tune a os max fp 25 motor - RC Groups
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Feb 02, 2006, 02:13 AM
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tune a os max fp 25 motor

hello i have a os max fp 25 motor tore it down to clean it. know it don't run very good help in tuning it how many tunes on the air screw in or out the motor has a air screw and a needle valve for fuel
Last edited by loonetoon; Feb 02, 2006 at 09:26 PM.
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Feb 02, 2006, 07:03 AM
Registered User

Um I'm assuming you have an RC carbie on it? It's not a control line engine?

I'd start with the fuel line, new check for stuff from the tank and make sure the carbie is clean.

Then flush out the engine with clean fuel.

Set the main needle at about 2 1/2 turns from fully in.

Set the Idle mixture (if it has one) so you can barely see a gap at the end of the thread or even leave it closed for the time being.

Check that the glow plug gives out a really bright red colour when the battery is hooked up.

Do you have an electric starter or are you flicking it with your finger ( if you are make sure you have a chicken stick )

Oh, to set the prop on the engine, you have the engine in its mount ( upright to start with it's easier to start ) and put the prop on so it's just starting to come onto compression at about 3 O'Clock, as you try and wind the engine over with your finger.

With good fuel, glow and the needle set the way I said you should have some bark from it even if you flood it to start it, just watch the kick back. Listen for the hiss as the fuel is burned by the glow plug.

After the engine fires, it may die, don't try and flood it again, flick it again, it should start. Once the engine is running, wind the needle in a couple of clicks at a time till it leans out, if it won't run but keeps stopping try richening it a bit by winding the needle out a few clicks till the engine runs by itself. Let the engine bed itself in for a couple of minutes and get a bit warm, before you move the needle around, in or out. Once it's getting warm, try leaning it out till it's just short of flat out ( you must let the engine bed itself in a little after reassembly).

Once you have the engine tuned with the main needle and the carby opened fully ( you can see right into the crank with the butterfly fully open ) then you can start to mess around with fine tuning the idle needle.

The idle needle is extremely sensative to being too lean and will cut almost instantly once you start to close the throttle, just work the rev's down opening the idle screw one or two clicks (maximum) at a time till the engine won't run without the glow plug driver on when it's ideling, then wind the screw back in to richen it up abit ( at least 2 clicks) it should now idle with the glow driver removed.

5%- 10% nitro in the fuel helps start a new / rebuilt engine, but don't over do it as you want to run it in a little bit rich for a couple of tanks first.

Oil, either castor or Coolpower synthetic at 18% to 20% is ideal for the engine at the moment.

Just a few things to check if it still won't run, is the head on with a gasket? Is the head on tight, but not tooo tight?

is the carby seated correctly? No air gaps? Is the needle correctly fitted in the carby? Is the main needle straight and not bent on the tapered end? How worn was the engine before you cleaned it?

did you scratch the bore when you cleaned the engine? Did you flush out all the carbon when you put the engine back together?

Let me know how you get on
Feb 02, 2006, 07:10 AM
Registered User

Again, here is a link to a picture of the engine in a trainer, if you look at the prop it is in the correct position to start the plane, the needle looks like it is set in the right position ( hard to tell ) and the idle screw isn't too far out letting in too much air.

And the filter is in the right place.

Good Luck

Feb 02, 2006, 08:12 AM
S.A.D. member
ivanc's Avatar
I'd recommend reading the sticky on top of the Engines forum: https://www.rcgroups.com/forums/showthread.php?t=205602
Your engine has an airbleed carb, so the idle screw works opposite to an idle needle. You screw it in to restrict the air at idle thus richening the mixture or unscrew it to lean the idle mixture.
Feb 03, 2006, 06:30 PM
Registered User
thanks that help alot
Feb 04, 2006, 05:56 PM
Registered User

I just read the link too and yes, it's very useful, he has a lot of good practical advice, I will be re adjusting all my engines the way homebrewer has suggested as it makes sense and is easy to do.

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