'83 K100 RT slight stumble on throttle from idle

[vitjakes]

Dear fellow brick-owners,


I've just got my hands on an '83 K100 RT with 164k km on it (WOW) that my friend bought and handed it over to me to service it. I've done all the fluids replacements and the bike seems to have some problems. It starts well, goes like a champ and everything, but it seems to be stumbling a bit when I give it a good amount of throttle on idle (both cold and hot) and it's accompanied by an audible rattle. He'll probably be selling it so I'd be glad if I could get it solved. From what I've read so far this could come from out-of-sync throttle bodies or air leaks which I have no idea where they usually come from on this bike. Also, sometimes idle stays higher (100-200 rpm) before it gets down, which could lead to air leaks.


Any ideas what I should check? Here's the clip. Thanks!

[vitjakes]

Oh and also when I'm taking off and gradually releasing the clutch, the whole transmission/drive seems to be a bit noisy / causing minor vibrations - how much is normal with these bikes?

[Laitch]

Any ideas what I should check? Here's the clip!

No clip. Are you using the Attachments and other options feature? Try again.

[Laitch]

I've just realized that the stumble isn't REALLY that much noticable, but it is there.

. . . the whole transmission/drive seems to be a bit noisy / causing minor vibrations - how much is normal with these bikes?

The stumble is not there for me and neither is the rattle but I expect my motorcycle to be noisier than a Prius and maybe don't notice what others would. How much vibration is normal is impossible for me to say. Some people are more sensitive to that than others. The bike should not vibrate so much it blurs your vision, I guess. or knocks off your helmet.

It is an old bike with a lot of mileage. Air leaks can be at crankcase ventilation, at throttle body manifolds and at the air box. Read this link. Visit the Technical Library Card Catalog for more information. All hoses might need to be replaced with the correct type. Is the air filter clean? The fuel filter should be replaced; the tank should be checked for debris and cleaned if necessary. Are the injectors working well? Are the spark plugs in good condition? How do the spark plug wires look? Do they light up in the dark?  :nono :giggles

Do all that, set the valve clearances then balance the throttle bodies. Read this document. It illustrates and explains most of the systems.

Take off the final drive and lube the final drive splines, lube the transmission output splines, remove the transmission then lube the transmission input splines.

Nothing to it!

Otherwise, just put clean fuel in it and ride it until it stops.  :hehehe

[Filmcamera]

I would be delighted if the hesitation on my bike was so small!

[vitjakes]

I guess I must be too used to the perfectly-tuned Japanese inline fours.

[Laitch]

I guess I must be too used to the perfectly-tuned Japanese inline fours.

I have no experience riding 34 year old Japanese fours with 168K km on them so I'll take your word for it. :giggles

[The Mighty Gryphon]

First, hesitation with throttle is a result of the MAF sensor in the air box.  When the throttle is snapped open it takes a few milliseconds to open the door of the sensor to allow for full air flow and there is further delay while the engine control unit gets the signal from the MAF and opens the injectors for more fuel. 

The delay is only really noticeable when the engine is unloaded.  When riding it disappears. 

Idle issue could be the crankcase breather hose.  It is a short zig-zag hose the goes from the top of the engine behind the airbox into the back side of the airbox right behind the throttle body for cylinder #4.  This hose is notorious for cracking and leaking where the clamps bear on it.  Fortunately it is a pretty inexpensive part and if it were my bike I would probably just change it now, because if it isn't bad now, it will be some time in the not too distant future. 

There is an anti-backlash spring on the output shaft that when it goes bad allows the gears between the crank and the output shaft to rattle a bit.  Again, this is most noticeable when the engine is unloaded.  Setting valve clearances and balancing the throttle bodies goes a long way toward smoothing out the rattle and some of the vibrations.  Be aware, the K100 does vibrate a bit, not enough to be a problem, but it is there.

There is another source of noise and slight vibration and that is the alternator drive coupling.  There are some rubber cushions in the coupling that can wear out over time and allow the metal parts of the coupling to rattle against each other.  There are many threads addressing this.  Search on Monkey Knutz or Monkey Nutz.

[Nine80seven]

All rubber parts connected to engine suspect.  Slight advance of timing may help?  Generally any bike that's over thirty years old needs to be carefully gone through, cleaned, adjusted, new parts, etc., etc., etc.  Payoff is they don't don't make them anymore and never will.  Love waving at new bikes as I go by.  "Whachya got there broheem?"  "Eighty four RS, see ya later!"

[milq]

I would be delighted if the hesitation on my bike was so small!

I thought the same thing! But the thread got me thinking and I grabbed a light to inspect the crankcase vent hose on my bike. Surprise! its in bad shape, looks like someone smeared some RTV on it around the clamp area in the past. From the bends that it makes I think I need to find a proper one, I doubt that heater hose or similar will make the bends.