B&S engine idles good, Smokes black at high speed.

I have an old Craftsman 6HP vertical shaft chipper/shredder that is giving me trouble. The engine idles great, but at any higher speed, runs really rich, making black smoke, and even spitting gas out the carb if I take the air cleaner off. It does not rev properly in the process, except when it first starts or as it runs out of gas.
The diaphram in the carb looks excellent. It looks new. I might have replaced it a couple years ago when I acquired this unit used. The gas valve it operates seems to operate also. If I blow into the fuel inlet, no air passes until I push up on the needle that opens the fuel flow. I even tried installing a layer of plastic from a bag "outside" of the diaphram (between it and the cover) to eliminate the possibility of any leaks through the diaphram, with no effect.
Would a bad diaphram allow the idle to work great, while higher speeds are all super rich?
The idle needle adjustment works properly. The high speed needle adjustment seems to have no effect, other than if I remove it completely, at which point the engine will quickly speed up like it should work for a few seconds before it then dies.
Any ideas? Is there something I haven't thought of that could be repaired on this carb/engine?
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It seems fairly obvious that fuel is bypassing the high-speed jet. Maybe the needle has worn down. If gas was not bypassing the high- speed jet, with the jet fully screwed in, the engine would die at high speeds, not run rich and belch black smoke. I would look at the highspeed jet needle and its valve seat and see if either are worn.
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The float could be stuck open. But If so you would have gas in the oil. If you do, gas in oil can thin it and ruin the motor. but gas pouring out the carb is likely the float. Does choke stay open, I would not run it till you go over everything and change the oil. How is you oil level now, if its up its the float.
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ransley wrote:

That would be a Tecumseh engine, not a B&S.
-Bob
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wrote:

While you're at it, get a complete carb kit and rebuild the whole thing. No sense tearing it apart piece by piece, and possibly having leaky gaskets that were reused. If it has a float, that may be sticking. Rip it apart, thoroughly soak the casting in a solvent, and make sure all the passages are clean (use an air compressor). Then put back together with all new gaskets and seals.
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On May 14, 2:45pm, snipped-for-privacy@myplace.com wrote:

He said it had a diaphram so I'm thinking it doesn't have a float.
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Any idea what happened to the cap, did it get sucked into the engine??? Is it flimsy enough that the engine just ate it up? Thanks for the credit for ponting to the high-speed jet.
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One other possibility: clogged cooling fins. That's what was causing a friends mower to over heat and die when operatied at "normal" throttle settings. It took 4 trips to the repair shop to have their untrained drop-outs fix it. Eventually, the owner discovered the real problem.
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g'day bob,
my policy don't look a teh diaphram replace it, once you pull it out and it dries that can make it worse.
black smoke usually means running rich, look for badly seviced air cleaner element, replace if it is a paper cartridge type. replace diaphram, the tune should not alter without human imput to move the adjustment jet screw, also the spark plug should be in best condition.
for me i'd be hinking something is wrong with the air cleaner setup, it has become fouled and is acting like a choke causing the machine to run rich.
keep us informed
we still get unleaded fuel over here that is ideal for low compression garden equipment engies would not suggest using E10 or other grades of unleaded fuel in these engines, my mower engine 20 odd years old and going strong.
wrote: snipped
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