The Tillotson HW-49A carburettor

Unlike the float bowl carburettor generally used on all our kart classes, the OK-N class makes use of a diaphragm type of carburettor that is equipped with a butterfly valve (rather than a needle and slide) that is opened and closed via the accelerator pedal cable.  The one chosen for use in the OK-N class is the Tillotson HW49A.  Internally, diaphragm carbs differ quite substantially from the float bowl carbs that we have become accustomed to, but still make use of the same basic principles of physics for their operation.

Principle of operation

The diagram below shows the innards of the HW-49A diaphragm carb and will assist with understanding the principles upon which it works.

For a float bowl type carburettor, the fuel pump sends fuel into the carb via the needle valve which is connected to a float that then keeps the level of the fuel inside the carb bowl at a fixed level inside the bowl.  This obviates any fuel starvation during throttle depression and the fuel/air mixture ratio by and large remains constant over the rev range.  On this diaphragm carb, once again the fuel pump does the same job, but the flow is then controlled via two diaphragms viz. a fuel pump diaphragm and a metering diaphragm.  The latter is of importance to us, because as the name suggests, it ‘meters’ out a certain quantity of fuel that is sent onwards for every intake stroke of the engine.

The quantity of metered fuel is then sent into the ducts containing the LO and HI speed needle adjuster screws before the fuel enters the venturi section of the carb where it is atomised by the incoming airflow.  Another interesting fact is that because all diaphragm carbs don’t have a float bowl and are always filled with fuel, they can operate in any orientation.  This is also the reason why chainsaws, weed whackers and the like make use of a diaphragm type of carb.

Disassembling & servicing the carb

To properly service the carb it must be removed from the engine before disassembly can commence.  Then referring to the diagram, remove the strainer cover screw, the cover, the cork gasket and strainer mesh.  The mesh and cover may contain some trapped particles of dirt that will need cleaning later. 

Remove the 6x screws that hold the fuel pump upper and lower halves to the main body, the fuel pump diaphragm, pump gasket, lower half, metering diaphragm and metering gasket.  Some of these items may be stuck to the metal parts, and any debris that remains behind will require removal later.

Next, the metering lever and its shaft must be removed using the following procedure.  Place your finger over the lever to hold it down whilst completely undoing the shaft’s hold down screw, because your finger will temporarily keep everything in place.  Then grip the lever on its sides and slowly remove the assembly in an upward direction.  Be careful when doing this because there is a spring trapped under the lever and you don’t want to go hunting for this if it jumps out.  Note that the front part of the lever has a slot that connects it to the needle valve itself, and one can either slide it off first, or remove the valve whilst it is still attached in the lever’s slot.  Then remove the spring.

The needle valve’s seat can be removed if it is going to be replaced.  If that is the case, one needs a thin walled socket spanner to do so.  There is also a small copper sealing washer that may or not remain in the carb body – if it remains behind, use a small screwdriver to fish it out.  If you don’t know the settings on the HI and LO speed adjuster needles, carefully screw them home whilst counting the turns on each of them, and make a note of the settings for re-assembly purposes.  Once done, each setting needle together with its spring and O-ring can be removed.  The O-rings may remain inside the carb body but they can be fished out with the aid of a small screwdriver.

You can change just the diaphragm and gaskets with a ‘DG kit’ or the diaphragms, gaskets, inlet needle and seat with a ‘RK kit’.  Tillotson recommends inspecting the diaphragms and gaskets after each race weekend and to change them at least once per season.  When inspecting the condition of the metering diaphragm, check to see if the convolution has lost shape and become flat because this results in incorrect metering of fuel and inconsistent carburetion.

For the fuel pump diaphragm check if the area around the fuel pump is ‘sagging’ (refer picture) which indicates it has become weak and as a result will not have the same pumping capacity as a new one.

Thoroughly clean all the parts and blow out all the passages with compressed air before re-assembly commences.  Also, it’s not a bad idea to dip the diaphragms and gaskets in some fuel as this aids with sealing the assembled carb.  Apply a small amount of grease on the O-rings of the adjuster screws prior to fitting them into the carb body, followed by the screws plus their springs.  Lightly screw each one fully home and then screw them out to their original settings.

Refit the needle valve’s seat plus its copper washer if it has been removed and tighten up with the socket spanner.  Then insert the needle valve into its seat.  Place the metering spring into its recess, and assemble the lever arm onto its shaft.  Slide the slot of the arm onto the needle valve and lower the shaft plus the lever arm into place ensuring the dimple on the arm (refer picture) fits into the top of the spring.  Hold everything in place by applying finger pressure to the top of the arm whilst fitting and tightening the hold down screw.

Ensure the fuel metering lever is flush with the body.  If it is raised or lowered, adjust it slightly by bending it very carefully.  This may involve removing the arm once or twice to get the setting correct.  If it’s set too low there may be an excessive gap between the metering diaphragm button and the lever that could lead to poor throttle response.  If it’s set too high it may come into contact with the metering diaphragm button and cause poor sealing at the inlet needle valve’s seat.  Once all of the above has been completed, it’s time to check the pop-off pressure.

pop-off pressure

The pop-off pressure can be verified by making use of an instrument as shown in the picture, and this can be done on a fully assembled carb, or one that has been opened for servicing.  For a fully assembled carb, simply disconnect the fuel line at the strainer entry and fit the hose of the instrument onto the connector.  By depressing the instrument’s plunger a few times, the reading on the gauge will keep on increasing until it reaches some value close to 80kPa (the pop-off pressure) and will then drop off slightly and hold steady at a lower value.  If the carb has been opened, connect one of the adaptors to the end of the hose of the setting instrument and insert its end into the duct as shown in the lower illustration.  Then, depressing the instrument’s plunger a few times will provide you with the pop-off pressure.

In both methods, once the pop-off has been established, the lower value should remain constant on the gauge.  If not, then the needle valve is leaking on its seat and will need replacement.

pop-off pressure adjustment

It is useful to check the pop-off pressure every race day to verify that all is still within the correct limits and that the needle valve isn’t leaking.  Altering the pop-off pressure requires removal of the metering lever to gain access to the spring and the needle valve as described previously.  The pop off pressure is adjusted by changing the pop-off spring.  These springs differ in length and the number of coils so fitting of a stronger spring will increase the pop-off pressure and vice-versa.

You may have read elsewhere that the spring’s pressure can be reduced by cutting it shorter with a nail clipper or similar instrument, or made stronger by stretching it.  This is not really good advice.  By cutting a spring’s length you are effectively letting it sit on an end that is not flat, and in doing so you cannot guarantee that it will act the same way every time.  Similarly, in order to lengthen a spring you have stretched it past its elastic limit.  The best solution is to use the correct OEM spring for the job.  From the factory, the carb is supplied with a spring termed ‘Inlet spring 37gram’ but there are a range of about six springs available with tensions ranging from 26grams to 48grams.

For small increases to the pop-off pressure, stretching is fine but take it easy with this.  You can also squeeze the spring slightly between your fingers to marginally reduce the pop-off pressure.  For the same wire diameter, the applied force required for the same deflection is directly proportional to the free length of the spring and indirectly to the number of coils (referred to as turns).  Differences in the OEM springs can be spotted by examining the quantity of turns, so as an example, a 31g spring has more turns than 37g, and a 42g spring has less than either of these.

Once on the track, if the engine revs don’t drop off cleanly and fast when you lift the accelerator pedal, this could be a sign that the pop-off pressure in the carb is too high, and is fixed by lowering the pop-off pressure.  If the engine is slow to accept the load when accelerating, it might be that the pop-off pressure is too low and it needs to be adjusted.

One can also visually tell the difference between the two types of screws by examining the tip of the screw itself.  Referring to the picture above, notice that the air screws shown have a fairly fine point on their tips, whilst the fuel screws have rather stubby tips.  Although either of these mixture screws remain in operation over the whole rev range, their primary function is for use in conjunction with the throttle screw to obtain the best mixture setting for the engine whilst it is idling.

INITIAL SET UP OF THE CARBURETTOR

For the HW49, and in fact on most diaphragm carburettors, the fuel mixture being fed to the engine is controlled using two setting needle screws on the side of the carb.  The one closest to the engine modifies the fuel mixture at LO revs and is marked with a ‘L’, while the one closest to the air filter adjusts the HI end revs and is marked with a ‘H’.  Being located in the fuel passages inside the carb, tightening any one of them reduces the corresponding flow rate and you get a ‘leaner’ mixture fed into the engine, whilst loosening has the opposite effect and you get a ‘richer’ mixture.  As described in another chapter entitled ‘Air density and jetting’, changes to the mixture settings are affected by the ambient temperature, air pressure, altitude and humidity.

Proper setting of the needles requires feedback from the driver on the track.  However, as a good starting point (if you have no other information), set the LO speed needle at two and a half (2 ½) turns out from its fully closed position, and the HI speed needle at two (2) turns out.  Adjust the butterfly closed limit screw on the carburettor so that it just cushions the butterfly when the throttle valve goes closed.  Do not use this screw for setting the engine idling speed whilst on the trolley as this will cause the engine to lean out in the corners and also when you lift off on the throttle.  Also, set the butterfly WOT (wide open throttle) limit screw so that the when the throttle pedal is fully depressed you are actually getting exactly 100% opening and not anything more or less than that.  Be aware that because the butterfly valve is capable of going past the ‘true’ WOT position, that this is the same as running with less than full throttle.

TRACK TUNING

Do a lap or two to get the engine up to its proper operating temperature.  Then commence with the LO speed setting as follows.  Floor the accelerator out of the corners - if engine response is a bit hesitant (the engine revs take forever to speed up and possibly there is a lot of smoke from the exhaust), it means that carb setting is too rich, and so you need to slightly tighten the LO speed needle (closest to the engine) until the kart achieves a decent acceleration out of the corners.  However, if the revs do not increase instantly and the kart only accelerates sharply after a slight delay (sometimes accompanied by a pinging noise) when you floor the accelerator, then the mixture is on the lean side and you have to loosen off the LO speed needle.

Once you have achieved a satisfactory setting for the LO speed needle, the next part of the operation is to set the HI speed needle.  By the time you reach the end of the straight, the engine (with the correct gear ratio) should be attaining maximum revs.  If that isn’t the case, it means that the mixture is on the rich side.  Therefore you need to tighten the HI speed needle (closest to the air filter side) so that the engine revs increase until you start braking or until the rev limit is reached.

If you can’t really discern any difference after the adjustment, you can use the following as a guide.  When on the straight, block off one of the air inlet tubes of the air filter.  If the kart then accelerates, it means that carb setting is on the lean side but if it slows down, then your setting is on the rich side.

The setting of the LO and HI speed needles can be compared to a clock-face where 1-turn is equivalent to 60minutes of rotation.  If you are so inclined and wish to do really fine adjustments, an adjuster as shown in the picture helps you to set these needles to precise locations.  Simply align the rotation arm and free-wheeling guide to 0 once the needle adjuster has been fully screwed home, and place the slot over the flat end of the needle adjuster.  Then, by rotating the main arm using the 2-minute increment markings, the instrument allows you to easily and precisely set each of the needles to your desired opening.

Some DON’Ts when servicing the carb

It is important to know what NOT to do when servicing the carb as damage to the body can hinder performance or even render it beyond repair.

• When tightening the carb top, don’t overtighten the central screw as this can crack the plastic cover and could result in a leak.

• If the carb has not been serviced in a long time, some gasket material may stick to the body of the carb.  This should be removed but don’t use anything sharp that could result in damage to the body and possibly affect the sealing.  Rather use a very fine wire wool to remove any unwanted material and thoroughly clean off afterwards.

• If/when fitting the inlet needle valve’s housing into the carb body, ensure that it isn’t cross-threaded as that will cause it to no longer seal and the carb will no longer function correctly.

• When closing off the LO and HI speed needles to get a starting point for setting them, don’t overtighten them because you are forcing a tapered steel needle against the soft aluminium body of the carb that can easily be damaged by using excessive force.

Exploded views

The exploded views of the HW-49A complete with part numbers, and also the kits available for the carb are shown below.

AVAILABLE KITS

  Emile McGregor - MSA Technical Consultant