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Just bought a bravo that had the engine and a prop overhauled 100 hours ago. At 2400 RPM it runs ok but lower RPM it vibrates, a lot. Has anyone dynamically balanced their engine/prop and had it smooth through 22-2400 RPM? Are there any tricks? 

Thanks 

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You might try a dynamic prop balance and see what happens.  What occasioned the engine and prop. overhaul?  If it were a sudden stoppage any number of problems should have been addressed including careful inspection of the counter weights and bushings.  I would take all the paperwork (invoices, work orders, etc) and logbooks to an experienced mechanic and have them evaluated.  Let say the plane slid into a frozen snowbank and the airframe was undamaged but the engine and prop needed major repair. A major overhaul can cure the problems if all the steps are followed.  Sometimes the owner wants to do a cheap overhaul and sell the airplane.  I am not saying this would be the cause of the vibration but you should check it out before you fly to far.

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It should be smooth at those RPMs if compared to our Bravo.  A dynamic balance is a great place to start.  We did that on our former (M20C) and it made a huge difference.  If you have a modern engine monitor take a look at the logs and see if things are running consistently at the various RPMs.

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I find other engine problems when balancing the propeller. Any good balance tech should be able to tell you if there is a problem that requires repair before the engine will balance correctly. Problems with the ignition system, fuel delivery imbalance, and intake leaks will result in the engine being too unstable to balance. The location and size of the weight needed will keep changing on every run.

Mechanical things like pushrods that are too long, and don't allow the valves to seat 100%, will also produce vibrations due to cylinder power imbalance. Loose pitch change forks in propellers also cause weird vibrations, since the blade angle changes randomly. Grasp each blade and try to rotate it in its' socket. It should move less than 1/16", and all blades should move the same amount. If you have one loose one, the prop will vibrate. Don't forget to look at the airplane's tail. Looseness in the tail is aggravated by propwash, and that develops a vibration felt in the seats.

While it's best to balance at the RPM used for cruise, the engine should still be smooth below that RPM. 

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Thank you for all the replies. This information s good to know.

As I was saying before, the plane had a prop strike 100 hours ago and the prop was sent out and overhauled and the engine sent to G&N for the teardown and inspection. At that time they installed new cylinders and reassembled the engine per Lycoming.

It's going into my mechanic this week and I have a call into our local company who has balanced a couple of my engines before and they were pretty smooth after.  

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  • 3 weeks later...

Late to the party, but just had my Bravo dynamically balanced, and it is now smooth through all normal RPM's -  When I bought the plane it was not terrible, but I could tell it needed a little help.. I just think it is better on the main bearings and longevity of everything in the machine - imo   Not an A&P , blah blah.. :)

Seriously though, I think it was $300 for the balance, and worth every penny

David

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@philiplane makes many good points that are all worth checking out before getting it done. After 100 hours the engine should have bedded itself in well enough to balance, but if you are going to do GAMIs, then I would suggest you get this done (or at least do the test to determine if you want to do them) before balancing the prop (or be prepared to balance it again!)

I had mine balanced a few years ago and very pleased with the improvement, but having changed the engine it is overdue to be done again

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9 hours ago, David_M said:

Late to the party, but just had my Bravo dynamically balanced, and it is now smooth through all normal RPM's -  When I bought the plane it was not terrible, but I could tell it needed a little help.. I just think it is better on the main bearings and longevity of everything in the machine - imo   Not an A&P , blah blah.. :)

Seriously though, I think it was $300 for the balance, and worth every penny

David

Nice pirep for dynamic balancing...

Best regards,

-a-

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