K100rs (1991) limited to 140kph

[Sparky]

Hi team, I am beginning to arrive at my wits end. My faithful machine is otherwise running well but hesitates at and will not exceed 140kph. I shocked the bejesus out of myself checking the sparkplugs; which I have replaced and trust that the coils are fine. I wish to note that plug two was loose in it's cylinder for goodness knows how long before I discovered and tightened it. I have diligently checked hoses et al. My question is could the loose spark plug have caused the problem, and if so what would be my next step to exact a repair. I really do not want to blindly purchase a new petrol filter, coils and so forth before exploring other avenues.


At, and consistently at, 144kph the machine stutters and will not respond to further throttle. Electrics are seemingly normal.


I have attempted to thoroughly search all related threads but are yet to find a meaningful solution.


Cheers team,
Shaun (Wanganui, New Zealand)

[Filmcamera]

Does this happen at that speed in any gear? Before then does it rev freely to 7000+? If not then it sounds to me like you have an injector not firing properly. 

[Laitch]

I wish to note that plug two was loose in it's cylinder for goodness knows how long before I discovered and tightened it.

Welcome, Sparky.

Put the year and model of your bike in the Bikes: box of your Forum Profile section so it will show up with every post. Post some photos of your bike, too.

A loose plug runs excessively hot. That can damage the plug as well as contribute to engine damage. Goodness knows how long is not an effective maintenance interval  for a high quality machine.

• First, check all the plug wires to be sure they are pushed down completely and firmly connected to the plugs.
• Remove the plug that was loose. Look at its electrode. There might be debris in it causing the missfire. Check its gap with a plug wire-gauge and let us know what it measures.
• Clearly label each of the other plug wires with the cylinder number of its plug, take a photo or make a drawing of how the wires are routed to make their connections with their plugs then remove all the other plugs and measure their gaps, too. Let us know.

[The Mighty Gryphon]

Is the problem at 144kph a pronounced flat spot, or does the engine gradually lose the ability to accelerate as the speed increases?  That may offer a clue.  Will the bike accellerate to redline in lower gears?

Have you checked the ignition timing?  The brick engine seems to be pretty tolerant of a wide range of timing, but there is a limit.  Some owners have had good results with a slight advance. 





 

[Laitch]

Some owners have had good results with a slight advance.

The terms advance and retard might be slightly out of reach right now. :giggles

[Martin]

Check the plugs after a top speed run. Ideally a plug chop is called for. This involves running at the problem speed, and cutting the engine off and at the same time puling in the clutch. You then pull over to a safe place and remove the plugs and read them.  Post pictures of the plugs if possible. Doing a plug chop takes a bit of practice, so if you are not confident in doing this. Just take pictures after a high rev run in second or  third gear, that way you stand less of a chance of getting booked. Another possibillity is a fuel restriction a collapsed fuel supply or return  line, blocked filter, restricted screen blocked pump.
Regards Martin.

[Sparky]

Thank you, good people, for input.


I took the machine for a jaunt and happily red lined through to fourth gear. The bike was still pulling at 160kph in fourth gear. I then dropped down to circa 100kph (60mph), engaged fifth gear and accelerated through to 170kph seemingly with no problems; that is to say 140kph was exceeded without incident. And with luck an end to my problem.


But just to be clear, could it be that the cylinder with the previously loose spark plug could have caused excessive fouling of the plug which has since burnt off; or perhaps a soiled injector, come right? Possibly?


Laitch, I put the 'cart before the horse' and in doing so failed to do due diligence, by reviewing the plugs appearence (I opted to replace the plugs). I have learned from my ways and will be more methodical in my approach to problem solving in future. Anyway you have a firm but effective bedside manner.


I will, no doubt be calling upon fellow Brickers' expertise in the very near future.


Cheers,
Shaun

[The Mighty Gryphon]

I suspect that with the loose plug you were losing that cylinder at high rpm and high loads.  Wide open throttle allows more air flow into the cylinder raising the pressure at maximum compression in the stroke, exactly the point where the plug needs to spark. 

The higher the pressure is in the cylinder, the more resistance there is to the spark.  Couple that with a bad ground(the result of the loose plug) and that cylinder will stop firing properly just at the point where it is needed most.

At partial throttle and low loading, the meager spark in the loose plug will usually be enough to fire the mixture.  That's why the effect isn't noticed at lower speeds.

Hope this makes sense to you.

[Sparky]

Yes, indeed it makes very good sense. Thank you for your thorough explanation.


Cheers.
Shaun