Most of the time the problem is that the carburetor is partially restriced with dirt, gum, or varnish and you are not getting enough gas for it to run with power under load.
Always check the spark plugs and the air filter first, but most of the time it is the carburetor.
Make sure that you are using fresh 89 octane gas. DO NOT use regular 87 octane gas as this gas has changed into garbage gas in the last few years.
Here is my comprehensive carburetor and fuel answer that may give you some ideas.
As engines sit or get older, fuel that is left in the carburetor can turn to gum and varnish and cause this and other problems. Also, any gasoline that was left in a gas can for a period of more than 30 days must be discarded because it also has begun to turn to varnish.
Today's gasoline's contain MTBE and alcohol. (Ethanol) They turn to "Junk and garbage" very quickly. Alcohol is partially water (H2O). And they call it "Oxygenated fuels! It is the oxygen that breaks down the organic compounds in the fuel and turns the gas to "Garbage" (Gum and varnish) The fuels we had just a few years ago had no alcohol in it and would store for longer periods of time before going stale... and fuel stabilizers do almost nothing to prevent the fuel from going bad with the changes in today's fuels.
Do not buy gas from the "Discount" Stations. The discount stations get a reduced price on gas because they may be buying fuel that is nearly 30 days old already. You may be getting fuel that's nearly stale right from the pump when buying from a discount station. Purchase your fuel from the well-known stations such as Shell, BP, Sonoco, Phillips 66 etc.
More than 70% of all of our repairs in our lawn mower business are due to this same issue. You most likely have dirt, gum, varnish...etc in your carburetor plugging up the small passageways and jets in the carburetor.
The carburetor must be removed from the engine. Clean all parts with carburetor cleaner and blow out all the small holes and passageways with compressed air. Remove all of the non-metallic parts since the carburetor cleaner will cause them to be disfigured decompose and plug the carburetor as time goes on.
Wash the carburetor cleaner off of the metal parts by washing them in warm, soapy water then rinsing with clean water. Dry them by blowing it off with compressed air. Make sure that all the passageways are blown dry before reassembling.
Reassemble using a NEW carburetor rebuild kit.
DO NOT TRY TO REASSEMBLE WITHOUT USING A COMPLTE CARBURETOR KIT! You will just end up having to do the job again.
ALWAYS clean the fuel tank and replace the fuel line when doing this repair or you may have to do it all over again. The inside of the fuel line disintegrates over time and these small pieces of rubber will plug up the carburetor too. Dirt and water from a dirty fuel tank will also plug up the carburetor. Find the Model, type and serial or code numbers off of the engine and take them to your local dealer to get the carburetor repair kit.
If you do the work yourself, take pictures or at least make a drawing of where all the linkages, gaskets, and component parts go. Correct reassembly is critical.
If the carburetor still doesn't work correctly, you may have to take it to someone who has an "Ultra-sonic" cleaning machine. This machine uses carb cleaner and ultrasonic vibrations to get the very small passageways clean when traditional methods fail.
Here is where you can get an inexpensive "Ultrasonic Cleaning Machine"
If you don't feel comfortable with this kind of repair I would suggest sending it to a reputable shop.
Here is a Carburetor rebuild process.
First make sure that you have the correct complete carburetor repair kit.
Now take special care when you take the carburetor apart..to make sure that you watch and remember exactly how the gaskets come off.
Remove the float pin ...it just pushes out...and remove the float..carefully.. so you can see how the float needle is attached...(Some of them have a small wire spring holding it to the float....you will have to reinstall this spring when you reassemble the carburetor.
Remove the float needle
Next...remove the float valve seat. . Look around the carburetor and see if it has a small idle adjustment needle...remove this needle if it has one. You are now done disassembling the carb. DO not take anything else out of the carb.
Get a good carburetor cleaner....preferably with a small hose you can attach to the nozzle of the can. Spray the carb cleaner all over the outside of the carburetor. this will loosen the dirt on the outside while you clean the inside. Now spray the cleaner through every hole, and nook and cranny you can fine on the inside of the carb.
When you are finished with the spray,blow all of the passageway clean with compressed air from an air compressor. Also blow all of the dirt from the outside of the carburetor.
Next. Take the entire carburetor to the kitchen sink......Ok maybe it would be better to get a big bowl...so you don't make your wife mad....and wash the entire carburetor with HOT SOAPY dishwater. This is to make sure that all of the carburetor cleaner is completely washed from the carb...(If it is not, you will deform the new gaskets and such that you install...even if it is dry.) Now rinse in clear watter and then blow the carb dry with the compressed air again.
Reassemble the carb using the identical parts out of the carb kit as you took out. install the float seat, then install the float needle and the float in the same position that you took them out. Check the float level after it is installed. Making sure that when the carb is upside down with the float in the up position, the float should be nearly level with the body of the carburetor. Now reinstall the bowl gaskets the same as you took them off. Then install the bowl nut with the small gasket under it. Reinstall the small idle needle if it had one.
There you are done.
Install the carburetor on the engine and you should be ready to go.
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Also check the fuel filters in the fuel line and the gas tank.
there is also a chance that the valves need to be adjusted. But try the carburetor first.