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Only idles at full throttle (not enough power)

#1

W

Whitebird

22" Murray 4.0 push mower (8-22541)
Date: 0718
Engine: Briggs & Stratton (data plate is gone)
Murray 8-22541 carburetor.jpg

Here's one that's beyond me (not hard to do, actually).

At full throttle (not just the lever, but the throttle is all the way open on the carb itself), the motor runs smoothly but at about high idle speed.

When I start it after letting it sit for an hour it fires right up to full speed, but after 10-30 sec. it dies or else slows down and runs at high idle. If I start it again right after it dies it will sit and run at high idle speed (but throttle is still open all the way). It will run like that for a couple of hours (I tried). No stuttering, no surging, just smooth idle.

Onset was sudden. Was working fine; then a week later gas was filled, oil added, and this started happening.

I've already disassembled and thoroughly cleaned the carb twice (wire and carb cleaner). The first time, it seemed to help (mowed well for 10 min). Gas is fresh. Fuel tank and filter are clean. Fuel line is old, but clear (flow seems good too). All linkages appear like they're working properly.

It has started to leak some oil from somewhere near the exhaust while running, but that's a different issue, I think.

What am I missing?


#2

C

chance123

Whitebird said:
22" Murray 4.0 push mower (8-22541)
Date: 0718
Engine: Briggs & Stratton (data plate is gone)
View attachment 15846

Here's one that's beyond me (not hard to do, actually).

At full throttle (not just the lever, but the throttle is all the way open on the carb itself), the motor runs smoothly but at about high idle speed.

When I start it after letting it sit for an hour it fires right up to full speed, but after 10-30 sec. it dies or else slows down and runs at high idle. If I start it again right after it dies it will sit and run at high idle speed (but throttle is still open all the way). It will run like that for a couple of hours (I tried). No stuttering, no surging, just smooth idle.

Onset was sudden. Was working fine; then a week later gas was filled, oil added, and this started happening.

I've already disassembled and thoroughly cleaned the carb twice (wire and carb cleaner). The first time, it seemed to help (mowed well for 10 min). Gas is fresh. Fuel tank and filter are clean. Fuel line is old, but clear (flow seems good too). All linkages appear like they're working properly.

It has started to leak some oil from somewhere near the exhaust while running, but that's a different issue, I think.

What am I missing?
Click to expand...

Sounds like your linkage is binding or hanging up on something


#3

W

Whitebird

chance123 said:
Sounds like your linkage is binding or hanging up on something
Click to expand...

Which linkage are you thinking?

The throttle plate stays wide open, which tells me the governor is working as it should (opening the throttle when RPMs drop).


#4

W

Whitebird

Ok, I'm learning more about how small engines work. Help me think through this and tell me if I'm thinking in the right direction.

The engine is not getting enough fuel/air mix; if it was it would run at full RPMs instead of at idle. Therefore, something is wrong with the carburetor because that's the part that gives fuel and air to the cylinder. The problem is not the throttle plate, since that swings wide open. What else could be wrong with the carb?

Bad needle or needle seal? But if the needle was bad that would result in it flooding because too much gas would flow to the bowl, right?

What else?

Thank you all.


#5

C

chance123

Whitebird said:
Ok, I'm learning more about how small engines work. Help me think through this and tell me if I'm thinking in the right direction.

The engine is not getting enough fuel/air mix; if it was it would run at full RPMs instead of at idle. Therefore, something is wrong with the carburetor because that's the part that gives fuel and air to the cylinder. The problem is not the throttle plate, since that swings wide open. What else could be wrong with the carb?

Bad needle or needle seal? But if the needle was bad that would result in it flooding because too much gas would flow to the bowl, right?

What else?

Thank you all.
Click to expand...

You say that "you are learning" so I might share that the throttle plate is wide open. I can only guess that you have the aircleaner off and are looking past the choke plate in order to see the throttle plate (butterfly) It would not be mixture adjustments. If it is running smooth, leaning out the mixture would cause it to die, and fattening out the mixture would cause it to fall on its face


#6

W

Whitebird

chance123 said:
You say that "you are learning" so I might share that the throttle plate is wide open. I can only guess that you have the aircleaner off and are looking past the choke plate in order to see the throttle plate (butterfly) It would not be mixture adjustments. If it is running smooth, leaning out the mixture would cause it to die, and fattening out the mixture would cause it to fall on its face
Click to expand...

Yes, that's right; I have the air cleaner off and am looking past the choke plate. My carb only has the idle adjustment (not the full throttle adjustment); I have played with that adjustment but with no change in the behavior.

It sounds just like I'm running it with the throttle mostly closed, but I'm not: the plate is all the way open (due to the governor sensing low RPMs).


#7

J

jlitzz

Although I am quite new to this forum, and lawn mower repair in general, I recently had some issues with my 6.0HP Briggs surging. I had also cleaned the carb several times with spray carb cleaner, various size wires, and compressed air, only to have the issue continue.

A member of the forum suggested soaking the carb to remove any stubborn blockages that are eluding the cleaning. After soaking my carb for 20 minutes, the symptoms disappeared and the mower runs like brand new. I would suggest that there is still some grime or gunk in your carb that is causing the runability issue. Try soaking your carb, I used Gunk dip, rinse with hot water, let dry, then spray with carb cleaner and compressed air. Because of this last issue, I probably will just soak every carb to avoid having to "clean" them more than once!


#8

W

Whitebird

jlitzz said:
A member of the forum suggested soaking the carb to remove any stubborn blockages that are eluding the cleaning. After soaking my carb for 20 minutes, the symptoms disappeared and the mower runs like brand new. I would suggest that there is still some grime or gunk in your carb that is causing the runability issue.
Click to expand...

That makes a lot of sense. I will try this. Thanks so much for chiming in!


#9

C

chance123

Whitebird said:
Yes, that's right; I have the air cleaner off and am looking past the choke plate. My carb only has the idle adjustment (not the full throttle adjustment); I have played with that adjustment but with no change in the behavior.

It sounds just like I'm running it with the throttle mostly closed, but I'm not: the plate is all the way open (due to the governor sensing low RPMs).
Click to expand...

If the throttle plate is wide open and the engine is running slow, there "has" to be a loud noise at the air input of your carb. Much like when you open your mouth and say "Ahhh". If you don't hear it, then something has to be blocking the passage way between the carb and engine.


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