Mercarb Rebuild

Mercarb Rebuild Disassembly

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Mercarb Rebuild Assembly

Disassembly

Mercarb Rebuild 1st remove the choke to fast idle cam rod and the pump rod.
Remove the fast idle cam screw.

Mercarb Rebuild - Rods Removed

Remove the choke thermostat (3 screws).
Remove the lever attached to the choke shaft. Be careful, this screw is easily broken.
The housing is attached to the main body with 2 screws, which you need to remove.
Remove the gasket.

Remove the 8 screw from the top of the carburetor. Note that one screw is a bit longer than the others.
Remove the Venturi (3 screws). The middle screw is different from the 2 outside screws. It should have a washer and a fiber gasket.
Remove the power jet, using a full bladed screwdriver so you don't chip the jet.
Remove the main jets, again using a full bladed screwdriver.
Remove the float by pulling out the float pin. Test your float by heating up a pan of water and immersing the float. Any bubbles indicate a leak.
Loosen the philips screw holding the accelerator pump to the pump lever and pull the lever out. Make a note how the pump attaches.
Be sure to remove all of the gasket material.
Use an awl to scrape any staked over metal from around the power piston. Push down with your thump, then let it go so that the piston pin snaps back. Keep doing this until the piston pops out.

Mercarb Rebuild - Throttle body Remove the throttle body by removing the 3 screws.

Remove the fuel inlet fitting & filter. Notice how this will go back together, spring, filter with closed end toward the carburetor and then the gasket, or washer.

Soak all parts in a gallon of carburetor cleaner following manufactures directions. Not following directions may discolor, or damage your carburetor.

Polish the pump well with crocus cloth to make it smooth. Ethanol gas has little lubrication which can damage a good pump. The pumps needs to glide smoothly.

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Assembly

The assembly process is the same as disassembly, only backwards.

Install the throttle body gasket and the throttle body using the 3 screws. Putting the throttle body on 1st will give the carburetor something to sit on.

Install the jets, using a flat blade screwdriver. The blade should be as wide as the jet so that they are not damaged. Note: What is important on a jet is the hole where the fuel goes through. As long as the hole isn't damaged the jet will be OK.

Don't forget the washer if they had any to start with.

Jets generally don't wear out, so replacing them is not necessary, even if you were able to find some.

Next install the power jet, again using a screw driver with a wide blade. Be sure not to bend the stem. This is where the power piston pushed down and if the stem is bent you will have a problem on power.

When the accelerator pump is pushed down, the fuel will be forced through the main discharge.

The main discharge requires a check ball so that fuel isn't siphoned through when not accelerating.

Install the check ball, spring and T, in that order. Stake the T slightly so it doesn't pop out. At this point you can bench test the accelerator pump circuit.

Install the venturi gasket (they sometimes need to be trimmed so they sit flat) and the venturi.

There are 3 screws, one is longer than the other 2. The longer one should have a washer and a fiber gasket under the washer.

This would be a good time to test the venturi main discharge. Pump the accelerator pump again with fluid (we use mineral spirits). You should see 2 strong squirts coming out of each side of the venturi into the bore. If not then you either install the main discharge parts wrong, or you have a clogged passage.

If you haven't done so, put the accelerator pump return spring in the pump well.

Install the needle & seat shield, then the bowl gasket.

Install the power piston and make sure it glides smoothly. Only vacuum move the power piston so it needs to be clean and smooth. Any silicon spray lubricant will also help.

Installing the Needle & Seat - Our carburetor kit includes a spring loaded needle & seat along with the solid type. Use the spring loaded with one exception. If you do a lot of sharp turns, you may get a lean condition and the solid type may be necessary.

Spring loaded needle & seat - install the needle, put the spring in the needle, finish up with the pin.

Don't forget the gasket under the seat.

Do not put any pressure on the needle. They are easily damaged resulting in a flooding situation.

Install the accelerator pump as shown.

If by chance you took the pump apart, slip the spring over the pump stem, then the retainer to hold the spring in place.

The older Mercarb carburetors used a straight shaft and will install a bit differently.

Install the float, using the float pin. The float pin goes through the mount, the float and the seat shield.

Buy your new brass float here.

Adjust your float so that the bottom of the float (top is upside down) is level with the bowl gasket.

Adjust the float drop using these directions. Turn the top right side up. Measure the brass float from the bottom to the bowl gasket.

What is critical about the float drop is that it drops enough to allow the needle to open, but not so far as to allow the needle to get cocked to one side.

Install the op assembly being careful not to fold the accelerator pump cup over.

One screw is slightly longer than the other 7. It goes where illustrated.

Tighten the screws a little at a time, starting with the inside screws and working outward.

Insert the choke housing gasket, then the housing, using the 2 screws as shown.

The choke lever goes on next. Position it so that it fits over the recess correctly. Be gentle you can't buy these.