CB250 carb probs :-(

Ducati750s · Borekit Bruiser Joined: 01 May 2005 Karma :

Just got an '81 superdream as a project bike, and I'm looking for a bit of advice.

If blipping the throttle from tickover, the revs die and it cuts out. But, if the throttle is eased really gently to start with, it will happily rev right up to the limit. Blipping it whilst the revs are already higher then it's fine also. It is just whilst it's ticking over around 1400rpm and you give the throttle the normal sharpish twist.

Any ideas?

I've cleaned the plugs and set the gaps. I've stripped the carb completely and the problem is still no different.

Any pointers?

Cheers in advance, Anthony.

tatters · Exxon Valdez Joined: 04 Jan 2004 Karma :

air filter?

exhaust blowing? (collecter box gaskets are a nightmare to replace if they go)
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Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Sounds like the mixture is not richening up when you open the throttle, and instead leaning off due to the extra air.

First guess would be a perforated diaphragm.

Not sure about the 250, but on the 400 there was also a rubber diaphragm under a cover on the side of the carb. For some fuel cut off I think (not sure), but these used to get holed and cause problems as well.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

Ducati750s · Borekit Bruiser Joined: 01 May 2005 Karma :

I've stripped the carb completely but wasn't aware of a diaphram! By 'diaphram' I'm thinking like a rubber seal type affare, simlilar to the rubber flap usually under a master brake cylinder cap? Pretty sure there wasn't anything like that.

In the top caps of the carb there was simply a piston looking thing with the needle hanging out the bottom, on top of which was a long large flimsy spring which located in a stub at the top of the cap. The piston looking part moves up and down in the cap, presumably to shift the needle position to control fuel flow. No rubber seals in the top part at all come to think of it.

mmm.

Nath · World Chat Champion Joined: 28 Jul 2004 Karma :

Sounds stupidly obvious, but are you letting the bike warm up first?
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Hard livin', hard luck.

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Had a quick look in the Haynes (not opened it in a decade). There are a few seals in there. One seals the cap on the top. There is also a seal around the needle on the slide.

Sorry, no real knowledge of these carbs.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

Ducati750s · Borekit Bruiser Joined: 01 May 2005 Karma :

Cheers guys.

I'll be stripping the whole bike down (the bike was bought to take to pieces to learn a bit more about maintenance) so will have another check over the carb as it seems likely this is the cause of the problem.

If it was easy, it would be no fun i guess Wink

ZaphodBeeble · World Chat Champion Joined: 10 Feb 2004 Karma :

Check the condition of the sparks with the spark plugs out of the engine. It could be that they are getting soaked in fuel when you open the throttle quickly. It's always good to buy new plugs too, they are the cheapest bits and can make all the difference. Thumbs Up

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Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.