Briggs & Strat wont idle stable, rough/low idle

MEDEL514

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I have a Briggs and Stratton 6.25 HP motor on a yard machines lawn mower. The motor will not go up to the operating RPM, It stays at a low rpm and bounces around a lot. I've already replaced the spark plug, clean and disassembled the carburetor, new fuel line, new air filter and screen, And I bought a bunch of other random parts such as new wheels, skirt, and blade.

Video of problem here:

I still cant figure out what's wrong, any ideas?
 

bertsmobile1

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Your engine is surging
Surging is cause by a fuel : air ratio that is too lean.
So either the fuel supply is too low or there is air leaking between the carburettor & the intake valve.

A common problem with these Briggs engines is the manifold cracking or coming loose.
 

MEDEL514

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Your engine is surging
Surging is cause by a fuel : air ratio that is too lean.
So either the fuel supply is too low or there is air leaking between the carburettor & the intake valve.

A common problem with these Briggs engines is the manifold cracking or coming loose.
Are you referring to the manifold on the carburetor itself? I took it off the engine and cleaned everything out, but I didnt notice any obvious cracks. Do you think it's time for a new carb?
 

ILENGINE

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I don't think is is as much of a carb issue is it is an engine speed issue. The tab with the hole in it closest to the front of the engine that the governor spring is connected to has most likely been bent from mowing around bushes etc and needs to be pulled back toward the front spark plug end of the engine until you reach the correct engine speed. This is better to be done with a tach to make sure you don't over rev past the safe blade turning speed which is lower than the normal 3600 rpm of the engine.

Since this engine is not set up with a throttle cable the carb most likely doesn't contain an idle circuit because it doesn't need one to run full throttle only. But if the engine is slowed down below a certain point without the idle circuit will cause a surging effect.
 

bertsmobile1

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There is a long plastic tube that takes the fuel from the carb to the opposite side of the head .
It passes under the magneto coil.
It is made from a thermo setting plastic so it will not soften from the engine heat but this material gets quite brittle with age.
There is a very small gasket under the 2 x 1/16" dia bolt that hold it in place.
The bolts are prone to coming loose and chewing up the gasket. Do not over tighten them
 

Scrubcadet10

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I don't think is is as much of a carb issue is it is an engine speed issue. The tab with the hole in it closest to the front of the engine that the governor spring is connected to has most likely been bent from mowing around bushes etc and needs to be pulled back toward the front spark plug end of the engine until you reach the correct engine speed. This is better to be done with a tach to make sure you don't over rev past the safe blade turning speed which is lower than the normal 3600 rpm of the engine.

Since this engine is not set up with a throttle cable the carb most likely doesn't contain an idle circuit because it doesn't need one to run full throttle only. But if the engine is slowed down below a certain point without the idle circuit will cause a surging effect.
X2 (y)
what ilengine said.
 

MEDEL514

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The tab with the hole in it closest to the front of the engine that the governor spring is connected to has most likely been bent from mowing around bushes etc and needs to be pulled back toward the front spark plug end of the engine until you reach the correct engine speed.
The engine its surrounded by plastic covers that cover the carb and spring area, (removed when I took the video) and they are not damaged, so I think it's safe to say that the tab is not bent.
 

MEDEL514

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There is a long plastic tube that takes the fuel from the carb to the opposite side of the head .
It passes under the magneto coil.
It is made from a thermo setting plastic so it will not soften from the engine heat but this material gets quite brittle with age.
There is a very small gasket under the 2 x 1/16" dia bolt that hold it in place.
The bolts are prone to coming loose and chewing up the gasket. Do not over tighten them
I checked the bolts and they were still tight. Also, the tube doesnt draw air from the carb, but rather the air filter box, so I dont see how this could cause a bouncy idle.
 

MEDEL514

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The tab with the hole in it closest to the front of the engine that the governor spring is connected to has most likely been bent from mowing around bushes etc and needs to be pulled back toward the front spark plug end of the engine until you reach the correct engine speed.
I tried to pull on the spring to readjust it, and I was able to increase the idle speed, but it was still bouncing around. Something else is off with this engine that wont allow it to idle smooth. Any other ideas?
 

ILENGINE

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Your video proves that it is an engine speed issue not a carb issue when the engine smoothed out when you increased and held the throttle on the carb. You are running around 1600 rpm on an engine that is designed to operate at 2800-3600 depending on blade length. Until the engine gets into that range it will hunt and surge. And you cannot properly diagnose anything else until that is corrected.
 
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