Chain saw fuel pump?

Old 11-28-01, 11:23 AM
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Chain saw fuel pump?

I have been attempting to revive an old Stihl chain saw (031AV) with little success. It has sat around for at least 8 years. It has decent spark, but I can't get fuel to the carb. Looking at the fuel line, I can see that there isn't enough pull to get the fuel up out of the tank. There seems to be a diaphragm type pump on the carburator, but I have no idea what drives it. My plan is to get a rebuild kit for the carb and replace the gaskets/diaphragm. Any other ideas? Also, any leads on reasonably priced Stihl parts?
Any suggestions would be greatly appreciated,
Old 11-28-01, 07:11 PM
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Hello: nickelD

There is a small hole going into the body of the engine. Look for it when you remove the carb. That hole goes down inside the engine to the crankcase. It applies vacuum when the piston rises up and pressure when the piston is going down.

This hole and that one in the carb is how the vacuum and presure causes the diaphgram to fluctuate and allow fuel to flow into it. Be sure both ends are cleared. Especially the hole in the carb during the carb rebuilding.

Also check for fuel flow out of the fuel tank. Check any fuel filters. Should be one inside the fuel tank. It should have a weight attached to the hose. The filter and weight combo should flop around in the tank freely.

It's possible the fuel tanks gas cap has a restricted vent.

To check this, remove the fuel line from the carb. Watch the fuel flow rate while doing this. If it diminishes over time as it is coming out, the vent hole may be restricted. If it stops totally, the vent hole is pluged. If all the fuel empties out, all is okay here.

If there is a restriction or blockage in the vent hole in the gas cap, the tank will drain until it pulls a vacumn in the tank. Then the fuel will slow down until it stops flowing.

Once you OPEN the cap or allow enough time to pass, the engine restarts because the tank once again gets air in it.

Removing the carb shouldn't be much of a problem once you gain access to it. Just be sure you make note of exactly how it is connected to the throttle linkage so you can reinstall it exactly as it current is.

Also note exactly how many turns out each fuel adjustment screw is currently set at. There will be two screws. A high "H" speed screw and a low "L" speed screw.

I would highly recommend you disassemble the carb on a cleaned and cleared off workbench top with plenty of light. These carb are often very tiny and have tiny parts which can get lost easily.

Carefully remove each piece of the carb and make note of exactly how it came off and in which order it came off. Also note which side or which way each part is facing or installed.

Failure to reinstall each part exactly as found will mean the entire carb must once again be removed and disassembled to correct the error.......not always a happy thought.

I suggest you only use aerosol canned automotive carb spray cleaner to clean and clear the fuel ports and jets. Automotive hot dip cleaners are much too strong for tiny carb parts. Plus the plastic tube that comes with the canned cleaner is an excellent means to flushout gums and sludges from the jets, ports, channels and under those welch plugs whithout removing them.

Use EXTREME CAUTION cleaning any parts with an automotive aersol parts cleaner product. Aersol liquid parts cleaners may be EXTREMELY FLAMMABLE!!! The manufacturers CAUTIONS should be strictly adhered to.

These products produce large quantities of flammable vapors. They must be used in a well ventilated area while being applied and until totally dry. Which means a safe area may not be the shop or garage, if there is any source of ignition!

Potential parts sources:

BILLIOU'S 1-209-784-4102

L.S.THORPE CO. INC. 1-617-776-2445

I do not have any affiliations nor vested interests in either of these retail parts suppliers. Provided solely to assist with locating parts.

Small engine parts can also be found at your local small engine and lawn mower repair shops listed in the phone book.
Old 11-29-01, 08:12 AM
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The impulse line that Tom is referring to is located on the right
side of the carb, and the long steel nipple on the top of the
carb is pressed into it. But with a saw of that age, the line could
be broken or have a lot of cracks in it. It goes to a nipple
at the bottom of the cylinder behind the carb box. Also look
closely at the fuel lines for cracks as well. Also the foam insert
in the fuel filter usually falls apart and the remains collect in the
carb screen. The carb on the saw is either a Walbro or Tillotson.
You will have to find the brand and model letters to buy the
correct kit like Walbro WA or Walbro WT Or Tillotson HU or HS,
but any mower shop should have them, but Stihl parts have to be through a dealer. Stihl keeps a tight grip. You might find them online, but they will cost a lot more. You have a great saw and it is worth fixing up.

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