[Plagg]

Hello, looks like a nice forum and community here. I come from the land of Ninja 250's, where all of my mates went up to their 600 Super sports, I took the route of an even slower bike haha, my new bike is a 1993 CB250 nighthawk that is pretty much cafe'd out. Love it so far, had it about a month. Always loved hondas and so proud to be on one. It's ran perfectly so far. The bike has about 8,500 miles on it. And I know it has spent times sitting for bits but that's about all I know of it's previous life. From what I can tell most things have been replaced or maintained really well. Came into this weird problem today, went to ride to an arrand and everything worked perfect as normal, went to warm her up pulled the choke and she settled in the rev's after about 5 seconds (as usual) so i push the choke off and let her idle there for about 5 mins then hop on and make my 20 minute highway ride flawlessly. Come back out from the store (spent maybe 5-10 mins in there) and go to start it up again (I never re-choke unless its been sitting over a couple hours) and it idles for maybe 10 seconds then dies. Just thought it was funny and caught me off guard so i kick it back on and it seems to do better, maybe 30 seconds.. then dies. I start to panic and kick it on and rev it a little to keep it going, then pull the choke on while i put my gear back on. After that I quickly hop on, drop the choke, and take off, trying not to let it rev low. Pull up to the first light and sure enough as soon as I touch the clutch it dies on me. I planned on going back home through the city but after fighting the bike 4 or 5 lights I stop to a gas station. At this point she's being a REAL PITA to get going. Full choke will barely turn it over and it dies right after --- can only keep it running with full choke and some throttle. Planned on getting gas cus I figured this was the simplest thing that could be the problem, but when I check the tank I still have a generous Half-Tank full. Remembering how all of my ninja 250 friends give me crap for putting premium in my small displacement bike, (which, on this bike I bought it with a half tank of who knows, topped that off with premium, and am on my second fill up of all premium,) so this time I fill it all the way up with regular. Nothing changed. Mess with the petcock on res, off, and on, No change. Mess with the idle, kinda helps if I literally cranked it all the way up but I know that's not a solution or gunna work. I so play the throttle and choke game to get it started and take off holding it from dying. Had it die the first light or too and had to restart forcing it to go, noticed at low rpms its also lurching really bad, with what I think is backfire? could be wrong on that one. I got frustrated and just limped it on the highway and took that straight home no problems 75-80mph in 5th gear the whole 15-20minutes, but as soon as I got off, same issue to my house. Parked her and now I'm at a loss. Gas? Could it be the premium full tank messing things up? Carb clean? but it ran so good before and took nothing to warm up. Spark plugs maybe? I know they're easy to check but it still accelerates really strong and maintains highway speed same as before. Something going on with the air intake/filter? Please give me anything you got, I'd rate myself as pretty comfortable when it comes to mechanical knowledge, until you get to engine internals, but very new to this specific bike, never turned a bolt on it besides changing oil and cleaning the chain. Appreciate anything you can give me,
Thanks, Trevor.

 

[J-T]

Sounds like a problem with the idle circuit. Have you adjusted the mixture screw?

Might have developed a vacuum leak, not familiar with your bike but if it has vacuum hoses one of them may have come disconnected or broken.

Welcome to the group.

 

[Plagg]

Sounds like a problem with the idle circuit. Have you adjusted the mixture screw?

Might have developed a vacuum leak, not familiar with your bike but if it has vacuum hoses one of them may have come disconnected or broken.

Welcome to the group.

Are you saying the mixture screw inside of the carb? Is this something that is commonly done to make it run better?
Gunna do a search and get a list of the vacuum hoses and look into that tomorrow morning, I would be so happy if just a hose fell off haha, but knowing my luck, probably not.

Appreciate the response though J-T, thanks! Open to more ideas I pretty much have all day tomorrow to tinker with the thing so keep throwing ideas at me.

 

[longdistancerider]

The mixture screw protrudes from the bottom of the carb closest to the engine. Looks like it has a limiter cap on it prevent more than 1 full turn in the fiche.

 

[Old Putz]

Agree with the clogged idle circuit theory.
You stated the bike sat for a long time. There's a good possibility that there's a bunch of deposits and general carburetor crap that may have broken loose or gotten stirred up from the bottom of the float bowl and clogged things up. It would probably be a very good idea to pull the carbs and open them up and have a look around.
Premium gasoline isn't "better" gasoline. Its higher octane number only means that it's more difficult to get it to burn. This is handy when you've got a high compression engine (prevents the fuel from spontaneously combusting due to pressure). If the bike isn't pinging under acceleration, you're just wasting money.
I bought a 250 Ninja when they first came out in 1986 (holy crap, 30 years ago!). Ended up putting a 1988 motor in it (they removed a good deal of crankshaft weight and the engine revved a good bit more quickly). Definitely a fun bike to toss around, and I had fun embarrassing guys on larger bikes when the road got really tight and twisty. Once the road opened up it was an entirely different matter...