Briggs carburetor

bus_driverNovember 21, 2011

At one time, I understood the Briggs carburetors rather well. But have never been inside the ones with no external speed control nor external choke control. Mine is a 4.5 HP vertical shaft that uses the brake to kill the engine and has a red priming bulb in the side of the carburetor. Mine "searches" with varying speed of the engine, a classic symptom of lean fuel mixture. No visible means of mixture control. My thought would be to clean the jet. The cleaners poured in the tank have not helped. So where is the jet and what is the easiest way to access it? Model 10H902, Type 0285, E1 0421855. It is a flat head engine.

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It's the short cylindrical part where the pick up tube meets the top cover. (under side)

    Bookmark   November 21, 2011 at 5:21PM
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Almost certainly a bad diaphragm. See these pictures.

3.75 Briggs carb rebuild

This is an easy DIY job to replace the diaphragm of the carburetor. A damaged diaphragm will cause engine surging, hard starting, and erratic high speed operation. You may also notice that the primer bulb isn�t working properly.

This is an overview of the carburetor and these types of carb-over-tank are typical of small Briggs engines. Note the two governor springs. The carb/tank assembly is held in place by a �" bolt to the left of the carb and a 3/8" bolt on the engine head to the right. Remove both bolts and pull the tank straight out with a rocking motion. If possible, the carb/tank assembly should be blown off with compressed air and all loose dirt removed. Use the proper safety equipment, especially safety glasses, before using any compressed air in any of the steps below.

In the highlighted area you will see the governor linkage still in the throttle plate of the carburetor. You will have to gently separate the carb/tank assembly from the linkage. It is a Z type linkage. It is not necessary to remove the springs or the other end of the linkage, from the governor.

Note that on the rear of the carburetor is found a white plastic ring and O ring. Sometimes these remain on the tube of the engine housing. If so, place them back into the carburetor body, as shown, before reinstalling the carb/tank to the engine.

Note the raised area of the diaphragm. This is caused by age, but more often, by ethanol fuels, which damage the diaphragms. A damaged diaphragm will cause engine surging, hard starting, and erratic high speed operation. You may also notice that the primer bulb isn�t working properly.

Shown here are the dip tube, which brings up gas from the tank, in the bottom left corner. Just behind that and partially obscured, is the orifice that compressed air can be used to clear the dip tube of debris. It also leads to the primer bulb, so any air must be low pressure and the primer bulb must be held in the depressed position to prevent it from blowing out. In the center top, you�ll notice the jet, surrounded by a wire mesh filter. In the bottom right is a spring. Be careful to not lose the spring or forget to install it during reassembly.

Remove the old gasket and diaphragm combo from the carb body. Install a new gasket/diaphragm on the smooth surface of the tank. I always blow out the tank with compressed air to remove dirt and any water. Be aware that there may also be old gas in the tank. I always empty the tank and let it sit in the sun until all the gasoline evaporates before applying compressed air. Align the carb and lower it down on top of the gasket, trying to keep the five screw holes in the gaskets aligned. Insert the screws and partially tighten them and install all five screws. Tighten each screw until it gently bottoms out. Then turn each screw another quarter turn. It is not necessary to use more than a few inch pounds of torque on these screws. Reinstall in reverse order. When installing, be sure to engage the breather tube with the rubber piece on the carburetor.

    Bookmark   November 21, 2011 at 7:32PM
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I am very grateful for the concern and effort to help me. Hard starting happened during the second year of ownership, as did frequent stalling. I thought it might be water in the tank that picked up at certain angles of tilt. A new spark plug and dumping the fuel tank improved performance. I guess that it is now about 6 years old with very light and limited use. Less than 3 gallons used in that time. And fresh gas purchased twice a year. It starts easily now and does not stall, the priming bulb works well. The searching/surging is the only problem now. The spark plug is the only service ever other than oil changes.

    Bookmark   November 21, 2011 at 8:39PM
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The surging is usually cured by changing the diaphragm, which is almost certainly the cause. The ethanol ruins them in short order.

    Bookmark   November 22, 2011 at 5:31AM
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Those are very very good pictures! There are also videos on youtube that show the replacement of the diaphragm. It's an easy job. I agree that it's most likely the appropriate fix.

One thing you could try though would be to slightly increase the tension on the governor spring. Look at the first picture and see the bigger of the two springs. Try bending the tab that the spring is attached to at the right of the picture. Just a little bit to increase the tension. If you do this while the engine is running you can listen and see if the surging goes away.

Good Luck, Chuck

BTW...if there's LOWES near should be able to buy the diaphragm kit there for about $3.00...cheaper than online; especially when you figure in shipping.

Their Item #219691 Part #5083D

    Bookmark   November 22, 2011 at 11:35AM
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