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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 David Gallagher on
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I wonder if you are now a little over-rich. If your plugs and exhaust are black and sooty, that points to over rich, as possibly do the roughness, lower RPM & EGT's. Arion has a tech tips page where they discuss tuning the Bing carb:
http://www.flylightning.net/jabiru-tech-tips.html
I had to go a jet size smaller on my 601 from what was supplied with the engine when I got it from Jabiru. It was a bit of an experiment, but didn't take too long to get dialed in.
Jabiru has their manuals that discuss carb tuning, but I didn't find them of much help:
https://jabiru.net.au/service/manuals/
Jabiru also has a support forum I have contacted in the past. They have been very responsive to me:
https://jabiru.net.au/service/technical-forum/
Dave G.
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Permalink Reply by Sam Cyphers on
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Hi Dave, thanks so much, I'll check this but one thing I THINK I saw was more black soot on the bottom of the plane. When something goes wrong with the plane you suddenly become very observant, but that only helps if you have a past reference. I'll take a better look.
What jet size did you end up using?
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Sam,
My engine came with the 255 main and 285 needle jets as stock. I purchased the next two smaller sizes from Bing before I even flew in anticipation of needing some. Initially my run-up’s were rough with black sooty smoke and I could not get very high in RPM. So before I even flew, I put in a smaller 250 main jet. I got good results at full power with this. A while after I made my first flights, I put in the next smaller 280 needle jet to bring up my cruise EGT’s, which I thought too low and rich. I liked my new cruise EGT’s, but now my takeoff EGT’s were too high so I changed the main jet back to the original 255 and that combination seemed to work pretty well. So my current needle configuration is 255 main and 280 needle. Your results may certainly vary.
Dave G.
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Heres a plot twist. Due to my lack of proper description, I failed to mention how Im reading the EGT's.
I have one gauge with a 2-position switch which flips between the left and right exhaust banks.
One side reads a normal 1100-1200 while the other side reads 200-400. Thats why I mentioned its possibly a bad probe.
Now I dont believe I can drastically change the routequantity of air and not change the fuel mixture, so I am still going to tune the carb because I still have a lack of power and most likely black soot, but somehow I have at least one exhaust bank still reading correct EGT's.
Any thoughts on what this combination might indicate?
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Permalink Reply by John Austin on
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I really think you should seriously consider 6 probe/6 cylinder EGT and CHT monitoring. With the air-cooled Jab 3300, you really need to know what each cylinder is doing! You'll be surprised at the variation of temps between cylinders (both EGT and CHT!) and how it changes with various configurations of jets, carb orientation, SCAT vs smooth bore air induction, etc.
John
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I completely agree with this. I have 6 CHT readings and I look those over often. I dont understand why the builder only added 2 EGT. I am looking into upgrading this.
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Mike Busch (Savvy Aviation) teaches that CHT's are the best indicator of stress on the engine (Jabiru seems to emphasize the EGT's more, but EGT's are just a number that varies wildly with location and orientation of the probe, but it's better than nothing!), so if you keep those CHT's within nominal parameters it's not likely you'll hurt the engine so it's great you have 6-probe CHT! However, for tuning purposes you really need that 6-probe EGT to truly tune the Bing - sometimes slight rotation of the carb dramatically affects the EGT's as well as the induction tubing, vanes, etc..
John
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Years ago, I followed the "Tuning the Bing Carb" instructions and similar to David's experience, tried different jets until I hit on the right combo and had fairly satisfactory results. However, being ever the experimenter who likes to "fix things that ain't broke!" - Ha! - I finally switched to a Sonex/AeroConversion "AeroInjector" TBI carb. The primary reason I did this was to gain a mixture control. Of course, the trend has for years been towards "single lever power control," but I liked the idea of a manual mixture where I can deliberately run rich for cooling in a long, hot climb, or lean as much as possible for extended ground taxi, etc. Also, I had always had a manual mixture on my previous two planes for thousands of hours and was just plain "more comfortable" where I was in control of the mixture vs the cv carb, or in more modern engines, the ECU!
However, the negative side is that the AeroInjector has a steep learning curve to get a successful installation - but if the installation is carefully and thoughtfully carried out, (at least in my case) rewards with you being able to set the mixture wherever you want it for virtually any situation, but a switch from the Bing is definitely not for the faint of heart! LOL!
John
N750A
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Hi John, I believe that carb is gravity fed, correct? Im in a Zenith 601 so Im low wing and the fuel draws directly from the wing tanks, I have no dashboard tank to facilitate an AeroInjector. Am I understanding the AeroInjector correctly?
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Well, I wasn't really recommending you convert, just relating my experience with switching to a carb vs fiddling with jets, but you are absolutely correct! The AeroInjector works best gravity-fed, and of course that was perfect for my STOL 750 with high-wing tanks. The only low-wing I know that is successful is a Sonex and it has a small header tank to supply the gravity feed, which you can't do in a 601. In theory, you could use fuel pumps and a regulator, but that adds another whole layer of complexity and I'm in the KISS - Keep It Simple, Stupid! - camp!
I'd definitely stick with what you have and follow the "Tuning the Bing" instructions, but you really need 6-probe EGT/CHT to get the best results. (I've never heard of a probe on an entire cylinder bank - in the Jabiru installations, the probe has to be a prescribed number of cm from the exhaust port to give a meaningful number and that means you're just monitoring that one cylinder ... where is the EGT probe located for the 3 cylinder exhaust bank???)
John
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Permalink Reply by Stephen R. Smith on
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Hello Sam,
I have been flying a 601XL with a Bing for about 2,700 hours. Initially I had problems with mixture being too rich. I managed to eliminate all the scat hose from the cold air inlet and that took care of the problem. I use radiator hose.
Inside the hose I have an aluminum air guide to helps the air make the corner with minimal turbulence.
The run from the cowling to the airbox is also radiator hose.
I have flown with the Bing and also the AeroInjector. I much prefer the Bing for its smooth and easy throttle operation.
Steve
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We discussed previously that the AeroInjector would not be an easy swap for the Bing in the low-wing 601, so that's not under consideration in Sam's case, but just wanted to add that my AeroInjector installation has a very smooth throttle operation. Don't know if it applied to your experience, but at some point AeroConversions added a Delrin gasket that lets the slide move more smoothly. Also, I designed and installed my own throttle reversing bell crank which gives an almost 3:1 mechanical advantage to moving the throttle slide. That, combined with the McFarlane "Vernier-Assist" throttle, allows very precise adjustments of the throttle - increments of 10 rpm are possible! But, as I said, none of this is for the "faint of heart!" LOL!
John
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