Topic: Runs fine, for 15 seconds.

[Don R]

   Besides a plugged exhaust or gas cap what would cause a bike to run great on startup then after 10/15 seconds it will start missing?
   Will a valve warm up and then begin to stick? Other than that, I'm thinking coil or condenser. When we got it there was a worn rubbing block on the 2-3 points. I swapped them for a good used set of tec.

[BenelliSEI]

I’d check for plugged idle jets. If you start up on choke, the engine will pull fuel through the mains. Then run rough at idle. Fried condenser as #2.

[Don R]

 Leakdown is about 4% all cylinders, all plugs spark when in the air, but the #3 plug looks like new. You may have nailed it. Once upon a time, I swapped two racks of carbs on and off of this bike until it ran good.  Now what will happen is, my brother will ride it home and it will sit until it's time for me to fix it again. His "bought new" K4 is sitting here with the carbs gummed up too. I may as well do them all.

[BenelliSEI]

Leakdown is about 4% all cylinders, all plugs spark when in the air, but the #3 plug looks like new. You may have nailed it. Once upon a time, I swapped two racks of carbs on and off of this bike until it ran good.  Now what will happen is, my brother will ride it home and it will sit until it's time for me to fix it again. His "bought new" K4 is sitting here with the carbs gummed up too. I may as well do them all.

Don…. I’ve tried everything. The only solution is fewer bikes and ride ‘em!

[scottly]

Does it run fine for 10/15 seconds, then start missing only when the choke is removed, or does it run fine for 10/15 seconds even when started without the choke? Can you wait five minutes or so, and then it repeats the behavior of fine for 10/15 seconds? If so, suspect a restricted fuel supply, like a semi-plugged in-tank filter, petcock, or maybe a kinked fuel line. Is the problem limited to a single cylinder? If so, you might try removing the bowl drain plug on that cylinder and check for flow. A plugged tank vent takes much longer than that, depending on how full the tank is...

[Don R]

   John is correct. I'm overwhelmed with carburetors even with avgas in them.
   At first, I had a cold #2 pipe, but the intake had no lash, now it's adjusted and the cold leakdown test looked a tiny bit better than the other cylinders, it's heating up, but the plug was wet.
 The #3 pipe is hot, but the plug looks new while the 1 and 4 plugs have gotten a little carboned up.
   Good looking spark on all 4 at least in the air, new plug caps, trimmed wires, relatively new Denso plugs. I'll go back into the carbs, one more time.
  While I was working on his bike, my brother was sitting in a pub. That dampened my enthusiasm a little.