Adjusting RPM??

[mhorowit]

Previously I mentioned that prior to major engine work, I would have to bring the throttle back at cruise to indicate 2150. Now I find in the pattern that I develop just over 2000 at full throttle. The prop has not been changed.

What factors could cause me not getting my old RPM?

Things I'm thinking about:
1. the throttle cable isn't properly adjusted
2. this mechanic may have used a different timing than the old guy.
3. there is an adjustment on the carburator (?)

Might there be something else?? - Mike

[Robert Lees]

Re: Adjusting RPM??

Generally, the only adjustment on the carb is the idle mixture, and throttle-stop positions. Sounds like the latter is more likely to be affecting you By removing the lower cowl (and if necessary, the air box), you should be able to see if the butterfly valve is closed at throttle shut and open at full throttle.

Do you have a mixture control?

Timing could make a difference...have it checked.

[alwaysoar]

Re: Adjusting RPM??

It could be timing, what's your mag drop. I took my carb apart and cleaned it with MEK-based carb cleaner, put in a new gasket kit and picked up 170 turns at static, and about 100 on climb out. Be sure you use the proper torque values when you put it back together--it could be sucking some air. If it's running well otherwise, it's probably something easy to deal with. Do the timing first, only takes a few minutes.

[Goneflyin]

Re: Adjusting RPM??

Mike, you mention in the "It's up " post that the oil pressure is low, how's the oil temp? Also I changed my accessory case and oil pump a couple years ago and don't remember anything adjustable. Ignition timing or air leaks are a possibilty, let's hope it's something something simple.

Bill

[Guest]

Re: Adjusting RPM??

Previously I mentioned that prior to major engine work, I would have to bring the throttle back at cruise to indicate 2150. Now I find in the pattern that I develop just over 2000 at full throttle. The prop has not been changed.

What factors could cause me not getting my old RPM?

Things I'm thinking about:
1. the throttle cable isn't properly adjusted
2. this mechanic may have used a different timing than the old guy.
3. there is an adjustment on the carburator (?)

Might there be something else?? - Mike

In addition to all the other good suggestions it might be old gas if the a/c sat for a long time. Are you in level flight and only reaching 2000 rpm? Dave.

[mhorowit]

Re: Adjusting RPM??

Generally, the only adjustment on the carb is the idle mixture, and throttle-stop positions. Sounds like the latter is more likely to be affecting you By removing the lower cowl (and if necessary, the air box), you should be able to see if the butterfly valve is closed at throttle shut and open at full throttle.

Do you have a mixture control?

Rob - The only thing that might be a mixture adjustment is a knurled adjusting screw on the pilot's side of the carb. I haven't touched it.

What I did do was adjust the throttle-stop position to allow the arm to go further forward. I now cruise at 2200RPM. By doing that, I haven't caused anything else to be out of adjustment have I?

I did not check that the throttle-stop position and a wide-open throttle were the same.

A buddy suggests because of the work on the engine internals, I may have picked up some friction which in turn effects my RPM, and after some time I'll see my RPM still higher. Do you concur? - Mike

[VictorBravo]

Re: Adjusting RPM??

As mentioned by someone else, the major engine work could have done a few things...

Weaker valve springs replaced with new stronger ones results in more internal friction.

Tighter tolerances on new crank and cam bearings result in more internal friction.

Rocker arm bushings add a surprising amount of friction...they are highly loaded.

New rings in the cylinders result in more friction.

More oil pressure means that the oil pump gears are workign harder to move more oil.

Someone could have backed off the timing a little.

The gaskets between the carb and spider, carb and airbox, intake elbows and intake ports, and the intake tube hoses, could be slightly misaligned and cause a very slight blockage in the mass airflow through the engine.

A new or changed exhaust system might have something to do with it.

Different brand of air filter element...induction pressure drop...less power and RPM.

DO NOT discount the amount of airframe drag that can change when you fool around with the coolign system. If someone made more or less air flow through the baffling by replacing baffle seals and what not, the drag of the plane changes. Cooling drag is a much higher percentage of total drag than most people think. How much faster/slower is your airplane flying than before?

Bill

[mhorowit]

Re: Adjusting RPM??

As mentioned by someone else, the major engine work could have done a few things...

How much faster/slower is your airplane flying than before?

Bill

Haven't looked yet.
Too much wind to evaluate.
Busy "tip-toeing' down the runway.
Eventually I'll look at the airspeed.
I'll let you know, but if I get a headwind, all bets are off. - Mike