Magnum .91 Four Stroke RPM Question - RC Groups
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Nov 08, 2017, 11:49 PM
Registered User
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Magnum .91 Four Stroke RPM Question


Greetings everyone! I just acquired my first four stroke engine. It's a magnum .91 that i purchased used. I'm running a Master air screw 14x6 prop with 15% Omega Morgan fuel and an OS type F glow plug. The engine is fully broken in from the previous owner and runs fine. I was only able to get a peak rpm of about 9270. I backed off the needle until i hit about 8900 RPM to keep it on the slightly rich side. I was under the understanding that I need to have an RPM between 9000-10000. I have not flown the plane yet (60 size Stylus). Am i doing something wrong? Is there anything I can do to bump up the RPM a little bit without leaning it out too much? Is it still safe to fly it at 8900 RPM? Thanks for any tips.
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Nov 09, 2017, 02:02 AM
Registered User
Yes it is safe to fly at 8900 rpm. Want faster? Install a smaller prop. But 8900 unloading to maybe 10000 in the air will work just fine.
Nov 09, 2017, 04:30 AM
Engine man
dmrcflyr2's Avatar
On a recent refurbishment of an ASP 91 I recorded 9870 RPM with a Master Airscrew 14x6, Enya #3 plug, and Omega 15% 4 stroke fuel. Magnum and ASP are/were made in the same Sanye factory in China so they are effectively sister engines.

ASP FS 91 First Run After Rebuild (4 min 33 sec)
Nov 09, 2017, 05:16 AM
Registered User
Quote:
Originally Posted by dmrcflyr2
On a recent refurbishment of an ASP 91 I recorded 9870 RPM with a Master Airscrew 14x6, Enya #3 plug, and Omega 15% 4 stroke fuel. Magnum and ASP are/were made in the same Sanye factory in China so they are effectively sister engines.

https://youtu.be/Z-zyj5qydEA
Looks like a non-standard muffler - that can make a difference...
Nov 09, 2017, 05:40 AM
the great Gassif´er
Maybe it is me (and I do not really know how the MA prop relates to the APC when comparing power consumption) but I have the feeling that that RPM is quite low.

My propcalculator seems to indicate that MA prop run slightly lighter than APC props, and my ASP 91 takes an APC 14 x 6 easily above 9500 on gasoline (Gasoline develops less power).

My suggestion would be to not lean it out, but check for clear intake and exhaust ports in the cylinder head, proper valve seal, maybe a new plug, proper valve lash, things like that.

But whether you choose to give the engine a go-over or not, it is not likely that something will go wrong if you run it in its current state, especially since you indicate you backed out the needle a couple of hundreds of RPM.
Nov 09, 2017, 01:20 PM
Registered User
marmalade1's Avatar
I have that same engine and plane combination!

I've been using a 14x6 APC for almost 3 years. I use Omega 10% fuel and get somewhere in the 9000's. It's been a while since I measured it. Just be sure it's a little rich (300-400 RPM) and enjoy a perfect combination. Remember it's not going to be a fast plane; that wing just isn't made for it, so don't worry about high-speeds and enjoy it for what it is.

I added 1oz or so tail weight. Check your CA hinges regularly, I've ended up using toothpicks to ensure mine don't come out (I think I used cheap CA).

Also, the UC was weak on every VH plane I've had. Add at least epoxy and tri-stock on both sides... it will come out on all but perfect landings and it's easier to strengthen it before it breaks and tears your covering and formers.

It's a wonderful flying machine. Have fun.

PD
Last edited by marmalade1; Nov 09, 2017 at 01:48 PM. Reason: Ok, maybe not 'all' VH planes
Nov 10, 2017, 08:40 AM
Registered User
Cougar429's Avatar
You mentioned the previous owner did the break in, but if done incorrectly or stored poorly the engine can suffer.

There are other factors that can affect your top end performance. Pop the cover and check your valve lash. With the valve(s) closed aim for .001--002" gap between the valve stem tip and arm. Too tight or too loose and you will lose top end.

Also, the age of the plug and fuel could play a part. Same with the prop. M/A tend to be rather flexible so if available try another with the same specs to compare.


You should reasonably look for a top end in the 9600 RPM range and may need to try different props to get in that range. I personally use the MA 13/8 on my 91 OS and Saito, with that same spec 3 blade on the 100's.

WARNING: As this is your first 4-stroke should give you one very important warning. Avoid having yourself or anyone else forward of the prop plane at anything above idle. Some are worse than others, but if lean at higher RPM these engines can come to an immediate stop within one rev, chucking the prop with high energy, Believe me, THIS IS TRUE!!!!!. Even double lock prop nut setups cannot guarantee keeping the prop when this happens.
Nov 15, 2017, 06:12 PM
Registered User
Thank you everyone for your suggestions. I wound up popping off the valve cover and readjusting the gap (which was off by a lot) and now she seems to be running fine with about 9900 peak rpm . Thank you again everyone!!
Nov 15, 2017, 06:17 PM
Engine man
dmrcflyr2's Avatar
I guess the lesson is the valve lash should always be the FIRST thing to check when an engine isn't running properly.
Nov 15, 2017, 09:22 PM
Engine FrankensteinerÖ
1QwkSport2.5r's Avatar
Quote:
Originally Posted by dmrcflyr2
I guess the lesson is the valve lash should always be the FIRST thing to check when an engine isn't running properly.
And set it tighter when the engine is cold. The lash increases as the engine heats up and expands.
Nov 16, 2017, 06:18 AM
the great Gassif´er
Quote:
Originally Posted by Deeman67
Thank you everyone for your suggestions. I wound up popping off the valve cover and readjusting the gap (which was off by a lot) and now she seems to be running fine with about 9900 peak rpm . Thank you again everyone!!
That is the minimum I would expect on that prop....
Nov 17, 2017, 06:34 PM
| Saitopunk |
Bunnyshooter223's Avatar
Glad it was something simple. It usually is, in my experience


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