OK, so what is causing this oddness? Running a bit funny.

[northwind]

Developed a bit of a running problem today, and I'm a little stumped. It's possible this problem's been there for a while but that I've just not ridden for long enough, this is the first time it's come up.

After about 20 minutes of typical mid-throttle riding on bendy a-roads, I develop a stutter. It sounds and feels like fuelling, in fact, it feels exactly like carb icing. I'm reasonably sure it's not a wet plug as mine is sealed up, and it wasn't that wet (plus, it didn't really feel like it, I think I was dropping some sparks but not all). So, what I'm getting is a mild surging and a very obvious loud misfire.

What's odd is that if I immediately roll off the throttle as soon as it starts, it fades away (though comes back a few minutes later) but if I carry on with a constant throttle, it gets drastically worse over 30 seconds or so.

Once it happens, the only thing that reliably fixes it is stopping- after a minute or so of idling or being switched off, it's happy. Oh, the idle is perfect by the way, even when it drops from heavy misfiring it goes straight to a stable idle.

But, it doesn't happen at all at higher revs- I managed 30 miles of high speed motorway without a flutter.

I've not changed the fuelling lately, and it all looked fine last time it was dynoed. It feels fine the rest of the time, nice quick pickup etc. Definately not a stuck choke. Oh, and I'd blame bad fuel but I refilled with only about 2 litres left from a different source and it still happened.

Any suggestions? I'm a little stumped... Going to check the plugs and air filter, and clean and balance the carbs tomorrow but that's just kind of random servicing, not really a targeted cure.

[Holdup]

Perhaps along with cleaning the carbs check the fuel lines incase theres sh*t in there?

[Jamiebridges123]

If you DID have bad/stale fuel, it may take several tanks to flush it out.. try using a few tanks of V-power before going much further. And yeah clean, flush and reclean the carbs..

[petevtwin650]

Sounds a little like the issues I had with my curvy Northy.

Put that down to the Factory Pro needles wearing the emulsion tubes.

[Grinch]

I'd check the signal generator, sounds a bit like my symptoms when it decided to start packing up.

[Bibio]

worn needle/emulsion, worn float needle, sticky float, water in the float bowl...

i'll go for sticky float or water in the float bowl...

[ricky]

im glad you mention this as this is what i have had the last few trips on the bike and was thinking i have a prob

watching with interest lol

[NedSVS]

Does it stop missing / stuttering if you wind it open from this midrange throttle position? It may have become too rich because of emulsion tube (needle jet) wear but the motor will handle a rich mixture better under load. I checked my emulsion tubes for wear by making pins from suitable size steel wire and filing / polishing the end to an elongated oval shape accurately measured with a micrometer. I found they were about 0.075mm (0.003") out of round at 32000 miles (almost half of which was done with Factory Pro stage 2 needles).

[embee]

....obviously check carb diaphragms too.....

[northwind]

It doesn't improve immediately under load, and the misfire's bad enough that it won't pull effectively up to higher revs so I'm not sure if that would actually cure it, since I can't actually make it happen :( Another observation- it's not heat/run time dependant, last night it happened after 2 minutes ride from cold. Until now it's always come on after more riding.

I think all the comments about the needle circuit are spot on, that's the obvious home for this problem. I would have thought that worn emulsion jets would be a constant issue? But maybe not. I've not had it opened up yet so I'll take a look today and see if there's anything obviously dubious, I've got some spare carb bodies so if worst comes to worst I can just swap bits.

Oh, the emulsion jets, are they replacable?

[NedSVS]

If the plugs are a bit wet with fuel it might take a second to clear under load. If your midrange mixture is borderline, then a change in atmospheric pressure or temperature could take it over the edge. If you have got a micrometer and the time and patience you could make the measuring pins I mentioned and this would also tell you if your spare carb bodies are in better condition. I was lucky, the spares I had bought for the carb heater kit left attached to them were unworn. Worn emulsion tubes can be offset by lowering the fuel level (to do this raise float height) but you will need to go up about one main jet size for about each 1.5mm lowering of fuel level. There are no mikuni emulsion tubes available. Factory Pro offer a replacement service, but it's too expensive to even consider. I have removed the emulsion tubes from my worn carbs and they are of quite a simple design that any good metal turner with a small lathe could make. Unfortunately I no longer have my lathe - had to go when I moved house. Anyway, I think there are enough low mileage carbs around to keep us going for many years yet!

[northwind]

Yeah, that's the thing, from their condition i don't think these carbs have seen many miles, whereas my old ones had done 40000 or so and were still in fine nick.

The mix should be spot on, I've not changed anything fuelling wise since it was last dynoed (apart from riding it, so the filter could be an issue of course) and while it had a ****-weak idle then, everything else was nice. It doesn't ride like it's fuelling badly, except when it goes wrong.

Oh, this is happening in basically all atmospheric and temperature conditions I've thrown at it, from fairly heavy rain/spray to bone dry to cold night riding... So while I wouldn't rule it out entirely, I don't think it's environment dependant, I think it's purely hardware.

[NedSVS]

If you're really struggling to find anything else wrong, and don't want to mess about with carb bodies or fuel levels, you could simply lower the needles one groove to rule out the rich midrange question. It may well not be the best overall setting but might help with diagnosis.

[northwind]

Nothing obviously wrong, finally got the carbs torn down tonight. Plugs are both fine, this definately isn't a wet front plug as it was bone dry and no corrosion, the sealant I use has held up.

Carb wise, there was a little dirt and varnish in there so I nuked it with carb cleaner, stripped and checked the slides and needles, which all look fine. The diaphragms looked a little... odd, nothing apparently wrong with them, no holes or perishing but they'd crinkled a bit so I've replaced those with known good ones from my original carbs. No fuel hose probs (I was half hoping to find a kink or a compression which could cause transient problems, but no). Chokes are fine too, I'd ruled them out already but it's nice to know. Filter looks perfect.

(my original carbs are missing some bits otherwise I'd just swap them on, I have 2 sets in the garage but neither is actually in working order :smt101)

So, threw it back together, too late for a test ride so I'll see how it does tomorrow. Fingers crossed...

[Domas]

Another idea : could be that your carbs don't get enough fuel. You can try running with fuel tap on free flow position. Maybe there is a hole somewhere in vacuum hoses.

I hope you sort it out soon.

[yorkie_chris]

Tap doesn't have a free flow position! It is vacuum only.

Also be careful using this, prime setting on some old suzukis is very restricted.

[Dave20046]

When was the last time the plugs were changed?

[northwind]

No problems today, but then that might just be because I only rode about 10 miles. I don't think it's a general fuel starvation issue, since you'd expect that to be worse at high revs, rather than going away entirely...

Plugs are fairly old, 7000 miles or so but they're NGK iridium and the last set did 20000 miles without loss of performance, they both look absolutely fine so I don't think that's the issue- might change them over if there's still a problem just to eliminate them.

Fingers still crossed ;)

[Dave20046]

No problems today, but then that might just be because I only rode about 10 miles. I don't think it's a general fuel starvation issue, since you'd expect that to be worse at high revs, rather than going away entirely...

Plugs are fairly old, 7000 miles or so but they're NGK iridium and the last set did 20000 miles without loss of performance, they both look absolutely fine so I don't think that's the issue- might change them over if there's still a problem just to eliminate them.

Fingers still crossed ;)
I'd change 'em just to rule it out, plugs are like what £5? I'd say it's worth a pop, would need doing soon anyway.

[northwind]

Unfortunately it's still doing it. Ned's observation on load seems to be spot on, it only happened once today in a 250 mile ride using a lot of the same routes as last time, but last time it was pretty bad. The only differences were that today it was dry, and there was less traffic, so I was carrying more speed and doing much less rolling about on an almost closed throttle.

Not quite convinced about plugs, it still seems to be more likely to be fuelling, but it can't hurt to change 'em. I'll rebuild one of the spare sets of carbs and fire them on first though, clean break...

[Bibio]

iridium plug gaps go out quite easily...

hows your battery for health?

also try tapping the float bowls when you are out and it happens, see if that helps.

are you sure that the carb heaters are coming on and no loose wires or corroded connectors..

[northwind]

Won't be carb heaters, some territory bikes come without them and they do fine (and this isn't carb icing weather, it was scorching today!) Battery should be OK

The trouble is, it'll only do this in the midrange and under some load, and unpredictably, so I can't recreate it in the garage. Bit of a pain that, a lot of the usual diagnosis tricks don't work. I can ride around it at least (actually, in some ways it's quite good, I'm a lazy rider and I tend to do too much riding at lower revs, now I can't!)

[Bibio]

it really really sounds like a sticky float.. or a bit of crud floating about inside float bowl..

[northwind]

Bowls are as clean as I can make 'em without a sonic cleaner... Floats seem to be moving fine, but then they would since the bike works absolutely fine for 99% of the time or more, the chances of it sticking just when I'm checking are slim I guess. But yep, the explanations that made most sense were slide diaphragms, dirt in the carbs, a float issue or the emulsion jet being worn, and that's probably still true.

Sod it, new carbs time, it's not worth spending any more time messing about on since I've got all the parts anyway. And it should fix any of those 4 at a stroke.

Thanks for all the feedback folks... I'm pretty handy but I do sometimes overlook stuff so it's really useful to have you watching over my shoulder.

[Dave20046]

iridium plug gaps go out quite easily...

Got to admit I did neglect to gap my new plugs, they weren't too far off what they're supposed to be though IIRC. Can it make much difference (before they go completely out) can't say I've even noticed tbh

[ricky]

Bowls are as clean as I can make 'em without a sonic cleaner... Floats seem to be moving fine, but then they would since the bike works absolutely fine for 99% of the time or more, the chances of it sticking just when I'm checking are slim I guess. But yep, the explanations that made most sense were slide diaphragms, dirt in the carbs, a float issue or the emulsion jet being worn, and that's probably still true.

Sod it, new carbs time, it's not worth spending any more time messing about on since I've got all the parts anyway. And it should fix any of those 4 at a stroke.

Thanks for all the feedback folks... I'm pretty handy but I do sometimes overlook stuff so it's really useful to have you watching over my shoulder.


out of intrest has you bike been smelling of strong petrol when you have finished your trip ie when parked up

[northwind]

Nah, no different from usual.