When a person is ill an instrument
that is frequently used is a thermometer. When an ultralight engine is
"ill" its thermometer is the spark plug. A properly tuned engine
will come out with a nice sandy brown or tan coloured spark plug. An spark
plug that comes out grey to white indicates a lean fuel mixture
while a plug that comes out black, wet or sooty looking indicates a rich
Now if you are looking for a problem the spark plug can also help trace
the problem. A spark plug that is dry after several attempts to start it
would indicate that no fuel is getting to the engine. While a spark plug
that is wet would indicate a rich or flooding situation.
Since the fuel that causes these two situations is supplied by the
carburetor lets take a closer look at this unit. Experience has shown that
usually one of two thing happen if the carburetor is involved in the
problem. First either something has worn over a period of time and has
gradually effected the performance of the engine, or something happens
instantly. Of course something can wear over a period of time and
fail causing the instant situation.
Now before we get further into this -
the following is for troubleshooting and tuning a BING carb. Before you
start into it you want to make sure that your problem is with the
carburetor. If the engine that you are working on has seized rings, or a
blown piston, wrist pin bearing, or faulty spark plug, cap or ignition
coil - you will be wasting your time trying to "tune or troubleshoot
In order to do the following properly you should have an EGT gauge
correctly installed with probes for each cylinder and a tachometer.
One of the easiest ways to check for damage to a two stroke engine is to
remove the exhaust manifold. With this done you can see the pistons, rings
etc. If there is any damage, blow bye, or ring seizure forget about your
carb and fix the real problem.
Your engine won't start and the plugs are wet.
This indicates excessive fuel in the combustion chamber. This can come
about IF YOU HAVE NO SPARK -which is not carb related. It can also occur
if the float level is too high, or if a component such as your float(s),
float arm, needle, seat are damaged or obstructed.
It can also occur if your "enriching circuit" has been used to
much, or you have primed the engine too much. Engines that are used
inverted will generally be found in this condition after a day or two of
storage, this is not a carb problem! To start this engine DO NOT prime it
- give it full throttle and crank it until it either starts or clears
Suggestions: The first thing that I do when checking for proper fuel level
in the carb bowls is to check the floats.In looking at the float I
am looking for damage, this usually occurs around the pin hole that goes
through the center of the float. To check the floats fill the float
bowls with fuel lay them on a flat surface and then place the floats on
the stems. Note where the float arms are in relation to the fuel. Let them
sit there for about 15 minute and then check them If
floats are not level with each other then and both arms are not even with
the top of the fuel them the float(s) are damaged and need to be replaced.
A float like this will cause an improper fuel mixture since it allows more
fuel in than it should. Also because only one float is working the other
is taking more of the load and it will sink.
Next turn the carb body upside down on a float surface. The two float arms
should be LEVEL with the base of the carb. You adjust this by bending the
CENTER tang where the needle rides. DO NOT bend the float arms.
If fuel is running out the over flow tube then your needle and seat has
either failed or an obstruction is preventing it from closing. To
repair this you will have to remove the float arms and needle and then use
compressed air to clean the passage way. It is suggested that you use a
plastic bag over the other end of the passage when you use the air. This
way you can verify what was obstructing the needle and that it has been
Your engine won't start and the spark plugs are dry.
Dry spark plugs usually can be traced to fuel not getting to the carb, a
plugged low speed jet, or the use of throttle when using the choke system.
So find out why fuel isn't reaching the carb - e.g. - turn the fuel on -
clean the low speed jet - when using the choke system the throttles MUST
BE AT THE IDLE POSITION - the choke does not work if the throttles are
even cracked a small amount.
In order to do the following properly you should
have an EGT gauge correctly installed with probes for each cylinder and a
Engine starts but won't idle.
You don't have the slides in the carbs set correctly - refer
to Tech tips 1 for the proper way to set up the slides. The low speed
jet is plugged - clean it. Your low speed airscrew is adjusted incorrectly
to the bing carb jet chart for proper airscrew setting .
This air screw changes the air/fuel mixture by changing the ratio of air
to the idle jet circuit. Turning the screw out with lean the mixture while
turning it in richens it.
After referring to the jet chart you have a position to start from.
With the engine warmed up and idling and the screw set to factory specs
turn the screw out very slowly - the engine should start to pick up rpm as
you lean the mixture out. If nothing happens the low speed jet is probably
plugged, or the small atomizer hole located in the throat of the carb
on the engine side is blocked.
Once you have the engine idling at its highest and leanest setting using
the air screw turn the screw back in 1/8 of a turn which will richen it up
just enough for the best performance.
Note it is usually necessary to do this three or four times a year,
do to the difference in air that you get in spring/summer/fall/winter.
Note that on the 582 Rotax I have found that 45 and 50 idle jet work
better than the standard 55 especially in winter. On the 582 the lower jet
sizes have been shown to improve starting and help eliminate the rough
A quick way of checking your mixture is to let the engine idle for a few
minutes and then apply a burst of throttle. If the engine bogs it usually
means your mixture is too lean. You can verify this by applying throttle
then choke - which richens the mixture of course if the bog goes away then
you have too lean of a mixture.
You can also change the mixture using the needle and clip - but remember
this changes everything from idle to 3/4 throttle. Raising the clip - thus
lowering the needle leans the mixture - while lowering the clip and
raising the needle richens it.
Engine RPM varies at and idle - won't
This is usually caused by an air leak. A failed intake manifold gasket. A
crack in the intake manifold. If you have had the intake manifold off, the
gaskets can fall down inside the cover. If you have had the cylinders off
and don't use the proper alignment bar during reinstallation you can crack
the intake manifold. A failed rubber O ring in the top of the carb cover
plate. The rubber stopper covering the primer inlet has failed or is
missing. Water in the bottom of the float bowl.
Engine sputter or misfires (0 - 1/8 throttle)
This area is controlled by the low speed or idle jet, and air mixture
screw. Check to make sure the jet is clean and the proper size. Check to
see the air mixture screw is set correctly.
Engine sputter or misfires (1/8 - 1/2 throttle)
This generally indicates a lean fuel mixture. Check to make sure your
needle and clip are in the right position and are not damaged. The air
screw still has some effect here.
Engine sputter or misfires (1/2 - 3/4 throttle)
This area is controlled by the needle jet and jet needle. Verify that you
have the correct jet and needle jet. Also verify that they are located
BELOW the white plastic retaining cup. If they are located on top of the
retainer the engine will run rough in all ranges, since the mixture is
wrong in all locations.
The mid range is controlled by the needle and needle jet. The needle jets
are available in various sizes with the larger numbers giving a richer
fuel mixture and the lower numbers a leaner mixture.
A reported problem with ALL bing carbs is with failure of the jet and or
needle clip. This can result in erratic running, lean fuel mixtures,
engine returning to and idle during flight.
Click here for the latest Update Information - with solution to problem!
To determine whether you need to
adjust the needle jets you require a EGT gauge and tachometer.
Many owners of 582's have reported high EGT readings in the 5600 to 5800
rpm range using the stock needle jet of 272 - using a 274 or 276
eliminates this problem.
Owners of older model 532 report having a problem keeping the engine set
at 5400 rpm. It would jump up and down from 5400 to 5800 resulting from
a lean fuel mixture. Again the larger jet usually cures this problem.
Engine sputter or misfires (3/4 to full throttle)
This area is controlled by the main jet. Check to make sure it is in place
and snug. Make sure there is no water in the bottom of the float bowl.
Make sure the needle and clip are in good working condition. If the needle
fails around the clip retaining area this will allow the needle to jump up
and down causing erratic running. Proper float level is also important
here to little fuel flow can cause an lean mixture while to must flow can
result in a rich mixture.
One area that is of concern is in higher altitudes and colder climates.
Higher altitudes usually require leaner fuel settings - lower size main
jets, while colder temperatures require increased jet sizes to compensate
for the thicker air.
Consult the Bing jet chart for proper jet recommendations.
Now if you are tuning your carb using the above these have to be present!
1. The engine has to be in good working order.
2. The engine has to have the proper prop load.
3. The engine, exhaust etc all have to be STOCK, as supplied or
recommended from the factory or knowledgeable/authorized person.
check out Setting up your Rotax engine
Flat blade screw driver (supplied in Rotax tool kit)\..
Phillips screw driver
Small adjustable wrench
To disconnect carburetor from engine loosen clamps holding carburetor to
rubber flange intake boot.Gently pull and the carburetor will separate
from the boot. To remove the top loosen the two retaining screws the top
of the carb will come off . To remove throttle cable, compress the
carburetor piston spring (1), push the white plastic spring cup
out of the carburetor piston (7), move the throttle cable side ways in the
carburetor piston, this allows it to release through the larger hole in
the bottom of the piston. Remove the jet needle (2) and retaining clip
(3). Be careful not to loose the rubber sealing ring located in the
carburetor cover plate.
With the top removed place the carburetor on the bench upside down. Slide
the bowl spring clip side ways and the carb bowl will come free. (It may
be necessary to gently tap the bowl, to break it free from the bowl
gasket). Carefully remove the gasket.
Disassembly - Carburetor
In the center of the carburetor covering the main jet (6) will
be a sieve sleeve, (this looks like a little filter) remove it (its
purpose is to stop foam from entering the main jet).
Looking at the top of the main jet you will see a number, this indicates
the size of the jet. For jets specifications refer to Bing
jet chart for proper jet recommendations..
Remove the main jet (6) . Remove the mixing tube, this will allow
removal of the needle jet (5).
Located beside the main jet is an orifice containing the idler jet (4).
Using a small screw driver (supplied in Rotax tool kit), remove the idler
Next remove the float pin, (7) this will allow removal of the float
bracket, or float depending on which type your engine is using.
Under the float bracket is the float needle and float needle clip remove
them. Two screws, one large one small, are located on the side of the
carburetor. The larger screw with the spring under it is the idle (11)
adjustment screw . The smaller with the rubber ring under it is the air
regulating screw (10).
The correct order of the jet needle,retaining clip, plastic spring cup,
and spring, in the carburetor piston.
Note: On some models the floats are part of the float bracket, while on
others the floats are separate, and move on two locating pins in the float
Reassembly - Carburetor
Using a general purpose cleaning solvent clean all the parts. Check the
main jet and idler jet orifices to ensure that they are not plugged.Using
compressed air blow through the vent holes, main jet, and idler jet.
Place the carb body on the bench inverted. Replace the needle jet, mixing
tube, and thread the main jet into the mixing tube.
Install the needle jet and needle clip, install the float bracket, or
float. If proper adjusted the float bracket arms should be level when the
carburetor is sitting inverted on the bench. Now turn the carburetor right
side up and check to make sure that the needle valve is not
sticking.Invert the carb once more.Install the idler jet.
Install the float chamber gasket, seeve sleeve. Install the floats
(depending on carb style). Install bowl gasket. Install float bowl and
lock into place with the spring clip. Turning carburetor over install the
spring and idle adjustment screw, Install the O ring and air regulating
For engine idle speed and air screw adjustment refer to
Bing Jet Chart
Install the throttle cable through the rubber grommet, then and top
throttle cover plate. Make sure that the rubber O ring is in place under
the cover plate. Install the carburetor piston spring. Install the white
plastic spring cup over the end of the carburetor piston spring, using the
cup compress the spring making sure the throttle cable protrudes through
the correct hole in the spring cup. Install you jet needle, and holding
plate into the carburetor piston. Place the throttle cable end into the
piston and down through the hole in the side. Once through slide the
throttle cable over, and up to lock it into position. Now allow the
plastic spring cup to slide down over top of the spring clip. Install the
unit into the throttle body making sure that the piston guide hole and
carb body guide line up. Tighten up the two cover plate retaining
screws.Slip the carburetor body into the rubber socket and tighten the
It is recommended that all old float system carburetors be updated to
current float system. Older system does not regulate as accurately as new
one , and older system leaks excessively through float bowl over flow.