Excessive RPM drop on magneto check.

My brand new Lycoming IO-540 on my RV-10 is having excessive RPM drops when I do the magneto checks. The plane has yet flown and the engine has maybe 15 minutes of run time, but from 1800 RPM I get 100 and 230 RPM drops on the magnetos. Lycoming's manuals says it should be no more than 175 RPM overall and 50 RPM between magnetos.

One thing suggested in the manual was leaning the mixture and trying again, which I will do next time I'm out there, but I always understood that exercise was for cleaning fouled plugs. I can't see an engine with 15 minutes on it having fouled plugs (or could it?).

Any insights? My concern is that I have some sort of hardware problem that could derail my flight schedule.

Start with the basics and check timing. Is it rough when you switch to the bad mag? Check wire lead positions are to the correct cylinders. If you do pull the plugs, check that resistance is <5K ohms. I would doubt they are carbon fouled, but possible one is oil soaked from preservation oil and not firing.

If it is not missing on one or the other mag, then I would expect timing to be the problem. If it is late, the rpm will increase.

Are any of the EGT's dropping when you do the mag check? Will tell you if you are truly dropping a cylinder.

Most likely scenario is timing. The non-disabled mag when you have the larger drop is too retarded or the disabled mag is too advanced. So, if you are running on only the left mag and get the big drop, either the left is retarded or the right is advanced. Double check your timing on both mags.

The other possible scenario is one of the 6 cylinders is not firing when running on the mag that is producing the drop. On the side that is causing the problems, test each wire (resistance test) and check each plug (looking for 1000 ohms).

gasman said:

If it is not missing on one or the other mag, then I would expect timing to be the problem. If it is late, the rpm will increase.

Click to expand...

A late or retarded advance will result in an RPM drop and an early or further advacned timing will result in an increase in RPM. For clarity, if 25 is the target, 20 is retarded adn 30 is advanced. Also, at some point further advancing will reduce RPM.

Larry

Timing. What no one tells you is that Lycoming runs them with test-cell mags and harness. After testing those mags stay behind. Lycoming slaps new mags on the engine, the new harness is still in a bag, and ships. You need to check the timing.