Engine performance mystery

[Eric Brown]

I have a Cessna 150L with a newly overhauled O-200 engine. The engine has been flown twice. On the second flight I noticed that the RPM, during climb, was only 2300 RPM. The engine ground static run with the overhauled McCauley 69-48 (climb) prop is 2500 RPM. Right in the middle of the C150 TC specs. After the climb out, I level out to cruise configuration with the throttle wide open all this time. The engine will only turn 2500 max. NOW, the really strange part. With the throttle still wide open, I can dive the airplane until the RPM reaches 2700. I then level out to cruise configuration and the RPM now stays a 2700. If I climb the airplane again where the RPM drops to 2300 and then level out, the engine will again only reach 2500 RPM, max. All this with a continuous wide open throttle. If I dive again to bring the RPM back to 2700 and then go back level cruise, I now close the throttle to reduce to 2300 RPM and then advance the throttle back to wide open and the RPM will return to 2700. I did these scenarios several times. Each time the RPM would be limited after a climb, but not limited by throttle movement.

Never in my 50 yrs of working on these things, have I ever seen a problem like this. Nothing in the Harry Fenton files. It's not a tachometer problem because I can hear 200 RPM engine noise difference. I have one possible theory, but I don't want to influence the thought process.

Hey folks .......... any ideas ???????

[FlyingRon]

Have you looked at the induction system for loose or collapsing parts that may be transiently restricting airflow in these circumstances?

[Eric Brown]

Well Ron, you were close. When I wrote this thread, I had not performed any investigation as it was the end of the day. It appears that I cried "mystery of the month" too soon.
What I found today does not fully explain all of the events, but it was more than likely the problem. The RH exhaust tailpipe brace clamp had slipped down the tailpipe and dragged the brace rod with it. The throttle control runs under the brace rod. With the brace rod lowered, the the throttle control contacted the rod when the throttle was pushed open. This in turn would not allow the throttle to open fully. There was a 3/16" gap between the throttle arm and the full open stop on the carb with the throttle pushed full fwd.

I had to reshape the brace rod and reconfigure the rod attachment to the tailpipe clamp to make plenty of gap between the throttle control and the brace. I'll try to attach some photos. Should have taken the Harry Fenten advice that, low power issues are usually found in the engine control rigging. Will check out on next flight.

[Eric Brown]

I guess my hopes were to great. The RPM problem still exists. I had lots of time to evaluate on recent 2 hr flight. It still turns only 2350 RPM during climb and only 2500 in level flight after the climb. If I reduce power to 2000 RPM, in level flight, and then advance the throttle to full, it will then turn 2700 RPM. If I climb and level again, only 2500. I re-checked the throttle linkage clearance and it's OK. The carb throttle fully opens. Now, in my experience, the climb RPM should be at least the same as the ground static RPM. In this case 2550. It acts as though the prop changes pitch. Mixture leaning had no effect.
My thoughts are coming back to my original theory concerning a possible crankshaft thrust bearing problem. The RPM decreases after the prop has been loaded (climb) and then increases with then the prop is momentarily unloaded (pwr reduction). An oil screen check showed to contaminates. Engine now has 4 flight hrs since OH.

At a Loss

[Eric Brown]

Here's a late thought. Has anyone had experience with the Marvel carb. one piece venturi and the type of fuel nozzle used. My carb. is a 10-4894 that has the one piece venturi mod, but retained the original fuel nozzle. Precision Airmotive SB MSA-7 only recommends the newer nozzle when roughness, caused by a rich mixture, is detected. As I found, leaning the mixture made no change, however there is an increase in engine vibration above 2300 RPM.
I did experience the richness problem in a C-172 (O-300) back when the one piece venturis were first mandated. In that case the richness was easily recognized.

Whadda ya think ?

[PJZajkowski]

What is the airspeed doing at 2500RPM/level Vs 2700RPM/level?

[Eric Brown]

What is the airspeed doing at 2500RPM/level Vs 2700RPM/level?

100 mph IAS at 2500, 105 mph IAS at 2700.

[PJZajkowski]

100 mph IAS at 2500, 105 mph IAS at 2700.
How about this idea... Your RPM (and power) is obviously a function of airspeed. Once you get the airspeed to 105 mph in a dive, you have 2700 rpm , more power (compared to 2500), and the airspeed can stay stay at 105 mph when you level off. Put it into a climb and the airspeed drops, rpm drops, power drops and when you level out, you can't get back to 2700 rpm and 105 mph because you have less power.

I remember someone telling me long ago that you can cruise faster if you climb slightly above your desired altitude and then descend. You get a little more airspeed, which gives you a little more rpm, which gives you a little more power. Of course this would only be for fixed pitch props.

[Eric Brown]

Gentlemen; Here is an update. I installed the new style "pepperbox" type fuel nozzle into the carburetor. It only made a slight difference in that after the climb at 2350 RPM (full throttle) and leveling off with 2500 RPM (still full throttle), the RPM will now slowly increase to 2700 RPM in about 90 seconds. If I climb again, the RPM drops to 2350.

NOW ! .... What is very interesting is that on take off, I will hold the brakes and open the throttle and get the ground static of 2550 RPM. Release the brakes and watch the RPM. The RPM will drop from 2550 to 2450 when the airplane accelerates to approx. 20 mph. As the take off speed increases, the RPM drops to 2350 by lift off. I then tried a take off roll where when I saw the RPM initially drop from 2550 to 2450, I would hit the brakes to stop the forward speed with the throttle still wind open. The RPM increased from 2450 back up to 2550 as the airplane decelerated !!!!!! What the ------ ?

My thinking at this point is that I may have to connect a manifold pressure gauge to see if there is an airflow change occurring in the induction system. The only mod I've made, related to the air intake, was to install a baffle seal around the cowling induction air opening to prevent ram air from going around the induction filter and entering the lower cowling. If the baffle is a problem, how is it that the engine does eventually turn 2700 ? Zoom in on attached photo to see air box baffle seal.

Completely baffled

[Bill Berson]

To get the 2700 rpm in cruise on a 150 you might need to climb an extra 100 feet above cruise altitude and push the nose down for a short dive to get the 105. My primary instructor showed me that 45 years ago but I haven't flown a 150 since.
Called get it on the "step".

[Bill Berson]

Might be prop getting vortex ring at static and needs forward speed to get a good bite.

[FlyingRon]

To get the 2700 rpm in cruise on a 150 you might need to climb an extra 100 feet above cruise altitude and push the nose down for a short dive to get the 105. My primary instructor showed me that 45 years ago but I haven't flown a 150 since.
Called get it on the "step".
Mostly OWT. Doing the pushover may get you to the steady state faster but the steady state is going to be the same whether you level off at the altitude or overshoot and dive.

[Bill Berson]

Engine runs hotter with friction until after 20 hours or so.

[dschaus]

Check exhaust for obstruction. Same for carb heat box. Check fuel system for proper venting and flow. Verify carb fuel inlet screen is clean. If carb is older, might think about exchanging for factory rebuilt. Or barrow a different carb to see if makes any difference. Check throttle shaft to throttle arm security. Last time I saw a similar RPM reduction problem on takeoff, it turned out to be a crankshaft bearing issue. #2 bearing support was found cracked. Most likely this is not the case since engine was just overhauled. May want to try an oil sample looking for elevated aluminum or bearing material. This is a tough one. Looking forward to see what it turns out to be.

[Eric Brown]

Gentlemen, I have an update. Thru the generosity of another airplane enthusiast, I was offered the use of a Tru Tach ll shoot thru the prop digital tachometer. Guess what ? ........ the engine runs fine. The Tru Tach shows 2560 RPM in the climb while the airplane tach shows 2350. In level fight, the airplane tach matches the Tru Tach after 4 min. When I climb again, the airplane tach drops to 2350 and the Tru Tach shows 2560. Again, after 4 min in level flight, the 2 tachs match each other in the 2000 to 2700 range. I have had the tach tach drive cable out of the tach drive housing. No anomalies. I re-greased the tach cable and reinstalled. I changed the cable housing routing just slightly. No difference. Rather odd behavior for a tachometer. Whadda you guys think ?

[rwanttaja]

Gentlemen, I have an update. Thru the generosity of another airplane enthusiast, I was offered the use of a Tru Tach ll shoot thru the prop digital tachometer. Guess what ? ........ the engine runs fine. The Tru Tach shows 2560 RPM in the climb while the airplane tach shows 2350. In level fight, the airplane tach matches the Tru Tach after 4 min. When I climb again, the airplane tach drops to 2350 and the Tru Tach shows 2560. Again, after 4 min in level flight, the 2 tachs match each other in the 2000 to 2700 range. I have had the tach tach drive cable out of the tach drive housing. No anomalies. I re-greased the tach cable and reinstalled. I changed the cable housing routing just slightly. No difference. Rather odd behavior for a tachometer. Whadda you guys think ?

Eons ago, I noticed some tach anomalies in my C85 (which uses the same tach setup). In my case, it turned out that the end of the tach drive shaft (on the back of the engine) had cracked.

9044

From your description, I'd say this isn't your problem...but the next time you disconnect the tach cable, slip a camera in and take a picture of the drive shalt.

Ron Wanttaja

[Bill Berson]

Could remove and spin the tach with a drill motor and confirm the changes with angle of attack.

[Eric Brown]

Another update.
I found an old used tach and installed it in the plane. It's not to accurate, but it was steady. It showed NO rpm variance in take off and climb. That, to me, eliminates the tach drive system.
I rigged up a table top tach drive test cable using a drill motor to drive it (as suggested). Connected the suspect tach to it. The tach DID show a variance in rpm when tilted in the up and down pitch angles. Connected an old tach to the test cable and it did NOT show a variance when pitched.
The problem tach is a new Mitchell Aircraft Products unit, p/n D1-112-5025 / model 98480-25. A new tach is on order. Will continue to update.

[Bill Berson]

Cool. Easy fix.

[Eric Brown]

Last update
The problem has been resolved. The new tachometer has eliminated the problem. To me, the tach came under the least suspicion because it was new. One for the record books. Never have seen a tachometer act in such a manner. Thanks for all of your suggestions.