Mag Check Issues, braintrust needed

[AlecT]

I’ll do my best to sum up the things I’ve noticed and actions taken. Grumman AA-1, the engine is a Lycoming O-235, and since purchase we’ve noticed the tach waver slightly when performing a mag check. Drop and differential is within limits, and with the age of the plane and gauges, we had chalked it up to being a minor gauge issue. I recently had the prop balanced, and the following analysis pointed in the direction of a “cylinder misfire” causing some extra vibration. I tested it myself after and ran it on the ground lean to clean off the plugs. After the first plug cleaning run up, I was looking for any EGT abnormalities, and after switching to left mag it stayed for about 5 seconds, then suffered a large RPM drop/choke. I tested it with both full rich and leaned mixtures, both of which caused a large drop on the left mag. The following week I called my mechanic out to look at it, and after a visual inspection of the spark plugs/cleaning, we ran it up to test. Once again no large drop or choke upon initial run up, but it seems after the first run up or after the engine heats up is when it creates the choke about 5 seconds after switching to left. my mechanic could hear the slight surge as well as seeing the tach waver around 25-50 rpm on L+R mags. Running on both creates a stable indication. They now plan on further exploring issues on the spark side of things, any ideas?

 

[AlecT]

Looks like it was deleted from the original post, but after the initial mechanic visit, the problem was diagnosed as magnetos being the issue, and new ones were installed. Unfortunately this did not remedy the issue, leaving us with our present day situation.

 

[Doug Reid]

Could be a bad spark plug or wire. Let engine cool down...start engine and run on problem mag only...after 30-45 seconds, shut down engine. Look for coldest cylinder by feel. Check spark plug and wire going to that cylinder.

 

[Dan Thomas]

All the plugs need to be removed, properly cleaned, and pressure-spark tested to 135 PSI. Anything less is inadequate troubleshooting. If they're Champions, look down the connector well and see if the little silver contact down there has a screwdriver slot in it. If so, those are the much-hated old-style plugs with the spring-retained resistor that causes many headaches. Dump them.

Many O-235s had two impulse mags. Does yours? This problem sounds a lot like a badly retarded mag.

 

[AlecT]

Thanks for the suggestions! Dan, I'll make sure the mechanic does a thorough check of the plugs. They're the fine wire champions although I'm not as knowledgeable as I'd like to be. I'll do a look-over when I head back over to the hangar and in regards to the impulse coupling, it has one, a slick 4371.

 

[GLDP]

If you end up changing plugs dump the Champions and go with Tempest UREM37BY’s. You’ll enjoy less time cleaning up lead fouled plugs and more time flying.

 

[Dan Thomas]

If you end up changing plugs dump the Champions and go with Tempest UREM37BY’s. You’ll enjoy less time cleaning up lead fouled plugs and more time flying.

And they're a lot cheaper than fine-wires. We used the UREM37BY plugs in an O-235 in a Citabria that had a horrible plug-fouling habit. Stopped that habit dead. The 235, at least in the Citabrias, runs pretty cool all the time, making fouling a hassle especially if it was in the circuit a lot, which it was.

 

[AlecT]

My 235 runs pretty damn rich, anything that helps keep those plugs cleaner sounds great to me.

And they're a lot cheaper than fine-wires. We used the UREM37BY plugs in an O-235 in a Citabria that had a horrible plug-fouling habit. Stopped that habit dead. The 235, at least in the Citabrias, runs pretty cool all the time, making fouling a hassle especially if it was in the circuit a lot, which it was.

 

[Domenick]

My 235 runs pretty damn rich, anything that helps keep those plugs cleaner sounds great to me.

I'm pretty sure you can adjust that with the red knob and a set screw on the carb.

 

[AlecT]

Hahah, I do use the red knob aggressively throughout all stages of flight. Some more resilient plugs are just another tool in the tool bag The mechanic consensus I've heard is that the carb set screw will affect idle, which mine runs okay at. Seems the rich mixture is highlighted when given power, and being in the Northern US I'm hesitant to spent the labor hours tweaking something that'll operate differently come October.

 

[chemgeek]

Some basic ground operation advice is to lean aggressively during taxi. That will help keep your plugs from loading up with lead or oil prior to the runup pad. I leaned my AA-1A aggressively on the ground and never had an issue with lead or oil fouling.

When troubleshooting start with the simplest/cheapest/most likely failure modes. Spark plugs are the most obvious place to start for misfire issues. I used to remove, clean and test mine myself at the local airport shop. It's ridiculously easy to do. I would occasionally find one that was faulty when doing pressure testing. And occasionally the cigarette connectors get dirty or the springs go bad. I eventually replaced the entire harness, considering mine was 15-20 years old and looking a little beat up. I used to use fine-wire plugs to minimize fouling, but found that with proper ground ops leaning I rarely if ever get lead fouling issues.

 

[Power cat]

The o-235 in the 2 place Grumman’s runs fairly hot unlike the same engine in other application. As the above poster stated I too lean aggressively on the ground to the point where the engine stumbles upon any application of throttle beyond taxi rpm and have perfectly clean plugs.

 

[AlecT]

Thanks for the feedback everyone! After another inspection/ops check, the mag timing was off, the problem is now remedied.

 

[John B Reed]

Thanks for the feedback everyone! After another inspection/ops check, the mag timing was off, the problem is now remedied.

You may also want to check the history of your magneto's in your maintenance records.