Missing 300 RPMs

Saturday, I flew my O-320 powered Super Cub to Oregon, seeing about 2500 RPM at max throttle, along the whole way. I'm still learning about this plane, and this was my longest cross country flight in this plane to date, flying about 2 hours. I flew at 5500', and although I kept pulling the throttle back to bring down the RPMs, pretty much every time I stopped looking at the tach, I crept back up to 2500 RPM (I must be a bit ham fisted).

Anyhow, yesterday when I began the trip home, all I could get was 2200 RPMs. I flew to KLS, then OLM, but even after carb heat, max throttle only got 2200 RPMs. During the run up, the mag drops were normal, and carb heat operated properly. I decided to hope that it had something to do with the rain, and I did clear a bit of carb ice once, so maybe the 'at-mo-fere' was doing something I thought... I had to leave the plane at OLM because of weather, and when I brought it home today under a broken sunny and dry sky, I still couldn't get more than 2200. I'm starting to think something is probably wrong.

When I got home, I started to chat with the local mechanic Phil. He had some suggestions, but seemed too busy to have a look. He loaned me an electronic tach (thanks Phil!) and my mechanical tach checked out, max I got it to read electronically was 2250 RPMs. Phil suggested I examine the carb to verify that it was hitting the full throttle stop; it was. He also said I should verify no obstuctions on the air filter; I didn't take it apart, but nothing seen.

This engine has an undesireable history. Although it has only about 400 hours on it, it entered service (with the rest of the plane) in 1980. The plane worked herring spotting on floats in AK for three years, before being damaged in a wind storm in 1983. In 1985 it was brought back to Washington State on a fishing boat. It was a project from 1985 to 1995 (replacement wings, fuel tanks, struts, etc.), and it was signed back into service in 1995 at about 320 hours. An almost immediate prop strike incident resulted in the engine being torn down, and having a bunch of service (including an AD), which has been described to me from the minimal documentation, and the reputation of the involved parties, as prudent. I'm inclined to believe that. I have been told that the crank was sent for magnaflux inspection, and checked out good. The minimal documentation seems to support this. After the prop strike, the previous owner operated it about 20 hours one year, 10 the next, 5 after that... When I bought the plane a year ago, it only had 40 hours on the rebuild after six years. Poor little neglected Super Cub!

I've put about 50 (tach) hours on it in the last year. I'm planning on flying more, but I was somewhat limited by availability of my instructor before I got my conventional gear endorsement. As far as I can tell, it's been operating fine, this is the first strange thing I've been able to detect.

Compressions on the last annual, in May, were "1-72/80, 3-75/80, 2-75/80, 4-75/80".

I was leaning at 5500' during the cruise down to Oregon. I still need to verify the mixture control hasn't slipped (it seems fine, and some leaning at cruise did seem to get an RPM rise, indicating rich of peak).

I'm calling my regular mechanic, but anyone want to take any guesses? Check the compressions again? What could cause a 300 RPM reduction in max throttle overnight? Can my carb do something like that internally? Slipping butterfly? Sticky valve?

Oh yeah, and by the way. Why was the overflowing 100LL dripping out when I was testing the throttle linkage bright red, rather than blue?

Rookie, I can't tell you why the fuel is bright red (unless you are dumping in lots of Marvel Mystery Oil in the gas) but one thing to check as per your missing 300 RPM's is the muffler. Sure it's still in one piece?

Look up inside the exhaust pipe with a flashlight (and or a long stick) if you see something covering the opening (push up there with the stick to see if it moves), then that's your problem. There should be a wire cage welded inside at the base where the pipe joins the muffler. This is to prevent a broken muffler piece from completely closing off the opening and the rather distressing results of no ehaust oultet.

Had this happen before as have a number of others I know. That's why the 100 hour inspection AD on the current system. I had to put on a new muffler (rebuilt one actually). Makes the new LEES system more attractive all the time.

Brad

Hey, I just got off the phone with my mechanic, and he suggested the same thing. At least it should be easy to find if that's what it is. I guess it won't even be that hard to fix, just a bit spendy. (It looks like the hardest part is scheduling an A&P this time of the year.)

The LEES muffler? It sounds cool, but how many times the price of a rebuild is $2925, plus crating and shipping? How much does a new stock system cost? It seems like a like an awful lot to pay. It does sound pretty trick, but I've seen lots of cool new trick racing parts for cars not quite work out the way they should.

Rookie...............you might also check the condition of your intake hoses. I had a similar situation where the engine was running super rich and didn't pull the proper RPM.s.......found the back side of the intake hoses cracked.

300RPM loss

I'd check the throttle cable...........especially since you kept gaining RPMS initially and then loss 300rpm's. Sounds to me, that the housing on the throttle cable might be creeping. There's a standoff clamp, that clamps to the motor mount and to the throttle housing....The cable might be floating in that clamp. I had a friend that happened too. Was so bad, that when he pulled back on the cable to land.............and then needed a little power to get to the airport, well.he didn't have any control at all....no power....and had to land in a field short of the airport......luckily nothing got bent on him or the plane.........was very lucky, because the field was very short and surrounded by trees. Throttle housing should also be clamped to the firewall just after comes thru the firewall on the engine side of the firewall. Muffler could also be a possibility as mentioned in other postings. Good luck and happy flying. "Brian"

It was the muffler

Throttle linkage was one of the things I checked before focusing on the exhaust.

I'll be positive when my repaired muffler is reinstalled, but I'm pretty sure that's what caused my problem. I'm sorry to report that a while earlier (I had forgotten this), I goofed badly, and while doing a mag check, mistakenly shut off both mags. I then made an even worse error by turning them both back on. I didn't realize at the time that the loud backfire I heard had caused a problem. I think something shook loose later, thus causing the overnight loss of 300 RPMs. Anyhow, I did look up the exhaust stack, there's a picture in the gallary of what I saw. Today I got the muffler out and sent it for repair/rebuild (see my other topic), and there's also a picture of the damaged insides that I just put up. Here:
http://www.supercub.org/gallery/view_photo.php?set_albumName=Supercubs&id=agc
At least I think the damage is from the backfire. I guess the flame tube can distort like this over time, so maybe I just cracked something loose with my error. I'll know for sure that this was the source of my problem when I get the repaired muffler installed and operational.

You mentioned that you had a storm to sit out etc. Perhaps your filter got water in it- could check that too. Your throttle creep sounds familiar. Mine creeps forward if I set it at anything over 2350 RPM below that it stays where it should. Also I assume your mag check shows equal drop on both mags??

I was thinking water, but that's not it at all, the correct answer was discussed in a separate topic, here:
http://www.supercub.org/phpBB2/viewtopic.php?t=257

OK
Didn't know if that was your problem or you were just going to have it fixed anyway. I've never had one run that good when the guts fell to the bottom. Usually 2000 rpm. If you send it to Dawley, you basically get a new muffler. Just the industries way of making almost new mufflers without a PMA-- just need to have one little piece that is original to be a repair. Bottom Line will be final price including shipping and what kind of reinforcements are put on the muffler.