Mag Switch Problem?

My RV-6A (O-320 150 hp carburetor) has suddenly developed a starting problem: Sometimes it starts right up, other times it cranks away without firing.

I noticed that if I stop cranking by allowing the key to spring back to "both", and then hit "start" again sometimes it will start up instantly, other times it just continues cranking away merrily without firing. So far, I have been eventually able to start the engine by repeated attempts, but it is hard on the battery.

Do I have a Mag Switch problem, or perhaps a mag issue? I realize that in the "Start" position, one of the mags is disabled to allow the timing to be retarded for start.

Mag checks, including hot mag check, are all normal, everytime. The only issue is on start. Anyone had this problem? Thanks!

It sounds as if your switch may be grounding the left mag in the start position. To check it, disconnect the "P" lead at the left mag and the wire running from the switch to the starter solenoid. Now check continuity between the left "P" lead (the wire, not the mag terminal) and ground with the switch in the start position. the "P" lead should NOT be grounded with the switch in the start position.
Be careful while doing this test as the left mag WILL BE HOT!
If you have 2 impulse mags, both should be hot (ungrounded) while in the start position.

mag switch problem?

http://www.aircraftspruce.com/catalog/elpages/ignitionswitchkit.php my rv starts much better after installing this switch kit.

PCHunt said:

I realize that in the "Start" position, one of the mags is disabled to allow the timing to be retarded for start.

Click to expand...

What type of mags do you have? You would only want to do this if you have one impulse-coupled mag and one regular mag. You would want to disable the regular mag during start-up. If you have dual impulse-coupled mags you should have both mags firing during startup.

Progress, however slight.......

Today I removed and disassembled the ACS mag switch. It had the red paint in the screws, indicating that the SB had been complied with.

The inside of the switch was pristine. I wiped it with a clean cloth, and lubed very sparingly with Lubriplate. Checked it with an ohmmeter, and all is working correctly.

Now thinking I need to check the left mag. Any ideas? Thanks.

I had a similar problem with a Cessna, the impulse arms on the mag were gummed up and wouldn't release when the engine was shut down. It wouldn't start because the impulse wasn't snapping when the starter turned the engine over. I cleaned the impulse and reinstalled the mag and it worked great after that. You can check this condition by turning the prop by hand and listening for the snap of the impulse coupling. If there is no snap sound then the impulse is hung up. Check this with the mag switch off and the mixture full lean. Be careful when turning the prop by hand as the engine could fire and the prop can kill you.