How to adjust typical diaphragm carburetors

On most Zama carburetors to help find the correct adjustment there is a small L beside the Idle mixture screw and H beside the High speed mixture adjustment. Normal adjustment for 2 needle carburetors begins with the L needle. This adjustment controls the fuel flow to the low speed or idle circuit. If the carburetor is too far out of adjustment to run, or has been disassembled, start with turning the L and H all the way in and then open 2 turns on each needle. Then turn the TAS screw or idle speed adjustment in all the way. When turning any of these screws be careful not to over tighten, this can cause damage to the carburetor.

Now you are ready to begin adjusting. Start the engine and allow it to warm up for a few minutes. Do not try to adjust the carburetor before the engine warms up as settings change while the engine is warming up. Begin with the L needle, turn the needle in clockwise and listen to the engine as you do so. The engine will speed up to a point then start slowing down again, the point of the highest speed is called optimum. This is at the most efficient point for the engine to run. When you find and set the engine to this point then adjust the TAS or idle speed screw. This screw adjusts the idle speed, you should try to adjust it so the clutch is not engaged or ringing. Note: when the clutch disengages the chain or string will stop moving. Some string trimmers do not have a clutch, so adjust these engines to a steady reliable speed. Once you have adjusted the idle speed it is necessary to recheck the L screw adjustment for the optimum point described above. It may be necessary to repeat these two adjustments several times to get the engine to run correct. The L screw may be opened 1/8 to 1/4 turn from this optimum point to help the engine to accelerate well to full throttle. The high speed adjustment procedure is similar to the L.

Accelerate the engine to full speed and search for the optimum point as described for the L needle. The important part to remember about adjusting the L or H screw is not to let the engine drop much on the lean side of this point, (screw in) you want to find the highest speed when turning the needle. However going too far past this point may result in engine damage. As with the L adjustment the H needle may be richened a little, but no more then 1/4 turn from optimum point.

If your carburetor has no slots for a screwdriver, it is adjustable only with a special screwdriver available from the original equipment manufacturer. Zama is not offering any of these tools for sale.

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Limit Caps:

As you are aware, California Air Resources Board (C.A.R.B.) and the EPA are regulating emissions on utility and lawn & garden engines. As part of this plan, manufactures have made many internal engine modifications and now install limit caps on carburetor adjustment screws to prevent carburetor mis-adjustment that would cause the engine to exceed emissions limits.

Because of the "Tamper Resistant" specifications defined by C.A.R.B. and EPA, carburetor limit caps cannot be easily removed by a "special" tool. The caps must be destroyed by cutting the plastic in order to remove.

Carburetor adjustment needles do not always need to be removed as a matter of "practice" in order to clean the carburetor. Solvents sprayed through the feed orifices will usually rinse any residue accumulated by improper fuel storage - without needle removal.

If caps are removed, they can be purchased through your service parts system. Carburetor setting and new limit cap installation may vary from manufacture to manufacture and from equipment type to equipment type. PLEASE CONTACT THE MANUFACTURE FOR THEIR SPECIFIC PROCEDURE.

Click here for information on Limit Cap Kit.

Pop-off pressure:

A lot of emphasis has been placed on carburetor pop-off pressures when in reality they are not an important issue to the service professional. In fact checking pop-off pressure can often damage the carburetor by stretching the fuel pump diaphragm and breaking the seal of the fuel pump gasket. Rather than pressurizing the carburetor to pop-off, Zama recommends that the carburetor should be pressurized at the fuel fitting to 10 p.s.i. It should hold this pressure for a minimum of 15 seconds.

Fuel Pump Diaphragms:

The tan Teflon coated pump diaphragm has been superseded by the clear mylar diaphragm. If your carburetor does not have a machined fuel pump surface you cannot use either the tan or the clear diaphragm. You must use the black buna diaphragm also found in the kits. All three diaphragms found in the Zama carburetor kits offer resistance to alcohol fuels.

Presetting the mixture screws:

Before carburetor L and H needles had limit caps the standard pre-set on the L and H needles was one turn open. When we had to begin using limiter caps we also decreased the needle sensitivity in many cases. So while we still try to have a preset of 1 turn, many needles fall between one and two turns open. Another thing we have tried to do is increase the clearance between the needle and the seat to make sure less debris collects there. This is because with the limit caps on it becomes much more difficult to remove the needles for cleaning. So while the limit caps have made it more difficult to preset the carburetor and to remove the needles, they have also made it less likely that the needles will get far out of adjustment, or that they will even need to be removed for cleaning.

L and H needles:

On many of the new emissions carburetors we reduced the sensitivity of the H needle to make it easier to precisely set the carburetor mixture. The H needle on some carburetors is now the same thread size as the L needle. This can lead to confusion if you take both needles out and do not identify them. Just remember, THE L NEEDLE IS ALWAYS LONGER!

Service Parts

Zama manufactures all our own gaskets and diaphragms. We do not purchase them from an out side supplier. We have increased the number of master distributors to make sure you get the parts and service carburetors you need. Always insist on genuine Zama Service Parts to ensure the quality of the work you do.

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