Chainsaw Repair

CHAINSAW CARBURETOR EVALUATION

PROVING THAT THE METERING VALVE SYSTEM WORKS

Put 10psi on the fuel inlet and if your gauge holds pressure. (If the gauge creeps down very slowly it is still usually OK). Put a spare tube on the metering diaphragm vent hole and blow, watch the inlet pressure instantly drop and hold remaining pressure if you just puff the vent hole once quickly for the final metering diaphragm check.

MAIN JET AND HIGH SPEED NOZZLE CHECK VALVE

Screw in the low speed Needle until it is a snug fit (Do Not Over tighten or you will damage the needle seat), then remove the high speed Needle, Use the red nylon tube from a WD40 spray can or similar and place it in the high speed needle valve hole (takes some fiddling to get a good seal) when you think you are seated beyond the threads in the needle valve hole, blow then - suck, you should feel the high speed nozzle valve opening and closing as you do this (easy to blow harder than to suck) if your not getting a response try sucking and blowing a little faster, if the clicking is still not noticeable, re-attach your WD40 can to the tube and squirt some WD40 in . . . (look in the venturi from the inlet and you should see the WD40 come out of the main jet. Then try the suck - blow test again (remember to clean the tube or you will get a mouthful).

TESTING THE LOW SPEED CIRCUIT

Remove the low speed needle and replace/close the high speed needle lightly. Put your WD40 can/with tube in the low speed needle valve hole tightly and spray with WD40. Look in the carburetor outlet with the throttle plate **** open, and you should see the WD40 coming out of the two or three low speed holes.

FUEL LINE

Check the integrity of your fuel line. First, that it is not plugged. Then, that it has no leaks. I usually disconnect the fuel line from the carb and see if I can blow back down through the line. Then I connect a extension to the end inside the tank and re-connect the line to the carburetor. You can run your 10 PSI test again on this end of the fuel line to see if it holds pressure as in the test of your metering valve.
   
SOME GENERAL NOTES:

The high speed nozzle check valve is protruding into the center of the venturi on the metering valve side. The low speed outlets are small holes each side of the throttle butterfly (engine end). The high speed needle is always closest to the air filter end of the carburetor and the low speed needle is always closest to the engine end of the carburetor.

Remember to not go over 10psi with your pressure test or you might stretch the pump diaphragm. Put both needles back in till snug then back off 1 and ¼ turns (most working Carb’s will at least try to start at this setting)

IF THE CARBURETOR PASSES ALL THESE TESTS, THE CHAINSAW SHOULD AT LEAST TRY TO START AND RUN NOW?

YOU NOW HAVE TO FINE TUNE THE NEEDLES WITH THE CHAINSAW RUNNING . . .

All this information was already posted in various places around the forum. I tried to make it short and sweet and in one place.

For a pressure tester, it has been mentioned that a old blood pressure tester squeeze bulb and gauge could be adapted for this purpose.

Also the cheap plastic MityVac normally used for just vacuum will easily make 10 psi through the port sicking out the top but there is no pressure gauge. You would have to adapt a Tee in the line with a low value pressure gauge. You can get clear plastic tubing at the hardware or farm store, black rubber at the auto parts, or use yellow Tygon from the chainsaw store. The farm stores usually have the low value pressure gauges and tees you can get at either of these stores.

MityVac makes a nice all metal combination pressure/vacuum with a combination gauge but at $70 - is not very cost effective for occasional use.

Remember that actually all you need to check the carb is 10 psi. And even to test a crankcase you only need 1/2 atmosphere pressure and vacuum. That would be 7 psi pressure and 14" vacuum. That would be close to +/- 50 KPa for you metric guys.

Some people use their air compressor with a regulator from the farm store.

Well, just because a carburetor passes these checks of the metering valve seat, the high speed nozzle check valve, the idle circuit, and that the pump diaphragm doesn't have a hole in it . . .  These tests didn't check that the pump check valve tabs were seating and not curled up, or that the metering valve diaphragm is not stiff as a board, but at least it helps.

There's nothing like getting out a new in box (NIB) carburetor, putting it on the saw, and you can not get any gas up to the saw because the metering valve is stuck in its seat from setting so long or the pump check valves have curled up.

The bottom line is . . . how well you can get the chainsaw to run.

Some of the Walbro HDA style carbs use a fine mesh screen rather than a one way nozzle as a high side (power) check valve.  On those carbs the high side needle test wont apply because the screen will pass air either way. 

Also, on the nozzle types, if you can suck air back through that check valve the check valve or nozzle will need to be replaced.

You can also check the tank vent while you are there, at the fuel line by attaching your vac tester and pulling a vacuum on the tank. Make sure the cap is on. The should be no build of vacuum if the vent is working properly. If there is you need to address the vent clean of replace, I have found that can mess up pulling fuel from the tank as well.