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Engine only runs with full choke on

B&S 18.5 HP engine only runs with full choke on. Have re-built the carburetor and still no improvement.

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You might want to check the fuel supply as it enters the carburetor.

Fuel Related issues that take a certain amount of time to appear
Depending on make and model of mowers:
a) Fuel tank outlet plugged (some have a screen filter,
some are 90' angle and plug with rust and debris inside the corner of fitting)
b) Fuel lines collapse when they are routed thru fasteners and between frame and seat decks
c) Fuel cap vent clogged
d) Fuel filter clogged
e) any fitting that has an orifice that can trap debris or rust will eventually become clogged
f) fitting on the side of the carburetor
g) some systems have a filter inside the line at the opening of the carb inlet
h) fuel pump
i) vacuum hoses to fuel pump
j) debris under inlet needle/seat on carb

Posted on Nov 12, 2009


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My murray weedeater want run off choke why?

Carburetor needs to be rebuilt. I won't even think twice about. the diaphram is on it's way out.

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It starts, runs, but after a few minutes it shuts itself off...any ideas why?

Look first for a plugged fuel line that is restricting the fuel from reaching the carburetor. There is usually a screen filter in the tank and inline filter in the hose and sometimes a filter at the connection to the carburetor. Disconnect fuel hose at the carburetor and opening shutoff valve confirm fuel freely flows from tank.

Old residual fuel can plug the filters because of the ethonal additive.
If fuel line is clear try maintaining the running condition by holding the choke in the partial choke position. Many of these units have a choke that you set and as the engine runs the choke slowly backs off. By retaining the choke on position and the engine continues to run is an indication that the engine is running lean,

This is likely a result of residual fuel evaporating and leaving residue behind in the carburetor. This residue coats the fuel passages and restricts the fuel air mixture resulting in the engine running lean. Maintaining the partial choke compensates for the lean mixture and the engine will continue to run.
To correct for this remove all fuel from the engine. Some unleaded fuel is available from the hardware stores($$) but will help to clean out the carburetor. I have found that by running clean fuel through the machines the carburetor in time will likely clean itself.

Next step if after some run time the unit doesn't improve is to add a carburetor cleaner into the gas. Try again running the unit until engine running condition improves.

If there has been some improvement but the engine is hunting not running smooth than an adjustment to the fuel mix screw on the carburetor will help. Record the position of the adjustment and open 1/4 turn max listening for engine to settle down. The adjustment may need to be returned to the initial position after running as the fuel passages continue to clean out.

If all is well then I would in the future follow manufactures storage procedures.

Last resort is to clean out the carburetor. I do not recommend this unless you have additional skill. It is easy to damage the assembly.

If all goes well then the link provides more advice then you will ever need.

I empty all gas from mine and change the oil after every use. I have sprayed WD40 in carb before storage. I use mind only for power failures.

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I have an older craftsman snow blower 10 hp. It starts ok with full choke, runs ok half choke, backfires with no choke-run position.

It's too lean; not enough fuel mixed with the air. Look for the main mixture screw--it should be a large screw with a spring under it on the carburetor. Run the engine until it's warm.

With the engine running, turn the screw outward (lefty-loosey) about 1/8 turn slowly and see if it improves. Slowly turn the screw back and forth until it runs best with the choke completely off. Then set it 1/8 turn richer (turn screw outward 1/8 turn) than the best position.

Reason for doing this: Conditions change, even the barometric pressure. Small engines run okay if they are a little rich but not if they are a little lean.

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Check all the linkage at the carburetor for problems. Also try pulling the throttle trigger and may sure it operating the carburetor.

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Afternoon Jacob;
If you must choke it for it to run it is to lean. Have you cleaned the air filter? Might be good idea. An air leak can also cause this inspect the intake for a loose or defective connection at the engine or carburetor.
If these are good try to adjust the "H" needle valve CCW (richer) a little.
You can find a very good description of adjustment here:
Carburetor Adjustment by
CW is leaner; to lean will destroy the saw. If over tightened closed (CW) the adjustment screw faces are easily marred; marred adjustments are difficult to impossible to set properly.
Clean the air filter; adjusting with a dirty filter can cause a run lean condition once cleaned and the saw run.

There are 2 wav (sound - idle & full – may not be highlighted) files that I find most helpful, I think you will too. Good Luck

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Sounds like something is clogged. It may need a new air filter or the carburetor may need cleaning out. Also I would check your fuel, if it is over 30 days old I would replace it. It may be losing compression and either need a new head gasket or the head bolts tightened when it is hot.
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