Slight stumble/fumble/bumble developed at 3000rpm

[jchesshyre]

My bike's developed what could best be described as a bumble when cruising on minimal throttle around 3000rpm, as if one cylinder is slightly misfiring. It barely affects power output/drive but I can hear it and just feel it, especially in 1st gear. I'm getting a smell of petrol as well when it happens.

Bike is stock ('98 so #48 pilot jets) except for Fuel exhausts with baffles in place.

I checked the fuel tap and filter, since this has been on my list of things to do since the bike is now at 59k miles. There is some what I could call scuffing in the edge of the diaphragm but it is not perforated as far as I can see and appears to operate correctly.

Carbs are perfectly synced and pilot screws are on 2 and 2 1/4 turns. I've tried them both half a turn in and half a turn out and it didn't affect this issue.

TPS is at 500 Ω.

I noticed that the rubber shroud for the front choke valve had come off the carb - I thought this could be the cause but putting it back has not cured it. Is it possible however that a fleck of crud has got sucked in to the choke valve during the time this shroud's been off? I think it will have been like that for a while, possibly since I changed the choke cable in December (since when I've done 10,000 miles ). I think the 'burbly' feeling of the misfire combined with the smell of petrol points to something to do with the choke but maybe an ignition misfire would also be able to cause this?

I plan to take the carbs off tonight and check they're all clean including the choke valves. I'll also check the spark plugs are tight (they've only got 6,000 miles on them) and will check the side stand and clutch switches and also the ECU connections.

Is there anything else which could cause this which I should check whilst I'm at it tonight?

P.S. it's not a carb fart - it does do them now and then but this is different.

[fabiostar]

is the choke cable routed the correct way incase its holding the front choke open a tad?

[jchesshyre]

I think this probably is the culprit, but I can't work out why. I'm sure I've got the cable routed correctly. But I have noticed that ever since I replaced the choke cable (with an OEM one I should add) I haven't been able to pull the plunger out quite as far as before.

I had the carbs off last night to look inside them and they're all fine. I'm wondering whether I've got the right-angled metal ferrule things (or one of them) that go around the choke needles the wrong way round, meaning the needle(s) isn't/aren't quite seating properly. It's a bit hard to tell as in the parts fiche they're covered up by the rubber covers.

[MacV2]

At the knob end...Under the rubber hood is a knurled plastic nut...Mabey thats just too tight to let the choke pull out all the way...

Loosen it a bit & try pulling your knob...See If it make adifference...

[jchesshyre]

Hahaha I'm not that familiar with pulling my knob.

Thanks for the suggestion but I've already tried that.

It's actually not just at 3000 rpm - it's basically whenever I'm riding on only-just-open-throttle.

[MacV2]

Hahaha I'm not that familiar with pulling my knob.

Thanks for the suggestion but I've already tried that.

It's actually not just at 3000 rpm - it's basically whenever I'm riding on only-just-open-throttle.

Ah well theres your problem...Just open the throtle more & that will cure it...

[jchesshyre]

Hahahah generally I find that results in an increase in speed, which can be problematic when there's a bus in front doing 25 with no room to overtake.

Does anyone know if the choke cable can be adjusted? There's a metal hexagonal bit behind the knob assembly which looks like it could be for that (not the big plastic nut, further back than that).

[alanfjones1411]

Hahahah generally I find that results in an increase in speed, which can be problematic when there's a bus in front doing 25 with no room to overtake.

Does anyone know if the choke cable can be adjusted? There's a metal hexagonal bit behind the knob assembly which looks like it could be for that (not the big plastic nut, further back than that).

change down

[jchesshyre]

Sadly I don't have a gear below first

Just to clarify, I'm talking about riding in town at 15-30mph in first (i.e. about 2500-5000rpm) when this is most noticeable.

[tony.mon]

The routing for the choke is often assembled wrongly.
The rear one should go up and over the rear intake stub.
The splitter goes between the carb intakes.

[jchesshyre]

Frustratingly I don't think I've done it wrong. They passes under the coolant hose between the carbs and the rear one loops back over the rear carb diaphragm cover. Does this sound right? Obviously a photo would help but I won't have a chance to get the airbox off till maybe Monday.

Should the wider end of the brass-coloured pipe outside each plunger face the carb or away from the carb? Mine are facing away i.e. with the thinner end entering the carb where the plungers go in.

[tony.mon]

Sounds right.

[jchesshyre]

Sorted it! Believe it or not, the issue was caused by the bracket for the choke knob being mounted (by me when I replaced the choke cable) so that the tab on the back wasn't quite up against the frame, so the knob assembly was (at most) about 5mm further out from the carbs than it should have been. Once I corrected this, I was able to push the cable assembly about 1cm further through the gap under the water pipe between the carbs. There must have been just enough tension for the (probably front) plunger to be *just* cracked open, or perhaps the slightly wrong positioning of the cables caused by it meant that the whole plunger assembly didn't quite seat properly. I guess I've noticed it more now that the air's getting warmer - I changed the choke cable in December.

[tony.mon]

Have you checked plugs?
One may well be sooted up.

[jchesshyre]

Just pulled them after some riding in town and they look like this:

Front

Rear

Although I in theory know how to read plugs, I think modern petrol blends and also iridium plugs change things somewhat. How do these look? On my other bike (CB500) I used to get nice tan coloured plugs and then at some point which I can't quite place they just looked grey from then on...it's been years since I've seen a nice coffee-coloured plug...could this be to do with rising ethanol content in petrol?