need help diagnosing

[1982cb750scnighthawk]

hey gang. nice to be here.
bought this bike a while back, was running fine. stored the bike for a year, and now it wont run properly.
I thought it was crappy gas so I replaced the petcock, drained and cleaned the tank, had the carbs sonic cleaned and inspected by a pro. reset idle screws at 1 3/4 turns. then for fun I changed the oil/filter, air filter, and replaced the plugs(gapped correct).

now the bike idles very rough at 1000rpm with the choke closed. when I open the choke the engine quits, even after bike warms up. If I touch the throttle the engine quits. If I start the bike (choke closed) with a little throttle, the bike will idle smoothly at 2000-2500rpm, as long as i hold the throttle. Either way if i open the choke, bike dies.

I`ve had the carbs out half a dozen times, to check them. floats ok(plastic), float valves ok, all jets clean and clear, no junk in float bowls, linkages working properly...nothing stands out as a problem. needles show some light wear, but bike guy said it was no problem.

I`m about to stick a fork in this thing...hope someone can help

[LukeM]

Don't give up...you're 90% there. :-)

I need to ask: open choke means actuating it, like from a cold start?  On the CB750s, to activate the choke, you pull on the choke knob. I would assume the same happens with your Nighthawk.

I'd double check the carbs.  Adding choke richens the fuel/air mix, making it easier to start. If it dies when you add choke, my guess is you're getting too much fuel or not enough air, depending on how the choke circuit works (some close off the air inlet to the carb, others use fuel enrichment). If it dies when you turn off the choke, then your carbs aren't getting enough fuel, and it stumbles and dies. Adding a little choke puts more fuel in the mix.

I'd pull the plugs and see how they look.  If it's very rich, they'll be black and shiny. If they're correct, they'll be light brown on the insulator.  I'm sure there are web pages that show spark plug color ranges.

Luke M

[TOOLS1]

It seems to me you have a lean condition as Luke said. I would check for a vacuum leak. Probably the rubber boots between the carbs, and the head.
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