Maximum static rpm too low. Low static rpm in Lycoming or Continental aircraft engine.

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Maximum static rpm too low. Low static rpm in Lycoming or Continental aircraft engine.

With fixed pitch airplanes low static rpm is caused by low engine power, inaccurate tachometer, or incorrect or mispitched propeller. While constant speed aircraft engines can suffer the same problems the diagnosis is more difficult.

If a constant speed propeller airplane won't reach red-line rpm in cruise then the propeller governor is holding back the propeller and your problem is not low engine power. At cruise (or descent) even an engine with low power will turn a propeller past red line because of the low engine loading. Governor has high rpm stop, propeller has low rpm pitch stop. Either the propeller or the propeller stop may limit maximum static rpm.

To determine which is causing trouble, open the throttle and move governor control back and forth slowly. If maximum static rpm is reached before governor stop is reached, propeller low pitch stop is probably limiting the rpm. If this is the case, turn governor high rpm stop out, loosen blade clamp outer bolts and rotate blades in clamps. Reduce blade angle about 1 degree for each 100 rpm increase. (1 degree equals 1/32 inch circumference at blade root.) After blade clamps are tightened, check to see if maximum static rpm is reached before governor stop is reached. After proper blade angle is set, reset governor high rpm stop. If maximum static rpm is reached at same time as governor high stop is reached, governor stop is probably limiting the maximum rpm. Readjust governor stop to obtain proper rpm. Turn stop screw out to increase rpm. Inability to attain positive high pitch can be caused by the high-rpm screw adjusted too far in, causing restricted arm travel. Remove control arm and rotate one serration clockwise. Back out high-rpm screw as required (one turn equals 25 rpm).

1. Incorrect system rigging.
2. Low engine power.
3. Erroneous reading tachometer.
4. Sticky pilot valve.

5. Insufficient oil flow to the propeller. With Lycoming aircraft engines you need to check main bearing clearance and rear plug in center of crankshaft for blockage. Remove propeller governor. There's a passage that goes down to the center of the front main bearing. The big hole indexes to the oil transfer tube which goes to prop governor. Take a differential pressure tester and with a rubber end push against this passage. Apply 40 pounds in and as oil blows out of bearing you will see the secondary gauge come down. After 15 or 20 seconds you want to see 5 to 20 pounds. If you get very little leakage, say 30 pounds, then this means that oil pressure from the governor can't get to the propeller to provide control. This is a quick and easy test to do after engine assembly at overhaul of your Continental or Lycoming aircraft engine.

6. Loose carburetor bowl attachment bolts allowing air to suck into carburetor.

7. Camshaft lobes ground down.

8. Weak or broken valve springs

Description of symptom

The major problem though is low engine power. I don't have experience with this engines performance prior to the annual. It came out of a 12 year storage and the pilot who flew it in stated it was low on power then. The mechanics say it isn't low on power but I pointed out the engine doesn't approach static R.P.M. stated by Continental with this engine and prop combination. In fact according to the power absorption curve for this prop it is lucky to be producing 50 horsepower out of a 75 HP rated engine and prop. It flys but just barely. When I tried to explain power absorption to the A&Ps I was met with blank stares. I noticed the plugs and exhaust are covered with black soot. The mechanics reset the carb and now it drips fuel. It drips even worse in flight attitude and I suspect this may be a contributing factor. Do you offer new or rebuilt carburetors for this A75. Bigger and better would be nicer if the engine would benefit from it.

I picked up 50 R.P.M. moving the muffler inner perforated cylinder away from the discharge pipe which was an apparent restriction. Now if I can find another 150 R.P.M. in static I will be happy. Any information on carbs and suggestions would be greatly appreciated.

Solution

I found someone more qualified to work on my J4,.... me. The problem was weak valve springs. 62 years of heating and cooling caused these 54# springs to lose temper and drop to 37#. The higher R.P.M. caused valve float and worked like a governor. Pulls like it is advertised to do now. I am a happy camper.

Thanks for your time.

Comment

Were the valve springs really 62 years old!

Around 18,000 feet you'll begin to breathe with more comfort, so pull the rip cord and see what's cooking. Start finding yourself a nice soft place to land suitable for a smart pilot like yourself. Parachute Sense US Navy 1944

since 1940

Maximum static rpm too low. Low static rpm in Lycoming or Continental aircraft engine.