht]

Haven't stalled it but it defiantly runs rough when pulling extended g's such as in a 360 degree steep turn. Have to lean it first and then richer up once the g's are gone. If I don't it gets too lean and egt and cht start to climb rapidly. No idea how to fix it as I think it is the increase in fuel head pressure causing it. A normal carb has a float and bowl that would regulate it. Not sure how the rotec or Ellison carbs combat the issue. The great plains and Hummel engines use a normal zenith carb. That has its own issues as you now need carb heat and some setups don't have cockpit mixture control.

Keith
#554

[sonex1374]

Full throttle EGT's can be mysterious and frustrating. Part of the trouble is that the temperature-mixture relationship is not linear. It doesn't work out such that lower EGT's mean richer mixture, and higher EGT temps mean leaner. In certain cases you can have high EGT's and still be really rich! You can verify this by leaning slightly on climb out - if your engine suddenly runs better and EGT's come down, you've running way too rich, and your high-EGT indication have tricked you! Now this won't always be the case, and if you're not actually rich, then engine will complain if you lean it further (CHT/oil temp climbing, rough running or stumbling). It's a pretty easy test to run to see where you might be on the WOT mixture setting. (My theory is that unburned fuel is still burning in the exhaust manifold after the exhaust valve has opened, but this is just a guess.)

What should a well-tuned engine do? It will take full throttle well with no sagging, stumbling or surging, temps don't run-away after just a few moments, and leaning the engine a small amount on WOT climb out doesn't make the engine complain, nor run noticeably better.

On my engine, that sweet spot is the high-1200's to low-1300's F on EGT for the rear 4 of 6 cylinders, with the front 2 slightly higher maxing at about 1300-1350. These are all higher than recommended by Jabiru, but the engine just seems to run better here than richer and cooler. Even so, I'll see 8.5-9.0 gal/hr on WOT climb out, which is exactly what Jabiru says the 3300 should consume at 3000-3100 rpm. I shutter to think what the fuel flow would be if I kept going richer and richer to chase the elusive 1256 F!

Jeff

[Onex107]

sonex1374 is correct in his evaluation of the AeroInjector needle adjustment. My research found the same conclusions. My post on the #3 needle is valid for the Aerovee, not for a Jabbaru. Any engine that uses more than four gal/hr. needs a needle that has a higher slope if the wot and idle sweet spots are to be within the slide movement. There needs to be a number 4, 5, and six for those engines. AeroInjector, are you listening.

[NWade]

Just a quick note: Page 19 of the AeroInjector manual specifically mentions that a #4 needle, and others, are available from Sonex.

[builderflyer]

Jeff,

Good food for thought. For "full power" as opposed to 3000 or 3100 rpm, the Jabiru manual specifies 10 to 10.6 gph which is what I see when at WOT during a climbout. I believe the high rear cylinder EGTS during WOT are valid in that the companion CHTs also follow suit. In response to your test to do some leaning at WOT, I've done that and find that the engine will run smoother and increase in rpm. But I've rationalized that that occurs because the 4 front cylinders are running too rich at the same time the back 2 cylinders are running too lean and not that the two rear cylinders are also too rich.

I know of at least one other 3300 pilot who experiences the high EGTs on the rear cylinders during climbout and what works on his airplane (not mine) is to throttle back a bit to bring those EGTs down. But hasn't anyone else seen this phenomenon? This appears to be but another example of what is so puzzling about these experimental airplanes...........the lack of consistency from one to another, even when, on the surface, all looks to be the same.

Art,,,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jab 3300 #261/old Aerocarb

[lutorm]

In response to your test to do some leaning at WOT, I've done that and find that the engine will run smoother and increase in rpm. But I've rationalized that that occurs because the 4 front cylinders are running too rich at the same time the back 2 cylinders are running too lean and not that the two rear cylinders are also too rich.

Quite possible, but the EGT traces should tell the story. If the back 2 cylinder EGT's don't rise as much as the others (or drop) when you lean, they're leaner.

I see this on our Aerovee. If I lean from full rich at full throttle, the back 2 EGTs drop while the 2 fronts rise. The engine rpm doesn't change noticeably for a while, but it's clear the rear cylinders are already running lean of peak even at full rich.

[builderflyer]

Good point........I'll give that a try tomorrow!

Art

[DCASonex]

Art,

If you have not already done so, try replacing the airfoil shaped flow divider in the intake manifold with simple 1/2" (12mm) round bar. That cut my WOT EGT spread about in half on my old Jab 3300, New Jabs and my Camit have two round bars in manifold and with that, middle cylinders tend to run highest EGTs

David A.