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Old 02-06-2010, 02:22 PM   #1
jhk94
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Default Help with '93 CB250

Hi everyone,

I am very new to motorcycling, and am just starting with a 1993 Nighthawk 250. I had it in the shop for something unrelated, and they told me it needed to have a valve adjustment. It was running good, but I figured I'd check out the clearances.

So I do the valve adjustment last night, seemed to go well. However, now when I start it up, it runs ok, but when I open the throttle, I weird throaty noise comes from the air intake area (or at least it seems that way), and if I roll the throttle when its coldish, it makes this noise louder and dies. Once its hot, it will run normally, but still make this noise when I roll the throttle. It is very weak in first gear unless I give it a ton of gas, and then runs okay until I hit ~45mph, when it then really struggles and slows.

What could be wrong? I thought everything went okay with the adjustment, but I can't figure out what could be going on!
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Old 02-06-2010, 02:26 PM   #2
longride
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You probably have a valve or two that is too tight and may not be making a good seal. Check the clearances again when the bike is COLD, and see if they are a bit on the tight side.
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Old 02-06-2010, 03:11 PM   #3
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Hmm, interesting. When i measured the valve clearances, they were pretty tight already, and the bike was running okay. So if anything, the valves are looser than before. Could too loose valves be the problem?
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Old 02-06-2010, 09:24 PM   #4
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Quote:
Originally Posted by jhk94 View Post
Could too loose valves be the problem?

No, it can't be.
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Old 02-07-2010, 01:09 AM   #5
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Looser is better, always (within the tolerances). Unless the adjuster loosened up, I've never seen valves get looser either - the seat always wears faster than the stem or followers.
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Old 02-07-2010, 11:28 AM   #6
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Ok, so I readjusted the valves. One was a hair tight, so I loosened it up and checked the others, they are all on the loose side of tolerances. Start it up...still same problem. It starts fine, idles okay, but when I roll the throttle, it makes a weird throaty/airy noise and dies.
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Old 02-07-2010, 12:01 PM   #7
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Did you leave a rag in the airbox? Seriously, something is wrong with the intake: bad seal on the airbox; loose manifold, something like that. Re-check whatever you removed to get at the valves. You'll find it.
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Old 02-07-2010, 12:51 PM   #8
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I checked that out...I never really did anything with the airbox when I was doing the adjustment. Yikes, this is terrible. Here, I had a perfectly okay running bike, maybe a bit out of adjustment, and now...
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Old 02-07-2010, 01:26 PM   #9
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I'm leaning toward what Ken sez: sounds like something is leaning your mixture out. I had a problem with my PC800 like this once after working on it: I'd left off a couple of screws on one part of the airbox. (D'OH!)

Not every engine is this sensitive, but some are really bad about it.
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Old 02-07-2010, 01:38 PM   #10
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The airbox makes sence to me, I diddnt comment earlier cause they are alot more into this then myself, but the way you described the sound it reminded me of running a lawnmower with the airbox completely off (and spraying ether to keep it running). Almost like its sucking air too fast ie my filter was off and you may be getting too much air in there yourself.
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