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4-Stroke Mounting Question

Old 11-29-2011, 10:13 PM
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Default 4-Stroke Mounting Question

After almost 35 years in the hobby (off and on due to working, now retired!) I will be installing my first 4-stroke. I just purchased a Hanger 9 Pluse 60 XT and planned on installing a great used Magnum 91 4-stroke I purchased here on RCU. But I have a concern.

I ran the Magnum out at our club field on a home made test stand just to get familar with it. I guess I was not ready for the vibration difference between it and all the 2-cycle engines I've had over the years. The Magnum was smooth when at idle (and boy, that idle was great) and smooth at the top end (about 9000 RPM with a 14/6 Voss prop) but there was a couple of spots in the low midrange that I was really surprised by how much it vibrated.

Is this how 4-cycles are, should I purchase a special 4-cycle mount (like the Du-Bro) or am I just worried about something that is normal. The guys at the field thought it ran great, just looking for reassurance I guess.

Old 11-29-2011, 10:57 PM
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Default RE: 4-Stroke Mounting Question


Welcome to retirement. I have found it has much to offer. And it can be extremely busy.

I think you may have a Vess prop? If it is, it is probably a wooden prop?

Generally I don't use wooden props on four strokes. The greater weight of fibre filled props is of great benefit to four strokes as they only fire once every two cycles and the greater inertia of the heaver plastic props seems to help them smooth out. They can be quite inclined to vibrate as their power pulse is, particularly in larger engines like yours, quite a whack and this can make them vibrate in a less than rigid airframe. I do not use soft engine mounts at all.

Having said that, I generally don't use wooden props as our field is at best rough and it is too easy to break props there.

I'd suggest trying some other props. The Graupner props are really good if you can get them. Master Airscrew are tough and work well on four strokes but the APC are probably the best all rounders but quite easy to break. Sadly, my favourite prop, the Bolly Clubman series, is no longer made. They were really great on four strokes.


Mike in Oz
Old 11-29-2011, 11:01 PM
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Default RE: 4-Stroke Mounting Question

That is somewhat normal. The only thing I would check is your low end, 4 strokes have a much higher tolerance on the needles when compared to a two stroke. I have seen four strokes with the low end 3 turns out of tune…it idled rough but it didn’t die. I would just make sure your low end isn’t too rich (start the engine, go to full throttle, tune, drop to idle…..let it sit for 30 seconds…..then gun it to full throttle….if it stumbles/hesitates lean it out, if it sounds like it is about to die richen it up.
Don’t think too hard…..try one way….if it doesn’t work….go the other way
Personally if it is running reliably don’t stress it, just fly it, if you like to tinker with engines (who doesn’t) then verify the low end is tuned

Good luck!
Old 11-30-2011, 04:08 AM
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Default RE: 4-Stroke Mounting Question

Homer, what is normally mistaken for vibration on fourstrokes is actually tortional impulses caused by the light weight engine and the widely spaced power pulses. A heavy prop such as a Graupner, MasterAirscrew K or Classic wil do wonders toward curing the shake. A rich LS needle makes it worse.

Woops, sorry I just repeated what Mike said. And yes the Bollys are great.
Old 11-30-2011, 10:19 AM
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Default RE: 4-Stroke Mounting Question

And welcome to the wonderful world of 4-strokes!

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