Tillotson HS Carburetor Repair


The demand for a miniature all position diaphragm carburetor prompted Tillotson to design and develop the “HS” series. This small compact unit, only a 1-1/2 inch cube, is versatile in application because of its lightweight and compact size. Even though the carburetor is extremely minute, it still comprises a self-contained fuel pump and filtering screen.

Information contained in the following pages is presented as an aid to understanding construction, operation and servicing of the “HS” series carburetor. Complete carburetors, repair kits and service parts are available through our dealers.



  1. Filtering Screen
  2. Venturi
  3. Pulse Chamber
  4. Fuel Chamber
  5. Fuel Pump Diaphragm

5A. Diaphragm Pump Inlet Valve 5B. Diaphragm Pump Outlet Valve

  1. Fuel Pump Body
  2. Fuel Pump Gasket
  3. Fuel Inlet
  4. Impulse Channel
  5. Throttle Shutter

11A Primary Idle Discharge Port 11B Secondary Idle Discharge Port

  1. High Speed Mix. Screw Orifice
  2. Idle Mixture Screw
  3. Body
  4. Metering Chamber
  5. Idle Mixture Screw Orifice
  6. Diaphragm
  7. Atmospheric Chamber
  8. Atmospheric Vent
  9. Diaphragm Cover
  10. Inlet Tension spring
  11. Fulcrum Pin
  12. Diaphragm Gasket
  13. Inlet Control Lever
  14. Inlet Needle
  15. High Speed Mixture Screw
  16. Choke Shutter
  17. Fuel Inlet Supply Channel
  18. Main Nozzle Discharge Port

The “HS” series carburetor is die-cast aluminium, consisting of three basic parts: metering body, pump cover plate and main diaphragm cover plate. This carburetor incorporates many of the components found in float type carburetors: choke, throttle, idle and high speed mixture screw and inlet needle.

Idle and high speed mixture screws are spring friction type and require no packing. These mixture screws are located on the right side looking toward air horn.

A rubber tipped needle seats directly on a machined orifice in the body casting. An inlet tension spring

exerts a pre-determined force on the inlet control lever, which holds the needle on its seat.

A metering diaphragm is subjected to engine suction on the metering chamber side and atmospheric pressure on the vented side.

Atmospheric pressure on the vented side pushes the diaphragm toward the inlet control lever opening the inlet needle to allow fuel to enter the metering chamber, from which it is then delivered into the mixing passages.

The inlet control lever hooks on the center of the metering diaphragm and a self tapping screw holds the fulcrum pin place.

Movement of the pump diaphragm is caused by pulsation’s from the engines, acting on it through an impulse channel. This movement draws fuel into the fuel chamber and a reverse movement forces fuel out of the fuel chamber through the inlet needle into the metering chamber.

An easily accessible fine mesh-filtering screen is located under the fuel pump cover and diaphragm.


Open high speed mixture screw one and one quarter (1¼) turns.

Open idle mixture screw one (1) turn.



Before disassembling carburetor it is imperative to blush it clean of sawdust and dirt by pouring gasoline over it and tools.

  1. Remove pump diaphragm cover screws and cover.
  2. Remove pump gasket and pump diaphragm.
  3. Remote filtering screen.
  4. Remove main diaphragm cover screws and cover.
  5. Remove main diaphragm and gasket (slide diaphragm towards mixture screws approximately 1/16th of an inch and pull up to unhook from the control lever).
  6. Remove fulcrum pin screw, pin, control lever and spring….
  7. Remove inlet needle.
  8. Remove idle and high speed mixture screws and springs.

Commercial carburetor cleaner can be used on all parts except diaphragms and gaskets. Before reassembling, rinse all parts in clean gasoline and blow off with compressed air. Do not use cloth, as tiny particles of lint adhering to carburetor parts will cause malfunction. Channels in the metering body can be cleaned by blowing through idle and high speed mixture screw orifices. Do not use wires or drills to clean orifices.




Starting an engine with the “HS” Carburetor involves the same methods used in a conventional float feed carburetor.

When starting a cold engine, place the choke shutter in the closed position and throttle shutter in a cracked or open position. As the engine is cranked, engine suction will be transmitted to the diaphragm fuel chamber through both primary and secondary idle discharge ports as well as the main

fuel discharge port, creating a low pressure area on the fuel side of the main diaphragm. Atmosphere air pressure on the opposite side will force the main diaphragm upward causing the diaphragm button to depress the inlet control lever, overcoming inlet tension spring pressure, permitting fuel to enter through the inlet seat, then into the fuel chamber side of main diaphragm, up through the idle and high speed fuel supply orifices and channels, and out the discharge ports to the engine.


When engine is idling, throttle shutter is partially cracked. Engine suction is transmitted through the primary idle fuel discharge port to the fuel chamber side of main diaphragm via the idle fuel supply channel. Again, the main diaphragm is forced upward by atmospheric pressure, depressing the

inlet control lever overcoming inlet tension spring pressure and permitting fuel to enter through inlet seat, and filling the fuel chamber. Fuel is then drawn up through idle mixture screw orifice and delivered to the engine through primary idle discharge port.


Fuel is delivered into and through the carburetor in the same manner as when the engine is idling.

However, as the throttle opens and the engine speed increases, more fuel is supplied to the engine by valving in the secondary idle discharge port located immediately behind the throttle shutter.

As the throttle shutter continues to open and engine Speed increases, the velocity of air through the

venturi creates a low pressure area at the venturi throat and diminishes the suction on engine side of the throttle shutter. When the pressure at the venturi throat is less than that existing within main diaphragm fuel chamber, fuel is drawn up through high speed mixture orifice and out main fuel discharge port into the air stream entering engine intake.


As the throttle shutter progressively opens from intermediate position to full open position, the air velocity through the venturi increases and fuel is metered up through high speed mixture orifice and main fuel discharge port in accordance with the power requirements of the engine. The action of the main diaphragm is the same as previously described with suction required to operate the diaphragm being transmitted through the main fuel discharge port.

As the throttle shutter progressively opens from intermediate position to full open position, the air velocity through the venturi increases and fuel is metered up through high speed mixture orifice and main fuel discharge port in accordance with the power requirements of the engine. The action of the main diaphragm is the same as previously described with suction required to operate the diaphragm being transmitted through the main fuel discharge port.



Be certain both diaphragms are installed correctly – the

pump diaphragm next to the metering body, then the

gasket – the main diaphragm gasket, then the main diaphragm. Evenly tighten all screws to insure complete


When reassembling the inlet control lever and spring, care should be taken to see that the spring rests in the well of the metering body and locates on


Under extreme conditions of

the dimple of the inlet control lever (as illustrated above).

CAUTION: Do not stretch spring. Inlet control lever is properly set when flush with the floor of the diaphragm chamber. If diaphragm end of control lever is low, pry up. If lever is high, depress diaphragm end and push on needle for proper adjustment. (See illustration). On some models the inlet control lever is hooked to both the inlet needle at one end and the metering diaphragm at the other. On other models the inlet lever is hooked to the inlet needle while the opposite end of the lever merely rests against the metering diaphragm.

Special care is required when reassembling these parts to insure proper operation.

clogged idle fuel channel and discharge ports, it

may be necessary to remove the channel welch plug. If so, it must be done very carefully in the following manner:


Drill a 1/8 “ diameter hole through the 11/32” diameter welch plug. This hole should just break through the welch plug. Deeper drilling will seriously damage the body casting and render the carburetor non-serviceable.

Carefully pry out the welch plug and clean discharge ports and cross channels. To reinstall, place welch plug in well, convex side up, and flatten with a flat end tool slightly larger than the welch plug. Nozzle welch plug can be removed in the same manner by using a 1/16” drill.











Dirt or foreign particles preventing inlet needle from seating.

Diaphragm lever spring not seated on lever dimple

Diaphragm improperly installed in carburetor

Idle mixture too lean.

Incorrect setting on diaphragm lever.

Diaphragm cover plate loose. Diaphragm gasket leaking Main fuel orifice plugged

Incorrect idle adjustment

Idle discharge ports or channels clogged.

Diaphragm lever set incorrectly.

Throttle shutter cocked in the throttle bore causing fast idle.

Dirty nozzle check valve or outlet screen.

Welch plug covering the idle discharge ports does not seal. This causes the engine to idle with idle mixture shut off.

Tank vent not operating correctly.

Leak in fuel system from tank to pump

Ruptured fuel pump diaphragm Main fuel orifice plugged

The 7/32 “ diameter nozzle welch plug, or nozzle check valve cage, is not sealing.

Remove, clean and replace

Remove lever and reinstall

Replace diaphragm or correct installation.

Readjust Reset

Tighten Replace

Remove diaphragm cover, diaphragm, diaphragm lever and high speed mixture screw. Clean out orifice by blowing through high speed mixture screw threaded hole.

Reset to best idle

Blow out with compressed air, or, if compressed air is not available, clean and flush with gasoline.

Reset diaphragm lever so it is flush with the floor of the diaphragm chamber.


Clean or replace.

Replace welch plug, following instructions outlined in service hints.

Clean, if possible, or replace Tighten or replace fittings or line

Replace Clean

Install new plug or

new cage


Set engine idling speed in accordance with engine manufacturer’s recommendation.


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