running problems please help!

Ok so I pulled the carbs out of my cb the other day just to check some things (not important) and when I put the bike back together it started running like a sack of sh*t.

 It was misfiring and coughing and there was a bit of smoke coming out of the exhaust (though not burning oil more like excess petrol) it wouldn't start idling and I had to keep on the throttle to keep it running. it also sounded like it was only really running on one cylinder. I noticed that it seemed to run a bit better if I disconnect the vacuum line from the tank for some reason but then it would eventually run out of fuel.

 I didn't change anything when I initially took the carbs off and have tried removing and reinstalling the carbs a couple of times now with still no luck. I'm quite mechanically minded but this has got me (and my mechanically minded friend) absolutely baffled. any ideas on what the problem is? I can't really afford to take it to a proper mechanic so would rather fix it myself.

Thanks in advance

Did you remove any spark plug leads and not put them back on properly?

I did remove the spark plugs to check them but I made sure to reinstall them properly and checked them again after the problem occurred

Try draining the carb float bowls (drain screw on the bottom of each with rubber hose below it). The symptoms sounds like a pilot jet is blocked but draining the carb bowls could be a good first thing to try.

I cleaned the carbs out and checked all the jets when I removed the carbs a second time with still no difference

And have you tried draining the bowls? 

Also I wonder whether the float height of one or both carbs was somehow disturbed when you had them apart?

i checked the floats which all seemed normal. I haven't tried draining the float bowls but I doubt that'll make any difference if I've already taken apart and drained the carbs

HT leads definitely the right way round? Apologies if these are stupid suggestions!

Did you adjust the idle mixture screw when you were doing the carbs? If so, did you set it back to the stock setting? If you did, is it possible that it was set to some other setting previously (ie. did you count how many turns out they were?)

Also - to get the carbs out you have to disturb the carb rubbers - on most peoples bikes these are getting quite old now and if they get split, holed or are just going a bit porous then they can cause all sorts of strange running problems. You can sometimes diagnose this by spraying some 'easy start' around the carb rubbers when the bike is idling - if the revs increase then the rubbers are dead.

Hi,
that sounds like a non-closing float needle valve to me. Sometimes the floater is blocked while installing the floater chamber. Is there any fuel leaking? In some cases, even fuel leaks into the cylinder, so remove the spark plugs and let the starter motor rotate for just one second (caution, keep your face away from the cylinder head as this might lead to a fuel fountain splashing out of the engine!).

flying brick talks sense.
The bike was running OK until you took the carbs off, so logic dictates that this has caused the upset. The symptoms sound like flooding ( sticking fuel valve ) or leaking manifold rubbers.

Maybe both ! 

finally got to work on the bike again today and still no luck. installed new spark plugs and made sure they were sparking properly, pulled the carbs and checked them out. but it still does the same thing; runs really badly (like almost on only 1 cylinder) with everything connected up and then runs better (back on two cylinders) with the vacuum hose disconnected. any ideas??? I had a look at the fuel tap as well and everything looked fine, no damaged diaphragms or anything.

please help

I'm having similar problems also - even created my own thread this morning http://www.cb500club.net/t3501-problems-starting#27661 so very interested.

Flyingbrick wrote:Hi,
that sounds like a non-closing float needle valve to me. Sometimes the floater is blocked while installing the floater chamber. Is there any fuel leaking? In some cases, even fuel leaks into the cylinder, so remove the spark plugs and let the starter motor rotate for just one second (caution, keep your face away from the cylinder head as this might lead to a fuel fountain splashing out of the engine!).

Im interested in this test - what 'should' happen on a healthy bike? no fuel? and I'm guessing allot of excess fuel spitting up is bad?

Also how would you rectify the blocked 'floater' (...trying not to laugh as I write that line)

On mine even after draining both chambers I removed the carbs, placed them down and in inadvertently tipped them upside down and even more fuel came gushing out.

Even before finding this thread (thanks sullivj) the HT question had been put to me so I've looked at a whole bunch of information to make absolutely certain despite my labeling that I have the coils connected correctly and to the plugs as I've had all of it out on mine