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cb250 not running

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Sir loin
L Plate Warrior



Joined: 10 Jun 2018
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PostPosted: 19:33 - 10 Jun 2018    Post subject: cb250 not running Reply with quote

Hi All.

I am the frustrated owner of a 1997 Honda CB 250t (aka cb250TWOFIFTY or Nighthawk), and I am having trouble getting it to run on anything other than tickover.
I have cleaned the carb and jets, checked seals are ok, changed fuel in case it was too old (the bike has been unused for a while) but as soon as I open the throttle a little it stutters and dies yet it will happily run on tickover without a hitch.

the carb is vacuum operated, the choke is called a starter enrichment device and there is a electrical connection on the base that connects to the main loom but I have no idea what it does.

Any sound advice/help would be appreciated

TIA, Sir Loin
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Teflon-Mike
tl;dr



Joined: 01 Jun 2010
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PostPosted: 20:05 - 10 Jun 2018    Post subject: Reply with quote

From memory the carb on the 'two-fifty' should be a plainslide Kein PD, so when you say the carb is vacuum operated.... do you mean its a diagphram CV 'constant-velocity' Keihin, or the... ahrggggg horrible CCV one with a diagphram operated slide 'assisted' by cable and cam?

If the last my advice is to ditch the damn thing and fit something off a CG125!!!!

I suspect it's a CV though from mention of electronically operated choke; not a fan U have to admit... of either very much..... But how many fellas have had a poke and a prod at the carb, trying to 'fix' it with muttering of 'cleaning the carb'.... the Keihin CV has a very condom-thin Diagphram bonded to the carb slide, and only replaceable as a unit.... which is effoff expebsive from oldondasisuz... (Dave silvers) A brand new Kehin CV for one of the chinky derivatioves is actually cheaper than that part.

B -U-T before getting too drastic... I would be checking the electric choke... I believe that it gets volts from a bi-metalic strip to turn choke 'on' and when strip gets hot, it breaks the circuit, choke gets no volts and turns off.... assuming that's the deal... bypass the bimetal, give it 12v straight from battery, and see if it runs OK like a manual choke making and breaking circuit to turn on or off....

If so, there's the answer... check connections and if good swap out the bi-metal... of go oldskool... and fit a manual switch!

If choke 'working' manual or on bimetal... then the ;like;ly culpret is, if CV carb that diagphram..... buy new carb, probably from one of the CBTwo-fifty engined quadz, from China.. its cheaper than trying to salvage wot you got.... swap jets if needs be to suit.

Have you got a Haynes? If not go look for one, and if you cant find one that covers the "Two-fifty" and probably a bunch of CD's and CM's, go look at the yankee Clymar offerings... they are, in many ways, i think 'better' than the more dumbed down Haynes, and if a gen US market Nighthawk, probably has more specific tech data.

But '97 model year hack-commuter cum-custom? It's 20 years old, and likely spent most of its life in the care, in the US of highschool kids who in some states didn't even need a learner's permit to ride it, ahile in the UK, probably as a grey-import, will have most likely survived a fair chunk of its life in the hands of miser-mile commuters spending as little on it as they could get away with, and/or 33bhp restricted licence holders, similarly spending as little as they could get away with on it, and run into the ground as a result... and/or pulled out of a shed and sold on to another with as little scrub-up to make e-bay pictures look good, and get cash for the thing.

Make sure, before you spend any money you know what you have, vis import, and that the thing is worth putting time, effort and money into as far as getting the value out of from use.... it is very unlikely to be worth a fraction of overhaul costs on the open market.
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cb1rocket
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Joined: 30 Jan 2010
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PostPosted: 22:20 - 10 Jun 2018    Post subject: Reply with quote

Its a vacumm operated cv carb. I have recently worked on one. Check that the slide from cv actually goes up and down. Also the crap from the tank has probably worked it's way back in the carb and clogged the jets again. You need an inline fuel filter on these.

If the cv slide is iffy check there isn't any tears in the diaphragm. Fix with tiny pieces of inner tube rubber and superglue if you find tears or holes or buy new
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Courier265
Nearly there...



Joined: 01 Oct 2017
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PostPosted: 22:57 - 10 Jun 2018    Post subject: Reply with quote

Hmm,

See Diagram,

First get a bowl.....

Part number 10 is a chamber which contains a foam filter, follow the pipe down near the exhaust, you will see a plastic bung with a clip, with pliers grip the clip and remove bung with your other hand. what should happen is that a load of gunk will pour out, hence the need of a bowl.

After that you can if you want try and remove that chamber and replace the foam.

Try running the engine again, see what happens, if you still have a problem, check the main air filter.
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Sir loin
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PostPosted: 00:26 - 12 Jun 2018    Post subject: Reply with quote

Thank you for the replies.

The diaphram looks intact and the slide moves freely when operated by hand but with the air box off and the throttle is operated it does not move, but i don't know enough about cv carbs to know if it's supposed to.I also cannot tell if the needle is moving.

The accelerator pump, for want of a better way to describe it, squirts fuel into the venturi when throttle is opened so that is working, and I can't remember if there is any foam in the air cleaner but I do get gunk out of the breather tube. There is a filter in the fuel tap, and it is brand new as I changed it just before laying the bike up as the old one had gone brittle and was falling apart.
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Sir loin
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PostPosted: 23:07 - 11 Oct 2018    Post subject: Reply with quote

finally got the bike running. the slow running (middle jet) was blocked so ended up running a micro drill bit through the jet by hand so bike now throttles up, albeit roughly, as it need new exhausts - the balancer pipe on the twin exhausts has rotted so airflow is uneven, and It also needs at least one new plug. the "starter enrichment" fitting is a threaded plastic piece that screws into the carb and this is worn out so need to replace that as well
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cb1rocket
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PostPosted: 14:13 - 12 Oct 2018    Post subject: Reply with quote

Good to see there is finally a closure, give it the good service it deserves then your be good to go Thumbs Up
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