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Briggs 21 HP engine runs poorly

#1

G

grumpajohn

Inherited a Troy-Bilt tractor from father-in-law which had a history of troubles. Continued with me. Engine starts fine, but after warming up, wants to quit and will do so on occasion. I can "wet nurse" with choke, but still runs poorly. Paid online mechanic who said to replace valves. Did so. No help. Replaced coils, fuel pump twice, fuel solenoid, filters and plugs, cleaned carb. Even went so far as to scrub cylinder cooling fins and removed sheet metal from front of tractor so as to improve cooling. No help. Runs fine for about 1/2 hour and then starts acting up. Engine is B & S 407777; 0229 E-1 21 HP Intek. What next?


#2

I

ILENGINE

Did you check fuel cap vent, and also make sure the tank doesn't have something clogging the fuel outlet, or something obstructing the fuel line limiting fuel flow.


#3

G

grumpajohn

ILENGINE said:
Did you check fuel cap vent, and also make sure the tank doesn't have something clogging the fuel outlet, or something obstructing the fuel line limiting fuel flow.
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Replaced fuel cap and fuel lines before. Also removed and cleaned gas tank. If that's a problem, why does the motor run well when cool? I'm thinking the fuel solenoid is faulty and read something somewhere to replace with standard bowl bolt. Any thoughts on this?

Oh, also insulated fuel lines thinking possible vapor lock. No change.


#4

I

ILENGINE

I am going to relate a story, and lets see if it could be relevant . Had a customer from a few years ago that owned a craftsman with a similar V-twin Intek. It would run about like yours and then would sputter, miss, backfire, and eventually quit until it cooled down. All the things that you have done was done to that engine. In the end we discovered that there was no shielding between the exhaust and the carb, and the heat from the exhaust was actually boiling the fuel in the carb. If you touched the carb after it quit, the fuel bowl would be so hot that you couldn't hold your hand on it. And you could hear the fuel in the bowl bubbling.

The fuel bowl should stay relatively cool to the touch.


#5

G

grumpajohn

ILENGINE said:
I am going to relate a story, and lets see if it could be relevant . Had a customer from a few years ago that owned a craftsman with a similar V-twin Intek. It would run about like yours and then would sputter, miss, backfire, and eventually quit until it cooled down. All the things that you have done was done to that engine. In the end we discovered that there was no shielding between the exhaust and the carb, and the heat from the exhaust was actually boiling the fuel in the carb. If you touched the carb after it quit, the fuel bowl would be so hot that you couldn't hold your hand on it. And you could hear the fuel in the bowl bubbling.

The fuel bowl should stay relatively cool to the touch.
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Like this idea a lot. It does have a heat shield above the muffler, but it still could be hot enough to boil the fuel. I"ll check that out maybe over the weekend and see if the bowl is hot.


#6

G

grumpajohn

grumpajohn said:
Like this idea a lot. It does have a heat shield above the muffler, but it still could be hot enough to boil the fuel. I"ll check that out maybe over the weekend and see if the bowl is hot.[/QU

HOO-RAY!!!!!!!! Ran unit this afternoon for over one hour; engine ran flawlessly. When checking the heat shield, noticed that it was in direct contact with exhaust pipes thus transferring heat directly to shield. Modified it slightly to clear pipes and add more air movement under the plate. Works like this thing has never worked before. Thank you for the direction - I'd have been lost without it.
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#7

I

ILENGINE

Glad my story could assist you in your repair.


#8

primerbulb120

primerbulb120

Interesting! ILengine, have you ever had this problem with string trimmers before? Specifically, the 31CC MTD's? I have several of them. All start and run fairly well until shut off. Then they start leaking gas out of the carb throat. Could this be because of the muffler heat?


#9

I

ILENGINE

Primer, don't think it is as much muffler heat since they normally are away from the fuel tank and on the opposite of the engine as the carb. I suspect part of the problem is related to the non vented/one way valve breather for the fuel tank, and pressurizing of the fuel tank that overloads the metering diaphragm and leaks through the main nozzle. Have seen it a few times on chainsaws that were left in the hot sun with a full tank of fuel.

Create the same effect if you reverse the fuel line on the primer bulb and push fuel into the carb, instead of drawing it through by vacuum.


#10

primerbulb120

primerbulb120

I haven't had this issue with the weedeaters that have carb and muffler on opposite sides. The MTD 31CC engine is a reed valve design, so the carb is located right above the muffler. Thus leading to my suspicions


#11

I

ILENGINE

The carb is fairly sealed off from the muffler on those but it is possible that the heat could be rising and vapor locking the carb. Fuel coming out of the carb after shut off seems more like a pressure problem forcing fuel through the carb. Primer bulbs that are not sealing and allow fuel flow backwards through them toward the carb will cause the symptom you are discussing.


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