LA 145 Engine Reving

[Klorinth]

Recently I have started having a problem with my wife's 145. It starts rough, usually with a bit of smoke, and then it continuously revs up and down. It's like you are pumping the gas in a car while in neutral.

I am no mechanic and I can't figure out what it is. I have no intention to start pulling things apart until I have an idea about what it might be.

Can anyone point me in the right direction????

 

[Klorinth]

Ok, answered my own question.
Likely a plugged jet in the carb. I had a fuel line get damaged last fall. Changed out the line this spring. It may not have been as clean a job as I thought.

Guess I'll change fuel filter as well. Maybe add a second one while I'm at it???

How do you prevent this from happening?

 

[jinman]

Ok, answered my own question.
Likely a plugged jet in the carb. I had a fuel line get damaged last fall. Changed out the line this spring. It may not have been as clean a job as I thought.

Guess I'll change fuel filter as well. Maybe add a second one while I'm at it???

How do you prevent this from happening?

I'm not so sure that your problem is a plugged jet. It sounds to me like your governor linkage may not be right or a spring may be broken. Look at the first photo below. This view below the carbs shows the governor arm clearly. When you increase the fuel control lever, spring tension overcomes pressure from this arm and the engine increases speed. As speed increases, the arm will have more tension and overcome the throttle spring to cause the engine to come to an equilibrium in speed. If the governor is adjusted wrong, the engine's rpm can overshoot the equilibrium rpm and oscillate in rpm above and below the proper speed. In the second photo, you can see the whole governor arm and mechanism because the carburetor's have been removed. Check for a loose or broken spring and also any bending of the arm that may have been done inadvertently or perhaps trash that is interfering with the governor's operation.

 

[meburdick]

I'm not so sure that your problem is a plugged jet. It sounds to me like your governor linkage may not be right or a spring may be broken. Look at the first photo below. This view below the carbs shows the governor arm clearly. When you increase the fuel control lever, spring tension overcomes pressure from this arm and the engine increases speed. As speed increases, the arm will have more tension and overcome the throttle spring to cause the engine to come to an equilibrium in speed. If the governor is adjusted wrong, the engine's rpm can overshoot the equilibrium rpm and oscillate in rpm above and below the proper speed. In the second photo, you can see the whole governor arm and mechanism because the carburetor's have been removed. Check for a loose or broken spring and also any bending of the arm that may have been done inadvertently or perhaps trash that is interfering with the governor's operation.

+1

Anything being out of whack or missing can make the motor "hunt" like that.

 

[Eric Barney]

Check the following: Will the engine idle without surging? If not, the idle passages are plugged. The fuel jets for idle are tiny, and plug very easily. This is much more likely than a governor problem. Fuel related problems are the most common of all lawn equipment run problems.

 

[Klorinth]

Thank you all for your responses.
I will take a look at the governor setup and see how it is. The engine does not idle though. If I drop the throttle to idle it sputters and dies. That was part of why I thought some garbage got into the carb. One if my dogs bit through the fuel line last fall and I had to replace it so I wouldn't be surprised if a piece got in.

 

[jinman]

Eric makes a good point. Be sure to check for contamination in the carb, but keep an eye on that governor's operation too. You can put your hand on the governor rod to see if you can damp out the oscillations. Also be sure to check the engine oil level. If the engine gets low on oil, it can cause surging through the governor because the governor is normally oil pressure driven.

EDIT: Here's a thread where I replaced the needle valve in my carb. It may be helpful for you during teardown.

Oops! That's not the thread with teardown. This is the correct thread.

 

[meburdick]

If the engine gets low on oil, it can cause surging through the governor because the governor is normally oil pressure driven.

Is this specific to this motor? I know that there are a couple of different kinds of governor systems in use, including one that operates off of *air pressure* created by the fins on the flywheel of the motor.

 

[jinman]

Is this specific to this motor? I know that there are a couple of different kinds of governor systems in use, including one that operates off of *air pressure* created by the fins on the flywheel of the motor.

You are absolutely right for smaller engines, but the larger engines don't use the vane type. The photos I posted showing a shaft going into the engine crankcase are of the actual engine on an LA145.

 

[meburdick]

You are absolutely right for smaller engines, but the larger engines don't use the vane type. The photos I posted showing a shaft going into the engine crankcase are of the actual engine on an LA145.

I wasn't challenging what you said... Was just asking if this particular style was specific to this motor. I know that most small engines use the flywheel to create air pressure because it's a simple model. And, I was curious as to what made this "different".

 

[jinman]

I wasn't challenging what you said... Was just asking if this particular style was specific to this motor. I know that most small engines use the flywheel to create air pressure because it's a simple model. And, I was curious as to what made this "different".

Yep, but once you get to the larger engines the governor is actually mechanical. On their website, Briggs has descriptions of the pneumatic governors driven by the flywheel vane and the mechanical governors driven by a gear on the flywheel with flyweights. The flyweights will work under all conditions, but having the proper oil level ensures free operation and the proper response to engine rpm.

 

[Eric Barney]

The governors job is simple. It limits the maximum RPM the engine will run. When unloaded the throttle is open less than 1/4, even with the throttle set to full. When the engine is put under work load, it will be slowed slightly, allowing the governor to open the throttle until the engine reaches the limit RPM. The type of governor that is in the crankcase is centrifugal. It is lubricated by oil, but does not rely on oil pressure to function. At lower throttle settings, the governor has an effect on the throttle. If the engine slows for any reason, it will open the throttle in an attempt to keep the engine running at the set RPM's

Enter a carburetor that won't idle. While free running a full throttle, (No load) the governor closes the throttle on the carb. to limit RPM. The engine starts to stall, and the governor opens the throttle to attempt to restore the set RPM. This causes the engine to rev too high, and the throttle closes. This cycle will repeat over and over until the engine is put under enough load to get the governor to open the throttle past the point of where the idle fuel circuit has an effect. (All fuel is entering from the main jet, which is much larger than the idle jet, and is less prone to clog.) This is probably somewhere around 1/3 or more of actual throttle opening. If you were to hold the carb throttle open at 1/3, the engine would be revving much higher than the governor is set to allow.

Bottom line: Restore your idle by cleaning the carburetor. Once the engine runs smooth at idle, it will run at any throttle setting without surging.