Zenith Aircraft Builders and Flyers
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3300 max RPM issue after intake mod
Hey all hoping someone has seen this issue. Here is my series of events.
- 3300 engine runs perfectly smoothly
- upon annual inspection I find the scat tube going from air filter to carburetor has failed at the very spot the carb and scat tube meet, meaning I had been drawing raw air into the carb.
- I replace the scat tube and I get a very rough running engine at high rpm, a known problem published by Jabiru where the scat tube creates disturbed air just before entering the carb.
- I replace the scat tube with the crosshairs air intake tube sold by Arion Aircraft which smooths out the airflow just before entering the carb.
- now I have engine max rpm issues where i have these issues,
- I can’t reach max RPM, even when I push throttle to the wall, I hit about 2800.
- even at 2800 I have engine roughness and not the smooth high rpm the Jabiru is capable of, though it smooths out at lesser RPM around 2300-2500.
- i am missing some power, I can’t cruise at my normal speed, it’s not climbing or cruising like it did.
- In other news, my EGT has taken a nosedive, down to 300-400. Yes this could be related. It could be a bad gauge which do go bad often. Any correlation?
Questions
- Is my air filter dirty? I’ll check soon but the massive drop in performance makes this seem odd given the FWF kit was done about 30 hours ago.
- Do I need different carb jets/needles now that I have a different air intake journey?
- is there another issue like jammed spark plugs (which were fine at annual) or some other issue that I’m not thinking of?
any thoughts are appreciated, it’s driving me crazy.
Replies to This Discussion
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Permalink Reply by Stephen R. Smith on
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John,
Its good to know that AeroConversions added the Delrin gasket. The plane I few had a heavy sticky throttle that made small adjustments difficult. I also found that I could not lean to lean-of-peak without the engine becoming much too rough. I have a HACman installed on my Bing and I can lean until the engine flames out without roughness.
Steve
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Permalink Reply by Sam Cyphers on
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Hi Steve, I have the Jabiru version of your modification (video below) which I believe is where the problem started. Adding this seemed to create the issue, given I was drawing raw air before this modification. Everything seemed to run perfectly fine until I added this. I do still believe David Gallagher from above is correct, I am going to continue to tune the carb over the weekend.
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Permalink Reply by David Gallagher on
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Sam,
I have the exact same carb intake rubber hose with aluminum flow straightener configuration that Steve Smith has. My photos would be carbon copies of his. I got my hose from intakehoses.com. They have thicker wall for vacuum collapse resistance than a typical radiator hose.
I only ran with a SCAT for a few ground runs as my engine performance was not good. Pete K., (retired from JabiruUSA, now Arion) told me to scrap the SCAT and go with the rubber hose. Sometime after first flight I made and installed the flow straightener. That in conjunction with the jet experimentation is what fixed the configuration I have now been flying with for probably 13 of my 14 years of flying the 601.
Good luck,
Dave G.
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Sam,
The modifications pictured address two different problems.
The first is that scat tube causes reduced airflow because of the rough surface. The reduced airflow means less power. You will never find scat tube or anything like it on a high performance car, for example. The designers know to how to minimize drag in intake systems. Sure there are intake paths where scat tube is a reasonable compromise in an airplane but there is no good reason to use it when you don't need to. Another cause of drag in an intake system is sharp bends in the path. Each 90 degree bend produces resistance that is roughly equivalent to three feet of straight pipe.
The second issue is uneven fuel distribution. That causes some cylinders to work harder then others. This problem is observable by looking at EGTs. If they differ significantly from one cylinder to the other, there is room for improvement. Having air enter the carburetor immediately after making a 90 degree bend results in a very uneven air/fuel mixture. Because the Jabiru intake manifold is so small, there is limited opportunity for the uneven mixture to normalize before being shunted down the 6 intake runners. On a 601 installation there is no way to avoid the 90 degree bend but we can help the air make the corner without becoming turbulent, so that it is nearly uniform when it enters the carburetor. That is what the aluminum is for.
Steve
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Some very interesting progress was made this weekend on the three issues that is good knowledge to keep in our minds.
1. Uneven EGT readings: In reading more about the Jabiru I came across uneven EGT troubleshooting, one of which was to tilt your carb to allow even air/fuel induction. I suddenly realized the elbow I installed (video above in thread) was not plumb with the carb. The vanes within the elbow were making a bit of an "X" shape when they should have made a "T" shape. I straightened it out, now my EGT's are perfectly balanced.
2. Fuel Mixture: A smaller jet size has been ordered and I will experiment with this. While my EGT's and CHT's are exactly where they should be, I am still seeing more black silt/residue down the belly of the plane. I believe this is due to the long and more restricted journey the air is taking to get to the piston. It was drawing raw air before but now takes a longer path through the newly cleaned K&N air filter, down the SCAT tube, through the intake elbow, then into the carb and down to the piston. The less air there is, the less fuel there should be. Thats the idea I will be experimenting with at least. More to come on this.
3. EGT Probes: Check out this video below, what an odd installation by the builder to only add 2 EGT probes when its clearly built for 6. I am looking into this but probably wont make this upgrade until next conditional inspection.
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Permalink Reply by John Austin on
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Looks like the engine obviously had 6 probes at one time - those 4 vacant positions look like they were closed-off with stainless rivets? - and maybe the probes eventually burned-out and were removed? Don't know if it has changed, but normally Jabiru's don't come with the EGT holes already present - the installer has to measure and drill the exhaust runners to install the probes. Also, for the EGT's to be meaningful, the probe locations need to be the same distance from the exhaust port on each cylinder - Looks like those 2 probes and the plugged hole locations are about same distance down on each runner, so at least that's a plus!
It's very doubtful "my EGT's are perfectly balanced!" Looking at only 2 of 6 cylinders, even if they're on opposite cylinder banks, is perhaps better than nothing, but your other 4 cylinders' EGT's can be wildly variant from those two. Hate to be so repetitious, but I wouldn't worry much more with tuning until you have 6-probe EGT monitoring. It should be really easy to add the 4 probes since the holes are already there - just drill out those rivets and insert the probes! Of course, you'll need a gauge that displays 6 probes - what gauge is being used to display the two probes?
John
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Permalink Reply by John Burns on
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Those look exactly like the rivets that were installed on my GEN 4 Jabiru 3300 to mark the recommended locations of the EGT probes. I would guess that the builder just never installed the other 4 probes.
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John B,
As I said, used to you had to measure, mark, and drill the holes for the probes. Good to hear that it is already done for you as I likely will upgrade to the Gen 4 when my late Gen2/early Gen 3 comes up for top end overhaul. Just out of curiosity, was that done by the Jabiru factory or in the US by Arion?
John
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The rivets were there from the Jabiru Factory. I just drilled them out (I have some practice at drilling out rivets) and enlarged the holes to fit the EGT probes. No measuring and exactly where the factory thinks they should be.
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Hi John, here is a dumb question, when you drilled them out did you do it with the exhaust connected to the rest of the engine? Or the exhaust connected to the rest of the muffler? I was wondering what happens with the drill debris that would go down the exhast and into the muffler.
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I did not have the muffler on at the time. Since it is only held on by springs it is easy to remove.
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John B,
That's great news! If all the Gen 4's have consistent EGT probe locations, it'll no longer be "apples and oranges" when people try to compare their EGT's!
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