Working on the fuel system of a TB685EC
I replaced the fuel lines on a TB685EC last October. The primer bulb was cracked, the tank had varnish in it, and the engine wouldn't stay running above idle no matter how much I adjusted the carb. What follows is the actual layout and the procedure I used to get it running again. No theory, just what worked. The fuel system on this engine is simpler than people make it. There are three pieces in the fuel circuit: the tank, the primer bulb (mounted on or near the carburetor), and the carburetor itself. The lines are standard vacuum-type fuel tubing, typically 3/16 inch ID for the main supply and return, with a smaller 1/4 inch line for the tank vent in some configurations. Here is how the fuel actually flows through it:
From the bottom of the tank, a fuel supply line runs out through the grommet in the tank. Inside the tank, that line connects to the in-tank filter, which is a small mesh sock on the end of the rigid plastic pickup tube. The line exits the tank and goes to the primer bulb. On the TB685EC, the primer bulb is a small round rubber dome attached to the carburetor body. The fuel line from the tank connects to one port of the primer bulb. When you press the bulb, it pulls fuel through that line, through the bulb, and pushes it out the other port into the carburetor inlet. A second line runs from the other side of the primer bulb back to the tank. That is the return line. Some setups also have a small crankcase vent line that routes crankcase vapors back to the carb inlet or to the air filter housing. It is easy to miss because it is thin and runs separately from the two main fuel lines. The check valves are molded into the primer bulb itself. You will not find separate in-line check valves on this model. The bulb has one-way flow built in: pressing it draws fuel from the tank and pushes it toward the carb. Releasing it creates a slight vacuum that draws more fuel in. If the bulb is cracked or the internal valves are worn, you lose that pumping action entirely. That is why a cracked bulb makes the trimmer impossible to start cold.
I found one thing that is not obvious from any diagram. The primer bulb on these engines has a specific orientation. One side of the bulb is the inlet from the tank, the other is the outlet to the carb. If you install the bulb backwards, the check valves block flow in the wrong direction. The engine may prime but then sputter out, or not start at all. Troy-Bilt marks the carburetor body with small arrows or fuel flow direction indicators near the bulb mounting point. If yours is faded, do not guess. Map it by tracing the original lines before you pull anything off. The carburetor on the TB685EC is a standard diaphragm-type carb, usually a Zama or Walbro equivalent. The fuel enters the carb through the primer bulb outlet port, passes through the carb inlet filter screen, hits the main jet circuit, and then the overflow returns through the carb gasket passage back to the primer bulb return port. That return path is why the primer bulb looks like it is in the middle of everything. It is the central junction point for the entire fuel circuit.
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How to replace the fuel lines and bulb
Remove the air filter cover and the air filter. The carburetor is right behind it. Unclip or unbolt the carb assembly from the engine. Do not disconnect anything yet. Take a photo of the routing from every angle. The lines look identical and it is very easy to mix them up when you put them back on. Drain any remaining fuel from the tank. Disconnect the spark plug wire before doing anything else. That is not a suggestion, it is the only thing that keeps you from starting the engine accidentally while your hands are full of fuel and tools. Start with the tank. Remove the two screws holding the tank to the engine shroud. The tank comes off with the fuel filter still attached inside. Pull the filter and rigid pickup tube out through the neck. Inspect the filter. If it is dark brown or clogged with sediment, replace it. A clogged in-tank filter causes exactly the same symptoms as a bad primer bulb: hard starting, running rich then dying, and poor high-speed performance. The filter on this model is a standard Troy-Bilt part, usually carries part number 610-0465 or similar depending on the production year. Check your specific model number plate, the one riveted to the engine shroud, and confirm before ordering.
Now the lines. The supply line from the tank to the primer bulb is usually the longer of the two. It connects to the tank fitting at one end and the inlet side of the primer bulb at the other. The return line goes from the primer bulb outlet back to the tank. The carb inlet line is the short one that runs from the primer bulb outlet port to the carburetor inlet. On many TB685EC units, the primer bulb and the carb inlet line are a single molded assembly, so replacing the bulb means replacing that short line too. I pulled the old lines first. They are brittle after a couple years of sun exposure. If they crack when you tug on them, that is a sign the entire fuel system needs replacement, not just the obvious broken piece. New lines are cheap. The whole set runs about eight to twelve dollars at any parts counter. Buy the 3/16 inch automotive fuel line, not standard vinyl tubing. Vinyl degrades fast when exposed to ethanol-blended gas, which is what every station sells now. Rubber or reinforced fuel line is the only thing that lasts more than a season. Reassembly order matters. Install the new in-tank filter onto the rigid pickup tube first. Make sure it is fully seated and pointing down when the tank is mounted. Feed the tank back into position and secure it with the two screws. Route the new supply line from the tank fitting through the grommet, then to the primer bulb inlet. Route the return line from the primer bulb return port back to the remaining tank port. Connect the short carb inlet line from the primer bulb outlet to the carb inlet. Double check every connection against your photos. Get it wrong once and you will be taking it all apart again.
Reinstall the carburetor onto the engine studs. Use a new carb gasket. The old one compresses and leaks air, which makes the mixture run lean and can crack the piston over time. A lean condition on this engine shows up as overheating and a high-pitched whining sound at full throttle. That is not normal and it destroys the engine if you ignore it. Fill the tank with fresh gas. I use 89 octane with no more than 10% ethanol. Higher ethanol content accelerates seal degradation inside the carburetor diaphragms. Mix in a fuel stabilizer if the trimmer sits for more than thirty days between uses. The TB685EC carburetor diaphragms are sensitive to varnish buildup, and that is what happens when gas sits and evaporates. Prime the bulb by pressing it ten to twelve times until it is firm and fuel is visible inside the clear tubing. Connect the spark plug wire. Start the engine. Let it warm up for a minute, then check for leaks around every fitting. A small weep at the tank grommet means the hole is oversized or the grommet is cracked. Replace the grommet, it costs about forty cents and takes two minutes.
Common problems and what they actually mean
The most frequent issue I see with the TB685EC fuel system is the primer bulb failing silently. It does not always crack. Sometimes the internal flaps just wear flat from years of flexing. The bulb looks fine but does not hold prime. The fix is replacing the bulb assembly, and on this model that means replacing the two fuel lines attached to it as a set. You can buy the bulb and lines together as a kit from most small engine parts suppliers. Part numbers vary but look for a kit listed for the TB685EC or the 358.793810 engine platform, which is the underlying engine used across many Troy-Bilt models. Another issue is the tank vent. If the vent line is kinked or blocked, fuel cannot flow out of the tank properly. You get fuel starvation that mimics a bad carb. Check the vent line route. It sometimes runs through a clip on the frame and gets pinched during reassembly. On my unit, the vent line had been routed through a hole that was too small and was partially collapsed. I swapped it to a larger grommet and the idle consistency improved immediately. If the engine runs but dies when you apply load, check the in-tank filter first. Then check the carb inlet screen. Then check for air leaks around the carb mounting bolts. An air leak after the metering jet causes a lean condition under load that fuel starvation also causes, and people mix them up constantly. The diagnostic path is: fuel flow first, then air leaks. Always in that order.
I replaced a fuel line on a TB685EC once where the original supplier had given me the wrong size tubing. The 3/16 line was loose on the carb inlet port and sucked air. The engine ran, but poorly, and I could not find the leak because the line looked installed correctly. The port on the carb was actually 5/16 inch, not 3/16. Upgrading to the larger line solved it. This is why tracing the original routing and measuring the ports before buying replacement tubing saves a lot of frustration. Measure everything before you assume the diagram is wrong.
Where to find the diagram
Official diagrams are available through Troy-Bilt's parts lookup. Go to the Troy-Bilt website, enter your model and serial number, and pull the exploded view for the engine and fuel system. The TB685EC uses the TB685EC engine number, which maps to a specific carburetor and primer bulb assembly. The diagram shows the exact line routing, part numbers for the primer bulb kit, the in-tank filter, and the tank grommet set. I rely on that for part numbers, not third-party sites, because Troy-Bilt has changed suppliers several times across the production run and the correct part depends on your manufacturing date. Third-party diagrams exist on small engine repair sites. They are generally accurate but sometimes show the pre-facelift version of the carburetor, which has a different primer bulb mounting pattern. If your primer bulb does not match the diagram, you are looking at a later carb revision. The fuel routing is the same, but the physical connection points differ. Verify before you order parts.
Bottom line
The fuel system on the TB685EC is straightforward if you respect the routing and use the right materials. Ethanol-resistant line, a new in-tank filter, a new primer bulb kit, and a new carb gasket will restore nearly every fuel-related problem on this engine. The common failures are the bulb, the filter, and air leaks from old gaskets. Diagnose in that order. Do not blame the carb until you have ruled out the simpler causes. If you have the model and serial number, I can help you track down the exact part numbers. Otherwise the Troy-Bilt parts diagram is the most reliable reference you will find. Read it carefully, measure your existing lines before ordering, and do not reuse old fuel tubing. It does not matter how clean it looks. Once fuel line ages, it degrades internally even if the outside is fine.
