Long story short: K100RS 1984

• Engine momentarily missing when quickly revving 
  
• Idle screw settings significantly different.
  

I have posted some of my machinations on other posts but I though I might collect things together here and bring the latest issue(s) to the fore.

Originally, I was pursuing idling issues - which might be labelled an air leak.
Bike might idle acceptably when first started but once hot, revs rose. I assumed this was due to "things expand when hot" ... so rubbers.

12 mths earlier, I had replaced rubber caps on TBs as well as idle screw o-rings
Serviced fuel injectors (filters, o-rings, flushed internals with carb cleaner)
New spark plugs. Caps pinched tight 

Replaced valve shims - all set to correct settings, 1 out by .05 (my error)
Replaced upper and lower throttle body manifold rubbers. (definite evidence of lower rubbers 'o-rings' flattened - though no evidence of idle fluctuation when LPG, throttle cleaner or WD40 sprayed at the base)
Cleaned TBs - removed and cleaned butterflies and reset to same amount of movement when forced closed (ie. they all contact the TB throat at the same time. TPS clicking when throttle just opened
Replaced oetiker clamps

Checked flow on each of the TBs - managed to get them generally the same ... however...
Screws should be within 1 turn of each other. Mine are: 
#1TB - 4 turns out
#2TB - 0 " "
#3TB - 3.5 " "
#4TB - 1.75 " "
Q: Am I to concentrate on getting the air flow the same or the screw settings? I would have assumed the former. So why the big difference ... valves not sealing properly?

At this point a test ride showed the bike running okay. Engine braking more noticeable. Still got an annoying vibration at 5000rpm - which makes me reluctant to keep it at this rate (though I did get it up to 6500rpm)

Did a compression test today and got the following settings
#1 - 160 (I assume it's PSI)
#2 - 170
#3 - 160
#4 - 180

When I had it all back together, I ran it and revved it and noticed a definite drop in revs when first quickly revving the engine up from idle - even cut out one time. This was a concern.

Elsewhere, I have the usual sprag clutch blues plus what sounds like a rattle coming from the engine (might be output shaft rivet play) but nothing that (I think) should be affecting idle issues.

Any things else I can address ... short of pulling the head and checking valve seats?

Any ideas appreciated

Regards

GF

What are the readings of your moto’s coils compared with those in the Troubleshooting the EFI document in this site’s Portal? Are those the original spark plug wires? Have the frame grounds and electrical connectors been cleaned? And what's the condition of the blasted Z-hose. 

Thanks for the prompt reply Laitch

Z hose new - but will replace (always good to have spare)
Earth grounds good (was in there 12 moths ago, earth ground from battery upgraded to thicker cable)
Battery good - fully charged

Coil readings not known and will hopefully have a look at them today or at least this week.

HT leads not new - so some parting of cash needed (are there alternatives to BMW?). Is there anyway to test these above and beyond the static bench test / OHMS readings? I was told that if the bike can accelerate from low speed while in top gear then the HT leads should be good because they are under load. 

I reconcile any frustrations with the reassurance that all mechanical issues are finite.

Regards

Guy

Yeah I wouldn't buy leads unless they were bad. 

Is there a photo in post 1?

For some reason I don't see some photos on my phone.

What's the history like have you had it long, been sitting long, high km's running good at some point?

Edit: I'd look to get the screw settings the same, like it looks like a leak at #2. The compression is OK so you shouldn't have to take the head off.

Early coils have given other owners problems. I think they had some sort of insulation issues on the high voltage section that caused spark leakage. It was corrected in the later versions.

Full throttle at low speed makes the engine run at maximum compression and cylinder pressure. The higher cylinder pressure when the spark plug is supposed to fire requires the maximum voltage to be delivered by the coils. If the insulation is failing, the voltage will leak and the spark won't happen. That is what your low speed 4th gear acceleration test is looking for.

As far as aftermarket plug wires, NGK has sets for the K100 that sell for about $60US.

Thanks for the replies

I was thinking that it might be electrical... the (previously) slight but consistent hesitation at idle is now appearing as a clear drop in revs when the throttle is "blipped"" at idle. The system is unable to immediately climb and can cut out.

I'm assuming that the fuel pump, injectors and throttle bodies will still be the same. 

My father in law (a life long tinkerer) suggested that the air/fuel mix is not providing fuel. However this would mean adjusting the screw at the air box. Considering that it ran satisfactorily previous puts some doubt on this.

My only question mark over the coils is that there has been a noticeable change since I changed the valve shims, upper and lower manifold rubbers. The only thing I have played with is removing the butterflies, cleaning and reinstating them...careful to get them as close to each other as possible.

Idle is currently 900-950 and once the revs are up bike is running fine.

If I am to replace the coils are any particular versions / years better? Are there after market versions? Can I use 4 bright red ones covered in lightning bolts intended for cars? 

Just checked online...motobins UK not showing any, munich motorcycles Melbourne Australia (wait for it) $500 each.

Regarding HT coils, does anyone have a link to the NGK site? 

I have three 1984s so electrical components are of similar age and potential problems.

Cheers 

GF

I suspect that the hesitation when "blipping" the throttle at idle is the result of the Jetronic not being able to identify the blip so it can inject enough fuel to prevent a momentary leaning out of the mixture. If you have ever worked with old school carburetors you will recall they had accelerator pumps that squirted a shot of extra fuel when the accelerator is punched. This extra fuel helped prevent the problem you are seeing with your bike by momentarily enriching the mixture. Unfortunately, the Jetronic doesn't have that function.

My bikes, especially the K75's all have that hesitation and are happier when I roll on the throttle coming off idle.

Update: Injector / Fuel system cleaner helped

Probably motivated by being a tight arse, I thought about proceedings ... even though I had serviced the injectors, used 98 octane fuel and assumed no air leaks (Correct ... I think), I recently put an injector cleaner additive into my car Isuzu / Holden Rodeo RA 2007 - designed to run on E10. The difference was noticeable.

I began imagining that, if "blipping" the throttle provided a lot of fuel that the bike couldn't burn then it might be flooding / choking itself.

So measured an amount (125ml for 20L tank if I remember) went for a higher speed ride and this seemed to do the trick. I was able to blip throttle and bike responded (though not like some modern Jap racer of course)

Of course, might have solved one problem but found another... revs wanting to climb when hot. As stated above, I would normally assume this was an air leak and as rubber expands when heated I would go looking at the manifold rubbers et.al)

However, previously I had to scrounge a throttle cable from another bike but it seems to be too short ... by about 2mm and the centre throttle screw is not bottoming out. Seems to get a little worse when hot. So this is the next target. 

On this front, I took the cable I wanted to duplicate to local mechanic who said that the little squared ferrule (throttle end) is pressed on. Any thoughts anyone? I would like to reuse the original outer. I got thinking I could buy an aftermarket copy (of the K100 throttle cable) and cut the other end off, measure to the right length and reattach the lower ferrule. I haven't seen the little ferrules available anywhere nor a way of attaching them. I guess a MIG welder and depositing a lump of weld then ground to size might work??

Winter's arriving and the summer jacket will be retired. Got caught out on the weekend but the zip had jammed so it was magazine up the shirt til I got home.

First world problems

GF

Don't weld it as the heat will destroy the strength of the inner and it will break very quickly. Buy an OEM from Motoworks or Motobins

Thanks Rick

Will do / not weld

GF

If you replace the throttle cable support bracket at the engine, as well as just the cable, that might fix your problem, especially if they are different model years. I had that problem once, because the bracket offset was different, but that was with RT models.

I think I've said before, a piece of brass tube 3.0mm OD x 1.0mm ID and about 8mm long soldered on will do the job of the crimped-on ferrules. The throttle cable on LFB has done around 40,000 miles like this.