My 3 year old push mower runs like crap. It looks like it’s getting a decent spark but I changed the plug anyway. The gas isn’t old or anything. It starts just fine and doesn’t stall, but it just doesn’t have power. It kind of comes and goes (more and less power/rpm), but is happening more often than not… What do I look at next?
Possible causes:
Old gas, bad spark plug, very thick/dirty oil.
Choke is partially closed.
Carburetor needs adjustment or service.
Ignition not timed correctly.
Air filter is clogged.
There is a lack of lubrication.
Valves are not sealing properly.
Piston rings are not sealing properly.
Head loose or head gasket blown or damaged.
Exhaust ports blocked (2 stroke).
Here’s a general lawnmower maintenance link. It might have something for you.
If its an old Briggs eng. , remove or clean the air filter for a test run . Throw a bit of oil in the fuel " like watered down 2 cycle mix " valves get sticky -rings get dry . As always the basics - Air -fuel -spark , see if the poles are rusty on the coil / flywheel .
Heli
Well it looks like I have my project for tomorrow. According to slowguy’s link it seems like it could be the carb. I’ll try your trick too. It is a Briggs…
Slowguy knows his small engines. It could be alot of things, including a combo. We used to go-cart race with the 5.5 hp Briggs and Honda engines. They’re like women - treat 'em good and they last a little longer. 
Take it to a small engine repair guy and get it refurbished.
Hey best of luck
There are a lot odd things on a Briggs , a plastic paddle as a governor . Try the mixture screw they can get dirt in the jet and all that is require is a one turn out and back in . Buy a cheap can of "Brake clean " at the auto parts store , to clean up all the carb. linkage and adjustments . There is an electronic ignition kit for the old engines .
Ok. It took all of 25 seconds to fix the symptoms but I’m not sure if this is the right fix. Correct me if I’m wrong but if you look at Figure 1 the following items are labeled:
A- Throttle
B- Idle Spring
C- Idle Spring
D- Adjustment
I can’t determine a functional difference between springs B and C? They both appear to be putting load on A to control the idle speed of the mower. Spring B is just plain weak. There is really no load on it when the mower is running. Spring C is doing all the work. D appears to be some sort of adjustment but unless something is bent it hardly moves. When the mower is running I can move D back and forth a bit but it doesn’t change the idle speed.
So the quick and easy fix is to put more load on one of those springs in order to keep the engine running faster. I assume there is a better way to adjust this but I’m not sure.
What do you guys think?
Figure 1:
A. is the governor arm ( it should swing freely )
b. Gov. spring idle
C .Run spring
D. Should be to the throttle cable or choke ? in don’t see a cable in the photo.
In theory A . gets flywheel / cooling fan air , which in or decrease throttle to the engine .its really made to keep the rpm down from over speeding / damaging the engine . On the flip side it keeps you from stalling in heavy grass , if the C .or B .spring is weak you will never get max power .
So you have two throttles 1. Your power lever 2. The big brother keep you safe ( we are overriding you ) A. is connected to .
Ok. I just rigged it so there is a tiny bit more load on C. I think I hit a happy medium where it’s not idling too fast but the power is great. The strange thing is I don’t see any possible way to adjust anything outside of manually tweaking the springs? Maybe the “correct” way to fix it is to just replace the spring. It just strikes me that there should be some sort of screw to adjust it. Thanks for your help.
If its all free and lubed , you can pull the spring to where it sounds good ( not screaming Max ) . Bend the hook end of the spring to shorten it just a bit with a pair of needle nose ( this is like riding a bike with a running chainsaw ) so guess-ta-mate the bend an move slow .