Hi all, I've done about 4K miles on my 1991 900ss carby and it's been faultless up until a couple of months ago when it started occasionally ticking over at about 1500rpm, a quick crack of the throttle and it would drop down to normal tickover at approx 1k revs, that went on for a month or so but now cracking the throttle open doesn't result in the tickover dropping, if I let the clutch partially out and semi stall it the revs drop to 1k and stay there until I open the throttle again , when I close the throttle again it's back to ticking over at 1500rpm again, has anyone got an idea what could be causing this?, thanks Mark.
Try easing the rubber twistgrip sleeve away from the actual twistgrip cable body/cover - it can gradually work it's way closer (or further away, depending on what it decides to do) to the cover, where it touches it and causes a samll amount of binding.
Thanks for the reply Arquebus, I've checked and the grip isn't binding, manually turning the grip to closed doesn't affect the tickover either , it's odd that stalling the engine speed down works, almost as though the engine is sucking a sticky slide closed, I think it's possibly some debris or gum inside a carb that's stopping one of the slides fully closing, I'll get the airbox off and have a look.
They are notoriously sensitive and awkward to set up correctly. Mine had similar symptoms to your when I first got it, though it used to idle much higher on occasion. Basically make sure all cables (choke and throttle) are clean, lubricated and routed correctly. As has been mentioned, also check the throttle twistgrip for smooth action. Next get the engine good and hot (go for a decent ride) but be prepared to adjust the idle stop screw immediately on return before it cools too much. It's really awkward to see and adjust and is VERY sensitive. A head torch and a long Phillips No 1 screw driver make it a bit easier. If you do it when the engine is not up to temperature, I found that the setting would still be wrong once it was! Once I'd done this all was well, though the bike won't tick over without a bit of throttle till it's hot. Better that than it racing away at 3k revs when you come to traffic lights though!
If you look at the left side of the bike and at the carb (the vert cylinder one) you can see, at the back of it you can see the end of the slider which is part of the choke mechanism. It is just above the rubber manifold connector. If you operate the choke lever while watching the slider, you can see it move. When you have 'opened' the choke and then 'closed' it, use a pair of pointy nose pliers and see if you can gently pull the slider a bit nearer to you. If you can, it is probably sticking open a bit. The slider can also be seen from the other side of the bike - it sits in two neoprene grommety things and it can get pretty filthy around them. A light squirt of WD40 with a nozzle tube on them may ease it. The best thing you can do for altering the idle is to get a remote adjuster - they were fitted to TDM Yamahas IIRC, although I made mine from a spare throttle stop screw and a spare trip meter adjuster; mounted on the frame.
Thanks for the replies, I checked out the choke mechanism, it was seating properly, gave all the linkages (choke and throttle) a clean with carb cleaner &WD40, checked cable routings, oiled twistgrip, took airfilter out, cleaned slides, liberally sprayed carb cleaner wherever possible, butterflies appeared to be seating correctly, slides nice and free, checked/tightened throttle return cable, tickover still high , added redex to fuel, went for a 40 mile ride, half way through I could get the tickover to drop by cracking the throttle, then it would intermittently either be revving high or be fine at the various junctions/ traffic lights I stopped at, by the time I got home the problem appears to have gone away , so who knows, maybe the WD worked it's way in on a linkage, maybe something dislodged, I was planning to tap the carbs with a piece of dowel, as you would with a sticking float when I got home, but I couldn't get it to replicate the high revs so maybe it's fixed?, or maybe not , I'll post again when I next take it out, cheers Mark.
As an engine starts to run out of fuel, the revs can rise quite a bit because of starvation / weakening of the mixture as such. I'm wondering if a filter on your bike may be getting a bit blocked. Also look at the manifold rubbers - they may have some small cracks / splits in them.
It's easy to get air leaks here as well if the rubbers are not seated perfectly. Carb balance could also be a problem
Well it's fixed, I can't really say what the fix was because I didn't do one task in isolation and then take it for a run, but I've been out a few times now and the high tickover hasn't reappeared, in fact it's a bit too low at bang on 1k revs, I'll have to turn the tickover screw in a fraction as I think 1.1 k revs is ideal, thanks for the tips all.
I had a similar problem with my T9 which turned out to be snagged throttle cabling under the tank after a service. After playing around with the cables the problem went away. Could have been a issue in your situation with the cable having now freed itself.
