Tillotson: Diaphragm carbs for karting

The name Tillotson is as synomus with the diaphragm carburettor as it is with Karting.

Though Tillotson today is very much an Irish company its roots go back to the early 1900’s. Founded in 1914 by Harry Tillotson and John Willys in ToledoOhioUSA.

Mr Willys left the company soon afterwards to form the Willys – Overland Automobile Manufacturing Company producing vehicles now known as “Jeep”. Tillotson initially produced float carburettors for this application and the then budding automobile industry. The Tillotson Manufacturing Company prospered and grew. Then when the industry matured, and car companies began vertical integration Tillotson explored other products in their field of expertise. It began concentrating on small two and four stroke engines.

In the late 40’s, the development of their diaphragm carburettor enabled engines to operate in positions other than horizontal thus boosting the already expanding small engine market. Tillotson was the first independent producer of the Diaphragm carburettor with applications such as power mowers, chain saws, motor bikes, outboard motors, snow mobiles, pumps, generators, trimmers etc.  Under the influence of a continually growing market, the sophistication of the diaphragm carburettor evolved as unique applications were demanded. B.C. Phillips the then Chief Engineer of Tillotson Manufacturing filed for and was granted over 30 related patents on this product line for the company.

Tillotson’s success resulted in Borg-Warner Corporation purchasing it in the early 1960’s and merged it with their Marvel-Schebler operations, forming the Marvel/Schebler/Tillotson Division. For the next twenty five years Borg Warner manufactured a variety of carburettor products at three factory locations: DecaturIllinoisUSA, BlythevilleArkansasUSA, and Tralee Ireland. The Tralee plant was established in 1972, first with an assembly operation and later as a full production facility with machining and assembly. Engineering, design and R&D design was soon to follow.

In 1983 Borg-Warner Corporation decided to sell its carburettor operations which were now totally consolidated at the Tralee facility. This happened in January 1985, thus the formation of Tillotson Limited, once again a self sufficient independent company reigning as the leading manufacturer of “Diaphragm Carburettors“


Located in Tralee, Co. Kerry, Ireland the plant contains 4,830 sq. meters of manufacturing, engineering and office floor space. The company employs 90 people being self sufficient with the capability both from a design/development stage through to manufacturing to bring world class quality carburettors to the market place.


Tillotson facility Tralee Co. Kerry, Ireland    


The core capability/speciality is in close tolerance machining and assembly of the various component parts requiring specialised skills.  Machining is typically, drilling, tapping, reaming, boring, milling, gun-drilling with some special finishing operations such as thermal deburring and cleaning. Machines vary from cam driven rotary index type to modern CNC.



 CNC Machining line




Typical tolerances held in the process are +/- 0.01mm. In the case of fuel jets the diameter is a reference only.

The assembly operations consist of conveyor lines with associated work benches, specialised fixtures and tool required to assemble the delicate small component parts. The size of the final product can be as small as 3.8cm cube consisting of over 40 component parts. The function of the pump, metering and adjustment system is checked at various stages during assembly. A final flow test which simulates the operation of the engine is carried out on all carburettors before packing/shipping.




Machined bodies awaiting assembly





Being an ISO 2001 : 2000 registered company Tillotson has in place an extensive quality management system which ultimately reflects in the performance of its carburettors.


Engineering is a key factor in the realisation of Tillotson’s objectives and in this regard their is a long history of innovation, design and development. A view widely publicised some years ago was that the 2 stroke engine would die with the introduction of strict emission regulations, but this is certainly not the case. Traditionally small swo-stroke engines were branded as nasty, smelly little things which make too much noise, but they do have a lot of attractions and in particular to the Karting industry. Because they have no camshafts, valves and so on they are cheaper to make. Two-strokes produce more power per cc then four-strokes. This makes them the preferable choice for Karting. It is comforting to know that CIK is of the same opinion. As we now know next year will see the introduction/homologation of new so called “long life” 125cc TAG engines specifically designed/developed for the industry and sanctioned by the CIK. These new engines will require a new carburettor and in that regard. Tillotson are in the process of completing designs for a totally new carburettor specifically for this application. In fact prototypes have already been flow bench and track tested under stringent conditions and as a result the major design parameters have already been specified for Junior, ICA and FA. The new carburettor series will be designated HW resulting in various models accordingly. Needless to say it will be a butterfly type diaphragm carburettor but because of the application particular attention has been given to well known problem areas such as the fuel metering transition between the low speed system and the main fuel delivery system where a “lag” or lean mixture area results in poor acceleration. Sounds familiar. Wide open throttle position has also been addressed by bench testing and optimising the air intake, venture profiles and shaft/shutter positioning to achieve maximum power. The carburettor body will be produced in a new single cavity high pressure die casting  die which will virtually eliminate porosity making this casting comparable with gravity feed bodies and those produced by forging. Tillotson will also CNC machine for accuracy and repeatability.  The intention is to produce a dedicated race ready carburettor which will from a cost standpoint have the benefit of being produced by a large carburettor manufacturing company with years of experience both in design/development and manufacturing. This new carburettor will have new dedicated tooling and the associated investment. Combine this with the development costs and you can see the long term commitment Tillotson have to the Karting industry.





Selection of Carburettors currently manufactured by Tillotson




Engine on dyno


Analysis of engine performance on the Dyno




So now we will have a new dedicated Tillotson carburettor to look forward to in 2007, but what of the future regarding emissions, two stroke engines and the Karting industry. Well, should this become an issue, Tillotson are ahead in this respect and have already designed, developed and are currently manufacturing the next generation fuel system for the latest Stratified Charged two stroke engines which meet current and up coming emission legislation. This no doubt will answer the sceptics with regard to the four stroke engine replacing the two-stroke for applications where they are ideally suited such as Karting. This mainly due to cost, reliability and the relatively simple design of the two stroke engine it is here to stay for now. However, should the industry be forced to make a change the new Stratified Charged two stroke engines it will give the following benefits. Increased power, lower fuel consumption, reduced emissions (as much as 70%) and reduced odour from the exhaust gases. Other benefits are a concentrated fuel mixture in the crankcase that lubricates the crank bearing and cylinder barrel. Also a quieter engine producing more power even at lower rev’s with a longer service life at least in other applications.

So how it this achieved in a small, light reliable two stroke engine.

It is all to do with the gas exchange process. Instead of pushing out the exhaust with a mixture of air and fuel from the intake this is done with clean air. In the current two stroke engine cycle any un-combusted fuel is exhausted during the gas exchange cycle with some of the exhaust gas still remaining the cylinder. The secret here is the introduction of a single or multiple flushing air ducts at the appropriate location where a barrier of clean air is introduced during the cycle at a precise time and forced into the cylinder between the exhaust and the incoming air containing the fresh charge. This blanket of clean air separates the burnt gases from the fresh charge. As a result part of the clean air charge is emitted in the exhaust and part remains in the cylinder. This process is known as dual charge or air head engine technology. It is that simple. The modification to the carburettor is the addition of a throttle bore required to throttle this clean air barrier.



Computer generated solid model of new Airhead Carburettor








Manufactured airhead carburettor showing the secondary air throttle





The throttle in the air bore is interlinked with the standard carburettor air/fuel throttle and this linkage is calibrated for precise timing for introduction of the air charge.

So what’s the long term future for the diaphragm carburettor/two stroke engine in the Karting industry? Based on the new generation air head engines it seems that this would be a natural progression if emissions legislation forces the issue. In this case the engine is still simple in design terms, reliable with a dual barrel carburettor as outlined above. Your average Kart owner will not be baffled by the upgrade in technology and will still have the option to adjust to his liking. Other options would be to introduce electronic control of some carburettor functions/adjustments and finally injection. These systems are available today, are reliable, expensive, requiring special skills/knowledge to adjust and set.

For now it looks like we are stuck with the humble or not so humble diaphragm carburettor for some time to come yet. How bad can that be?

We can now look forward to the new generation of “long life”125cc engines and the new Tillotson HW carburettor for next year.