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What causes what appears to be vapor lock (i.e. runs for 2 minutes and then dies only to restart after cooldown) on a Briggs & Stratton lawn mower engine?

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Tracking down fuel problems.

  1. Make sure there is plenty of fresh fuel in tank. High Octane less than one month old. Water in the fuel system will often cause hard starting, low power, backfiring, surging or stalling
  2. Is there a cut off valve at the gas tank? If so check that it is open or working if it is electric.
  3. This step comes from repeated personal experience with JD gas tanks. Remove fuel line from filter and remove the filter, gas should pour out of the hose coming from the tank. (look at the filter before removing it, it should look at least 1/2 full of fuel.) If gas does not pour out remove gas tank cap. If fuel flows now, you need to unclog the cap vent or replace the cap. The vent is that tinnnny little hole in the center of the cap. If the fuel still does not flow then blow air from the filter end of the hose back up through fuel line to blow out any blockage inside the gas tank.

    Do your self a favor, leave the gas cap off and turn the outlet air on your compressor down below 20 psi. If the tank is nearly full you will blow gas out of the tank. Do this outside if possible; at the very least you need a well ventilated area. After blowing air from the air filter back to the tank, you may have to reverse the process and pressurize the tank to get the fuel started. I place my hand over the open tank and hold the air hose between the joint of my thumb and forefinger to seal the tank, and then squeeze the trigger. Once you have a good fuel flow out the line to the filter, put on a FRESH FILTER. Try cranking the engine. If it still does not start move on to the fuel pump and carburetor
  4. Check the fuel pump. The fuel pump system on the twin Koehler and the Twin Briggs (and others) are almost identical. Watch this excellent, eight minute, video to learn how to check the fuel system, including the pump, up to the carburetor. . Small Engine Repair Checking Vacuum Fuel Pump Fuel System on Kohler Twin...
  5. Note: My engine was starting hard and dying in the field this year (March 2015). While doing a valve adjustment, I found a hairline crack in the bend of the vacuum hose powering the fuel pump. It's the short hose running from the valve cover to the fuel pump. The fuel pump will not work if this hose is cracked.
  6. Many systems have a fuel shut off solenoid. This is a small cylinder on the bottom of the carburetor. It has two wires running to it. It should make an audible click when you turn the key on. If it does not click you will have to replace it or track down an electrical problem.
  7. If this has not solved the problem you may have to remove and disassemble, clean, or rebuild the carburetor at this point. You may have water, dried fuel (varnish) or trash in the carburetor bowl. Before removing and disassembling the carburetor try draining the bowl. Or, take the bowl off, careful not to drop the small parts inside. Clean the bowl and replace. I often remove the bowl on small engines and save the hassle of a complete carb removal.

NOTE: Connect a volt meter across the two battery posts. If battery voltage drops below 9 volts when cranking or while the engine is running, the fuel cut off solenoid will not function. That means the engine will not start or will die soon after starting. So check your battery voltage before, during, and after cranking. If the battery voltage is good before but drops below 9 volts while cranking this is generally a sign the battery has one or more bad cells. Replace the battery. If the battery is in fact good, and the voltage is still dropping, you may have a bad starter motor which is drawing too much amperage.

If you find this fuel checkup does not solve the problem, repost your question with updated info and I will walk you through the next step. Tracking down a spark problem; yes, your problem could involve the coil or any part of the ignition/spark system. (Not as likely as fuel)
Small Engine Repair Checking Vacuum Fuel Pump Fuel System on Kohler Twin...

Posted on Apr 11, 2015

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When that happens loosen the fuel cap and see if it stays running. If this is a gravity feed system make sure you have the correct fuel filter on it,the clear paper filters can allow vapor lock to happen,put the small briggs and stratton filter in it,which has a small chamber and a stainless screen.Also make sure there is no excess fuel line that would cause a dip or a high spot for a vapor lock to occur.

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It could be vapor lock. Vapor lock is a condition where the hot gasoline turns into vapor bubbles either in the fuel line or carb. When it happens, the vapors will cut off fuel flow thru the carb, essentially causing the engine to run out of fuel. You should re-route or isolate the fuel line away from the heat of the engine and install an inline fuel filter close to the carb. Also make sure that the cooling fins are not clogged up with trash or something that is causing the engine to overheat. I would also check the gas cap. Make sure the vent is clear and clean around the gasket in the cap. This can sometimes cause similar problems. Hope this helps and good luck.

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