'82 accelerating poorly. Losing one cylinder?

[TheRadBaron]

I just finished up with all the repairs on my '82 and got it out of the shop.  It starts easily whether hot or cold, idles smoothly and consistently, and runs great going down the road as long as you go really easy on the throttle.  However, if you give it even moderate throttle and put the engine under load it falls on its face.  If you hold the throttle in the same position once it starts running bad, the bike just goes "bwaaaaaaaap" and continues to have no power.  However, if you slightly roll off the throttle the bike runs much better and will accelerate.
I've ridden a lot of twin cylinder bikes and it really seems to me like it's losing one cylinder abruptly when put under load.  I've checked everything that I can think of.  I've changed plugs, synched the carbs, checked resistance on the spark plug boots, looked at the accelerator pump nozzles to make sure that they're working, made sure there are no vacuum leaks, etc.
The only thing that I found out of the ordinary is that the front spark plug is sooty and looks rich, while the rear plug is nice and clean.  Does anyone have any ideas?

[fret not]

Coil wires?

[Lucky]

Flapper stuck closed?


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[TheRadBaron]

I don't think that the flapper is stuck.  When I have the air box off it works when I put suction on the tube.
I think that I will check the coil wires.  My XS650 exhibited the same symptoms when it had a partially fouled spark plug.  I would spark when the engine was under light load, but stop sparking when the load increased. 
Another thing that occurred to me is that I never took the emulsion tube out of one of the carbs when I was cleaning them.  The tube was stuck in the carb body and there wasn't anything to grab onto to get it out.  Is there a clever trick for this?  I'm thinking about drilling a small, shallow hole in the top of the tube and trying to use a small EZ-out to break it free.  However, I don't know how thick the brass is on the top of the tube.

[fiatracer]

can you power through the flat spot? sounds like a bad case of Vision stumble! notorious in the 82s

[VisionMeister]

get after that emulsion tube.
I had been chasing a similar problem for my V for six weeks. It had run great for the last six years and all of a sudden went off similar to your description.
I kept finding small problems and putting it back together, thinking (hoping) this would be the solution, only to still have it fail to pull through it's powerband properly.
It was running rich on the front cylinder
some things I discovered:
small cracks in the brass downtubes from the upper carb body; replaced the upper body with a good one. I could only see these with a magnifier.
small nick in the seal on the enriching plunger face.
I had cleaned the carburetors without pulling the emulsion tubes. When I did pull them I broke the front bowl gasket. I tired to run it with the broken gasket, It wasn't leaking gas, but, it was leaking atmosheric pressure and ran terrible.
I replaced the gasket and every thing cleared up and the V is running the best it ever has.

[TheRadBaron]

Thanks for the tips.  I'll get after that emulsion tube next. 
It won't power through the flat spot.  It's such an abrupt and complete loss of power that I doubt it's Vision stumble related, though I'm not a Vision expert....yet  ;)
Hopefully that emulsion tube is the problem and I can get it sorted out soon.  It's such a nice bike to ride other than the obvious problem.

[Lucky]

Two tricks for the tube:
1) use a worn cutting disk to cut a slot in the top, use a screw driver to spin it & free up the crud.
2) drill a hole in the top, fill with PBlaster. insert a screw & yank it out like a bad tooth in an old whore...


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