C90 Not making full power?

[7985]

I have a Continental C90- 12F engine in a Cessna 140, 97 hours SMOH. This aircraft has been grounded for 13 months due to a malfunction that has me and my mechanic completely stumped. I am desperate to find a solution to this problem and I hope that your expertise can help.

On June 26, 2024 I flew this airplane for 1.2 hours without any issues.
A week or two later, I attempted another flight which ended with a rejected takeoff due to limited engine power. At full throttle, the engine would not develop more than 2200 rpm during the takeoff roll.
Over the next several months my mechanic and I performed numerous tests and interventions which did not resolve the problem.

The engine runs smoothly from 0-2200 rpm. EGT's are balanced and nominal across all cylinders.
We tested the tachometer using a hand-held tach and found that the aircraft's tachometer is accurate.
Spark plugs were examined and compressions measured to reveal no problems in the cylinders.
The magnetos were examined and the timings were checked.
The throttle control was checked to ensure that the throttle is wide open when the throttle control is set to full.
The air filter was removed during an engine test to ensure that the filter does not restrict airflow.
The intake manifold seals were inspected to ensure no leakage.
The fuel line was disconnected from the carburetor to ensure clean, unobstructed fuel flow.
The carburetor gaskets were replaced.
The carburetor was sent to a specialty shop for overhaul

[6643]

What prop are you running, and what is your static RPM?
Are you sure the carb heat isn't on, or getting sucked on at high throttle settings?
Can you get a full throttle intake manifold pressure reading?

[8342]

Does it have the O-200 muffler?
Check that the exhaust is clear and no obstructions.
What were you getting for RPM before the noted loss of power?

Rick

[7985]

Thank you very much for the feedback and suggestions. I hope that getting more and better brains on the problem will finally produce a solution.

To answer your questions:
The prop is the same prop that I have used for 8 years.
Static rpm is supposed to be 2475.
I have taken the plane to the runway and run it to takeoff speed at full throttle to test dynamic rpm. Only got 2200 rpm.
The engine delivered full power for years until it suddenly stopped doing so.
I have tested the engine with and without carb heat, full rich and lean and done mag checks -- no joy.

I have not examined the exhaust system, and I am not sure if my mechanic did. I will check this out, but I have to wonder how something could have gotten into the exhaust or muffler. Also, all four cylinders are performing similarly (same EGTs), so I think a blockage on one side would result in an imbalance in the EGTs.

[6643]

To answer your questions:
The prop is the same prop that I have used for 8 years.

Right, but what prop is it? e.g. McCauley 1A90 CF-7152

Static rpm is supposed to be 2475.

No, that's your RPM limit. Static RPM is the RPM you get at a standstill with full throttle. The requirement depends on what prop you have and it is a good indication of whether or not the engine is making full power. It is a requirement, and if you can't make the required static RPM the aircraft is not airworthy. I always do a static RPM check at the end of my run-up, before entering the runway. If you can make maximum RPM at a standstill, or you exceed it at full throttle, then there's something wrong with your prop (or your tach).

For example, a C90 with the McCauley 1A90 CF-7152 prop has static RPM limits of not less than 2050 and not more than 2350

The engine delivered full power for years until it suddenly stopped doing so.

How are you determining that? A proper fixed pitch prop installation will not allow maximum RPM until you're into the range of cruise airspeed, certainly not before lift off speed. Otherwise, you'd be exceeding red line all the time.

I have tested the engine with and without carb heat

Is there a difference between with and without?

[8413]

John is correct. My C90-14F turns 2150rpm static with a McCauley 1B90-CM7152 prop. On my 140A, I can just barely get to 2475rpm full throttle level at about 1500'MSL and about 115mph indicated. I don't have any suggestions to troubleshoot your problem other than to give a datapoint on what is normal for a C90. If you were actually getting 2475rpm static you would overspeed your engine shortly after becoming airborne although you would have really really good climb performance.
Josh

[6643]

I've split the recent posts about the C90 from the 7 year old thread that was an unrelated problem.

[6643]

The effective pitch of a fixed pitch prop is reduced as airspeed increases, so, engine load is reduced with increasing airspeed. If your prop could turn 2475 RPM at takeoff speed, any increase in airspeed beyond that would reduce engine load, and increase engine RPM beyond red line. Reducing throttle to limit engine speed would result in reduced airspeed, so your maximum cruise speed would be limited to speeds at or below lift off speed. Clearly, not practical.

If you had an adjustable pitch prop you could adjust the pitch down to the point where you could produce rated power at maximum RPM, then increase the pitch as the plane accelerates to maintain full power without exceeding rated RPM. With a fixed pitch prop, on the other hand, you're stuck with less than rated power until you reach the upper range of your cruise speed.

How much time do you have in this airplane?
What were the conditions (temp, pressure, altitude, etc) on the day when you experienced the problem, and what were the conditions for the previous flight.

[7985]

I always got at least 2300 rpm during the takeoff. Cruise at 2300 - 2350.

Prop is a Macauley IB90 that was re-conditioned by the previous owner to a climb prop.

I have more than 300 hours in the airplane.

[6643]

What is the pitch of the prop? The actual length would also be useful. (Not all 71" props measure 71 inches.)

What is your static RPM? If you really have a climb prop they you should expect something in the upper half of the allowable range. (2200-2350).

What was your airspeed when you saw 2200 RPM on your takeoff run?

Is there a difference between with and without [carb heat]?

What is different between then and now? It seems you've eliminated the engine as a possible explanation.

Oh, did you notice anything unusual during the previous flight?