Bike only runs right without air filter

[cbpaul]

As title my CB250N superdream wont rev over 5500rpm unless I take out the air filter and it runs spot on . I washed out the air filter to remove the oil and have to open the air screws 4.5 turns to run ok but not 100% book said 1.5 turns. I have stripped the carbs many times and cant find a problem and I know running without air filter will run lean any ideas

[N cee thirty]

what colours the plugs,maybe alter the needle height in the carbs to get the right mixture
____________________
'00 Aprilia RS50 > '92 Honda CG > '99 Yamaha Fazer > '91 Yamaha RXS > '79 Suzuki X5 > 01' Honda Cg > 07' Honda Cg > 82' Kawasaki Z200 > suzuki gsxr 400 gk73a > honda vfr 400 NC30 Mod 2 Passed 09/06/2011

Jewlio Iglesias wrote: I actually did vote BNP once

[yuri2085]

Perhaps the fuel mix is rich?

-which is what rob was getting at.

[cbpaul]

Thks guys as for the needle there is no adjustment i.e extra groves in needle. and yes the mixture is very rich with filter giving shooty plugs . I have unscrewed mixture screws nearly to the point of falling out so I think it can get any air when running on main needle.

[yuri2085]

If you still have a problem you may find you get more knowledgeable help in the workshop section.

[Frost]

Don't open the air screws 4.5 turns, your just hiding the problem. Open them up 1.5 like the manual says, then start looking for the actual problem. Provided that the air filter isn't clogged, put it back in.

Check the needle heights are as they should be. Take the carbs off and scrub them clean (the insides that is) Poke the jets through with a bit of wire or a needle. Check the diaphragms for splits and holes, like wise the airbox and inlet rubbers. Grease up the seals and reassemble it. If that fails to fix it, then it's time to look at more complex and expensive causes.

[iooi]

[cbpaul]

New air filter and yes it should be in gear oil I will try a second set of main needles and reset air screws and see what happens . I might have to get a used pair of carbs to see if that does the problem.

[Bru]

Running rich.. sounds like your bypass valves are leaking. On the lhs of each carb body is a small cover, hiding a rubber diaphragm and spring (which can fly out). I'm sure you've seen it, having stripped the carbs already.

Have you inspected the diaphragms closely? Any creases could hide holes; any holes will cause the valve to fail, and allow excess fuel downstream of the jets, so causing over-rich running. I believe you can buy replacement bypass valve kits for around £10 on Wemoto or similar.

Your 'air' screw is actually a mixture screw, which really does meter a mixture of fuel and air from two separate channels. So, opening the screw allows more air and fuel into the engine.

[N cee thirty]

is your main jet the right size ? maybe a previous owner changed it up and never moved it back
____________________
'00 Aprilia RS50 > '92 Honda CG > '99 Yamaha Fazer > '91 Yamaha RXS > '79 Suzuki X5 > 01' Honda Cg > 07' Honda Cg > 82' Kawasaki Z200 > suzuki gsxr 400 gk73a > honda vfr 400 NC30 Mod 2 Passed 09/06/2011

Jewlio Iglesias wrote: I actually did vote BNP once

[fiery tupp]

[Pretendra]

Has the problem been solved, if not try checking the float height. which should solve this problem.

[Pretendra]

Has the problem been solved, if not try checking the float height. which should solve this problem.

[cbpaul]

No luck yet I set the floats at 15.5 mm is that right about to give up bikes all ready to go such a pain .

[Pretendra]

I had the same problem on my bike, initially i went by the book 'Haynes' and had set it to 15.5mm as u mentioned. but the problem persisted, after trying all the stunts possible to lean the mixture i started increasing the float height and now it runs good at a float height of 20 - 21mm. this is provided you have checked your ignition system and is working fine.
i wonder why this happened and for sure people at haynes are not bums to put those figures in, guess could be because of incorrect float needle dimensions..... i may be wrong. but hey my bike is running much better than before. earlier my plugs used to foul and short because of rich mixture and did not rev, but had a good lower end.
Now this i what i experienced, you could try increasing the height in increments and see what happens whats there to loose. though over leaning should be avoided.

[Bru]

Have you taken each bypass diaphragm out and checked them? A crack in the diaphragm will run your engine extremely rich, and no amount of fiddling with the carb needles and jets will remedy that.

Float height will account for some, but not much of your described symptoms.

[cbpaul]

Thks pretendra I will try the float trick this weekend as for a bypass diaphram thay are only fitted to the cb400n. I have 4 sets of carbs and no luck but i will keep trying. swapped cdi stater and coil but no change. what bike do you have Pretendra?

[Pretendra]

i agree bypass diaphrams are on the 400's. I have a CB250N superdream. which was in a crappy state when i bought it, after 6mth's of restoration, engine rebuild and paint work it was road running.
I too had electrical problems, my magneto stator coil was not producing enough primary current for the CDI. i got a good salvaged stator coil from ebay and replaced the HT coil with a new one.

[Bru]

Interesting. Sorry if my advice was based on incorrect information; I found a parts drawing on cmsnl.com, for a 250N which shows bypass valves on the carbs.

[cbpaul]

Thats alright Bru no problem I need all the help I can get thks again. Good to see you have got your bike sorted Pretendra have you got any photos as I am a bit of a superdream nut . I have 1 250NA and 1 400NB AND 2 400NCS must be mad. Going to work on bike sunday and play with the carb floats again wish me luck.

[Pretendra]

Bru theres nothing to be sorry about, that pic on cmsnl.com is misleading. It had kept me wondering for quiet some time wether my bike has one or not when i had initilly seen the exploded view.
then i read some where that it had come only on 400s to prevent misfire during diceleration.

[garyd]

Could the "air screw "actually be a fuel screw???
____________________
CRF230 all you need

[Pretendra]

I think to a certain extent garyd is right. I guess logicaly since the screw is located at the end of pilot circuit it restricts the flow of premixed airfuel which happens at the pilot jet immulsion tube, if the screw was at the opposite end it would have controlled the amount of air that went to the pilot circuit.
According to Haynes manual for a CB250 it should be set to 2 turns.
But again the pilot circuit operates only untill the throttle is shut, as soon as the throttle butterfly opens the main jet comes in to effect.

[cbpaul]

Bike looks very tidy Petendra I like the front mudguard what have you used. Didn't get time to work on bike today but I will let you know what happens you say you changed stater do you test with a volt meter when running. Just got to up load photos so I will have them on 2nite thks paul.

[cbpaul]

Forgot to say jet size 105 main 72 secondry as haynes book dose'nt say is this right.