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Basically I'm getting too much fuel in the combustion chamber....allowing that fuel to bypass the cylinder to the crank case. Thought the problem was a Carb issue with a stuck open needle or blocked needle...but i'm actually replaced the carb twice on this mower......last time being this week.....which didn't fix the problem. The mower works fine when I manually turn on or off the fuel switch to meter fuel to the carb.......aka I run the mower then shut the valve off a couple of minutes before I want to empty the bag so the carb drains (aka starve the engine for fuel). The problem arises if I leave the Fuel shutoff valve on......after I shut down to change out the bag for maybe a minute or less....I go to restart and I get some flooding (and normally a backfire)....most of the time it starts up no problem with a little black smoke (too rich a mixture..aka fuel is still leaking into the cylinder). The longer I wait the more it floods and have to shut off the fuel valve (petcock) and pull several times before it slowly claws its way back to life. After it starts up I just just modulate the fuel valve on or off.

Im tired of trying to trouble shoot this when I thought a simple carb swap would solve the problem (that being a needle not seating to shut the fuel flow to the carb)...but that is not the case. I dont feel like spending more $$$ on carbs...obviously...if it doesn't fix the problem....and I'm tired of changing gassy oil.

Because i'm new here I can't link to the other forum I was troubleshooting on...... but its over on Jack's Small Engines forum.

THE only way for the fuel to get into the crankcase, is through needle in the carburetor. If it's not seating right, the any gravity flow system, is going to do as you described. Maybe you got new carbs that had the same problem.

If you've ever worked in Quality control, you'll know how easy it is for several bad parts to pass through inspection. (because all parts aren't inspected. If you bought two within a short time period, and they both had needles that didn't seat properly, then chances are, you got two that didn't get inspected before they were shipped out.

BTW, is your fuel cap clogged up? If it is, it could build up pressure in the fuel tank and carb.

PTmowerMech said:

THE only way for the fuel to get into the crankcase, is through needle in the carburetor. If it's not seating right, the any gravity flow system, is going to do as you described. Maybe you got new carbs that had the same problem.

If you've ever worked in Quality control, you'll know how easy it is for several bad parts to pass through inspection. (because all parts aren't inspected. If you bought two within a short time period, and they both had needles that didn't seat properly, then chances are, you got two that didn't get inspected before they were shipped out.

BTW, is your fuel cap clogged up? If it is, it could build up pressure in the fuel tank and carb.

Click to expand...

Actually most times fuel in the crankcase comes from a leaking float valve not cutting off the fuel.
The float overflows out of the air vent hole.
On older mowers this was outside the carb , but pollution laws forced a change of position to inside the carb.
On Hondas it is usually quite high but on Briggs it is very low.
In any case, unless the vent is higher than the fuel tank the fuel flows out the air vent hole dribbles down the carb and fill up the engine

You need to thoroughly clean the tank. Replace the lines. Clean the petcock. Add a inline gas filter. You are getting trash into the carburetor. The only way gas is getting into the engine is from the carburetor. The float is sticking.
I would also put in fresh gas. I would take a look at the gas in a glass to see what is in the system.
BTW gas lines do break down adding particles to the system.
Do this and re clean the float needle and this will cure your problem.