Hot Starts

[GAHorn]

Anytime a "mag check" (1700 RPM runup check) is performed and there is NO mag drop on one or both mags.... it's time to suspect that the mags are not grounding with the switch. Immediately perform a "hot mag" check by switching both mags OFF to ensure the engine shuts down.
Remember, EACH mag MUST indicate a "drop" when one is selected off or one of the mags is not grounding properly.

Depending upon the exact make/model aircraft and/or mags and or mag-switch, a complete check should be performed at 25 or 50 or 100 hours (depending on make/model). A safe thing to do is simply check it and log it every 25 hours minimum.
It's easy to do: Switch both mags OFF...and the engine should die. Start again and perform a regular magneto drop test in which each mag should indicate a drop in RPM at the specified RPM (1700 in a standard 170). Up to 200 RPM drop is acceptable, and the two should not be more than 50 RPM different from each other. No roughness indicating a "miss" should occur.

If one mag drops, say 50 RPM and the other drops ZERO... then somethings wrong.
If both mags drop exactly the same amount... that's OK.
If neither mag drops at all....then somethings wrong.

If the key can be removed from the switch with either magneto operating...then the switch is bad or the key should be re-cut. (The key should only be removeable in the OFF position.)

PS: One more thought: I don't like to switch the mags off, letting the engine begin to die, then switch mags back "ON" to prevent complete shutdown. I believe this abuses the crankshaft counterweights on floating crankshafts. If you deliberately...or INADvertently switch mags OFF...then let it DIE completely. THEN simply make a re-start (and shut it down with the mixture if desired..or go fly if that was merely a mistake during runup.)

[bagarre]

What about the mag check during your run up ?
The rpm drop indicates the mag grounded properly.

I've never had an issue with hot starts and only need to prime a few shots if it's below 40 degrees.

Funny, I have the opposite problem as it won't shut down once warmed up. The mixture cut off is against the stop but it keeps chugging as it pulls a little more gas I end up pushing the throttle in a little to get it to quit. My carb could use a good going over.

David, You could also check your primer it may be leaking internally.

In hopes of it just being the primer, I bought new o-rings and re-built the thing.
...but the motor keeps chugging along. Just as the prop is about to stop, it gets enough fuel from somewhere to kick and throw the prop around again...it can go on all day if I don't push in the throttle enough to make it die.

Since I'm in the early stages of an IO-360 swap, I'm hesitant to put any money into the carb for a leaky mixture.

[minton]

It smacks of a leaky or mis-rigged mixture valve given you've looked at the primer although I would take the primer outlet line loose to make darn sure.

[n2582d]

Mike, Here's how to start it when the battery is dead.

[blueldr]

George,
I was taught, early on, to switch the mags to "OFF" and then back to "ON" at idle to make sure that they were safe in the "OFF" position. I have been doing it for about seventy years now on engines from about 65 to 4000 horsepower and, up to this point, I have not heard of any ill effects to the engine. I also have never had an engine kick when it was being pulled through by hand. I can't immagine that doing so would kick the counterweights any more than a normal start.

[minton]

In the past I've come across two hot mag situations and been totally surprised. One fresh out of annual.

[GAHorn]

George,
I was taught, early on, to switch the mags to "OFF" and then back to "ON" at idle to make sure that they were safe in the "OFF" position. I have been doing it for about seventy years now on engines from about 65 to 4000 horsepower and, up to this point, I have not heard of any ill effects to the engine. I also have never had an engine kick when it was being pulled through by hand. I can't immagine that doing so would kick the counterweights any more than a normal start.

It's hard to disagree with an experienced old geezer like yourself, blEldr... I was only repeating what my own sage, mentors of your vintage always taught me, and it seemed to make sense to an imaginative lad such as myself to avoid the supposed "thrashing back-and-forth" of the counterweights. (Of course, none of those sage mentors demonstrated your success at longevity so perhaps their survival skills were never up to YOURS.

(Thanks for the input.)