CB250 carb/idle issues

[chrisjpartrid...]

I've just bought myself a 1983 Honda CB250n Superdream project. It has sat for ten years in a shed and so I'm expecting issues.
Overall the bike isn't looking to bad - I've given it a new battery and the only electrical issue seems to be the horn.
The body isn't amazing but there are no major rust issues to worry about, though the paint is showing it's age.
It's done 35,900 miles but it's a Honda so should be fine.

https://i1129.photobucket.com/albums/m511/chrisjpartridge/CB250n%20Superdream/B6yvFQmkKGrHqZh0EycQkIBkTBMybuwlMfw_3.jpg
https://i1129.photobucket.com/albums/m511/chrisjpartridge/CB250n%20Superdream/B6yypiWkKGrHqEOKikEyV3VvOQwBMybvyzs2_3.jpg

Today I've set to getting it running, and along with the new battery have changed the oil and filter before even trying to turn it over.
Have also given it new plugs and changed the air filter... which had disitergrated!

After flushing the tank and carbs, running some clean petrol through and so on I found that the carbs were leaking quite heavily.
There is a brass nipple on one of the carbs that was gushing fuel everywhere and after some research suspected a stuck float.

I have now removed the carbs and found the following:
Both pistons stuck
3 out of 4 jets blocked
1 float valve stuck
Lots of varnish and gumming old fuel.

I've cleaned it all out with solvents, rag, whole can of carb cleaning and finished off with compressed air. With everything put back together all seems to move as it should.

I have refitted the carbs to the bike and am now trying to get it running. Got no joy at first but helped it back to life with a little easy start.

Now for the part I'm stuck on...

I have got it running but there is no way I can get it to idle.
With the choke fully open (after warming it up a little) you can get it to just tick over roughly but that's it.
With the choke out again, you can rev it up - it struggles mid range but past 7,000 RPM it sound good and goes well.

Choke in and you'll get nothing.

So my question is what to do next?

[LongJohn22]

First thing you need to check for are air leaks on the engine side of the carbs. Are the O rings between manifolds and heads still in one piece? They can get damaged when refitting the carbs. Are the manifolds clamped tight to the carburettor? When you stripped the carbs, did you notice the little rubber bugs fitted inside the float chamber on the air jet? If they're missing the engine will certainly run rough. Good luck.

[chrisjpartrid...]

Ok;

I have replaced the O-rings in the inlet manifolds.
Rubbers arond the inlets look good and I checked the rubber bungs.

I guess a good way to check this would be to hook up some vacuum guages but I can't see were I'd connect them as there are no screws on the inlets?!

[paddlesat16]

The screws for the carb balancer to screw into are at the back of the head just in front of the inlet stubs.

Regards
____________________
I once saw a bloke shagging a donkey.... in Saudi arabia. Theory Test Passed 26/10/09 Mod 1 Passed 26/4/10, Mod 2 Passed 7/6/10 Current Bikes Suzuki GSX1400 K5, Aprillia Pegaso 650, Suzuki DR350T.

[LongJohn22]

Paddlesat, I just read the first part of your signature, are you sure this is something you wish to boast about, or is there something you want to tell us?

[paddlesat16]

Hi Longjohn

Sure i'll tell the story.....

Many moons ago as a young soldier in the first gulf war....

we happened on this town called Hafa Al Batan, so as not to disturb the local population we would drive in convoy around the town as opposed to down the main street.

We came across as what would be described as the allottments...

We found three men and a donkey...

Picture the scene...

One guy holding the donkeys head by a halter
One guy watching
One guy stood on a box giving the poor donkey what could be described as throwing a cat into an entry

My convoy fell about laffin and takin the piss, and so did an American convoy coming the other way... Somewhere out there there are pictures
____________________
I once saw a bloke shagging a donkey.... in Saudi arabia. Theory Test Passed 26/10/09 Mod 1 Passed 26/4/10, Mod 2 Passed 7/6/10 Current Bikes Suzuki GSX1400 K5, Aprillia Pegaso 650, Suzuki DR350T.

[LongJohn22]

I was demobbed back in 1973, I never came across anything to compare with that. 10/10

[paddlesat16]

Hi Longjohn

who was you with, I was 3RRF gone but not forgotton
____________________
I once saw a bloke shagging a donkey.... in Saudi arabia. Theory Test Passed 26/10/09 Mod 1 Passed 26/4/10, Mod 2 Passed 7/6/10 Current Bikes Suzuki GSX1400 K5, Aprillia Pegaso 650, Suzuki DR350T.

[LongJohn22]

REME VM, spent most of my time on heavy armour with the cavalry, 11th Hussars, 13/18 Hussars, 2RTR, 4 Fd Regt RA. All a very long time ago now, still go to 11th Hussars annual reunions.

[paddlesat16]

We were next to 3RTR in Hemer(my last posting) Iserlohn Garrison, Happy memories of drinking in brothels and racing for the ferries to get back to blighty for the weekend
____________________
I once saw a bloke shagging a donkey.... in Saudi arabia. Theory Test Passed 26/10/09 Mod 1 Passed 26/4/10, Mod 2 Passed 7/6/10 Current Bikes Suzuki GSX1400 K5, Aprillia Pegaso 650, Suzuki DR350T.

[Moo.]

Have you tried adjusting the idle screw any? Could also be the carbs waaay out of balance, i had problems when trying to balance mine, wouldnt hold an idle for shit.

Only way i did it was pull out choke to start it, link up the carb balancer, sort it enough to idle, choke back in then match them up.

Could try get hold of someone with an ultrasonic cleaner, and stick your carbs in there, they do a good job
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A2 Passed 18/6/10

[Robby]

First off, can you edit the first post so the pics aren't side by side. Hate having to scroll across to read the thread.

Now, the problem could well be an air leak, but here is an alternate theory:

After 10 years the tank is probably quite rusted inside, and the fuel has pulled the loose rust out of the tank into the carbs. If the tank is rusty, then treat it - looks in nowhere_elysium's thread in show and tell where I outlined how to de-rust a tank.

Once it is de-rusted, fit a paper inline filter to catch any more crud getting washed through.

Whilst the tank is off doing the de-rusting process, the carbs need to be pulled apart again. Expect to see some rust (maybe rusty water) in the float bowls. If it the carbs run accelerator pumps, then strip them. You'll also have clogged mixture jets, so remove them. Best way I know to clean them at home is to use a can of carb cleaner with the long straw nozzle attached and poke it right into the nooks and crannies in the carb and spray, wear eye protection because it will squirt everywhere and the stuff stings.

Then let the carbs dry, brake cleaner does take a while to evaporate so leaving them in front of a fan heater for 20 minutes does a good job.

Reassembly to factory spec, making sure you remember the accel pump O-rings between the float bowl and body if it runs them - omit these and you'll be pissing fuel out of the back of the carbs when you open the throttle, still runs ok though.

Seriously though, if the tank is rusty then de-rust it. Don't use a tank liner, it costs more than a gallon of phosphoric acid and is likely to fail. Takes ages for the epoxy to go off as well.

[chrisjpartrid...]

Pics moved!

The rust theory isn't a bad one - I did flush the tank with clean fuel but I can admit that it's a little grim inside still.
I've ordered some inline filters, but the trouble is that there's not enough space to fit one.
I've seen in the manual that there's a filter built into the tap, so maybe I'll check that next.

I'll get the carbs off again later in the week and recheck them.
Thinking I'll rig up a bottle and hose next time with clean fuel and see if I can just get it to run at least that way.

I can't find the post on tank cleaning - is it just a case of soaking it in acid and flushing it or is there more to it?

[Robby]

To derust the inside of the tank, you use phosphoric acid.

Take off the fuel tap and pull out the internal mesh filter. If the tap screws on, plug the bit it screws onto. If it is held on with two bolts, make up a little plate with a bit of steel and bolt it over the hole, using some inner tube as a gasket.

Get a gallon of phosphoric acid (40%) off ebay, mine cost less than £20. Carefully pour it into the tank, then fill it up to the filler neck with hot or preferably boiling water - lot of full kettles.

It will fizz a fair bit. It probably only needs 20 minutes, I left mine for 4 hours or so. The phosphoric won't harm the steel, and didn't even attack my paint. Even so, if you do get any on the paint then wipe it off and rinse plenty of water over the affected area.

Once you think it's spent enough time de-rusting, drain the acid into a bucket and rinse the tank through a couple of times with a hose. Refit the fuel tap, pour in a small bottle of meths, put the cap on, and swirl it around lots. Meths will pick up any water in the tank. Now fuel cap and tap off again, drain the meths, then put it somewhere warm to dry out fully - this won't take long, I would leave it in front of a fan heater for an hour.

Then for the important bit - get the fuel tap back on and fill it with fuel. Tanks seem to rust up very quickly straight after de-rusting, but after a while with fuel in the stabilise. If you're going to store the tank empty, I spray in a lot of WD40 followed by a pint of cheap engine oil and swirl it around so that all of the surfaces are coated.