Alrighty. I have a head scratcher, or maybe not. The Champ has been a bit finicky this summer. I have chatted with my mechanic several times and he is coming up to my hanger to do the annual in two weeks. He is about 2 hours away and we are 40+ miles from the nearest shop, so I am trying to diagnose as best we can before he gets here. Looking for thoughts or prior experience. The plane is a 7AC with an A-65 and Stromberg carb. The carb has about 250 hours since a rebuild.

From the beginning: Generally the plane starts fine. A couple shots of prime, 2 blades with the mags off and it usually pops off on the first or second blade. It can be a pain when it's hot, but I have always been able to get it started. If I flood it, I clear by turning backwards 16 blades at full throttle and start at idle. Roughly a month ago it was fairly hot (high 80's) and humid. I flooded it trying to start it. Could NOT get it started after multiple attempts. Walked away and stole my friend's 172 for the afternoon. Went back the next day and still couldn't get it started after multiple attempts. Called my mechanic and on his advice, we checked the screen in the carb - clean, fuel flow - plenty, turned the gas on to see if it dripped through the airbox just sitting there wondering if maybe the float was stuck - nope. Pulled the plugs and checked for spark, all good. Put it back together and tried starting it. Finally got it started by holding 3/4 throttle. It ran fine. Taxied around, did a run up, checked static RPM and idle, all good. Flew it around the pattern a couple of times and called it a day. Chalked it up to hot & humid and started flying regularly and everything was just fine until a few days ago. Went for a flight with Mrs. Burns. The plane started just fine, but when I locked the primer, the backing nut was a smidge loose, so I tightened it, not realizing I aligned the little slot with the tab on the plunger until later... We were high in the pattern for landing so I throttled back to idle (Carb heat on). Usually I clear the engine a couple of times, but we were plenty high so I got a little lazy. I realized during my slip the engine had quit. In reality it had probably quit while I turned base (There is a downside to noise cancelling headsets). It windmilled until about 40mph. Landed no issue, explained to Mrs. Burns why we had to push it to the hanger and started checking things. That's when I noticed the primer was out about 1/2". Figured I had just flooded it due to that and my mechanic thought that made sense. Problem solved. Yesterday, it wouldn't start again. It acted like it was flooded. Tried multiple times with no results. Went home for lunch prepared to steal the 172 again, even put gas in it. Came back and gave the Champ another try, Low and behold it started after a couple of attempts. Ran it through a bunch of throttle settings, all good. Decided to take it around the pattern a few times. During run up I got no drop on the right mag on the first try. Figured I just missed the spot in the switch. Did another and had good drop on both mags. The plane ran fine, Idled, with and without carb heat, no hesitation, good static RPM, all good. Did a few laps and landed. Turned the gas off and waited for it to quit. Got antsy waiting and turned off the mags, it kept RUNNING. Turned the gas back on and did multiple mag checks. The right mag did not shut off. I checked the P-lead, all good. My thought is that I have two issues. A carb issue and a switch issue, but I would welcome any other thoughts. Is the carb is set too rich (I have no idea if that can be changed), is the float possibly sticking or have some other problem that's flooding the engine. For the mags, my money is on the switch, not the mags. I have no idea how old the switch is. The mags have roughly 300 hours on them - old Eisemanns and I have never had a bad mag check since I have owned the plane. I do plan on checking them again this week. It seems pretty unlikely that the mags would quit at the exact time I throttle back. ANY thoughts would be appreciated. Is it also possible that the flooding and mag issue are totally separate and a coincidence? Sorry for the long winded post.

Pete

The Stromberg is a painful carb, especially for hot starting. I was never able to get 65, 75, 85, or 90 hp Continentals propped when still hot from shutdown, without the tail tied down and considerable throttle setting. They just need the air when hot and the closed butterfly doesn't let enough air in. Otherwise, little Continentals want to be dripping a bit from the carb and smelly to start. They don't want as much prime as a Lycoming, however, especially when cold. The big advantage to having a starter is that we can set the throttle near closed and crank through some blades. With the Stromberg carb, throttle setting is very critical. Closed for max slurp and open for hot start. Usually a quarter inch or so open is the right setting for cold start. An advantage to propping is we can smell the gas is there and hear the slurp when it is just right for cold start. Hot start is just tricky. It ain't going to go until the throttle is open just right.

A note on propping difficulty. Low compression makes it really hard to get enough gas but not too much. Be patient. Be strong. It will eventually start.

I had this issue with o-470 years ago, it was the mags. Impossible to start hot. New mags fixed it

Sounds like you narrowed the problem a bit.
Just check the basics. Spark-Air-Fuel
When you said the carb was rebuilt, who did it? They are simple but finicky to set up. Was it just rebuilt on the bench or was it sent to someone who knows? Rebuilt has a FAA definition.
The Strombergs are great and dependable. They just need tlc on occasion. The pin on the float does wear. Do you have a steel rubber tipped needle or a delrin needle?
Unvair has some SB data you can read online.
There should be a 380xxx number on the carb itself, just confirm you have the right one for your ship. You should be able to find the application for your motor. There are different carb set ups for the small Continentals. Don’t let anyone tell you they are all the same.
Does yours have the back suction mixture control? Is it making full travel if installed?
Yes it works at altitude, not on the ground, not as an idle cut-off.
If you have a carb with one that is wired off, make sure it is in the full rich position.
Mags are simple, maybe your e-gap is off? get some mag base gaskets on hand when your mech shows up. It shouldn’t take a long time to check.
Good luck,
Report back what solves the problem.

Cheers,
Dave

Thanks Dave,

That is great info. It was overhauled at a shop in Illinois. Delrin tip. I will let you know what we find. It may be a few days, we are stupidly busy work wise through the holiday.

Pete

So, it’s looking like a mag issue, specifically the left mag. Spent a bunch of time with an ohm meter the last couple of days and everything checks out good, but the left mag, which has the impulse coupling appears to have an internal short. Going to pull it and have it looked at. That would in theory explain the hard starting, possibly quitting on final and the engine not turning off. If anyone thinks I am on the wrong track or has any input I am all ears.

Pete

So, I think we have it solved. Got the left mag inspected. Turns out it had a loose wire inside. Specifically the wire that goes from the points to the condenser, which explains a lot, including the intermittent part. Everything else looked good. Had the annual today, put it back on and set the timing , buttoned the cowling up and she fired right up. Did a few mag checks, static run up and shut it down with the switch. Everything worked great. Didn’t get to fly it, maybe tomorrow after I get all the inspection covers back on. So, hopefully this was the only issue. Theoretically every thing that happened can be explained by this, we will see. Learned a lot about ohm meters and how useful they can be. Also learned that it’s a good idea to shut the thing down with the mags once in a while and if it doesn’t shut down, it may be something other than your switch or p-lead. This was just unusual enough where hopefully it can help someone diagnose an issue in the future.

Pete

Thanks for reporting back, Pete, Hopefully this helps someone in the future that might have this combo.