Fuel Mixure

[Udi]

I can't get the fuel mixture in my Rotax 532 lean enough. I have already replaced the main jet twice (the first one was the recommended size for my altitude - 5000 ft [158] and the second - one size smaller [155]), and moved the jet needle to position #1 - the leanest position. The EGTs show 950 degrees at the mid-range (4000-5400) and fall to 900 above 5400. Below 4000 RPM the EGTs fall quickly below 900 degrees.

Should I get even smaller main jets? The next size down (150) is recommended for flying at 10,000 ft!

I cleaned and oiled the air filters, so I don't think they are the problem. Maybe they are???

The needle jet and the jet needle are the normal sizes for this engine, and they look ok.

Any opinions why my mixture is so rich?

BTW - I bought the engine from a guy who used it at sea level. When I replaced the sparks plugs (before the first time I started the engine), they were black, indicating that the engine was running rich also at sea level.

Thx.

Udi-

[gyropilot]

I can't get the fuel mixture in my Rotax 532 lean enough.Udi,

First, are you *sure* your engine is running too rich? How sure are you those EGT readings are accurate?

You mentioned the plugs showed signs of the engine being too rich at sea level, but I'm not so sure you should trust what you saw. You don't know how the engine was operated immediately prior to being shut down. If it was idling extensively prior to being turned off and the idle-mixture was set too rich, you could have this situation. It wouldn't necessarily be indicative of what's happening at cruise and full power settings.

Before assuming the EGT is telling you the truth, you should verify it by checking your plugs for color and appearance. Clean or replace them then go fly. Check them immediately after landing (without idling very long) to get an accurate as possible reading.

Good luck,

John L.

[Aussie_Paul]

Better still, turn the engine off and glide down to a landing and then check the plugs.

Aussie Paul.

[mceagle]

Udi,
Both the needle jet and the jet needle wear and initially it is not perceivable by eye. The only way to check is to "size probe" the jet and to measure the needle at different stations. I think a straight edge on the needle should reveal a slight convex surface, and certainly not a concave surface (along the length of the needle)
The jet needle and needle jet are responsible for the lower power mixture settings (up to cruise) and the main jet sets the full throttle mixture.

[Udi]

John - I have verified the EGT readings with a voltmeter. Still, I guess the K-couples might be bad, although that is unlikely, or there may be some losses in the wiring. Both couples are reading about the same, so I tend to think the couples themselves are ok. Your point is good though – I do not completely trust any instrument.

You are absolutely correct that I can't assume anything with regard to the engine operation before I owned it. I just added this bit of information for general info.

Tim, I too have suspected worn jet needle and needle jet. I don't have instruments accurate enough to measure the jet, but I can try and check the edge of the needle. Maybe I should just invest in two new pairs.

Here are some pictures of my spark plugs after about 1.5 hrs since new. I don't know if that is enough time to make conclusions. What say you?

[Udi]

Another one:

[mceagle]

What I think is that you have a better camera than mine. - Colour looks good though.