What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Engine rough low rpm RV-8

A

aviator921

Member
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
 
Rough

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!:eek:

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.
 
aviator921 said:
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
Click to expand...
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.
 
Check intake tubes

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:rolleyes:
I know you said all you did was swap props, but.....
 
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?
 
Last edited:
aviator921 said:
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?
Click to expand...
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.).
 
Last edited:
Since the only thing changed was the prop could it possibly be a problem with a surging governor?
 
Mixture

Get it up to operating temp, set idle speed where you like it and adjust the idle mixture until you have at least a 50 rpm rise when you pull the mixture to idle cutoff. If its really rich you might have to do this a couple times to get the idle speed where you want it. If you get more than 100 rpm rise your mixture is too rich at idle speed.
You might not get the lighter MT prop to idle as slow as the heavier Hartzell prop.
 
Last edited:
So im still trouble shooting and its driving me nuts. Readjusted low end mixture, checked for induction leaks none found on the intake tubes but I did find 2 small tears in the scat duct from the air filter to the throttle body.(Is that something that can cause rough low end?) Im still getting 100 to 200 rpm drops in the 700-1500 rpm range. Mags are good. Installed a new sniffle valve. This is happening even at first start of day ( 90f-100fambient) but could it be im boiling fuel at the injector lines. I checked the temperature after shut down and the lines and spider valves were at about 160f.. Is that enough to cause vapor issues? i once had a leak from the red cube fuel flow meter so not knowing at the time, i used white teflon tape. I know big no no.. Could there be pieces of it at the filter screen going into the fuel servo? One thing to note is the engine runs perfect at high power settings..
When at idle with mixture full rich, My number 1 egt reads aprx 50-100f colder than the other egts but when I pull mixture back, number one gets equal to or about 20-50 hotter than the others.. Any info is greatly appreciated. Thanks

David
 
Rough idle

I just got thru similar problem with my 8a drove me nuts for 2wks finally pulled flow divider cleaned out with air gun, did not see anything but it worked.
Brand new io380 180 hp
 
HI , YOU MIGHT WANT TO CALL LYCOMING, THEY HAVE AN 800 NUMBER ,
I DON'T WANT TO SOUND LIKE A KILL JOY, BUT THE NEW PROP. MAY NOT BE COMPATABLE WITH YOUR ENG, IF I REMEMBER RIGHT THE LYC I0-360A1B6 HAS COUNTERWEIGHTS, AND MAY NOT WORK WITH YOUR ENGINE AND PROP. ALSO THERE IS A LYC S/B 569A YOU MIGHT WANT TO READ IT .:)
 
Thanks for the heads up but to my relief I did find an STC for that particular prop on my engine. Its STC # SA02461NY for Mooney which is where my engine came from. I can now sleep thru the night :)
 
You must log in or register to reply here.
Back
Top