Hello there fellow riders!

I revived my bike 6 months ago it's a K100RS 16V which had been stopped for nearly 5 years! Since I have it I noticed a strange noise (toc-toc-toc) everytime it's running on low RPM (2000 RPM).

It stops when the motorbike is superhot (red line with the fan blowing), but when it comes back to normal operating temperature or cold it's there again.

I think it's the valves but I found something strange when I was checking the air intake, theres 4 caps for the vacuometer and the 1st one on the left is missing and there's a rubber hose on it going somewhere inside!!!! I'll post a picture and the video of how the bike sounds.


Hope you can help me!!!

The vacuum caps are supposed to be there and the vacuum hose goes to the fuel pressure regulator behind the throttle bodies.

As for the noise, grab a bloody great big screwdriver, place the pointy end toward the noise you're hearing. Put your earhole up to the handle part and have a good listen (like a doctor does) for where the sound is actually coming from. Sound travels in a motor so pay attention to the area.

If it's valves, remove the rocker cover and check the clearances as prescribed in the service manual. Unlikely that loose valve clearances would extinguish their noise at revs or when hot, but...

A 16V model will require cam removal for replacement of the shim under bucket if clearances are not within spec. But first, have a closer listen.

Two Wheels Better wrote:The vacuum caps are supposed to be there and the vacuum hose goes to the fuel pressure regulator behind the throttle bodies.

As for the noise, grab a bloody great big screwdriver, place the pointy end toward the noise you're hearing. Put your earhole up to the handle part and have a good listen (like a doctor does) for where the sound is actually coming from. Sound travels in a motor so pay attention to the area.

If it's valves, remove the rocker cover and check the clearances as prescribed in the service manual. Unlikely that loose valve clearances would extinguish their noise at revs or when hot, but...

A 16V model will require cam removal for replacement of the shim under bucket if clearances are not within spec. But first, have a closer listen.

Hello there, maybe not loose... but what about if they are tight? I've tried the screwdriver thing and the sound (to me) comes from the rocker cover... I'll try to open the cover and measure the valve clearance... Maybe they went tight...

Don't panic!
A few years ago I was looking at a K100RS 16v with a view to buy and the owner (a bike mechanic!) told me that there may be a problem with one of the valves as it was making a loud clicking noise at tickover.
Out on the road, no problem. Made me a bit nervous and I walked away.
He contacted me a couple of days later and told me the problem was solved. One of the plug caps wasn't tight on the plug and it was arcing and making the noise. Price had gone up too!
Remove the inspection cover and make sure the plug caps are ok before you get too serious.
My fingers are crossed for you.

Thanks you very much for your answer, Comberjohn.

It sounds a bit loud for a spark its more of a metal thing but for sure I'll take the cover off and press the caps to see if it goes away!

If not... I'll take the cover off and check clearance of the valves...

I'll keep you all updated if I find something!

Thanks again!

That's my point, Adrian.
It sounded like someone was tapping on the side of the engine with a small hammer. It was quite a loud noise.
Brick engines rarely have valve problems despite decades of neglecting to check valve clearances.
If I was a betting man, I would put money on it.

Better a loose valve making noise than a tight valve contributing to less compression. Tight valves rarely make noise unless something catastrophic has happened.

The injectors click at idle. Not to worry if it's them, it's just the fuel injection's metronomic symphony.

The noise is quite loud... It doesn't sound as round as when the bike is really hot on red line with the fans on... So I guess it's not injectors ((

Another problem the motorbike has is that if I put too much on the throttle and I cut it to 0% it goes under the idle and sometimes it shutdown. But maybe this is CO related?

adrian_52 wrote:Another problem the motorbike has is that if I put too much on the throttle and I cut it to 0% it goes under the idle and sometimes it shutdown. But maybe this is CO related?

What you describe here is likely to be the exhaust valve clearances are too tight. I had this exact same problem but worse. Worth checking as its easy to do but swapping valve shims requires a special tool. Search the forum. There's plenty discussed about this and where to get the tools with little cost.

Re spark plugs... make sure you have the connectors screwed onto the spark plug tops so the leads make proper contact. Without the connectors, the lead plug seems like its attached properly but the contacts aren't touching... then you get arcing.

Good luck

I would say it is valve clearance and that when it gets realy hot the stems expand just enough to bring the clearance down to a level where it wont click. However when you have the cover off also check the cam chain for wear on the tensioners and chain and possible sprocket damage.

Hello once again, I'm not sure of what those tensioners look like, as far as I know they are in the front of the bike, not on the left side, right?

I guess to know if they are worn I have to take all the camshafts out right?

I never did any kind of operation like this... 

The bike has 68000 km.

I'll get some courage and take that cover off, after I buy the silicone seal, and the feeler gauges...

Worth taking a look at this video from Chris Harris on Youtube.

Go easy with the silicon. There's a special product for that. Chris Harris' video on changing valve shims explains how to get the best seal on the valve cover using minimal gasket silicon. I don't use any on mine and I have no problems. You just got to make sure you have a very clean rubber gasket and all sufaces are totally free from oil. You need to mop the oil from the areas inside where it pools so it doesn't migrate to the seal surfaces when you replace the gasket & cover. I even degrease the seal and metal surfaces sparingly with brake cleaner. Getting the right tension on the cover bolts is important as well.
Once you have it all clean (ie all the old silicon cleaned off) its far easier to take the covers off and on without fussing with loads of silicon.

Hello there, dear K riders.

After surfing the web a little bit, I found people with same problems as me on low revs, it seems the problem comes from somewhere around the chain tensioner.

I will take it out hopefully, I noticed the hall sensor is a little tricky, if anyone has any advice please I'd be more than thankfull!

Thank you very much.

Hi there, hope this is isn't too late (or great if it is because you've fixed it!).

I had a very similar problem - loud knocking when cold and at low revs. Turned out to be split rubbers between the injectors and the cylinder head. Found this out after replacing the cam chain tensioner and doing the valve clearances (only 2 exhaust valves needed altering).

I found it accidentally when I tried to tighten the clips on the rubbers whilst the engine was running. I slipped and accidentally pressed against one of the rubbers and the knock stopped!

Cheers