So my O 470 is dropping about 110 RPM on each mag during run up. Cold starts are also rough and its taking a few minutes to fire on all cylinders. I assume it's time for a compression and timing check. Anybody have any thoughts or experience you want to share? Thanks!

Clean and gap ur plugs

How long since last magneto service? Points and condensers do wear out. Any roughness? Or just smooth even drop?

Even drop, pretty much the same on both mags. very rough on cold start though. That's kinda making me think compression issues.

I wouldn't lose any sleep over 110 rpm, even drop on both mags. If I recall correctly Continental allows up to 175 rpm drop on the O470L.

Flynengr

flynengr wrote:I wouldn't lose any sleep over 110 rpm, even drop on both mags. If I recall correctly Continental allows up to 175 rpm drop on the O470L.
Flynengr

I think so, but not sure. On my O-470R, I see 125 all the time when it's warm out, 150 when it's cold. When it is really cold, I get 200 unless I add carb heat to bring it backup. I doesn't seem unusual to me. My plugs are clean & gapped, 74+ compression all around, and the timing is right on. I have not inspected the innards of the mags, but they light up my spark tester fine. The rental I used to fly was the same way. My TCM O-470 manual offers 150 RPM as the max drop. Lots of folks have shown me even 300 RPM on cold mornings on big bore carbed TCM's without carb heat, and 150 after pulling heat on.

Does anyone here have a different experience with a carbed O-470?

I've worked on these engines for 40 years and have one on my C180. Many people sand blast their plugs. This makes them look real clean but taked the sharp edges of the center pole. Not a good idea, can ruin low time plugs. The blaster has a pressure tester on the machine. If the spark is strong then fades under pressure plug is no good. I clean mine with a pick and wire brush then swap top to bottom as lead will build up in bottom plugs. If mag drop is even on both mags I very much doubt if problem is mags or leads. Each mag powers top plugs on one side and bottom on the other. Have you tried leaning the mixture before checking mag drop, could be too rich. Usually I suspect a cylinder problem when drops are even, small crack or sticking or burned valve but yours sound OK. Hope this helps.

One more idea. Check the adjustment and wear on your carb heat gate. Is it partially on? If you have an intake air filter, is it dirty. I don't use one as mine stays on floats so no need. Check your carb heat scat hose, they will collapse with age restricting air flow. This will show up with a larger than normal drop when you check carb heat on runup.

flyingzebra wrote:So my O 470 is dropping about 110 RPM on each mag during run up. Cold starts are also rough and its taking a few minutes to fire on all cylinders. I assume it's time for a compression and timing check. Anybody have any thoughts or experience you want to share? Thanks!

You've got some good advice on spark plugs already.

If it hasn't already been done, cut a piece of foam pipe insulation (the split stuff you find at Home Depot to insulte pipes in your house) and tie wrap it around the induction crossover tube at the front of your engine. This is the small diameter tube that runs between the two induction logs on each side of the engine.

Without this crossover tube insulated, these engines sometimes run rough in cold temps. Depends on the cowling and how cold, but doesn't have to be screaming cold. Doesn't cost much in any case.

MTV

Thanks for the very good help all. I've just never had a motor drop that much and your experience is great to draw on. I'll try the carb heat thing and, next time the cowling is off, will try insulating the crossover tube as well. For now though, it sounds like the consensus is to just keep flying!

RPM drop aside, you sure you don't have a sticking valve? If it's "not hitting on all cylinders when cold" shut it down while still running rough and see if all six jugs are making heat. A stuck jug will be cold.

Gump

I have an O470J with about 1300 hrs on it. Last compression check ranged from 64 to 74. Mag checks drop are routinely in the 150 rpm range. There is no roughness, and the drop is the same between mags. Moreover, I watch the engine monitor (switched to the normalized mode), and EGT on each cylinder goes up as I switch from L to R, and the EGT's go down when switched back to both. If I saw an EGT drop on any one cylinder during the L or R position I would be much more alarmed.

I have the roughness after start issue as well. After starting all cylinders fire up, and it begins to smooth out. However, shortly after it gets running on all six cylinders, the #5 cylinder quits, and the engine runs rough. In order to get the #5 cylinder back running again, I have to lean the mixture out until just before the engine begins to lose RPM. At that mixture setting the #5 cylinder fires up again. I continue to let the engine warm up, and taxi with the mixture setting as lean as possible and still keep the engine running.

When I get ready for run up I set the mixture full rich and run the RPMS up to 1700. Then I lean to maximum RPM keeping the same throttle setting then turn the mixture rich a couple of turns. When I follow this procedure I usually get a good mag check.

Most of the time I run auto fuel (mogas). However, when I run 100LL it is especially important to idle and taxi as lean as possible. If I taxi and idle at a rich mixture setting I experience a big increase in fouled plugs, engine roughness, and bad mag checks.

Anybody have any ideas why my #5 cylinder quits shortly after startup?

DEGJR.

I taxi my 470 extremely lean. Big mag drop if I don't richen on runup. Also use primer on all starts. Summer and winter. Good tips here on plugs and valves. One new item I learned this year, the after market condensers put in many Bendix at 500 hour check are crappy. They will cause all kinds of strange symptoms that are wildly inconsistent. Cheap to change out compared to valves though. MTV is right about the cold crossover tube.

The local magneto shop (Aircraft Magneto Service in the Seattle area) has a troubleshooting guide entitled "Is it The Mag?" (http://www.aircraftmagnetoservice.net/id27.html) The second paragraph says "most bad mag checks are sparkplug related". That's been my experience too.
I'm not a believer in "bomb" (pressurized) spark plug testers. Per that troubleshooting guide, a better idea is to test resistance through them with an ohmeter. New plug spec is 800-1200 ohms, replace if it reads over 5,000. Spark plug life is generally figured to be around 500 hours, although in my experience, a Bendix mag will continue to spark a bad plug longer than will a Slick mag. Also in my experience, sometimes a mag check at 1500 rpm or so will seem OK, but checking them one at a time at cruise or full power in flight will turn up a problem.
Although it would be logical and nice if the mag's could go the life of the engine, the condensors & points aren't up to it. 500 hours seems a bit soon to me, but definitely after a 1000 or so they''ll be needed.

Another thing to keep in mind is that just because your differential cylinder test is good dosen't mean that your rings aren't worn somewhat. If a little oil is getting by them to your plugs, giving you the mag drop, the oil will also fill your rings to hold the pressure. Hope I worded that properly.

I suspect that I live near you, and I think that might be pertinent to this subject. There are times when the weather is wet for long periods of time that my 180 (O470L) will run like crap (even and equal mag drop, and even more than yours) for a while on start up. Carb. heat on or off doesn't seem to make much difference. Get it warmed up and all is fine. I guess it just has to bake out all that moisture. Or maybe just has to get warm enough to make the Gremlins uncomfortable..........

Where ya based, Zebra? I'm in Port Townsend.

KBVS. Pretty sure I met hotrod150 at the Spruce Goose a while back. Flight instructing and commercial Piloting? I think I was flying the ugly green/white 206 then... Now it's the ugly orange/brown/white/missing paint entirely in spots 180.
Hmm, do planes reflect their owners like dogs do?....