suppose i should know this but the last several times i've checked mags i have a drop of both left and right mag of 300rpm. they both drop equally but 300 seems a bit much. any ideas what this is about or am i just stupid and not to worry.

yellowbird wrote:suppose i should know this but the last several times i've checked mags i have a drop of both left and right mag of 300rpm. they both drop equally but 300 seems a bit much. any ideas what this is about or am i just stupid and not to worry.

I agree. 300 rpm is a little much. I was taught that anything more than 125rpm should be looked into. For some reason your ignition isn't making full juice and it might be time for an R&R.

Register and vote Republican and that will fix it. Just try it for 10 years.

Tim

qmdv wrote:Register and vote Republican and that will fix it. Just try it for 10 years.

Tim

Agreed

My votes on plugs

Ssounds like both mags are incorrrectly timed.

I always try for a 75-100rpm drop, what was your drop before you noticed this? If it's getting worse then start with plugs, if it's always been high then I agree, bad mag timming. Does it start ok?

Read your POH and/or checklist. Depending on the engine, mag drops should be no more than 150 to 175 on either side and no more than about 50 difference.

DO NOT fly an airplane with that much mag drop-get it to a mechanic.

MTV

In every aircraft I can remember flying, the POH/AFM says that the drop shouldn't exceed 125-150 RPM, with a max differential being 50 RPM. I'd have it looked at too.

How long since they been o/h'd?
Gary

Check the plugs first
Then go from there.

Don't overlook rich mixture.

I would look at all of the above...if the engine drops rpm and get's rough, look at plugs, check mixture while on one mag or the other, have the crank to engine timing checked, have the internal mag timing checked (points opening to mag coil timing), points and condenser in the mag checked, Have the spark plug wires checked for leakage, intake manifold vacum leaks, etc. I agree, look in the POH to see what drop is acceptable, if a simple test cannot find the problem get a mechanic to look at it. You may be betting your life on a timebomb that might let loose at any time.
HC

What was your mag drop before it went to 300rpm? Did it go there gradually or all of a sudden?
One bad spark plug would not cause this problem.
I would pull and clean all plugs and retime your mags. It is easy and something you could do in an hour or so.
I would also look at your tachometer. Is it accurate?

Good Luck

flyer

The odds of spark plug fouling causing BOTH mags to drop symmetrically is pretty unlikely, I believe. Find a mechanic--I'd bet it won't take a good mechanic half an hour to figure it out. Fixing it might take longer....

MTV

it seems the last few times i've checked the mags they have been reading slightly less each time. think what i'll do is run the engine up to 23 or 2400rpm and lean it out to see if that helps with the plugs. maybe even do it to each mag, and then check the mags again. hopefully it's only a plug or timing problem. i tried the republican thing to no avail so i'll try something that should work. thanks

This is a little silly, but make sure you're running your mag checks at full rich after leaning it out to clear the plugs. The big drops you're seeing on both mags could happen if you're running your mag check while leaned out far enough.

Don't ask me how I know...

When I was shopping for an airplane a couple years ago, I checked one out belonging to a guy at the airport I flew out of. He told me how, one time during a runup and mag check, on one mag the engine wouldn't run at all ... so he switched to the other mag, and took off.

I didn't buy his airplane.

Oregon180 wrote:This is a little silly, but make sure you're running your mag checks at full rich after leaning it out to clear the plugs. The big drops you're seeing on both mags could happen if you're running your mag check while leaned out far enough.

Don't ask me how I know...

Ditto. I was about to taxi back to the hanger after a mag check on a cold day and I thought to richen it up. I usually have my mix screwed out about a knuckle for my 5000' strip but that day when the mags were dropping I punched the density altitude computer and it said 2800. I went full rich and the problem went away.

Permissable mag drop on a Franklin is 125 to 225 rpm with a max difference of 75rpm.
Carb heat system can also cause the bigger drop.