Author Topic: cb250rs carb question  (Read 298 times)

spooky

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cb250rs carb question
« on: January 02, 2022, 11:01:14 AM »
On the bottom near side of the carb float bowl there is a drain screw, if this is removed should petrol run out with tap on or even with tap off ? I am wondering if there is something in float bowl that is occasionally blocking it , I don`t want to take float bowl or carb off until I know, 
Classic efi squadron blue, Conda 250, Terrot 125, Negrini 50,

themoudie

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Re: cb250rs carb question
« Reply #1 on: January 02, 2022, 04:54:12 PM »
Aye Rufus,

Back in from a canter on the Ducati, a wee bit blowy and rain coming in fast from the west, but not a bad run.

Now for your carbie query! Although I have never had to deal with one these beasties, I suspect that they are not disimilar to other Honda carbs. If the fuel tap from the tank is turned 'OFF', you should be able to drain the carb bowl by slackening the drain screw. N.B. these have a taper and a rubber 'O' ring seal to keep them fuel tight!

So, yes, you will have an initial fuel flow, with the fuel tank tap 'ON' or 'OFF'. But, in the 'OFF' position, the fuel flow should only be the contents of the fuel line and carburettor float bowl.

Spare parts prices on the CMSNL site aren't cheap, but then what "Classic" Jap parts are these days ?!  :(  A complete carb is £405 and a rebuild gasket kit is £28-74 inc 20%VAT, but no P&P.

I thought AMAL parts were expensive, but not compared with these. However, these were a better carb and have a diaphragm pump in them.  :-\

A good exploded diagram for the carb is available on the CMSNL website and this can be enlarged in a separate tab.

Here is the link: Honda-CB250RS-1980-a-England_model_carb

Here is the separate link to the exploded parts diag: Honda-CB250RS-1980-a-England-carb_exploded_diagram

Fowlers, Dave Silver, WeeMoto for parts.

Good health, Bill

spooky

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Re: cb250rs carb question
« Reply #2 on: January 02, 2022, 06:06:11 PM »
Thanks Bill, interesting, when i take drain screw out nothing flows, with tap on or off  ?, if I pull the fuel pipe off the carb petrol runs well from tank,  something stuck somewhere ?
Classic efi squadron blue, Conda 250, Terrot 125, Negrini 50,

themoudie

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Re: cb250rs carb question
« Reply #3 on: January 02, 2022, 11:41:21 PM »
Aye Rufus, sounds like sludge etc in the float bowl. Be warned that float needle is no cheap!

However, with a wee clean and rebuild you might find those missing Freiberger or Haflinger horses. Or should that be a Noma uma?  ;)

Good health, Bill

spooky

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Re: cb250rs carb question
« Reply #4 on: May 09, 2022, 01:27:46 PM »
Well after quite a while of reasonably smooth running, a niggle... Last time out it started easily as usual, but after a couple of miles it spluttered then stopped, restarted again  then the same again a few times.  Got home and took carb off, stripped and cleaned , everything looked ok but now as soon as float bowl fills it runs out of the overflow. Took it to bits again and checked float , again all looks good but it must be a sticky float ? I tried following instructions to check float level but it`s beyond me.  Am I right to think the only culprit could be the float ? It does start and rev ok but all the time fuel is running out of the overflow.   ?   does anyone have BrendanO`s phone number ? I had it then lost it, he did have a spare carb as i remember...
Classic efi squadron blue, Conda 250, Terrot 125, Negrini 50,

iansoady

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Re: cb250rs carb question
« Reply #5 on: May 09, 2022, 03:37:21 PM »
That drain screw doesn't need to be removed completely - just slackened off. The petrol drains through the little spigot on the bottom of the float chamber. I've just bought a new drain screw from NRP carbs at a whopping £10.80 including postage. They also have jets, needles etc. https://nrp-carbs.co.uk/. I have to say they're pretty poor at responding to queries - eg the acceleratpr pump diaphragm on my CB250 carb was quite distorted and they couldn't tell me whether their replacement was E10 proof.

It does sound as though either the float or the needle is sticking. They are quite simple carbs and I'd pop it off and make sure everything is clean - maybe a bit of muck is caught on the needle valve seating? The float height is 14.5mm measured between the base of the float and the joint face with the carb upside down.

I found it easier when replacing the carb to take the rubber adaptor off the cylinder head and fit this to the carb then fit the whole lot to the head although the bottom bolt is tricky to get a decent spanner on and mine was very tight - I ended up having to use mole grips as the head was rounded off already.

See, I'm now an expert after a whole month with the little Honda!
Ian.
1952 Norton ES2
1980 Honda CB250RSA

spooky

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Re: cb250rs carb question
« Reply #6 on: May 11, 2022, 09:06:09 AM »
Carb is really easy to get off due to it not being in a honda frame, i`m still not convinced float height is correct but i have ordered a new needle valve , when that arrives i will try it before looking at float again,  carb kits are out of stock at NRP anyway
Classic efi squadron blue, Conda 250, Terrot 125, Negrini 50,

spooky

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Re: cb250rs carb question
« Reply #7 on: May 15, 2022, 04:08:51 PM »
New float valve needle  fitted, fuel seems to have stopped running out but that`s probably because it`s not getting there !  definiteley coming out the tank but not going into carb ? is this a float problem ? maybe too high ....
Classic efi squadron blue, Conda 250, Terrot 125, Negrini 50,

themoudie

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Re: cb250rs carb question
« Reply #8 on: May 15, 2022, 06:40:54 PM »
Aye Rufus,

Is there an inline filter/s on the feed from the tank? Empty tank and dismantle taps from tank, giving all a thorough swill out and then blow through with compressed air.

Is there a fuel filter between the taps and the float bowl inlet? Has it's guts collapsed? Replace.

Has the fuel pipe between the tank taps and the carb collapsed inside, due to age/ ethanol attack? Replace the hoses, with E10 resistant hose.

Is there a filter between the float bowl inlet and the float bowl? Check, clean/ replace.

If all of this is clear and fuel flows from the end of the fuel pipes or into the float bowl with no float and float needle in place, then assume float is likely at wrong level. Ian in his post of "May 09, 2022, 03:37:21 PM" gives the float height and how it is measured, "The float height is 14.5mm measured between the base of the float and the joint face with the carb upside down."

If the old bat still decides not to run, let us know and we'll have another grey matter stirring session! ;)

Good health, Bill