# String Trimmer Flooding



## waynemo (Jul 24, 2016)

I have repaired many string trimmer carbs and every now and then, I get one that will just flood the engine and wet the plug without starting. I check for spark and compression and the problem seems to be in flooding and wetting the plug. 
 I don't understand how gas gets into the combustion chamber so much. I assume it has to go thru the tiny hole into the combustion chamber and does not seem likely so much could get through there so easily. I adjust the idle and high speed screws a turn out and have tried adjusting them further to no avail. 
When you squeeze the primer bulb I wonder how much of the gas is pushed past the needle settings. I thought that the primer just puts gas from the tank to the carb entrance and did not appreciably shove it into the combustion chamber past the needles. 
I would like one of you guru's to educate me as to what is the likely cause of gas flooding the combustion chamber! I think I have a pretty good idea of the other carb issues.
I thought the gas is held in the carb until the diagram opens the needle valve after impulse from the engine compression. Only then does it flow past the carb.


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## finaprint (Jan 29, 2006)

Most of those carbs used on small 2 strokes are the 'pumper' type, they have a set of mylar or rubber diaphragms that actually pump the fuel into the carb like a small fuel pump. Time and ethanol if used in your local fuel make those little flapper segments of the gasket get harder and then they do not close nearly so accurately, it often throws the fuel metering way off the map. The small holes in those carbs are way big enough to solidly flood the engine, typically they may not need but maybe 1/4 the amount of fuel that could come through them. You cannot even picture the amount of fuel one of those at idle needs it would be so small. There is a spring loaded valve in there that is a demand valve, it's setting is ultra critical and off even the slightest can make therm flood too. That valve sets up the fuel amount in toto that the engine uses up and down the operating range. The valve in effect is the same as fuel level setting on a float type carb, think what float level set way too high does...............it floods. The valve is supposed to feed just enough fuel for proper running under load but no more, if it feeds too much then the amount fed then overwhelms the mixture screws, which are in effect the jets on bigger carbs. Again, rich and floods. They give settings for those valves but sometimes you have to tweak a bit off spec to get what you want. Normally if never touched you do the same, DON'T touch the setting, it can be h-ll getting it back. As little as .005" in difference can screw them up.

Ethanol allowed to set up in a carb over even one winter can harden those flappers, I tend to empty my tank and then start engine up to run the carb dry to keep it empty, it then starts much better next year.


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