Idle too high - can't get it down.

[Yorkshire Geek]

This started off with a carb sync. Adjusted the slightly rough idle to the right level then hooked up the carbtune to find that cylinder 1 had nipped off behind bike sheds for a fag and the others were all over the place. After giving it a good talking to (i.e. adjusted the screw) all four leapt together in exactly the right place (265mmHg) and the engine said "FREE AT LAST!" and more than doubled it's RPM to 2800.

With that done, I set out to bring the idle down to the right range (1250ish) but even with the screw fully out it's running at 2800. There is a few mm of play in the throttle in case that's relevant.

Any suggestions as to my next steps? Sadly the Haynes and workshop manuals assume that the previous owners did at least SOME maintenance but as I dismantle things it becomes apparent this is not necessarily the case, lots of little jobs that are made harder by being the first person to do it for a decade or so.

[jaffa90]

Go on, what bike, year, mileage and service history???

[Yorkshire Geek]

'92 XJ600S, 42k miles, service history debatable. Only thing I can guarantee is it's had new plugs, new oil, and valve clearances adjusted (hence slowly working my way through the entire service manual).

[AndyCF]

Air leaks ? That can make the idle speed rise.

Check carb fitment (gasket or rubber tubing etc as appropriate) between carb/engine carefully.

[Robby]

Check the pilot (mixture) screw settings. If they are wound way out it can raise the idle on tickover.

[Yorkshire Geek]

Will go exploring with a can of WD40 later in the week then. Not messed with the mixture at all so hoping it's simpler than that.

[Robby]

You haven't messed with the mixture, but a previous owner may well have. It's just a case of screwing in the mixture screw(s), counting the turns. Standard setting is usually around 2 turns out, I've come across lots of bikes where they were either screwed all the way in (because screws should be screwed in tight?) or screwed out 4 or 5 turns (because more fuel makes it faster, innit).

[Tankie]

If you use vac gauges this can sometimes happen .
Pull off the carbs and set them up by eye ie so they all are open the same amount at idle, then as said reset the pilot air screws if you have fiddled with them.
The reason the gauges lie is that unless all the valve clearances are perfect, there is no air leaks , and all cylinders are breathing the same they will lead you up the garden path with the results you have
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Maybe someone has tried to adjust idle by the throttle cable? There must be adjustment, often at both ends.
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[iooi]

[P.]

Inb4 jaffa 1000rpm and it's all fixed.

OP. Spray stuff around inlets near carbs and see if it revs higher.

Air leaks there if so, cracks in old rubber boots.

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[jaffa90]

[quote="Paddy."]Inb4 jaffa 1000rpm and it's all fixed.
My xj900 was 1,000rpm idle,
https://www.motorbikespecs.net/index.dyn?flag=2&model_id=5834959&make=Yamaha
Also my Fazer 1,000 was 1,000rpm,
https://www.motorbikespecs.net/index.dyn?flag=2&model_id=2147997&make=Yamaha.
Don`t know the 600 divi.

[A100man]

[Monkeywrenche...]

[Tankie]

[Yorkshire Geek]

Thanks guys - plenty for me to go at there once the weekend arrives. Will report back on results!

[jaffa90]

[P.]

Oh shush. You know better than the creators of the bike. Of course you do.

Come get my GSXR and Triple to idle at 1k.

[Yorkshire Geek]

I think mpd72 may have had it - there is a few mm of free play but I'm wondering if that's slippage of the grip before it engages the throttle. If I twist the grip forwards rather than back the engine speed drops so it seems the throttle cable is incorrectly adjusted now the engine is properly set up (possibly for the first time in years).

Will see what happens when I get chance to adjust it properly.

[Yorkshire Geek]

Ok, further data. I tried adjusting the throttle using the midline adjustment and it made no difference. I then tried completely disconnecting the throttle and throttle return cables at the top end, this made no difference to the idle but did confirm that pulling the throttle return cable lowers the idle to the expected range.

I'm not quite sure how that setup works but the throttle return is non-adjustable so I'm a little unsure on my next step now.

[iooi]