When the Power Seems Low

The engine just doesn't seem to be acting normally in power output. What's the first thing to check?


The inability to achieve normal power output (e.g. poor static power) can be an insidious, difficult-to-troubleshoot condition that ultimately affects almost every small engine sooner or later. Sometimes, the cause can be pinpointed quickly, especially via multi-probe engine analyzers. But many times it can’t. The best bet is often analyzing all the power instruments that you do have, not just one.

It is more frequently encountered in engines that have not reached TBO but have weeks to months of sitting idle between flights with sometimes as few as 20 hours a year or less of operation. Also, flying past TBO may increase the occurrence of this condition. Static power is relatively easy to check with a fixed-pitch prop at standard atmospheric conditions. It should be done at every annual.

The first and most obvious sign of a problem with any engine is a reduced rate of climb compared to book figures (obviously taking atmospheric conditions into account). This is more noticeable than an airspeed loss. This is more obvious to those who have an operating history with their engine.

For example, my Bonanza went from 1400 fpm with only me on board to 1000 fpm in identical conditions. Something was not right. It had high time (1640 hours accumulated over 16 years and weak, essentially original cylinders).


What do you do when you know something’s not right, but you don’t know where to start to look for problems? Consider the obvious first.

If it’s a hot day, or you’re visiting a high‑elevation airport, density altitude may be the culprit (and leaning the mixture on the taxiway or runway may be the solution). If the plane recently came from the maintenance shop, ask the mechanics if anything was done that might have affected power output.

Perhaps the mag timing was reset. Maybe push rods were removed and reinstalled in the wrong locations (swapped), which could easily reduce maximum power. Maybe a shop rag was left in the air scoop or scat tubing left disconnected or loose. (Don’t laugh; these are all based on true‑life occurrences.)

Maybe you don’t really have an engine problem at all: Have you checked your tach and manifold pressure gauges recently? It’s easy to check a manifold pressure gauge: With the engine stopped, the gauge should indicate ambient pressure (29.92 inches at sea level, standard‑day).

The tach is almost as easy to check. The old trick of strobing the prop against a 60‑Hz fluorescent light source at night works; at 1,800 rpm, your two‑bladed prop should appear “stopped” by a 60‑Hz light (your 3‑blader will appear to have six blades), if your rpm is dead‑on.

Obviously, if the prop blades are creeping in the forward (normal) direction, your tach indicates low. If they creep in reverse, the tach shows high.

There’s a third way to check tach accuracy, and that’s to use the odometer or “hourmeter” portion of the unit (which is gear‑driven) as a crosscheck of the needle indication. If you know that your tach gearing is set up to flip the hour-meter over every hour at 2,310 rpm, then you can simply time how long it takes for a tenth of an hour (or longer) to roll by on the counter drum, divide the indicated time by the actual time it took, and multiply the result by 2,310 or 2566 (or whatever the magic number is for your tach) to arrive at the true rpm.

By far the easiest and most accurate tach check is to use one of the handheld digital tach checkers. The $49 unit is much less accurate and more difficult to use than the $275 unit on the market. You get what you pay for—use the better unit.

Assuming your tach and MP gauge are not to blame, what’s next? A static run-up to check book values is a good idea for a fixed-pitch prop. Bear in mind that variations in day‑to‑day static rpm are inevitable, since static rpm is affected not only by temperature and altitude, but oil viscosity, air filter condition, wind, humidity, etc.

Static rpm will not equal takeoff rpm in any case, since the engine turns faster with air going through the prop disc. But if you’re fairly sure the engine is power deficient, based on rate‑of‑climb or other indications, there are a few things you should check immediately:

First, if it’s a carbureted engine, be sure the carb heat door is rigged properly. This means, among other things, that it should open and close fully before the knob hits the panel stop in both directions. Also, baffle seal material in the carb air box could be missing allowing for air leaks.

If the panel stop is limiting carb‑heat, throttle, prop, or mixture control travel (rather than the physical stops at the carb, injector servo, or prop governor), have a mechanic assist you in re‑rigging things. You want a quarter‑inch or eighth‑inch of “cushion” at the panel with the power knobs all the way forward. Cables should not slip in their mounting points at all.

Nothing robs power like full-time carb heat, so be sure to check this possibility out. The mere fact that you get an rpm drop on run-up with application of full carb heat doesn’t mean the flapper door isn’t hanging partially open with carb heat “off.”

Likewise, check your alternate‑air door if you have one. Alternate air is engine‑compartment air, and while that air isn’t as hot as carb heat, it’s generally much warmer than OAT (and may be drawn from a low‑pressure part of the engine compartment).

Remove the air filter and be sure it isn’t hiding a rat’s nest (or some other nest) blocking the air scoop. The FAA’s Service Difficulty Report database is full of reports of gaskets becoming dislodged, and new air filters coming fresh from the factory covered in glue.

Carburetor or injector over-richness or over-leanness is easy to check: Note the rpm rise when the mixture is retarded slowly to ICO (idle cutoff). Do this at idle, and also once at off‑idle (1,200 or so rpm).

Some rpm rise is desirable, but more than 50 or 60 rpm in a carb‑equipped plane—and 25 rpm in an injected plane—usually spells the need for further checking. (Check your manual for the specific value.) If raw gasoline comes out the intake system sniffle valve after engine shutdown, you have a problem!

If you suspect an overly lean engine, try this: At various rpms, with the mixture full rich, slowly apply carb heat. Any increase in rpm with initial application of carb heat verifies the presence of an abnormally lean condition.

Broken baffles in mufflers and exhaust collectors that use such baffles is a common cause of power loss. If you have a plane where such breakage can be spotted by shining a light up the exhaust pipes (e.g., Beech Bonanza), do a little light‑shining; otherwise bring a rubber mallet and listen for telltale rattling noises as you gently bang the muffler. Be sure any Powerflow exhaust systems are properly maintained and the ICAs followed.

While you’re snooping around, eyeball the entire exhaust system for leaks, cracks, bulges, and blown gaskets. Any of these will give rise to a noticeable change in the engine’s “feel” in flight. Discrepancies should be corrected before further flight.

Along with exhaust pipe and baffle distress, mag timing discrepancies are probably a major cause, if not the major cause, of unexplained power degradation. Probably half the mags in this world are timed too far in the advanced direction, and the other half are timed too far retarded due to little or no maintenance. Here, we’re talking about external—or mag‑to-engine timing, but internal timing (E-gap) plays a critical role, too. Both have to be correct to get maximum spark.

When internal timing is off, external timing is usually off as well, plus the spark tends to be weak. Late or retarded timing is caused either by cam follower distress (internal) or improper timing procedures (external), which generally means somebody got lazy and decided to sight across the pulley timing marks rather than use a top‑dead-center locator and prop protractor. (Parallax viewing errors in timing a mag by pulley marks can give rise to errors in either direction of up to 6 degrees.) The engine makers confirm a huge percentage of engines returned have big timing errors.

Retarded timing shows up as a larger-than‑expected drop in rpm during single-mag operation on run-up. But the true confirmation comes when you perform a for‑real timing check with a TDC locator and prop protractor (and buzz box or mag hot‑light).

If the discrepancy was large enough to cause a noticeable power loss, you probably should insist on having the mag removed and taken apart for an internal‑timing (and general conformity check). Something improper may be going on. Now’s the time to catch it.

Incidentally, the timing specs for your engine will often be found on the engine data plate, but they are not guaranteed to still be correct or updated. Check the latest documentation. As an example, take the Continental O-200‑A. On some its data plate may be wrong. (Continental changed the timing of the O‑200 to 24 degrees at one time. AD 96-12-6 has the current info on the O-200, and MSB 94-8D has the latest info on properly timing TCM engines in general. Always check the latest service information, as timing can be changed by the factory for one reason or another. That’s why it’s so important to have the latest data. If in doubt, call the factory, Lycoming or TCM.

Obviously, cylinder compression plays a role in engine power output, so if a problem seems to be developing, stop everything and have a compression check done with the proper gauge. Generally, you won’t notice the power decrease from the cockpit until the majority of your cylinders have sunk below 40/80.

We know of some operators who flew into shops with 10/80 or 20/80 on a number of cylinders, and were surprised to learn that something was wrong. What it means is that if compression is bad enough to affect power output, you should be able to feel it in your biceps as you check compression by the strong-arm method (pull the prop through).

Note that TCM contends that lower than expected compression at the ground check is not indicative of the power output problems as long as they meet the values prescribed in the latest TCM compression test bulletin and you perform a borescope test to further verify the cylinder is OK. You also must use a compression tester with a calibrated orifice.

Soft/worn cam lobes are, or have been, a problem in certain well‑known engines, such as the Lycoming O‑320‑H, O‑360‑E, and TIO‑541‑E (not to mention the occasional O‑235 and IO‑360); certainly, flat cam lobes will degrade power measurably.

The thing to do if you suspect valve train involvement is pull all rocker covers at once (easy enough—it doesn’t take long) and look for mischief while somebody carefully (slowly) pulls the prop through. If you’ve got the time, remove all the rockers and pushrods (labeling everything so they go back in the same locations). Check for a worn cam, if still stumped.

Pushrods that are bent, mushroomed, shortened, or don’t roll straight on a flat surface are a problem, obviously. Flat lifters are rare, though by no means unheard of. (A&P supervision required for this check on certified aircraft.)

Lycoming and Continental provide for dry‑tappet clearance specifications that can be checked without extensive engine disassembly. (Still, plan on about three hours of shop labor.) In some cases, this check makes sense. The engines we’ve heard of with this problem have tended to be Continental IO‑470s.

Since almost all aircraft engines employ hydraulic lifters, the meticulous setting and checking of valve clearances that characterizes, for example, motorcycle maintenance simply doesn’t apply here except for one engine. You may recall that the Lycoming O‑235 and O-290 series uses solid tappets, and therefore require close attention to hot and cold valve clearances, etc. Consult the latest version and any supplements of Lycoming S.I. 1388 for details.

Another valve‑train area to check: springs and keepers. In Lycoming engines, exhaust valves have cap‑like rotators that can come off; and of course, springs can and do break or lose temper. Both Lycoming and Continental have had problems over the years (sporadic and rare, but problems nonetheless) with valve spring temper. Check valve spring tension.

This has been a particular issue with C-145 and O‑300 engines. Buy new springs, as propping them up with spacers or shims is a less desirable procedure.

Finally, don’t rule out float‑saturation problems in carbs (there are relatively new bulletins out on these), or diaphragm problems in Bendix‑injected equipment. Fuel‑saturated carburetor floats will ride low in the bowl and give a variety of rough‑engine and rough‑shutdown problems. Check this possibility last, however, rather than first, unless you have reason to suspect the carburetor initially.

This article originally appeared in the February 2014 issue of Light Plane Maintenance magazine.

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