What am I missing? Cub runs for a few seconds then dies

[fiddler]

So out of the clear blue the cub started dying after a few seconds of operation. It acts like it is starving for fuel. I took off the gas line at the carb and the fuel flows freely, so that appears not to be the problem. There is no sediment in the bowl so my fuel and tank appear clean. I took off the after market carburetor that I got from yesterdays tractor and checked the screen and the float. There is nothing plugging the input screen. I removed the bottom plug from the carb while it was on the tractor. When I blew in the input line while it was full of fuel I was blocked like the needle valve was closed. then when I drained the fuel from the bottom plug on the carb I was able to blow into the input line freely. so that appears to be working properly. I am going back out to check the ignition system. (Magneto). I can't imagine what would cause this from the ignition side. That stuff is usually go no go.... But.... any ideas?? It acts like it starves for fuel, then after a few seconds off it gets enough fuel in the carb to run for awhile. I am just baffled.....

[Mht]

It sounds like you have some trash in the main jet on the carb (the small brass bolt head on the side of the carb). If it has a little trash in it as soon as it starts the trash will block the jet and it will cut off. Once it cuts off the trash will no longer be pulled tight to the jet and fuel will flow through it again and the process will repeat itself. Take the jet out and let some fuel run out and then take a small wire and clean the small hole in the jet to make sure nothing is blocking it. It only takes a very small piece of trash to block the jet

[Glen]

Hi,
The main jet is the 6 sided brass colored part low on the side of the carburetor, that looks like a bolt head. It screws out.

Below are pics of the main jet, and cleaning it with a wire.
Don't use a wire bigger than the hole, or force a wire through it, the hole has to be a certain size.

The gasket on the jet can leak if it is old, and you reuse it, but you said the carburetor is new, so it probably will seal. I have used a little sealer on both sides of the gasket if it leaks.
Tighten it gently, it can break if overtightened.

[fiddler]

So I took the carb apart and blew out all of the orifices with compressed air. Never saw any problem, but upon reassembly it runs great. It probably was something in the main jet... Too darn small for my old eyes. (I remember when I hit my late 40s and I was doing some micro-electronics engineering with a teeny tiny mixer that I had to assemble to a small microcircuit. All of a sudden it stopped working and I was on my way down to put it under a microscope when one of my young co-workers said that the gold bonding wire on the local oscillator side was broken,,,, I couldn't even see the microcircuit and he could see the bonds, (like human hair thickness). I started feeling older then.... Thanks guys!

[MiCarl]

Not at all unusual to see nothing. I used to clean motorcycle carburetors for a living and frequently never saw the culprit. Just the other day my wife's Mantis tiller quit. Cleaned the carburetor that looked clean as new and now it runs like a top.

The ethanol that is in most gasoline grabs onto moisture. When it gets saturated it will precipitate out into little almost clear blobs. Impossible to see them if they're lodged in a jet.

[fiddler]

OK I am gonna install fuel injection next... (just a joke).

[Nelson 634]

Check for an intake leak some where.

[TOOLMAKER]

Try a new condenser . I'v had them go bad and that sounds like what u are describing

[fiddler]

Know what? Just because I have seen that problem a few times here on the forum...I am going to order a condenser (Why do they call capacitors condensers anyway??) just to have in case. Thanks for the inputs guys!!