3300 Jab problem

[tx_swordguy]

Rick,
I think your problem is a main jet problem (too small probably). Not for sure on your model of bing but If I am correct I believe you will need to start at a 255 main jet. If that is what you already have go up one perhaps two sizes. Not sure what the numbering will be but probably 258 and 260-ish. The jets are only about $8-10. not exactly cheap but a super easy thing to try. All you should have to do unclip the bottom bowl and you will see the main right there. Unscrew it and swap. done
Mark

[Rick524]

Thanks Mark, I have no experience with Bing carbs and no spare jets on hand so thought I'd try
what I can now, instead of waiting for parts. Where can I order replacement jets/needles from?

I'm going to try the air straightener tomorrow to eliminate that possibility.


Thanks for the help!


Rick

[DCASonex]

Rick, Do check that sense tube location, but if no joy may consider the following:
Too small a main jet might explain the problem, but ironically so can a too small needle jet. I encountered the latter with the needle and jet that came with my very early CAMit 3300. It did not cause mine to stop when fully opened, and seemed okay on the ground run, but all EGTs went red on takeoff. (Fortunately a Sonex with a 3300 will still climb when throttle setting is reduced.) The two jets are in series, and fuel passes first through the upper needle jet and that jet's open cross section area is reduced by the needle. The needle should be fully retracted when at full throttle, but its tip still remains in the jet. If the resulting open cross section area is less than needed, you can enlarge or even remove the lower main jet and it will make no difference. I installed an older set from my Jabiru, which was close, but then made a modified set by slightly (by a few 0.0001") reducing the diameter of the swelled portion of an old Jabiru needle and paired that with a 2.80 jet that was bored out to 2,82 mm.

David A.

[Rick524]

Thanks David, good info. I was busy watching the head temps and fuel pressure when doing the runups, but
didn't notice the egts going into the red. Would seem its not going lean and points more to the sense
tube or turbulent air at carb mouth. Ill know more later today.


Rick

[Rick524]

Couldn't find a suitable air filter to fit the carb, to try a runup with just a filter and sense tube inside it,
so I moved the sense tube into the existing air filter and did a few runups. Major difference! Full throttle
hit 2800 RPM, a 150 RPM increase. At full throttle, engine starts to run rough after about 5 seconds and will
smooth out again if I back off to about 2500 RPM. It wont stall out at full throttle as it did before.

The egts were about 400C and all even. I don't hold it at full throttle very long so maybe the egts don't have
a chance to rise much. It still seems like its starved for fuel and maybe the jets are the next thing to try.


Rick
Sonex 524

[peter anson]

OK, this is a real long shot, but how old is your engine? My very early Jab 3300, serial 33A-116 had a similar sounding problem when I first started flying, but not as bad as yours. It ran beautifully until I went to full throttle on climb out, when it would misfire until I eased the throttle slightly closed. Gave the impression of running lean so I spent lots of time changing jets, including going waaay too rich, without being able to fix the problem. I was at the point of thinking maybe I should just limit how far the butterfly opens, when I talked to someone at Jabiru and found that is exactly what they did. They gave me a new part, the little pulley that the throttle cable attaches to, which has the full throttle stop in a slightly different position. Turns out that the earlier part was allowing the throttle to open too far and the shaft that the butterfly attaches to was causing turbulence.

Since this is unlikely to be your problem I think you are on the right track, but it shows how a very small thing can have a huge effect.

Peter

[Rick524]

Hi Peter, the possibility of a throttle stop has crossed my mind, but don't think that's what I need, my engine is SN 2087, around
2009 vintage. You're correct about small changes resulting in large differences. I taped the sense hose in the filter
with a 3" pigtail and wonder if at full throttle, the roughness I see isn't just the sense tube bouncing around in the filter?
I'm going to mount the sense tube permanently today and see how it runs. After that, its on to the main jet, I'll see if I can
drill it out very slightly and see any improvement.

Rick

[sonex892.]

Hi Rick. Just for reference on the jet size my motor is serial 2121 and it has the same 255 main and the 285 needle.

Steve
Sonex 892

[sport65]

Rick if your a taildragger, make sure you get the aircraft level before you check the fuel bowl level. Dump the fuel bowl after the tail is level before first test, and after each test checking the level. Use your boost pump to fill it for each test. I had a similar issue, but got mine in the air before I new it, like you said pulling power made it run smooth again. I could not find how to do this testing accurately, so I found out the hard way. The carb is very sensitive to that 12mm. To high a fuel level let the fuel pump slowly kill the engine when at idle, to low a fuel level starved it of fuel at full power(even within the acceptable range of the fuel bowl level). Hope this helps with your problem.