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jhk94 · 02-06-2010, 01:22 PM

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!

longride · 02-06-2010, 01:26 PM

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.

jhk94 · 02-06-2010, 02:11 PM

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?

longride · 02-06-2010, 08:24 PM

Quote:

Originally Posted by jhk94

Could too loose valves be the problem?

No, it can't be.

The_AirHawk · 02-07-2010, 12:09 AM

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.

jhk94 · 02-07-2010, 10:28 AM

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.

Kenneth_Moore · 02-07-2010, 11:01 AM

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.

jhk94 · 02-07-2010, 11:51 AM

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...

The_AirHawk · 02-07-2010, 12:26 PM

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.

A Star Ride · 02-07-2010, 12:38 PM

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.

02-06-2010, 01:22 PM	#1
jhk94 Registered Member Join Date: Feb 2010 Posts: 6	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!

02-06-2010, 01:26 PM	#2
longride Super Duper Mod Man Join Date: Nov 2001 Location: Anywhere they let me Posts: 10,484	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. __________________ I'm a knucklehead

02-07-2010, 12:09 AM	#5
The_AirHawk Founding Member Join Date: Jun 2007 Location: Livin' in a Van: down by the Mariana Trench Posts: 10,891	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. __________________ *Parfois, on fait pas semblant* Sometimes, it's not pretend Oderint Dum Metuant Let them hate so long as they fear политики предпочитают безоружных крестьян Politicians Prefer Unarmed Peasants Nothing to see here, Citizen. Move along now...

02-07-2010, 11:01 AM	#7
Kenneth_Moore Registered Member Join Date: Jun 2007 Location: VIsiting the GIft Shop in the Pit of DIspair Posts: 7,118	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. __________________ www.kennethmoore.org

02-07-2010, 12:26 PM	#9
The_AirHawk Founding Member Join Date: Jun 2007 Location: Livin' in a Van: down by the Mariana Trench Posts: 10,891	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. __________________ *Parfois, on fait pas semblant* Sometimes, it's not pretend Oderint Dum Metuant Let them hate so long as they fear политики предпочитают безоружных крестьян Politicians Prefer Unarmed Peasants Nothing to see here, Citizen. Move along now...

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02-06-2010, 02:11 PM	#3
jhk94 Registered Member Join Date: Feb 2010 Posts: 6	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?

02-07-2010, 10:28 AM	#6
jhk94 Registered Member Join Date: Feb 2010 Posts: 6	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.

02-07-2010, 11:51 AM	#8
jhk94 Registered Member Join Date: Feb 2010 Posts: 6	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...

02-07-2010, 12:38 PM	#10
A Star Ride Registered Member Join Date: Oct 2009 Location: central Va Posts: 1,126	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. __________________ White Trash Hick from Appalachia