carb tuning, need advice

[MuZ]

Hey guys, I need someone with more knowledge about carb tuning than me to offer advice on a problem I'm having with my wife's 97' CB250.

I got the bike off a mate for nothing in a not running condition, said I could have it for the better halves first bike if I got it running. Turned out it was the cdi and then bike ran after that and I put rego on it.

Once I could give it a proper ride I had all types of dramas that turned out to be air leaking in the intake manifold. After I sorted that out the bike has run sweet except for a total dead spot from 6000 - 7000 rpm. If your on a flat bit of road it struggles through it then takes off when it gets to 7K. Down hill no dramas but uphill not a chance of getting the tacho past 6K.

I tried removing the baffle/end cap things from the exhaust and it run noticeably better but was still far from perfect. next I increased the jet sizes from 35/112 to 38/115 and it got worse.

I know next to nothing about carb tuning and its got me confused as hell. any advice at all is welcome.

(btw whatever the end result, the baffles are staying out of the zaust, it sounds shitloads better without them)

[Yella]

With out an A/F meter and not knowing what carb is on it , it's hard but sounds rich. Try going back to the original jet and dropping the needle if possible or going to a smaller main jet.

[Yella]

Just a thought have you check the diaphragm

[slingy88]

I would be stripping the carbs and replacing all o rings and checking the diaphrams like Ian suggested. It sound to me like the usual problem of old carbs with leaky seals etc. Get the basics sorted before playing with jets otherwise your just chasing your tale. Check the float height as well as that has quite an effect on mid range fuelling. Also, what is the air filter setup out of interest?

[MuZ]

Its got the stock air box and filter setup. The diaphragm is in good nick, and the needle height is non adjustable. I stripped the carb and gave it a good clean when I was chasing the first problem that turned out to be leaky intake manifold, however I didn't replace anything just cleaned the years of gunk out of it. now its been mentioned, I kind of feel silly for taking to o rings at face value, ill order a carb kit today, go back to the original jets then fuck around with things from there if there is no improvement.

Thanks for the help I'll let you guys know how I go once the carb kit turns up

[slingy88]

If it's running stock airbox and stock exhaust, then it should be on stock jets. Especially if it has been running ok and then starts playing up. This almost certainly leads to an issue with something. Either the o ring around the float seat, or the o ring on the pilot screws. Or, there is still Crap in the jets or passages. Can take a real soak and compressed air to clear some times.

[MuZ]

So the New carb kit turned up yesterday and I threw it in last night. first thing I noticed is you don't need the choke on for 10 minutes to get it to idle, you can turn the choke straight off. its also sounds a heap better with no hesitation. would have taken it for a test squirt but the 30knt wind and the burst of hail made me run inside to drink scotch instead where I dedicated my evening to watching videos and reading articles about carb tuning in an attempt to educate myself. once I take it for a test ride ill post up the results

[slingy88]

Yep, keep us posted. At least with the single carb arrangement, there's no need for balancing

[MuZ]

Well there was enough of a break in the torrential downpour we've had of late to take the cb for a squirt yesterday, it is much better. It was a little hesitant between 7 & 8k but went away as soon as it was properly warm. In hindsight I should of known beeter and ordered a carb kit the moment I was handed the bike, but lesson learnt. cheers for the sound advice

[slingy88]

No worries, it's all learning and often it's easier being on the outside just looking at facts lol