cb250rs running rich/lean (fixed)

[salem1987]

Hi,

Ive had this issue for a long time now and have run out of ideas!

Its a Honda CB250RS with pod filter (i also know this is silly but it came like this) standard jetting. Valve clearences spot on, carb spotless.

Basically:

The bike will not idle or cruise at 1/4 throttle very well at all, it surges and sounds like a 2 stroke- 4 stroking if that makes any sense.

The plug is black so im assuming its a rich condition.

I have tried different jets running without a filter etc with no success.

When the bike is cold and kicked off it will rev very high and will continue too until i either turn the idle speed down or the engine warms right up.
As the engine warms up i have to turn the idle speed back up to get some kind of steady idle speed when stationary.

More often than not when kicking off i will get back fires from the carb or flames from the exhaust, subsequently getting a fouled plug in the process and having to get the brake cleaner out to dry it out.

anyone got any ideas?

Cheers

[Robby]

Check the inlet rubber for splits first.

It is hard to get a good read off the plug because the 250RS runs an accelerator pump on the carb, meaning it always run very rich on acceleration.

They aren't as sensitive to pod filters as some bikes, because the carb isn't a CV. That said, an airbox is cheap on ebay and worth fitting.

Outside possibility, if the inlet rubber is fine, is a tear on the diaphragm in the accelerator pump (round cover on the bottom of the float bowl, 3 screws). If this has torn you can really cock up the fuelling. You can also get big problems if the float bowl has been off the carb and the little O ring on the pump shaft wasn't replaced. Pisses fuel out too.

[Suntan Sid]

Have you set the pilot air screw to factory standard turns out?
Even without the airbox at least you will have a base point to start from.
According to "Haynes", if the standard setting is not producing the right result there is a method to determine the optimum setting.
Starting with the standard setting for the pilot air screw, warm the engine up and set a faster than normal idle.
Screw in the pilot air screw, (counting the number of turns), until the engine note changes, it will sound like it's running a bit rough.
Screw out the pilot air screw, (counting the number of turns), until the engine note changes, it will sound like it's running a bit rough.
You should now have a number of turns between these two extremes.
Screw the pilot air screw back in to exactly half way between the two.
That should give you a base setting!
If this doesn't solve anything, as Robby said, buy an air box, it'll run better anyway!
____________________
"Everybody needs money, that's why they call it money!"

[salem1987]

thanks,

I have also checked the inlet rubber for leaks and it seems pretty air tight. Sprayed a bit of brake cleaner over it while running and no change in RPM.

The accelerator pump diaphragm is something i will check tomorrow!

Have run the bike with it disconnected and connected with the same results though...

No fuel pissing so guessing the O rings ok, but will check too.

Pilot screw is at standard setting, have played around with it loads too. Doesnt seem to make much difference at whatever turns i have it set at. Or even with it completely removed!

Could float height affect the bottom end of the rev range also?


and yes an airbox is on the xmas list lol.

[stinkwheel]

Christmas list?

They cost less than a round of drinks.

£19 delivered on ebay. That one even looks like it has the rubber attached.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[salem1987]

Christmas list- meaning buy one myself for Christmas as its that time of year ... Was barely a joke I know but yeah?

[temeluchus]

Does it have a manual or vacuum fuel tap?

Worn float needle and a vacuum tap behaves pretty similarly to incorrect float height adjustment.

If the choke plunger and seals are worn it will run quite rich as well.

Often you can buy a little kit of parts with new seals for the carb, a wise investment for an old machine. Saves fucking around tuning around variable air leaks.
____________________
Some shite cruiser. Now with guns and FREEDOM!

[Robby]

The brake cleaner trick for inlet rubbers isn't foolproof. You're best off taking off the bike and having a look and a squeeze under good light. Do that while you have the carb off for a proper overhaul.

[salem1987]

thanks for the info. Been away this xmas and just read these responses.

Turns out the slide in the carb was 180 out. Due to the design of the carb its possible to do this without it sticking half way up the throat.
So the recess in the slide was facing the inlet valve and not the filter. Hence the richness etc!

Unscrewed it, spun it 180 screwed it back onto the arm and the bike sounds like a different engine has been put in it!

Sorted.
Cheers