I just bbought a very nice 1989 RS which starts poorly and also falters and idles poorly now and then. When it’s above idle it runs fine. Any ideas guys?

Air leaks and varnish from old fuel are the usual causes of what you are experiencing.

Check the breather hoses for cracks and leaks.  Make sure there are no leaks in the bushings above and below the throttle bodies.  Run a couple tanks of fuel with injector cleaner.  Balance the throttle bodies.

Point-Seven-five wrote:Make sure there are no leaks in the bushings above and below the throttle bodies. 

This. Spray some starter fluid around the bases of the throttle bodies at idle. If the idle increases slightly then the boots below the TBs are cracked and leaking air in. Not uncommon as these bikes age.

That looks like a nice specimen.

Thanks, needs a few jobs sorting out, mirror, silencer rattles like crazy and the rough idle. I will then go front to back replacing oils, greases etc and do the valve clearances, plugs and filters plus a spline lube. The last owner had it 15 years and didn’t do much on it. 54k miles.

Update, I spent the last 2 days doing a few service items. Here’s what I found....no fork oil in it whatsoever! no wonder it was head shaking on the ride home. Refilled with some 10w, repacked the wheel bearings and adjusted head bearings. The brake fluid was a disgrace, black and old. Bled it all through and feels much better. Did a rear spline lube, splines in good shape probably due to the small amount of oil in the arm. Bevel drive oil was black also, replaced. Replaced gearbox oil and the old didn’t look great either. Regreased clutch cable barrels. That was yesterday and today made up a good exhaust from the ones I have and refitted that. Fitted the new battery and went for a blast. The faltering has now stopped and the bike feels 100% better than it did when I bought it. Got some new Pirelli’s on the way and just have the motor to freshen up with a flush, some injector clean, plugs and filters. Valve clearances to check and make sure the fan works.
Although I really enjoy bringing bikes back to spec I really don’t get why people let them go for the sake of a bit of tlc. 
For some reason this Rs feels a lot quicker than my standard K100.
I’ll keep an eye on oil levels to determine which seal/seals are weeping but my guess is the gearbox output.

Odd. The RS bevel drive has a lower gear ratio than the standard K, so (in theory) it should be a bit slower. Only a bit, mind. Someone could have adjusted the timing - that would make it pull harder in the midrange.

[Edit] I think it's a lower gear ratio. I may need correcting on this... if so, I blame Jamesons and tha fact that it's well into the early hours of the morning and I should really be in bed.

The RT/LT are 2.91 and the RS is 2.83 ratio but those monolever rear drives can be anything from 2.83 to 3.20 and could be made to 3.39

X2 on the Duck's comment , really nice bike Nobbylon ! , the heatshield on mine broke very early in the bikes life , caused an awful racket , I replaced the exhaust as soon as I could with a Staintune unit , is that the original paint & seat ?

John Re

Melbourne Australia

The tank, fairing front gaurd and rear seat unit are original paint. The side panels and mirrors have been redone. As far as I know the seat is original.
Rear drive is the correct 31/11 for an RS giving 2.8181.....
My standard K is a 32/11 drive 2.909
I’ll check the timing when I do the motor this week. Possibly it feels quicker due to less wind hitting me but it definately feels like it’s pulling stronger. I can’t wait to load it up with camping gear and head off to Belgium on a lads beer tour in a couple of weeks

This is my KRS , an 87

https://i.servimg.com/u/f58/16/85/95/43/pc040014.jpg

keep up the good work Nobbylon !

John Re

Melbourne Australia

Nice colour and pipe John. Did you fit those wheels? 
I checked the valves this morning and against my expectations considering the state of oils and forks over the weekend found them all in spec. Inlets all 15 and exhaust are 25,30,30,30. It’s also had new plugs fitted recently. Warmed her up to check fan is working and it’s fine so now just a flush, a filter and fresh oil. The tiny (so far) oil leak under the box has to be rear main or front gearbox seal. This means that I also have either rear box or front bevel seal weep or both! will ride 1k and then decide on a leak proofing plan.

nobbylon wrote:The tiny (so far) oil leak under the box has to be rear main or front gearbox seal.

It's probably the clutch nut o-ring.  You can do it when you pull the transmission to lube the clutch splines.

Yeah Nobbylon , 2.5x18 three spoke on the front , 4.5x18 three spoke on the rear , a few of the guys on the Forum have done the mod , lets you use a 160/60/18 radial on the rear , it is all mapped out on the thread started by Two Wheels better a while ago , see "wider wheels on a K100"

John Re

Melbourne Australia

JR_K100RS wrote:This is my KRS , an 87

https://i.servimg.com/u/f58/16/85/95/43/pc040014.jpg

keep up the good work Nobbylon !

John Re

Melbourne Australia

yep she's a beauty john

a fine example

Flush with my atf mix now done, sump off and cleaned but nothing in there of note. Re filled with 20w50 mineral. Checked the stubs with quick start but no leaks so I’m counting on the wynns injector clean doing it’s thing. Just went out for a nice blast, runs smooth and pulls cleanly and strong to the red line. The only glitch is the first start from cold which takes a couple of attempts. Once warm it idles perfectly. Just tyres and mirror glass to fit now. I’ll check throttle body balance tomorrow.

Thanks Charlie

John Re

Melbourne Australia

Syncronized the throttle bodies yesterday. 1 and 4 lean, 2 and 3 were rich. It was way out so someone had been messing . The rubber domes on 1 and 2 were split and the regulator hose on 4 was loose. All sorted and starts much better from cold. Thankfully the butterfly screws still have paint on and haven’t been touched.