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Discussion Starter · #1 ·
So my friend went out and bought a 1983 Honda Nighthawk, cb650. It ran well enough, but he thought it was misfiring so he decided to change the plugs. He put in new plugs, and of course, now the bike won't run - it seems to be running waaay rich and the plugs are getting drenched. With starter fluid, the bike will idle until the fluid is consumed, then die.

He told me he's tried leaning the carbs, but isn't having any luck, and I think his carbs are way out of whack now. By the way, this bike has FOUR carbs... one for each cylinder.

I told him I'd comeover to look at it, but I have no idea how to get four carbs close to tuned to even get the bike running rough. Does anyone here have any advice or links to working on this old bike?

Tia,
 

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Super Duper Mod Man
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1. Get a service manual.
2. Take the carbs off and get them CLEAN!
3. Get all settings back to factory spec.
4. Put them back on and synchronize.
5. If all that fails, sell it to the next guy and get something that runs right.
 

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The Toad
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You can't 'lean' those carbs without taking them apart. The adjustment screws only affect idle mixture. They should be turned carefully all the way in until they just kiss the seat. Then turn them out 1 1/2 turns. That should be the initial setting. He'll need a manual to do the full adjustment procedure.

But really, longride is right. The carbs should be pulled and cleaned and set to factory spec. A float or two are likely stuck open and flooding the engine.
 

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Aging Cafe` Racer
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Not to be rude here but niether you nor your friend seem to know what you're looking at. The best thing you can do at this point is take it to a Honda or independent mechanic and have them put it right for you.
 

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The Toad
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Yeah.

Not to be rude here but niether you nor your friend seem to know what you're looking at. The best thing you can do at this point is take it to a Honda or independent mechanic and have them put it right for you.
But it's more fun to encourage them to dig into it. Maybe they'll learn something. Maybe they'll end up with a reassembled bike with a handful of little bits left over. Maybe they'll just torch it.
 

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MODERATOR X
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Make sure you put gasoline in it, and not diesel. Also, having worked on CB650's, you might want to put your hands in a shrinking machine, so they're about the size of the hands on a spider monkey. Also, when putting the carb rack back on the manifold, boil the rubber manifolds in hot water for a few minutes, so you'll be abble to slide the rack back on without ripping one, or buggering them up. The 550/650 is put together real tight, with little room to work with. Good luck.
 

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If the bike was running well enough on the old plugs, I'd try a couple of things. The first is obvious, and that's to throw the old plugs back in, that really shouldn't make any difference at all, but I'm curious. My next thought is that something got "adjusted" in pulling the plugs out. Any chance he accidently got the idle screw adjustment cranked way the heck up by accident, or maybe the choke lever? Don't know why, but something else had to change. I'd take a flash light and take a close look at what they might have changed in getting at the plugs? I recall having a similar problem on an inline 4years ago and finding that somehow I'd created a leak between the carbs and the airbox (obviously it was lean not rich) at the boots when pulling the tank off.
 

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Discussion Starter · #11 ·
Not to be rude here but niether you nor your friend seem to know what you're looking at. The best thing you can do at this point is take it to a Honda or independent mechanic and have them put it right for you.
Not being rude - I've only adjusted carbs on a running bike before, and my friend has never worked on carbs, but this is more of an opportunity to learn something new, so if taking the carbs off is the way to go, we can do that.
 

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The Toad
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Hokay

Not being rude - I've only adjusted carbs on a running bike before, and my friend has never worked on carbs, but this is more of an opportunity to learn something new, so if taking the carbs off is the way to go, we can do that.
If you are serious about this then first follow bbtown's advice and look for something that got pulled loose or out of place when he changed the plugs. Maybe a cable got pulled out of its seat or a wire is loose. If everything looks in place and it still won't run then suspect the carbs. A manual will give you the adjustment and overhaul procedures. They usually have troubleshooting tables as well. Good luck.
 

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Aging Cafe` Racer
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You did put the plug boots back on the right plugs didn't you? gapped correctly, proper heat range and so on? since your problems began after changing the plugs, it's logical to assume the problem lies there. You may well be chasing a rabbit down a hole with the carbs, if they worked before the plug change they should work afterward. As previousley noted, check every connection, hose and wire for something that inadvertantly got disturbed.
 

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You did put the plug boots back on the right plugs didn't you? gapped correctly, proper heat range and so on? since your problems began after changing the plugs, it's logical to assume the problem lies there. You may well be chasing a rabbit down a hole with the carbs, if they worked before the plug change they should work afterward. As previousley noted, check every connection, hose and wire for something that inadvertantly got disturbed.
Or, all the rust, sediment, dead bodies, etc. in the bottom of the tank got stirred-up and is now sticking in the float-valves, causing them to hang-open.
 

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Airhawk and Sarnali have good points, the only way the carb would be involved from a plug change, is inadvertantly. Sediment could have been stirred up in the tank when removing and reinstalling, which then made their way block something, but that probably wouldn't make things rich (unless it did make it to the float mechanism as Airhawk said). A throttle or choke cable on the carb being disturbed is more likely IMO, but I think I went there previously. Check the cables, check the carb boots (especially on the airbox side), check the plugs for gapping, check for unplugged wires (maybe a vacuum tube got knocked off?), look for signs of problems in the airbox, is there a mixture of gas an oil sitting in the bottom? These kind of problems are usually fairly easy to find if you duplicate the process you went through to begin with. Did your friend notice that when he pulled the tank it seemed stuck? Some times a tab will hook a cable, especially when filled with gas and harder to lift.
 
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