I've got an old OS .40 Max FP from about 1992 that's never been run. It has
three adjustment screws on the carb. The main setting screw is large and
"clicks" like the newer OS engines do when it's adjusted. The other two are
smaller and just have springs pressing against them to hold the position. I
don't have the manual and before I try and get it from OS does anyone have
suggestions on how to set all the screws for the run-in and then thereafter?
I have the same engine on an Eagle 2 trainer I just bought to have an extra
plane. The large screw with the indents is the needle valve and adjusts like a
regular one. The othe side of the carb is the air bleed adjust. It works the
opposite of a standard low speed adjust, by letting more or less air in. As to
the 3rd, not sure.... Andy
We can make a box of wood.....FLY!!
For future reference download the manual for the 40LA from O.S. MAX's
site and read the instructions for adjusting the carb. It has
essentially the same air bleed carb as the FP did differing only in
the position of its remote needle valve .
Re your FP.
LHS needle valve adjusts high end mixture. Turning it in (clockwise)
leans the mixture, out richens. You adjust this with the carb barrel
fully open and accept the resultant mid-range, which with O.S. carbs
and air bleed carbs in particular is generally pretty good.
RHS screw is air bleed screw. It adjusts the idle mixture. It works in
the opposite sense to the needle valve. ie: turning it in (clockwise)
richens the mixture (closes the air hole), and of course the opposite
leans. You adjust this with the carb barrel against throttle stop set
with an initial visible intake gap of about 1-2mm.
Top of carb is the throttle stop screw. Screwing this in (clockwise)
increases the intake oriface gap between the throttle barrel and the
carb body. Like its name implies, you use this to set where the carb
barrel stops rotating at the low end. In the instructions, you set a
theoretical 1-2mm gap.
In practice, if you set the throttle stop there with the engine at
idle and TX throttle lever fully closed which is as desired, then you
won't be able to shut the engine down by retarding the throttle trim
lever down from neutral to close the intake as the barrel is already
physically up against the throttle stop screw.
So with modern computer radios it's common to actually set the carb
barrel position where the intake is just closed off entirely or at at
point where it won't sustain combustion using the throttle stop screw.
After doing this and setting up the mechanical linkage from the sevo
as accurately as you can, with the TX throttle stick fully closed and
TX throttle trim set to neutral, you re-set the desired barrel idle
gap (1-2mm) using the Sub-Trim feature of your TX.
You then retard the TX throttle trim lever to fully close the throttle
barrel, limiting servo travel to coincide with the physical throttle
stop position with End Point Adjustment (Futaba) or ATV in the case of
JR. This is to prevent the servo straining or jittering in trying to
move the throttle arm (& barrel) beyond the point limited physically
by the throttle stop screw when the trim lever is used to fully close
the barrel at the termination of the flight in the pits.
Finally to bench run it in, set the throttle stop about 1-2mm open for
idle and adjust from there if req. N/V initially about 2½ turns open
without muffler pressure, 1½-1¾ with. The air bleed will have been
factory set and you won't need to adjust idle mixture initially unless
you've fiddled with it or are at 8000' AMSL. Nice engine, reliable
idle, easy to tune and powerful in class.
The large one on the right of the carb (as you look at the engine from the
front) is the high speed needle. Turning it in leans the hig hspeed mix.
A small screw on the upper front of the carb, with the head facing to the left
(as you look at the engine from the front) is the low speed air bleed
adjustment. Turning it out leans the low speed mix.
The third screw, with the head pointing up and slightly rearward, is the
throttle barrel adjustment. It limits how far the carb barrel can close. For
most R/C applications, it should be set so the barrel will close completely.
If you turn it too far out, the barrel will be sloppy and may fall out.
"There's a Hun in the sun!"
One other thing you can set for your particular application is the relationship
between the throttle barrel and the throttle arm that the linkage attaches to.
There is a large philips screw in the middle of the arm that can be loosened
and that angluar positon of the arm can be adjusted. Just be sure to get it
real tight after you set it!
My Eagle2 came without a computer radio, so I adjusted the positions with
the arm to get nearest to a full range of motion with the trim set to close
the barrel completely. So far, even though it's old and sat around for a while,
it runs fine and pulls the Eagle around fully adequately for it's intended
Good luck with yours, let us all know how it works out for you. Andy
We can make a box of wood.....FLY!!
Thanks all for the info, your explanations are the exact setup of the
Like a monkey in a room with a red button I have already fiddled with the
low speed air bleed screw adjustment. Any suggestions on how many turns out
I should set that one so I don't run it too lean?
Actually, I did. :-)
On most air-bleed carbs, the air hole is too small to begin with. Even with
the screw backed out all the way, the idle mixture will probably still be too
rich. At least, that's been my experience with air-bleed carbs. On this type
of carb, there is no interaction between the HS needle and the air-bleed screw.
I have recommended, and have seen recommended by engine gurus, drilling out the
air-bleed hole to further lean the idle mix.
"There's a Hun in the sun!"
Then you got it back to front.
Turning the air bleed screw out will lean the mixture, and backing it
out all the way will ensure the FP's idle is excessively lean. Not the
reference datum desirable to start with. Even our inquirer is smart
enough to figure out it's best to start out rich.
It ain't on the FP, and we are talking about the FP in the particular
not the elusively imaginative "most".
Sorry but it just has to be said. That is utter C R A P.
We'll put it down to your obvious unfamiliarity with the venerable
40FP or ever having actually tuned the idle on an FP carb...nor for
that matter many Japanese air bleed carbs if you are suggesting Mr
Enya and Mr O.S. don't have the elusive "most" of them pretty well
sorted by this time? :)
Looking at my FP carb as we speak, its air bleed screw is screwed in
so that insofar as I can gauge within a bee's dick, 1/3 of the air
bleed hole is obstructed. It's flown many an instructing hour upon
hour such that its idle can be confirmed tuned. It idles consistently,
reliably and stably. Ne'r a flameout whether throttle closed during
approach, spinning or prolonged idling at the holding point. The size
of the hole in the FP carb is inarguably adequate, and that air bleed
carb not only works, but works well for this engine. One can only
recommend a datum starting point of setting the screw ½ way in (or out
depending upon your preferred frame of refference) as originally
suggested and working from the known rich setting there.
As for generalised statements about air bleed carbs. My empirical
experience doesn't support your hypothesis. The air bleed idle type
carb fitted default to my AAC Enya CX45, which is itself no slouch of
an engine, is the equal and perhaps arguably superior in terms of low
speed idle stability and transition to the mixture screw type idle
carbs on my various O.S. engines of similar class. eg: .46FX & 46SF.
Though I couldn't in honesty actually denigrate their idle mixture
screw carb idle and transition performance either. We'll talk about
Super Tigre MAGs another time. FWIW, that Enya CX 45 is my absolute
favourite sport engine of the Schnuerled tapered piston TBRs in that
.45-.47 capacity class...no mean feat when you look at what it's up
Transition isn't an issue on well designed and manufactured air bleed
carbs set up either by those with ½ a clue or someone who can follow a
simple set of written instructions from the OE manufacturer. A shitty
carb, air bleed or otherwise is a shitty carb. But the FP's doesn't
fall within this category IME.
Simply unnecessary with the FP carb.
re: iguana's post:
amazing how actual experience is so much more valuable than "what i heard".
in years spent tinkering with these engines, i also found that putting the
screw in the middle was the best place to start. at least then you have some
place to go, in or out, and certainly among "the proven" engines, the fp was
OK, you wanna get personal? Tell me I don't know what I'm talking about? HA!
You talk a lot, use big words, and say little. I repeat, it's been my PERSONAL
experience with air-bleed carbs. No hypothesis on my part. I don't pull this
stuff outta my butt. Anything I say, I've either witnessed or done.
You got lucky with your FP. I stand by what I said. Let the original poster
try it my way. If it doesn't work, then I'm wrong. If it does...
"There's a Hun in the sun!"
Well written, the experience of many.
As a standard work shop setting at sea level, I use a glass head pin which
is just under 1/16 inch diameter and wind the screw in until it just touches
the pin. Never start with screw more than 1/2 way out.
refer also to
"Engines,Plugs,Tuning, Petrol, Turbines, Mufflers, Cleaning, Anodising etc"
sub section =
"Carburettors, Tuning & Performance Testing, Glow Plugs." amongst many
others are for example =
Cen Racing - good tutorial on tuning, care & maintenance
Extend Engine Life - M.A.N article
How to Rebuild Your Nitro Engine - RC Nitro Magazine,***
Installation of Model Engine Bearings
K&B owners guides - Carbuettor tuning etc
Modifying Your Engine - Expansion Chambers etc
Performance Testing of Engines - Martin.Hepperle
Perry Pumps & Carburettors
Prop and rpm performance figures - RCFAQ
Running In Engines - Brian Hampton
Setting the Idle Mixture _ Brian Hampton
The Technology of Glow Plugs
Tuning and setting Car & Truck engines***
Alan's Hobby, Model & RC Web Links
this is what i love about your posts. it apparently doesn't matter to you that
we are talking about a specific engine, and a specific engine that has worked
well for a whole bunch of people without having to resort to voodoo wizardy to
make it run. its enough for you that there is an airbleed carb and that is
enough to invoke the generalizations. so maybe you might read his post with a
little more thought to the specific nature of the inquiry, and the mountains of
experience that support the response. just my two cents.
And that's what your reply is worth, two cents. I do not rely on "mountains of
experience" heresay from others. I rely on what I, personally, have
experienced. Every air-bleed carb I've ever tuned ran rich at idle, unless the
bleed hole was modified.
"There's a Hun in the sun!"
You'll probably take this on board with your ego instead of your
intellect as well, but then, that's your cross to bear.
You need to acquire discernment.
Getting "personal" would be indulging in ad hominem 'argument'. I
didn't do that. Nor did I imply or state that you're either a bad
person, or that your argument was stupid. You've since managed to do
that on your own.
What I did say with that sentence was that the point made in the
preceding sentence by you was FALSE. I addressed and denied the
veracity of it relative to the specific case in point, supporting it
with fact rather than unsubstantiated, generalised supposition. Quite
succinctly and with a contemporary word of common usage which I
normally abhor, but chosen deliberately because it couldn't be
confused even by someone limited by the comprehension and literacy an
8 year old which you seem to prefer?
Now your "know it all" nose is simply out of joint as illustrated by
the following sentence. Got a revelation for you. None of us know it
all...even if comfort zones are reinforced and egos fed by such
Err...in this quite specific case, not me, the empirical evidence, the
facts and O.S. MAX all speak adequately enough for themselves. All
I've done is illustrate them.
I appreciate that you appear to prefer the rejection of valid
information when it presents you with what you perceive as an
embarrassing loss of face? Get over it. A way to avoid compromising
yourself in future is to think before you speak, and then speak from a
position of knowledge or *pertinent* experience rather than either
conjecture or dogmatic opinion. You had the out diplomatically offered
by my first retort posed as a question. Your ego chose to don the
adornment of Grand Master - poor choice if you'll forgive the liberty
of me saying so.
Talk a lot. Well yes. That's the essence of interpersonal
communication unless one is scrawling in the journalistic sense
to the lowest common denominator of literacy, attention span or
intellect. Big words? My childhood was one of TV deprivation, and when
people's literacy in general extended beyond the 64 point 4 word
headline. OTOH, I can be abruptly succinct when appropriate, and I at
least know when it's wiser to remain silent.
on my part.
But it is until you present some, any, quite specific anecdotal
evidence' other than what you allegedly read somewhere, or refer to in
generalised terms akin to the ubiquitous "they" (or the equally
elusive "someone" (else)) who should always "do something".
In the case in point re the air bleed carb of the O.S. MAX 40FP, you
are in point of fact WRONG. In the case in point of the ENYA CX45 your
generalisation is similarly contravened. As such, the evidence itself
contradicts your belief, providing rebuttal of your generalised
experience viewpoint. According to the rules of logical deduction,
this means your belief is based upon a false premise, irrespective
that disclosure of this is distasteful to you. Pick up the 'clue',
absorb it, learn from it and move on.
Well on this occasion, you're apparently intent upon trying your
hardest to make it appear you did.
Not in this particular case you haven't - quite apparently. Hey, its
OK. You don't have to WIN to be a good person.
That statement is just so silly and unworthy of you.
Then apparently Brian Winch who reviewed the engine, whose article was
syndicated worldwide, and hundreds of thousands of other "lucky" 40FP
owners did to?
I'm sure you do. But if you think about what you've just said, both
it, and what you said, are rather silly positions to adopt.
I'm sure Mike G's adult enough to assimilate the information presented
and make up his own mind without either of us telling him how he MUST
chew his steak. :)