Bendix Mag SF-4 issue - 400 rpm drop on right mag/sparkplug.

[Oklahoma flyer]

I was on the list looking around for information on Bendix Scintilla SF4's and found your post. After reading your post and hearing the suggestions about the plugs possibly being the trouble. I wondered this why not switch the bottom and top plugs? Then if the problem follows to the other mag then you know you have a bad plug. If it stays the same then the problem must be the magneto or harness. We used to do this all the time in the Air Force, if we found a suspected bad part we would often swap parts and see if the problem followed.

Dan

[Oklahoma flyer]

..

[Oklahoma flyer]

[skyking3286]

Update, I've flown it twice to check on it. Yes, after a 5 to 10 min. warm up the mag checks are perfect. Absolutely no issue at all. Easy to start, idle is great, no real issue at all.

Upon landing after an hour flight, right mag is terrible. Rough, at full throttle it's only 1600 rpm, maybe creeping up to 1900. This last flight I let it run until it stopped. Backfires, rough. Pretty poor. Spark plugs on the right look just fine.

So gang... is the coil bad?

And the name of that fellow that fixes SF4?

[Richard Murray]

Mark,

My limited knowledge and experience suggests coil is shorting internally after sustained heat. The old swap technique has also worked for me with many things.

Why not swap only the mags that way you eliminate the harness and plugs. If the problem follows the mag I'd put money on the coil.

Richard

[clipperfixer]

I sent you a PM with my friends cell number. I told him he might be getting a call from you.

[skyking3286]

The plot thickens...

I changed the plugs, keeping them on the same cylinder on the engine, top to bottom, bottom to top, L mag to R mag was the plug rotation.

Then I started it up and ran it. Started out with 100 degrees and up to 170 on the oil temp, rpm at 1500 with a little run up to 2000 which is my normal static rpm at full throttle. The test ran for 20 minutes, which is about as long as I like to run it on the ground. OAT was 65 degrees.

Interesting. Every two minutes I did a mag check, L to both, R to both. No rpm drop, no rough mag, no issues at all. After a little bit at idle it shut off just fine with the switch, so the P leads were fine. I would say it's fit to fly. So next step is to take it up after the low pressure system hovering over Seattle moves east, and let it run for an hour in the air and then check if the mag issue reappears and if so, does it move with the plugs or stay on the right mag.

thanks for the PM on the mag fix... that's still in the mix, but not quite now. At least until I figure out what the plugs are doing. Found out that you can't use a digital
OHM meter on the plugs, since there isn't enough voltage running through it to make the resistance an issue. Need to find a old fashioned needle meter.

[Oklahoma flyer]

Mark,
If this next step leads you to supsect a coil I wanted to let you know that you could actually swap coils fairly easy without removing the mags. That way you don't have to break gaskets and re-time the engine.
Just remove the four screws on top the coil and condenser wire to the p-lead post and then the 2 bolts/screws holding the coil down. I have actually replace a coil before without removing the mag. FYI also Aircraft Spruce carries coils for about $132 each or Fresno for $97 each, but with Fresno you have to mail payment then wait on your parts or COD them. It just depends on how much of a hurry you are in the recieve your parts.

Dan

[Al Hatz]

Fresno for $97 each, but with Fresno you have to mail payment then wait on your parts or COD them. It just depends on how much of a hurry you are in the recieve your parts.

FYI, Fresno has entered the twentieth century and now take credit/debit cards over the phone.

[skyking3286]

Yes, the next step is condenser and coil!

We have a thermal trough over Seattle now and it's going to hit 90 degrees this afternoon. This morning I took the Chief up for a 42 min flight, about half of what I was doing. I know I 'm good for 20 min on the ground, so I wanted to heat things up a bit and test the spark plug wire theory, since I had moved the plugs bottom to top on each cylinder. Mag check at start of flight is perfect.

Nice flight, oil got up to 192 degrees, which is about as hot as it gets in Seattle at 2000 feet MSL. We had a inversion at 2000' feet which bounced me around a bit as I passed through it. Our U of W weather professor remarked about it on his blog (September 11th 2013 entry at http://cliffmass.blogspot.com/) That Chief ran smooth as glass the whole time, landed on the grass runway at Harvey, got off at first turn off of the runway and immediately checked the mags.

Left mag is fine. No real drop, smooth. So there goes the spark plug theory. Bottom plugs are now on the top and doing their sparkie sort of work just fine. Both mags.
Right mag, hold breath. There it is, about 120 rpm, which goes to a 150 rpm drop, smoother than before but if I let it run very long on that one mag, it starts to loose even more rpm and run rougher. So I'm thinking that the spark is marginal and the coil and condenser need to be replaced. Thanks for the heads up on the coil removal and replacement. It looks like a straight drop in and it's nice to know it is. I have an annual inspection in November.

Hello Fresno.

[skyking3286]

Ordered NOS coil from Fresno Airparts... this is what you get.
Funny, a coil that is new but older than my plane!

Flight experience since last post: Flying .2 around the pattern gives no right mag issues.
Run up is perfect at all times. Flying for .7 does it in cooler weather, but not as severe as late summer flights when it was warmer.
I have both the condenser and the coil. Might try just the condenser, which is the thing
a lot of folks have said is the first to go and heat affects. Annual inspection is on Tuesday.

[Richard Murray]

Mark,

Inquiring minds want to know was the can still vacuum sealed after seventy years?

[skyking3286]

I'm worrried about that too.. I'll share the results after the annual inspection on Tuesday! I'm not going to pop the can until I need it!

[EDGEFLY]

Mark,
Don't dwell on the negative, that can may have a beauty inside-Like your Fuschia on the table with it.

Dale

[skyking3286]

Update:

The annual went well. Here's the mag as we took it apart. I think these might be the only Bendix SF4- photos on the net!

First of all, no swoosh when we opened the can. I think that old musty smell is a wonderful old smell, BTW.

http://vimeo.com/80692042

Here is the new old stock coil (L) and the installed one (R) that was put in the mag in 1968. You can see where the carbon brush has made it's mark on the old coil.