Questions about magneto's on 7ECA Citabria

[Taildragger]

I own a Citabria 7ECA with an Lycoming O-235 engine and Bendix magnetos. Periodically during run up my right mag runs rough and and drops by 250 to 300 rpm. I typically go ahead and fly it anyway and after running awhile I check the mags again and usually the right is good and has a smooth 50-100 rpm drop just like the left. I was told by a mechanic that this is due to lead fouling of a plug that my engine is going to be prone to this since it is the lower compression O-235 and I am running 100LL with more lead than is needed. I started using TCP in the fuel which seems to have helped but it still has the rough drop on the right on initial run up more often than not.

A few of the questions I had regarding mags:

1. If this is lead fouling why does it always only occur with the right mag and never the left?

2. Is it possible to have a weak mag or is a mag typically good or bad?

3. How do I know if I need to get the mag rebuilt and if I do where is the best place to get this done and what is the ballpark price I should expect?

Thanks!

 

[kyleb]

Shut the engine down when the engine is misfiring. Pull the 4 plugs that are fired by the mag with the big drop. If one of them is fouled, then you have a fouling problem. Maybe that cylinder runs richer or cooler than the others and you can fix the problem temporarily by cleaning the plug and longer term by installing a hotter plug and by leaning aggressively on the ground. The O-235 in my old Tomahawk was a notorious plug fouler but we learned to lean it on the ground and that really helped.

Oh, if you pull the plugs and none are fouled, you probably have a mag problem.

 

[Taildragger]

Thanks for the feedback. Yes I always leaning aggressively for taxi which has made a big difference. Do you know what hotter plug you would run for that particular engine? I believe I am running Champion 40E's currently.

 

[comanchepilot]

Yep - 90% plus chance it is a lead fouled plug on the bottom of the cylinder.

You might have a cylinder running a little cooler than the other 3 meaning that the plug fouls at low rpm and cooler temps.

Can you clear it using a lean mixture and higher RPM usually? Then its likely lead fouling. It could be as simple as using a 32 plug instead of a 36 or going to a 36 from a 38 . . . you want the plug to run a little hotter which burns off the lead preventing fouling.

I had a problem with fouling but replacing the plugs with hotter plugs, properly timing the mags and having good plug wires made all the difference in the world.

 

[Taildragger]

Thanks for the feedback!

 

[Dan Thomas]

The O-235 is famous for running too cold and accumulating lead on the plugs. You might have a colder plug among the four plugs fired by the right mag. Look for an REM38E instead of the 40E that should be there.

Best solution for that engine is a set of REM37BY plugs. They can't foul like 38's and 40's. They fixed our O-235 fouling problems.

Dan

 

[bnt83]

Best solution for that engine is a set of REM37BY plugs. They can't foul like 38's and 40's. They fixed our O-235 fouling problems.

Dan

This is the best solution. , in a pinch, a guy could just put 4 of them in the bottom spark plug holes.

 

[Silvaire]

Yes that engine is notorious for this and it's always the right mag because it is always a bottom plug that fouls. The BY plugs eliminate the well in which the lead accumulates and I believe are rated as a "hotter" plug due to the lack of heat sinking metal in the body. (that doesn't mean they have a "stronger" spark)

The only thing that is not ideal about that solution is the cost. If you want to defer that cost until the time when you actually do need to buy new plugs you could get one of these for $30 and just clean the lead out of the bottom plugs every 25 hours or so. Maybe make an arrangement with your mechanic - you'd also need a torque wrench.

 

[Taildragger]

Thanks! Probably a stupid question but is there any premature wear on engine or other negatives to using these 37BY plugs?