(I'm not the author, but this is an excellent resource.)

Engine and Prop Adjustments JAN 2010

Picking the right combination of engine, reduction unit and prop is
discussed in a different article. This article tells you how to adjust the
engine and prop correctly after they are installed. Its also a good guide
to things you should check on a used plane and before you first start a
new engine.

A lot of problems can be traced back to inaccurate instruments. It is very
common for tachs to have large errors. Borrow a Tiny Tach or an optical
prop tach and compare readings to check your tach over the whole range.
It is impossible to correctly adjust the prop with a bad tach. Be certain
that your EGT and CHT gauges read correctly. Useing the wrong type of
thermocouples or wire for them is a common mistake. It's also real easy
to enter wrong numbers or settings for electronic displays. See my article
on thermocouples for more information.

Check for water or crap in the carb bowls. Make sure carbs are mounted
square with the engine since a tilted carb can cause one cylinder to run
hotter than the other on some engines. Cracks in the carb mounting
sockets can cause lean mixtures so replace them if they are brittle.
Check for adaquate fuel flow to the carbs. Any time you notice higher
than normal EGT or if the plane won't reach normal full throttle RPM
install a new fuel filter before you waste a lot of time looking for other
problems. See my article on fuel systems for more info.

Each carb has an idle jet, a needle jet with a jet needle and a main jet.
All have tiny numbers on them which tell you the size. Check to be sure
they are the factory recommended jets and the jets are the same in both
carbs because its common to find that a previous owner has installed
different jets. The factory recomendations are almost never wrong so
its very important that the factory jets are used during testing.

Check that all throttle cables are routed to drain out any water inside the
cable outer jacket. Water collecting in a low spot can cause corrosion
or freeze in the winter to lock the cable in one position.

Pull the throttle all the way back against the lower throttle stop. Where
the cable enters the top of the carb there is an adjustment that is covered
by a rubber boot. Slide the rubber boot up the cable to get it out of the
way then gently pull upwards on the outer jacket of the cable. There
should be a tiny amount of free movement before you feel the inner cable
start to move the slide upwards. That tiny amount of free play is critical
be sure that the cable is not holding the slide up off of the idle speed
adjustment screw. If you have trouble feeling it you can remove the air
filter and look inside the carb throat to watch the slide move. Loosen the
lock nut and turn the adjustment screw until you have that tiny amount of
free play if needed. Lock the adjustment and work the throttle back and
forth a few times to be sure the cable outer jacket ends are fully seated
in their sockets correctly then pull the throttle all the way back against
lower throttle stop. Double check that the amount of free play is still
correct at the top of the carb before you reinstall the rubber boot.

Push the throttle all the way forward against the upper throttle stop and
look in the carb throat. The slide should go up far enough so that it does
not block any of the carb throat opening. Its okay if it goes up just a
extra but if it goes up too much it will bind and put a strain on the
cable. You should install some sort of upper throttle stop to prevent that.

Dual carbs must be mechanically matched or one cylinder will have a
higher EGT than the other and/or you can not get a smooth idle. Pull the
throttle back against the lower throttle stop. Where the cables enter the
tops of the carbs there are adjustments that are covered by rubber boots.
Slide both boots up the cables to get them out of the way. Loosen the
locknuts and turn both adjusment screws down two turns. That makes
sure that the cables are not holding the slides up off of the idle speed
adjusting screws.

Remove the air filters and use the smooth ends of drill bits as round
feeler gages to check the clearance between the bottom of the slide and
the bottom of the carb throat on each carb. Pick a drill bit that will
slide into the smaller of the two openings and use it to adjust the idle
speed screws on both carbs until both openings are the same. Your
carbs are now mechanically matched. To keep them matched you must
ALWAYS turn the idle speed screw on both carbs the same amount
when makeing idle speed adjustments. NEVER adjust just one idle
speed screw.

Make sure the throttle is pulled all the way back then go back to the cable
adjusters on top of the carbs. Screw them out to take out ALMOST all of
the free play. You check that by feeling how much you can lift the outer
jacket before you feel resistance. It takes a delicate touch but you need
just a tiny amount of free play to make sure that the slides are not held up
by the cables when the throttle is pulled all the way back. If the free
is not the same on both carbs then one slide will start to rise before the
other so this adjustment is critical. Tighten the locknuts and work the
throttle back and forth a few times to be sure that all the cable outer
jacket ends are fully seated in their sockets correctly. Double check
that the free play is still correct on both carbs then slide the rubber
over the adjustments. If you have done it correctly both slides will start
to move upwards at the same time and the bottom of both slides will be
flush with the top of the carb throat just before you reach full throttle.

Set the throttle wide open and check that both slides go up far enough
so that they don't block any of the carb throat openings. Its okay if they
go up just a little extra but if they go up too much they will bind and put
a strain on the throttle cables. You should install some sort of upper
throttle stop to prevent that.

Because the jet RPM ranges overlap you should make the idle speed
adjustments first and work up to higher RPMs. If any large adjustments
are made it will be necessary to repeat the idle adjustment procedure to
fine tune all adjustments. Make sure a clean air filter is installed before
makeing adjustments.

There is an AIR MIXTURE adjustment screw which will fine tune the
amount of air at idle speeds only. The IDLE SPEED screw limits how
far the slide can come down to close off the air flow. If you aren't sure
which screw is which you can see the end of the idle speed screw
sticking out inside the carb air inlet if you remove the air filter. A
above the slide pushes the slide down against that screw.

To prevent engine shake and gearbox chatter our engines need an idle
speed of around 2000 RPM so warm up the engine and adjust the idle
speed screw for about 2000 RPM. Remember to turn BOTH idle speed
screws the same amount to keep the carbs matched if the engine has
dual carbs.

Adjust each air mixture screw to get the highest RPM at that idle speed
setting. 1/8 of a turn on adjustment screws can make a big difference so
keep the adjustments small. You will have to go back and forth between
dual carbs a couple of times to get the best possible idle because they

As the mixture gets better the idle speed will increase so adjust both idle
speed screws the same amount then fine tune the mixture screws again.
Keep at it until the engine idles smoothly at the lowest speed which has
minimum engine shake. Check the cable adjuster on top of both carbs
after you finish to make sure there is still the same tiny amount of free
play on each carb.

Be aware that it's easy to get an arm cut off by the prop and anything you
drop may go through the prop. I usually tie a short safety rope around my
waist and the base of the wing spars so that I have to stretch way out to
reach the adjusting screws. That way I can't forget and move toward the
prop. Fortunately you won't have to do this again until you notice an idle

Now you need to know how the factory recommends that your engine be
used. For Rotax engines download the operators manual at


The figures used below are for the Rotax 503 so look at page 10-1. If
you have a different engine you need to change those figures to match
your engine manufacturers recommendations. If you have a fixed pitch
prop all you can do is hope that it is the correct prop but all the other
below will still apply.

The Rotax manual lists a "takeoff speed" of 6800 RPM for no more than
5 minutes and a "cruising speed" of 6500 RPM with no time limit.

Its common practice for aviation engines to have a normal operations
limit and a higher takeoff limit so what Rotax should have said was "The
engine can be safely operated at 6800 RPM for five minutes but you
must reduce the throttle to 6500 RPM or less after that."

The RPM that a given engine/prop combination can reach is greatly
influenced by the air speed and any disturbance of the air flow into the
prop. During discussions you will hear several terms used so you need
to understand those terms.

During static testing the plane is tied down. There is no airflow into the
prop so the engine can not reach full RPM.

During the actual takeoff the plane does not reach full speed. There is
reduced airflow into the prop so the engine can not reach full RPM.

During climbout the plane does not reach full speed. There is still
reduced airflow into the prop so the engine still can not reach full RPM.
In addition the use of flaps or even the planes design may block or
disturb at least part of the airflow into a pusher prop.

If you set the prop so that the engine turns 6800 RPM during the actual
takeoff your engine will seriously over rev once you build up more speed
and/or finish the climbout. To prevent that you should adjust the prop to
allow 6800 RPM at full throttle during straight and level flight. This
you the maximum airspeed and keeps your engine below the maximum
factory recommended RPM during all phases of your flight.

Rotax describes 6500 RPM as "cruise speed" but if you actually cruise
at 6500 RPM your engine won't last very long. Its a general rule of thumb
to cruise at the RPM where your engines power curve produces 75%
power for best life. For the 503 thats around 5200 RPM.

Engines are expensive and pilots are cautious so many of them try to
adjust their prop to give lower full throttle RPMs thinking that will be
easier on the engine. That actually works an engine harder because
the prop becomes a bigger load than the engine was designed to
handle. The ignition timing, port timing and carbs are all designed to
work best with the engine loaded just enough to reach the max full
throttle RPM specified by the manufacturer. If you really want to be
easier on your engine simply give it less throttle after the prop has
been adjusted correctly. Loading the engine down so it can't reach
the maximum RPM at full throttle is called "over prop" or "lugging".
It puts an extra strain on all engine and reduction unit moving parts
which will shorten their life.

The first step is to adjust the prop to a pitch where you will have a safe
first test flight. We know that the prop load will be reduced once air is
flowing into the prop at high speeds so the engine will turn faster in
The amount depends on the type of prop, diameter, pitch, etc. To
prevent exceeding the max RPM in the air we need to first adjust the
prop for about 6300 RPM at full throttle while the plane is tied down.
The exact final adjustment will be determined by flight tests later.

Tie the plane down and monitor the EGT and CHT temperatures as you
slowly apply throttle. If the RPMs exceed 6300 before you reach full
throttle, stop and add more pitch to the prop. If you reach full throttle
RPMs stay below 6300 stop and reduce pitch on the prop. If the EGT
exceeds 1150 stop and find out why before continuing tests. If the CHT
exceeds 430 stop and let the engine cool down before continuing tests.
Maybe you need to think about adding some more cooling capacity.
Note that the operators manual lists maximum EGT as 1200 and
maximum CHT as 480 on page 10-1 but we want to stay below those
figures during our early tests.

This is also a good time to watch for surprises like leaks, fuel starvation,
and overheating problems. It's better to find them now than find them in
the air.

Prop adjustments and carb adjustments will affect each other so read
this whole section before any test flights and make all the adjustments
in the sequence given below.

During the first phase of first test flights the pilot needs to determine
full throttle engine RPM that is reached during straight and level flight.
should also monitor EGT and CHT closely the entire flight because they
may try to exceed the maximum allowed until the plane has been fully
checked out and the prop is correctly adjusted. After each flight make
small adjustments on the prop until the engine turns very near 6800 RPM
at full throttle during straight and level flight.

During the second phase of flight tests the pilot needs to check the
needle jet and jet needle operation by flying for a couple of minutes at
each of several engine speeds between about 3000 RPM and 5500 RPM.
There will be variations but all EGT readings must be in the normal range.
If most of those engine speeds show a high EGT try moving the clip
down on the needle to provide more fuel for the whole mid range.
Repeat test flights until you find the clip position which works best over
the whole mid range.

You may find a small range of engine speeds which have a high EGT
while all the other mid range speeds are normal. Avoid running in that
small range of engine speeds until all other adjustments are completed.
An incorrect prop or prop adjustment is almost always the reason.

During the third phase of flight tests the pilot needs to check the main
jet operation by watching the EGT during full throttle operation in straight
and level flight. If the EGT gets too high reduce throttle until it cools
down. You MAY need a larger main jet but if the engine could not turn
near 6800 RPM during the test the real problem was the prop pitch.
Adjust the prop and try another test flight.

You may have to tinker a little more with the needle clip to get the EGT
correct over the entire mid range after you have fine tuned your prop.
That may mean some minor compromises but try to keep the EGT as
close to ideal as possible over the most used RPM ranges. Double
check that the final adjustments will not let the EGT run above 1150 or
below 1000 at any RPM.

Remember that air temperature and altitude will have an effect too. I
have to adjust my needle one notch every spring and fall to keep mine
set correctly but four thousand feet of altitude change doesn't change
my EGT enough to matter.

There are four paths fuel can follow to get through the carb. The first is
the choke. Our choke is not an air flow restriction like on cars. Our
is a manual "on" or "off" lever which opens a path for a tiny amount of
extra fuel to flow into the throat of the carb to enrichen the mixture for
better starting. Always turn it off after the engine warms up enough to run

There is some interaction and overlap between the three main fuel paths
but generally speaking, the IDLE JET will control all of the fuel flow at
speeds. The JET NEEDLE is attached to the carb slide and it moves
into and out of the NEEDLE JET. That combination controls almost all
of the fuel flow in the lower midrange, most of the fuel flow in the upper
midrange and some of the fuel flow above that. The MAIN JET provides
the additional fuel needed from the upper midrange to full throttle.

Way too often people jump in and start changeing jets attempting to cure
some problem that isn't really caused by the jets at all. The factory isn't
stupid ... they choose the jet sizes for a good reason and it is very rare
for them to be wrong. Before you even think about any jet changes you
should double check that the prop is adjusted as described above to
provide the correct load on the engine. Maybe your tach reads wrong.

Many jet changes are done by misguided people attempting to get more
speed by adding pitch to the prop. You might get away with a little bit of
extra pitch but if you get weird EGT temperatures or weird fluxuations in
RPM in a narrow RPM range you have gone too far with it and jet changes
won't help much because the engine is overloaded.

If you were able to get a significant speed increase with too much pitch
you will be much better off to use a slightly shorter prop with that pitch
your engine can reach the correct RPM.

Some other things to keep in mind before you tinker with jets.

IDLE JET If you were ever able to correctly adjust the idle mixture then
there is no reason at all to change the jet size. Blow the trash out of it
replace it with the same size.

NEEDLE JET If the midrange EGT temperatures were ever correct
there is no reason at all to change the jet size. It might have the hole
enlarged from wear against the jet needle. Blow the trash out of it or
replace it with the same size.

JET NEEDLE If the midrange EGT temperatures were ever correct
there is no reason at all to change the needle size. It might get bent, it
might get worn in the taper area, it might get worn in the clip grooves on
top or the clip may need adjusting. Replace it with the same size.

NEEDLE CLIP If the mid range EGT temperatures were ever correct
the most it will need is adjustment one notch up or down to allow for
seasonal changes in air density.

MAIN JET If the full throttle EGT temperatures were ever correct there
is no reason at all to change the jet size. Blow the trash out of it or
replace it with the same size.

Go over all the adjustments above because parts wear together and
cables stretch during their early life. After you get the engine broken in
good it will produce more power and that might mean changes.

After you are more familar with the plane you might consider this trick.
The prop is still producing a certain amount of thrust at idle speed. That
means more "float" distance and longer roll outs which can really matter
in short field landings. After correct adjustment as described above I
back the idle speed screw out a little for a very slow idle. I use a little
of throttle to bring the engine speed back up so the engine doesn't shake.
On final I can pull the throttle all the way back for a steeper but slower
glide. The forward speed of the plane helps the prop windmill fast
enough to prevent engine shake and the prop has a large amount of drag
instead of a small amount of thrust. As the plane slows down on rollout
I add just enough throttle to prevent engine shake. It can make a big
difference in short field landings and more experienced pilots may prefer
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