QuestionHi i have a Sthil FS 85 brush cutter. problem it won't start. I change rings, engine block and carburetor. But engine will not start. If i remove the spark plug and pour some gas on top of piston , place back spark plug and start it will but it will only run for a couple of seconds.
Answerashram
# Fuel tank is empty or shutoff valve is closed, or fuel line or fuel tank cap vent is clogged.
# There is water in the fuel.
# Carburetor is overchoked.
# Carburetor is improperly adjusted or needs service.
# Ignition system or its wiring is defective or ignition switch is off.
# Deadman or other cutoff switch is open or defective.
# Spark plug is fouled, improperly gapped, or damaged.
# Engine compression is poor.
* Is there some in the fuel tank? If it is near the bottom, add enough so that there is no doubt about there being enough to reach the outlet pipe regardless of any slant on which the lawn mower is located.
* Make sure any shutoff valve is open.
* Check for a clogged fuel filter, if there is one. There may be a sediment catching screen at the bottom of the tank as well.
* If your engine uses a primer bulb, does it feel like it is doing something? There is a distinctly different feel when it is actually squirting a little gas into the intake pipe. Check that the rubber hasn't deteriorated but if many pushes still doesn't do anything (and you're sure there is gas in the tank and the engine hasn't flooded from TOO MUCH gas), the carburetor and/or fuel line may need cleaning.
* If you are using gas from last season, discard it and start fresh. While old gas will usually work in an engine in good condition, this is not always the case, especially with one that has seen better days. The more volatile fractions evaporate leaving behind higher flash point gas. Why add another unknown factor to the puzzle?
* There may be water in the gas. If the carburetor has a drain plug, operate it to rid it of the bottom layer which would have the water. If there is no drain, repeated pulling on the starter cord should eventually clear any reasonable amount of water.
The optimal air:fuel ratio is around 14:1. This must be lower for a cold engine and thus a choke plate or other means to increase the richness of the mixture is usually provided. A choke plate restricts air intake forcing more gas to be sucked into the cylinder. A primer bulb effectively squirts gas into the intake pipe to augment the normal carburetor action. Some carburetors have no choke and no primer but incorporate a small gas reservoir which fills when the engine is off and provides some extra when starting.
To much air results in a mixture that is too lean, burns too quickly, and can result in engine damage over extended periods of operation.
* Check that any choke is not stuck in the open position and not doing its job.
* The carburetor may need adjustment or cleaning.
Too little air results in a mixture that is too rich - there will be loss of power and possibly black smoke from the exhaust. This could be due to several factors:
* Check the air filter. For testing, it can usually be removed to see if the engine will start. However, do not run it for an extended period of time without a properly functioning air filter in place. Some are designed to be washed and reused while others must have their elements replaced.
* Check that any choke is not stuck closed. Though needed to start cold, if the choke remains closed, the engine will not restart and will quickly stop (truly choke!) due to an overly rich mixture.
* A defective carburetor may also cause the mixture to be too rich or too lean.
Spark
