Running Too Lean?

[Bevan]

Would an engine even start or run with a timing advance error of 50 degrees BTC?

My dual Pmags have always idled well, lean, one only or both.

Bevan

[Carl Froehlich]

Originally Posted by Bevan

Would an engine even start or run with a timing advance error of 50 degrees BTC?

Yes - and amazingly well.

Carl

[Jbon]

Originally Posted by Carl Froehlich

I offer that engines with electronic ignitions are more sensitive to lean conditions at idle than mags, and the more the timing is advanced at idle the more the issue will present itself (based on some personal fuel injector trouble shooting).

To the original issue, are you really sure you have the timing right? For Lightspeed and pMag I know the timing is set at Top Dead Center (TDC) - not 25 degrees before TDC like a mag. I don't know what Electroair tells you to do.

Is this the first run with the ElectroAir? If so, did the problem exist when running mags?

Carl

You're in agreement with Electroair regarding lean mixture. I have to admit I don't understand it. I get the concept of lean mixture misfire, I just don't understand why it only manifests itself when running on EIS but not a mag. Perhaps you can explain this to me.

Electroair has you jumper the module so that the there is no advance when checking the timing. I observed a perfect TDC reading w/ the timing light.

Yes, the engine has always had the EIS on the right side. I've only had the airplane for 10 hrs., and I didn't notice this large a drop early on.

[lr172]

Originally Posted by Jbon

I'm aware that the idle circuit is not a player at run up RPM, but it is pretty clear to me that there is an issue with the idle circuit. Turning the mixture screw out 5 or so turns should make a difference in idle mixture. With mine, it does not.

What makes you sure that there is an issue with your idle circuit? I have never seen an RPM rise when leaning at idle and mine is great. My idle is very smooth, I can get as low as the 500's in RPM and I have no stumble on acceleration (very smooth). I initially set it at 2.5 turns out. I progressively richened it maybe 1.5 turns or so through my phase I, as I was getting a stumble on acceleration. I also was trying to get an RPM drop and finally stopped when the engine was running well. Be sure to stretch the spring or add a washer to insure that you keep tension on the screw.

I would not get hung up on the RPM rise. I frankly don't believe that Dynon (my RPM indication source) reacts fast enough to see the RPM drop and I honestly didn't try very hard. There are many other ways to adjust carbs based upon sound, feel and performance.

Larry

[lr172]

Originally Posted by Jbon

You're in agreement with Electroair regarding lean mixture. I have to admit I don't understand it. I get the concept of lean mixture misfire, I just don't understand why it only manifests itself when running on EIS but not a mag. Perhaps you can explain this to me.

Electroair has you jumper the module so that the there is no advance when checking the timing. I observed a perfect TDC reading w/ the timing light.

Yes, the engine has always had the EIS on the right side. I've only had the airplane for 10 hrs., and I didn't notice this large a drop early on.

A spark is a spark is a spark. It is true that mags have weak spark energy at low RPM's and I suppose there is a possibility that a strong spark is more sensitive to lean conditions, but that defies the principals as I understand them. The spark timing is absolutely a factor.

I still believe that the issue you outline here (RPM drop differential) is all about timing and not mixture.

As I mentioned before, advancing the spark timing will increase RPM and vice versa. Let's look at how this affects RPM drop at run-up. If your mag is firing at 25 and your EI is firing at 35, you can estimate your effective timing at 30* (remember, two fires are lit and meet in the middle to form complete combustion). When you pull out the mag, your timing decreases from 30 to 25 (35-10) (a single fire must now go all way to the other side, not the middle and I am guessing at 10 here for the explanation) and the RPM goes down due to the reduced effective advance. You put the mag back in and timing goes back to 30 and RPM goes up. You now drop the EI and your timing goes down to 15 (25-10). Your RPM drop will be greater (lower RPM) than above because your advance is 10* more retarded than above.

As you can see from above, the timing of the EI at your mag drop RPM has a lot to do with the differential RPM drop. If your differential is larger than it used to be, that points to a timing drift in one or both or your ignition systems. On EI, that could be the overall timing reference, as set at calibration or related to the programmatic way in which it dynamically changes the timing (could be their system, but more likely a vacuum/MAP input that they use as a basis for adjusting the timing along with RPM).

I will add that RPM drop can be affected by several other ignition related problems, such as a failed or weak plug, failing wires, etc. Mag drop differentials can expose issue at an individual cylinder level in addition to the ignition "system," so you must be cautious in interpreting test results.

EDIT: I'll add that a more powerfull spark could ignite the mixture faster, especially when lean, providing a small effective increase in advance. However, it would be a small difference. I used to have two mag and my RPM differential was always in 10 RPM range. When I added my EI, that became 20-30 RPM. However, I built my own advance table and set the advance at 25* @ 1800 RPM to give me good comparison readings for my run-up.


Larry