Fixing The Fuel System On Pro Saws

Does your saw run fine when bucking, but lean out in an undercut? Did your brother in law leave fuel in the saw he borrowed from you two years ago? Do you think the water you just found in your gas jug is the reason your saw won't run? If any of this sounds familiar, you are probably ready for a carb rebuild. Symptoms such as these usually indicate it's time to pull the carb, clean it out, and install a kit.

In other cases, carb problems are more difficult to diagnose. Carburetors get blamed for all kinds of problems - many they don't actually cause. When you are having fuel system problems, it is important to first find the culprit before you do any repair. Maybe the carburetor isn't the problem. Fixing a carburetor that isn't broken causes unnecessary spending and unproductive down-time.

Carb Problem Or Something Else?

Since there is no way to bench-test a carb, a great way to test one is to run your saw without it. How do you do that? If you have a matching spare saw or partner that is willing to loan you the carb from his matching saw, trade him and see how your saw runs with a "loaner" carburetor you know is good. Most carburetors are held in place by just a few fasteners, so an exchange is pretty easy. If your saw runs perfectly with the loaner carb, you have found the problem.

But, lets say you switch carbs and the problem persists. Now you have learned the carb is not the problem. Where do you look next? The most likely culprit is an air leak. Many pro saw users don't realize the fuel/air mixture coming out of the carb actually travels into the maincase before it goes into the combustion chamber. This means, if there are any air leaks in the bottom end, it will contaminate the charge and/or make the motor run lean.

Checking For An Air Leak

Which parts can cause an air leak? There are many of them. They include: both main seals, the crankcase gasket, the crankcase itself (cracks), the cylinder head gasket, the cylinder base (cracks), the impulse hose (the tube that runs the fuel pump in the carb via crankcase pressure/vacuum), and the list goes on. Fortunately there is a method to test all of these components at once. See the page on Troubleshooting An Air Leak for more information.

Fixing A Carburetor Problem

Now, lets assume the carb is the problem, The first thing is to disassemble it. You can start with either the pump section or the metering section. Yes, on today's pro saws, carburetors have fuel pumps built into them. It is this pump that allows you to run the saw at any angle. If this pump was not there, when you turned your saw to the side, it would run out of gas because the tank would be lower than the carb.

Disassembled Carburetor

It is best to disassemble a carburetor on a shop towel so you can keep track of all the small parts.


Since the parts are small, lay a lint free towel on the bench and disassemble the carb on it. That way, little screw or clips won't get away. Chances are the carb will leak a little gas, too, so the shop towel helps absorb the mess.

During disassembly, carefully inspect each part. Look for diaphragms that are worn out or dissolving. Look in the inlet screen for signs of debris that got past the fuel filter. Look at the needle for signs of wear and/or leakage. Look for passages that are plugged. Look for adjustment needles that have been screwed in too tight. It is always good, if you can identify problems. If you don't, the carb is probably not going to work any better when you put it back together.

Throttle shaft wear test

The pen points to where you may find throttle shaft wear. It is not uncommon to see wear in the butterfly valve itself, too. Both conditions occur mostly to high-time carburetors.


It is also a good time to check the butterfly valves. (The flap in the throat of the carb. There's two, one for the choke and one for the throttle). Although you won't be removing them, look around the edge for wear. Also look for wear in the throttle shaft. Each time you squeeze the trigger the butterfly has to open. In time, the shaft and butterfly will wear causing another type of air leak.

Reassembling A Carburetor

Now its time to reassemble the carb. Start by installing the needle valve. Make sure the pivot arm is properly set. Never pry on the arm to set the height with the needle seated. This is hard on the neoprene end.

Next, soak the diaphragms in a little fuel and then install them. Tighten the screws evenly and be sure the diaphragms are not wrinkled.

Last, install the low and high jets. Pre-set them to where the saw will start and run. Consult the owner's manual for this setting or set it to where you remember it was set. If you don't remember and don't have a manual, a general rule is 1 1/4 turns on each jet to start.

Preadjust Carburetor

Pre-set the adjustment needles to a position where the saw will start and run. Once you get the saw running you can fine tune it. Never close and tighten these needles. Tightening these needles against their seats will damage them.


This is a good time to pressure check the carb. Apply a small amount of pressure to the inlet needle to see if it holds. 6 lbs is enough. If it doesn't hold pressure, take the carb back apart and see why. Before re-installing it in the saw, flush the fuel tank, check the fuel hose, impulse hose, heat block and intake boot, and change the fuel filter. Once this is done, re-install the carb.

Pressure Test Inlet Valve

A small amount of pressure in the fuel inlet line tests the sealing ability of the inlet needle valve.


Fuel Filters

The fuel filter on the left shows debris on its surface. Compare it to the new clean filter on the right. Fuel filters should be changed regularly and be part of any fuel system repair.


Now is also the time to replace or clean the air filter. You should also inspect the air filter assembly for damage, misalignment, or evidence of debris leaking past it. Once the saw is back together, start it, adjust by ear, listen for any indication of the old problem. Once it is warmed up to operating temperature, do the final adjustment with a tachometer.

Our Advice

If you attempt a fuel system repair and are not confident the problem is resolved, don't run the saw. Fuel system problems can damage or destroy the saw. Bring it in to us or take it to a capable technician and get fixed it right.

We have fixed thousands of air leaks and carburetor problems. We also have the special tools, like the dyno for stubborn or hard to find problems. We have never been beat! If you have any questions, contact us. Helping you get the most out of your pro saw is our job... That's why we're the Pro's Choice.

Got more questions about fuel systems on pro chain saws? Call or stop in.