HP Measuring Tools & Sensors - Recent Questions, Troubleshooting & Support

The fuel mixture screw only regulated the air and fuel at tick over. normal running is governed by the main jet needle and slide. Check spark plugs, clean carburetor and check timing. Also check for manifold leaks An air fuel mixture screw is a special screw on an engine's carburetor that controls how much air mixes with the fuel. Adjusting this screw changes how fast or slow an engine idles and how smoothly it runs.

How to Adjust an Air Fuel Mixture Screw: 7 Steps (with Pictures)

Fixes and videos https://www.google.com/search?q=briggs+and+stratton+3.5+hp+running+rough ..

TIMING IS OFF

Small Engines - » Valve Specifications-Briggs and Stratton (small-engines.com)

There are over one hundred models that start with 40h777. Try checking this site: http://www.smallenginesuppliers.com/html/engine-specs/briggs-engine-specs.html

https://www.youtube.com/watch?v=EIETGOSQYWc

https://m.youtube.com/watch?v=_7v1B8ebCA8

Your symptoms seem to be describing a fuel restriction. This engine has both an in-line fuel filter and fuel pump. In a clean safe area you will want to see if there is a restriction inside the fuel tank. Pull the fuel line off after the fuel filter. Does it run free? *Use some type of plastic container to temporarily capture the fuel. Do Not use styrofoam. If fuel freely flows the pump will be suspect. If not, remove the filter. If fuel flow does not occur then the issue resides inside the fuel tank. Please let me know if there is anything else and what you discovered. Good luck.

Check oil level. If it's high and smells of fuel then likely the carb float is stuck or debris in inlet valve.

If oil level ok, then possibly a main shaft seal is out.

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Does it run properly? may need valve adjust https://www.answers.com/Q/How_do_you_adjust_valves_21_hp_Briggs_stratton_intek_v_twin

Does it run properly? may need valve adjust https://www.answers.com/Q/How_do_you_adjust_valves_21_hp_Briggs_stratton_intek_v_twin Otherwise, timing.

You can google your question for answer. Or you could read instructions that came with engine. After you refill crankcase you may have oil left over, just set aside in garage, Big hint https://www.briggsandstratton.com/na/en_us/support/faqs/browse/mower-oil-type-and-capacity.html

Just google it. Why are asking?

Change the spark plug for a new clean one. Make sure you are using the correct gas/oil mixture.

Read this forum https://www.doityourself.com/forum/outdoor-gasoline-electric-powered-equipment-small-engines/457808-ok-increase-rpms-briggs-stratton-mower.html

indicates that the needle is passing fuel and not shutting of properly filling the sump with oil that is passing the rings and leaking into the exhaust port

the cylinder with 115 probably has a burnt valve or incorrect valve clearance the blow by will be normal as the sump oil is being sprayed around and when you open up the sump it will be thrown out both cylinders should be 130 -160 psi

check the linkages may need to adjust the governor or spring --also check idle not to high -and air fuel ratio is correct

Oil Recommendation

• SAE 30 40° F and higher (5° C and higher) is good for all purpose use above 40° F, use below 40° F will cause hard starting.
• 10W-30 0 to 100° F (-18 to 38° C) is better for varying temperature conditions. This grade of oil improves cold weather starting, but may increase oil consumption at 80° F(27° C) or higher.
• Synthetic 5W-30 -20 to 120° F (-30 to 40° C) provides the best protection at all temperatures as well as improved starting with less oil consumption.
• 5W-30 40° F and below (5° C and below) is recommended for winter use, and works best in cold conditions.

Follow the link http://www4.briggsandstratton.com/miscpdfs/RNT/Engine%20Specifications%20Chart_ms3992.pdf