Magnum 120FS RPM issue
 

Hello Guys,
I am not getting what I think should be proper high RPM from my Magnum 120FS.
The book says 2,000 - 11,000 RPM range. I top out at 7200 RPM.

Fuel Byron 15% Nitro 18% lube.
15 X 8 MA K series prop.
Low idle is great 1800 RPM.
This is both on the plane, and on the test bench.

Here is the issue:
I have this on my modified Senior Telemaster (2 piece wing w/wing tube, detachable tail, bombbay, widened landing gear)
I don't have the pull with this 120 as I did with the OS .91FS.

I am not expecting the RPM's like a 2 stroke, I am not a dummy. Just not sure this particular 4-stroke is running well enough.

ANY THOUGHTS?

RE: Magnum 120FS RPM issue
 

2 HP ought to pull about 9000 W/a 15 X 8. You're only making a little more than 1/2 of that according to this [link=http://personal.osi.hu/fuzesisz/strc_eng/index.htm]HP/THRUST CALCULATOR[/link].

Is it broken in? What is the valve lash? Bad fuel? Too rich on the HSN?

Something isn't right for sure.

RE: Magnum 120FS RPM issue
 

With a MA 15/8 K Prop, I'm turning 8500 rpm after 2 gallons of 10% nitro, 20% oil. I would pull cam cover and make sure the com its indexed correctly and check your valve clearances. It will only turn about 8000 before its broken in. With the piston at tdc the cam dot should be in line with the push rod tubes. engine may not be properly tuned, either to rich or lean.

RE: Magnum 120FS RPM issue
 

in this order:
fuel mixture strength
excessive carbon on valves
sticking valves
broken valve spring
camshaft timing off

RE: Magnum 120FS RPM issue
 

My magnum 120 ran like this and I discovered that the cam was advanced one tooth.

I retarded the cam for the mark to be in line with the lifters at TDC and it ran as good as it was supposed to.

The overlap must pass through TDC.

RE: Magnum 120FS RPM issue
 

Inspection shows the "dot" in-line with the pushrod tubes, well actually it is 180degrees or "opposite" of the tubes. For example, the tubes enter the top, and the dot is at the bottom, yet in a straight line with the tubes. IS THIS CORRECT?

RE: Magnum 120FS RPM issue
 

Some Magnums time at 6 o'clock while other time at 5:30.  Best way to check is overlap at TDC on the exhuast stroke.

RE: Magnum 120FS RPM issue
 

Yes. The camshaft has half the rotation speed of the crankshaft.

RE: Magnum 120FS RPM issue
 

Yes 'opposite the tubes' is fine too, rotate the prop 360 degrees and the dot will now be next to the tubes. It's the same thing.

I've had examples of these Sanye engines with the timing dot position a little bit variable. As W8ye and Kimhoff say, a good way is to confirm the timing dot is to check the valve overlap.

Replace the cam cover (loosely is fine in the meantime as long as the end of the cam is supported). Remove the rocker cover and the rear cover ( carb comes off first). Rotate the engine until you can see that both rockers are open, and then get them so they are both level (open the same amount), now look at the crank pin. It should be at the very top (12 o'clock) or very slightly before 12. One tooth of on the cam will be noticeable.

While you have the rocker cover off check for sticking valves or broken springs like Pe says. Also check the push rods are seated correctly in the adjusters. Check valve lash (gap).

You can remove the muffler to inspect for carbon without removing the head.

It's not overheating is it? That can make them run slow.

You could try much lower (5% or even no) nitro, it may be preigniting sufficiently to lose RPM. Normally you would know if hat was happening though because the engine would sound bad and try to kick the prop off. Can happen though so might be worth thinking about if nothing else works?

Best of luck,

Dave H

RE: Magnum 120FS RPM issue
 

Dave, The engine does have a hard knock at lower RPM (idle or jst above) So your thought about 5% may be helpful as well.
I can't say enough about how I appreciate the thoughts of you fella's answering my post so well. THANKS THANKS THANKS!
I am out of town until Saturday afternoon, so I'll have to wait until then to get deeper into this problem.