Running at redline

[Speedo]

I need some diagnostic help: I run an O-320 B3B (160hp) in my PA-12. The engine is approximately 600 SFOH. For roughly the first 550 hours the maximum achievable RPM in a 60 mph climb out was around 2450. Recently, though, I have been seeing 2700 RPM in a 60 mph climb out. This suggests to me the following possibilities: a) I have a miracle engine that gets stronger with time, b) I have a miracle prop that loses pitch with time, c) the plane's tach is wrong, d) the throttle formerly could not travel its full range, but now can, e) there was some other constriction in the fuel flow which has somehow corrected itself, or f) none of the above. If you know a diagnostic test I could perform to ascertain what has caused this change, please pass it along.

FYI, I have checked the plane's tach against an optical tach, and have found the plane's tach reads 50 rpm high at 2500 rpm.

Also, I have not changed anything about the engine or the prop.

 

[StewartB]

What are your mag drops? How is your CHT? Has your mixture to lean at cruise changed? What is your carb heat doing to rpm's at run-up? Have any of these changed?

 

[Jerry Burr]

Freightliner.

Hi Tim. Quite a few years ago, a fellow in California did a test on a J-3 with three engines. A 65,85 and a O-200. The tests showed the O-200 to be the worst engine of the three. That's as far as power. It took me a while to understand why he came to that conclusion. Here is what I came up with. It depends entirely on how your engine was rebuilt. In this case the O-200 was held to closer tolerances than the smaller engines. If you have an engine that is all new parts and they are (tight), it isn't going to make power for a long time. It will also run hotter longer. The Fed's were not going to approve my PA-11 cowl because of high CHT's. It took 300 hrs for them to come down, and nothing was changed. Another example. When I was a lot younger we rebuilt a flathead in a 49 Ford. By the book. It wouldn't pull itself out of a wet sack. We had a meeting with a little too much brew and deciced to just blow the darn thing up. We drained the oil and water out and drove home from the shop. The next morning realizing that it was our only transportation, we replenished the vital fluids. From that time on it was the fastest flathead in town. We probably put 10,000 miles of wear on it that night. I think that you just had a very good overhaul, and it's just starting to really get broken in. It will no doubt last you a lot longer than engines that are strong (loose) to start with. If it gets a lot stronger you can always add a belly pod, to slow it down. Think of the freight. Take Care. Jerry.

 

[PA12driver]

Jerry,

Good info, but if you are thinking you are talking to PA12driver? sorry wrong guy? I have never been that fortunate to gain 200 rpm!! I have had the Cylinder temp gauge start showing lower by about 50 degrees?? Probe and connections, baffling and cowl seem unchanged?

I think I was at your party when we drained the oil!! (Probably don't remember huh?)

Tim The other PA12 pilot!!

 

[Jerry Burr]

Oops.

Sorry Tim. I had a senior minute. Hope the PA-12 Pilot picks up on it. Thanks. Jerry.

 

[PA12driver]

Have have them regularily! Happy Thanksgiving! Going flying with 5 other cubs tomorrow, I hope to be able to take some good pictures, I will post some new ones if I get some good ones!

Later,

Tim

 

[Speedo]

Reply to Stewartb

Stewart: Mag drops are normal, do not have a CHT, but do have an EGT. The EGT climbs as high as 1470 on climbout, and I am throttling back to around 2500 rpm as soon as I get to 300' AGL to keep temps from getting higher. While flying yesterday the engine speed went as high as 2750-2800 on climbout (while pitched for a 65 mph climb). Go figure.

I'd like to think Jerry Burr's theory is right, but it sure is counter-intuitive: 600 hours is a long time for a break in!

I'll be changing the oil shortly, and will be holding my breath until the oil analysis comes back. All prior analyses have been OK, and I've control charted the results, and the control charts show no evidence of abnormal variation.

Let me know if you have any other ideas that might explain this change.

 

[PA12driver]

PA12pilot,

What prop are you running? I have an 0320A2B modified to 160hp and currently running an 82/41 borer prop and I get 2450static and as soon as the wheels break it will start upward and will easily do 2700 at a 60mph climb. I usually trottle back a little on extended climb. I have a single CHT in the #3 cylinder and it used to run consistantly at 300F? For some reason I am now showing 260f consistantly and I can't get the cyld temp over 300 even when leaned to just slightly rich of peak.

Have you taken an optical tach with you in the plane and tryed at different RPM. Sure sounds like a tach??

Are your cruise speeds any different then before?? How about takeoff performance?

Tim--PA12driver

 

[supercub]

I know you said you checked your tach at 2500, have you checked it at all ranges. Sure sounds to me like a tach problem, amazing that you're getting 2700 on climb out............must have a very flat prop to turn redline on climb out. Good luck and hope it's nothing serious.
Brian

 

[Speedo]

Prop is an 82x44, optical tach check at 1700 and 2500

The prop is an 82x44, and I've used an optical tach at two power settings: at 1700 indicated the optical tach read 1600, and at 2500 indicated the optical tach read 2450. I don't think these are significant errors.

At a constant power setting the tach typically reads smoothly - that is to say, there is very little variation in the tach readout. However, it will occasionally drop 50 rpms for a fraction of a second. I noticed this twice during yesterday's 2+ hours of flying.

I can remove, clean, and relubricate the tach cable, which might make me feel better, but which probably won't have any impact on the problem.

 

[supercub]

I'd still check the tach with an electornic strobe checker, one that can check the full range of operation.........we use one in the shop all the time. My tach in my cub is right on below about 1000rpms, and is off about 75rpms at 2400..........With you seeing a drop of 50rpms intermittently, still sounds suspicous. Anyway, it's an inexpensive check to make. Good luck......
Brian
PS: The one we use in the shop is about the size of a small portable am/fm pocket radio. We just set it on the dash facing forward towards the prop, has a setting for 2 or 3 blade prop.