Engine rough low rpm RV-8

[aviator921]

Hello to all. I just recently replaced my hartzel C/S 2 blade for an MT 3 blade composite. I have the IO 360 a1b6 mooney engine. After the new prop, ive been noticing a rough idle and instant drops of about 200 rpm in the 800 to 1200 rpm range. Runs smooth at high power. Mags are good. Tried different mixture settings..Could it be because the new 3 blade is 12lbs lighter or something with the way the prop is mounted relative to the crankshaft? Maybe my idle mixture needs to be adjusted? Thanks in advance.

David 194PT

[mannanj]

My Mooney IO-360-A3B6D with the McCauley prop is placarded-"No continouous operion between 1500 and 1900 rpm below 15 inches manifold pressure".

It sure doesn't like to operate in this range and you don't have to look at the guages to know when you hit this range!

It's possible your prop may have some effect on the harmonic dampers on the crankshaft similar to what happens on mine. You might try re-clocking the prop on the crankshaft to see if you get some relief.

I'm not giving this as a difinitive answer, just another lead to chase down. Lycoming did a lot of research when certifying these engine/prop/airframe combinations as to harmonics and you're in unknown territory.

[Low Pass]

Originally Posted by aviator921

Hello to all. I just recently replaced my hartzel C/S 2 blade for an MT 3 blade composite. I have the IO 360 a1b6 mooney engine. After the new prop, ive been noticing a rough idle and instant drops of about 200 rpm in the 800 to 1200 rpm range. Runs smooth at high power. Mags are good. Tried different mixture settings..Could it be because the new 3 blade is 12lbs lighter or something with the way the prop is mounted relative to the crankshaft? Maybe my idle mixture needs to be adjusted? Thanks in advance.

David 194PT

You've answered your own question. If changing the prop is really all you've done, and it ran good before, then it's almost certainly an underlying condition that's being exposed now that you have less mass spinning on the front of your engine. Are you 100% certain it didn't do this before?? Does it do this warm and cold? I wouldn't bet on it being a dynamic/harmonic condition with it suddenly loosing rpm. My money is on fuel delivery. But you just need to go through normal troubleshooting procedure looking at plugs, wires, timing, etc.

[Bob Martin]

Check for a loose or leaking intake tube. more air is lean mixture, too lean is rough at low rpms....don't ask me how I know this
I know you said all you did was swap props, but.....

[aviator921]

Thanks for all the responses. I gotta double check but i think its better when cold..If that were the case, why is it rough when hot? I will check the intake tubes. Is it an unsafe condition if the new prop and crankshaft aren't in correct balance? Can it wear out the bearings?

[Low Pass]

Originally Posted by aviator921

Thanks for all the responses. I gotta double check but i think its better when cold..If that were the case, why is it rough when hot? I will check the intake tubes. Is it an unsafe condition if the new prop and crankshaft aren't in correct balance? Can it wear out the bearings?

Balance isn't black and white. Does it feel like you're flying in an old helicopter with your new prop? Might be a concern! Seriously, I really doubt you could put a new MT on a previously smooth running engine and get vibration that was destructive. (Doesn't mean you should field balance one day though.) The kind of imbalance that would wipe a journal bearing in one of these engine would be scary-bad. Like unable to read the instrument panel.

Hot and cold matters on many counts. Mine hesitates a little when advancing the throttle when cold. Not so much when warm. Many, many variables there. But absolutely make sure you begin making changes referencing one condition (warm oil, warm engine, etc.).

[aviator921]

I will note the temps next time I'm out.. As for vibration I'd consider it very smooth.

[billdianne]

Since the only thing changed was the prop could it possibly be a problem with a surging governor?

[aviator921]

hmm never thought of that. What would cause a surging governor? Would that require a rebuild?

[Low Pass]

Originally Posted by aviator921

hmm never thought of that. What would cause a surging governor? Would that require a rebuild?

Governor doesn't control in the 800-1200 rpm range.