Bogging down in 4th/5th?

[Mudskipper]

My CB250RS seems to have a fuelling issue when going along in 4th or 5th gear. The revs bog down, feels/sounds like fuel starvation, but it doesn't conk out if I slow down/change down.

It starts fine, runs absolutely fine through the first 3 gears. I recently cleaned the main jet, needle jet and pilot jet and got a load of gritty crud out of the float bowl.

What should I check? Filter? Plug? Float needle height? (If so, how do I do this?)

Anything else?

It's bothering me cause I can't take it for too long a run in case the bogging turns to conking out (like it did before) and I have to push it back. I need it to be running reliably before I can use it properly.

Cheers
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CBR125|||GSXR400|||CBR400|||CBR400|||CB250RS|||GSXR750|||CB250RS
"You're clumsy, you eat too much and you behave like a 12 year old boy. But you know what? Every once in a while, you find a thumb."

[neil.]

Yeah I'd check the air filter out and the rubber connecting it to the carb for cracks/holes. New plug won't hurt if it's old but I would inspect the current one and see what colour it is, should be a dark beige/tan. If it's black/sooty or pure white then it's running too rich or too lean respectively. Check out the free/easy/cheap stuff first then take a look at the ignition system (assume it's coil/CDI?) once you're 100% sure everything else is okay.
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CBT February 2008 | A2 June 2008 | Yamaha YBR125 (written off) | Honda CBF125 (current)

[Mudskipper]

Cheers, just had a look, filter is fine but the jubilee clip on the airbox rubber is loose. It's also corroded and won't tighten or undo.

Is this likely to be the cause? I'll snip the clip off and fit a new one if so. I assume a normal clip of the correct size will be fine (ie. don't need a special Honda one)?
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CBR125|||GSXR400|||CBR400|||CBR400|||CB250RS|||GSXR750|||CB250RS
"You're clumsy, you eat too much and you behave like a 12 year old boy. But you know what? Every once in a while, you find a thumb."

[Robby]

It's odd that you're only getting the issue in high gears. Try shifting into 3rd early and using a large throttle opening to see if it bogs down.

An electrical problem is possible, a new plug, HT lead and plug cap is cheap and easy to do if you haven't already.

Do you have a fuel filter? The gritty crud came from the tank, so more could have been flushed down into the carb. Again, and inline fuel filter is 2 quid off ebay. To fit it you just snip the fuel line in the middle and slide the filter into the tubes.

My initial thoughts point at more crud in the main or needle jet, a constricted fuel tap reducing fuel supply. This is based on the problem occuring when you are trying to get more power and using a larger throttle opening, which is when the needle and pilot jets are in play. It could also be a torn inlet rubber. I rode mine with a massive split in the inlet rubber for 300 miles without realising, and I was getting similar problems to you combined with a rough idle. I had other carb problems at the time, so put it down to them.

Something else just occurred to me - when you cleaned the carb, did you undo the 3 screws that contain the accelerator pump diaghragm on the float bowl? If some water gets in there then it rusts the bottom of the accelerator pump shaft and fills the whole area with rusty crap. This means that you don't get the rich mixture required for accelerating, particularly noticeable in higher gears.

[Mudskipper]

[Robby]

You might be able to do it with the carb in place with a very short screwdriver, but you run the risk of rounding off the screws, so I wouldn't. Take off the float bowl as well, so you can make sure the very small holes between the float bowl and the accelerator pump chamber are clear.

[Mudskipper]

Bit of a bone question but...8mm universal inline fuel filter?

Edit: and 6mm copper core HT lead?
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CBR125|||GSXR400|||CBR400|||CBR400|||CB250RS|||GSXR750|||CB250RS
"You're clumsy, you eat too much and you behave like a 12 year old boy. But you know what? Every once in a while, you find a thumb."

[Robby]

Yes and yes.