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magnum 91fs rpm ? - 6/19/2012 12:56 AM   
skypiratescotty


 

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I just got a clean, used Magnum xl 91fs and love it. It starts easy, has great transition but I'm not sure I'm getting full RPM's out of it. It idles perfectly at about 2600-2700 and I'm only getting 8000 on the top. It's turning a MAS 14x8 K-series on 15% CoolPower that I added about 30 ml of oil to a gallon. My buddy flies only YS engines, is a pattern master pilot, and highly recommended
the additional oil. Iv'e been flyin glow for about ten years but i've only used 2-strokes and this is my first attempt at a 4 banger. I should mention that I bought a used tach so as to measure rpms and the guy said he checked against another for correct results but who knows. If i didn't have a tach, I would say "man, this this has awesome idle and immediately screams to WOT" and it seems to have plenty of power.

I guess my question is two-part......1)does it really matter if my reading is low and it still has enough power to fly a plane to my liking or 2)should i just get a new tach to see if i got bamboozled?

Scotty

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RE: magnum 91fs rpm ? - 6/19/2012 3:03 AM   
w8ye



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Status: offline 		Check the cam timing. If the cam is advanced one tooth with the dot straight up and down, the engine will ifdle great but the top speed will suffer.

The cam timing belongs in line with the lifter. I like to push the lifter up out of the way and have the dot in line with the cam follower hole with the piston at top dead center.'




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RE: magnum 91fs rpm ? - 6/19/2012 4:43 AM   
MetallicaJunkie



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Status: offline 		i remember getting over 9k with a 15x4apc prop.... you might want to search the TACH section, i believe i posted some number over there

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RE: magnum 91fs rpm ? - 6/19/2012 1:50 PM   
skypiratescotty


 

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Status: offline 		Thanks for the pic!
It looks like my cam lobe is more in the 7 o'clock position where yours is closer to 8 0'clock. is that the problem and if so how would i change it?

thanks
scotty

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RE: magnum 91fs rpm ? - 6/19/2012 1:54 PM   
SrTelemaster150



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quote:

ORIGINAL: MetallicaJunkie

i remember getting over 9k with a 15x4apc prop.... you might want to search the TACH section, i believe i posted some number over there



I get 10,000-10,100 W/a 15 X 4 Zinger W/my Saito FA91.

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RE: magnum 91fs rpm ? - 6/19/2012 1:55 PM   
SrTelemaster150



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quote:

ORIGINAL: MetallicaJunkie

i remember getting over 9k with a 15x4apc prop.... you might want to search the TACH section, i believe i posted some number over there



9k W/a 15 X4 would drop considerably W/a 14 X 8. Perhaps 8K W/a 14 X 8 isn't alll that bad W/this particular engine.

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RE: magnum 91fs rpm ? - 6/19/2012 2:00 PM   
w8ye



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quote:

ORIGINAL: skypiratescotty

Thanks for the pic!
It looks like my cam lobe is more in the 7 o'clock position where yours is closer to 8 0'clock. is that the problem and if so how would i change it?

thanks
scotty


What you should be looking at is the timing reference dot and no0t so much the lobe itself? It can be up or down but must be in line with the lifters.

Typically on these, the cam, is positioned with the lobe at 8 O-Clock and not 7 O-Clock. This makes them have poor high speed performance.

If yours is more retarded, that is good for High speed ability. and no farther adjustment is necessary?



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RE: magnum 91fs rpm ? - 6/19/2012 2:02 PM   
SrTelemaster150



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quote:

ORIGINAL: skypiratescotty

Thanks for the pic!
It looks like my cam lobe is more in the 7 o'clock position where yours is closer to 8 0'clock. is that the problem and if so how would i change it?

thanks
scotty



Where is the timing mark & are you sure you are @ TDC?

If the answer to the 2nd question is yes & your timing mark is towards 5 o'clock then your cam timing as advanced. That will give great idle & low end TQ, but it will hurt the HP on top end.

To change the cam timing, remove the rockers/pushrods/tappets, pull out the cam, rotate it 1 tooth clockwise, re-insert & check your marks. Do all this W/the engine locked @ TDC on the compression stroke.

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RE: magnum 91fs rpm ? - 6/19/2012 2:39 PM   
skypiratescotty


 

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Status: offline 		the timing mark was just like the one in the pic above and with the glow plug out it looks to me like the piston is at TDC. Not sure i wanna go pulling this thing apart so i may just leave it alone and fly it the way it is.
thanks
scotty

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RE: magnum 91fs rpm ? - 6/19/2012 2:46 PM   
SrTelemaster150



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Status: offline 		I have run the standard formula 15% Cool Power for years in my Saitos & never felt the need to add oil. It has plenty of oil to start with. Perhaps more than needed.

Maybe you should try un-adultered Cool Power & see if your RPM improves.

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RE: magnum 91fs rpm ? - 6/19/2012 2:59 PM   
SrTelemaster150



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quote:

ORIGINAL: skypiratescotty

the timing mark was just like the one in the pic above and with the glow plug out it looks to me like the piston is at TDC.



W/the glow plug installed & the throttle closed, S-L-O-W-L-Y rotate the engine to TDC on the compression stroke. It should lock in to TDC as the compression leaked down during the S-L-O-W compression of the air in the chamber W/a bit of (even) rotation in both directions as the piston is on "dwell" @ TDC.

In essence the chamber is devoid of any compression & rotating it gently in either direction will create a vacume in the chamber, creating resistance to further rotation in either direction..

You can then gently rotate the crank back & forth while observing the cam timing mark. If your cam timing is correct, the mark should vary evenly on either side of 6 o'clock.

While this method is not as accurate as using a degree wheel & piston stop, it is accureate enough for initial set up of electronic ignition timing & your purpose.

If your timing is not correct it isn't that hard to correct the timing. R&Ring the rockers, pushrods, lifter & camshaft is not all that complicated. The whole thing could be done in 20 minutes or less.

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RE: magnum 91fs rpm ? - 6/19/2012 4:07 PM   
w8ye



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Status: offline 		The finding of top dead center by feel is not explained completely.

With the plug in the head and you go to the compression stroke, feel for that dead spot at top dead center as explained earlier. The exact center of this dead spot area is Top dead center and where you should look at your cam timing

I WANT TO EMPHASIZE THAT THE CAM DOT POSITION AT TOP DEAD CENTER IS NOT STRAIGHT UP AND DOWN BUT IS IN LINE WITH THE LIFTERS! This is close to straight up and down but there is one tooth difference. I prefer to raise the exhaust lifter and place the dot at the lifter position to be most accurate.

Another way to verify your cam timing position is to find the center of the Top Dead Center on the compression stroke. Now turn the prop one full turn (360 degrees) to top dead center. You will now be on the overlap stroke. Both valves must be partially open as the piston passes through top dead center on the overlap stroke. 

Actually, the center point of the valve overlap should be about 5 degrees before top dead center. This is the point where one valve is almost closed and the other is almost open the same amount. You can verify this as to site across the tops of the rocker arms will be parallel.

You can also open the carb, close off the pressure nipple and blow through the muffler and out the carb through the entire overlap interval. As I said before, the exact center of this overlap interval should be about 5 degrees before top dead center on the overlap stroke.

If you meet these parameters in the overlap stroke, your cam is timed correctly.


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RE: magnum 91fs rpm ? - 6/19/2012 6:51 PM   
blw



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Status: offline 		Cool Power has 17% synthetic lube http://www.morganfuel.com/coolpower_performance_detail

I doubt that changing oil content will do much in the way of rpms. I add a couple of oz of castor to Omega 15% and have never noticed any engines underperforming.

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RE: magnum 91fs rpm ? - 6/19/2012 11:05 PM   
carrellh



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Status: offline 		If it pulls the plane the way you want, don't worry about rpm.
My brother and I have several of these engines. Never tried a 14x8. With a 14x6 prop, and 15% fuel, they turn from 9,000 to around 10,400. It's a little annoying that there is so much variation from one to the next. Our OS 91s are more consistent and all turn over 10,000.

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RE: magnum 91fs rpm ? - 6/27/2012 3:25 AM   
estradajae


 

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Status: offline 		I guess a 14x8 prop is the reason why you are getting about 8K rpms...I get around 10200 rpm on my ASP 91 with a 14x6 scimitar prop.

If the engine is running OK as you say (you tell it from hearing it already)...it is probably right.

I fly at 2500m OSL and use omega 10 on everything nowdays.

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RE: magnum 91fs rpm ? - 6/28/2012 12:01 AM   
skypiratescotty


 

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Status: offline 		thanks for the replies.....i've been thinking about it and i'm at sea level on the coast of Florida and the air is thick. Plus it should gain about 20% rpm when flying so it should be good to go.

Scotty

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RE: magnum 91fs rpm ? - 6/28/2012 3:16 AM   
jeffie8696



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Status: offline 		Its also quite surprising how much difference in performance when the engine decides to break in.


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RE: magnum 91fs rpm ? - 6/30/2012 12:19 AM   
Ed_Moorman



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Status: offline 		Scotty,

Do you fly with the Eglin club? I switch back and forth flying there and at Holly Field. I have a Mag .91. I had it on a Ultra Stick 60, but swapped it for a Tower .75 2-stroke. I've had several Magnums and they ran well. I now have it on a Douglas Skyraider, but haven't flown it in a good while.

If you're out at Eglin, look me up and we'll talk Magnum 4-strokes.

You can't miss me. I'm over age, over weight and out of step. I also were a red ball cap and usually a red polo shirt.

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RE: magnum 91fs rpm ? - 6/30/2012 1:23 PM   
skypiratescotty


 

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Status: offline 		Hi Ed
Yes sir....well, i flew at Eglin from around 02-07 then moved to Maryland. I just moved back to Destin in late Feb and rejoined the club but haven't had a chance to go flying yet. I run the parasail boat "SkyScraper" out of Dewey Destin Marina on the north side of the Destin bridge and work everyday so i can save enough dough to take off all winter. This time of year I just tinker with engines and dream of my next end of season treat to myself (new plane)....

I really tried to make it to the Memorial Day fly in but a relief captain wanted $200 to cover me for the day so I put on flip flops and went to work. I really wanted to show off this nice little scale J-3 done up in the New Mexico National Guard colors....

Well, 2 more months of season and then i can go fly every morning till next March. I never went to Holly Field but look forward to meeting ya out at Test site C-3.

Scotty

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RE: magnum 91fs rpm ? - 6/30/2012 8:08 PM   
slopoke


 

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Status: offline 		your description sounds like everyone at our club.

slopoke

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