1991 K75S Knock

[Dubuque]

Hi,

New to the forum so this is my first post/question.  I got a 1991 K75S back in May and recently noticed a knock when I get to 45-50 mph.  I can hear it and feel it through the handlebars.  I thought it might be the valves out of adjustment but did a valve adjustment and the knock is still there.  Above 50 mph it is hard to tell if it is there.  I did check back thru old posts but cannot find anything other than comments on valve adjustments.  FYI, I put new tires on in July and took a 1K round trip but did not notice this knock then.  The bike now has around 44K on the clock.

Any ideas?

Thanks in advance!

Tom

[The Dude]

Dubuque,welcome!
We will need more info.At 44k miles this is not expected from any mechanical source,except ,maybe the transmission shock absorber spring broken.Its a spring loaded cam.If so you can ignore it.
Valve adjustment no.The four cylinders models can have similar noises but at much higher milage.
So,is it happening on the overrun,loaded up or coasting?Top gear?
Throttle butterflies synchronised?
Can you replicate it without moving at speed?
Give us as much info as you've got on bike history,even an audio file may help.
All the best.

[rbm]

Does the knocking vary with engine speed (i.e. gets more frequent as the engine RPM increase) or road speed (i.e varies in direct proportion ot the speed of the motorcycle)?  Does the knock persist if you pull in the clutch at speed?

[Laitch]

It seems to me like a wheel was out of balance and the vibration was acting on some other part to create a sound. It might even have been a loose front axle or loose rear wheel bolts. The act of changing the tires seems to have corrected it all. Enjoy it while you can! :giggles

[Brad-Man]

The way the OP worded and arranged his post seems to be confusing.

If in fact it is still knocking I would ask if at the speed he notices a knock while cruising if he shifts down one or 2 gears if the knock stays the same?

If it does it is not engine related.

Knowing that I suspect would be a big relief.

[The Mighty Gryphon]

Brad-Man's question is excellent.  We need to know if the knock is MPH or RPM related and whether it occurs when the engine is pulling hard, engine braking, or coasting. 

Is the sound a dull knock like a hammer on a big piece of wood, a sharp metallic clank, or more of a rattle? 

I had a clanky rattle between 1200 and 2000 RPM in my K75RT which turned out to be the balancer shaft.  Took the engine apart to find it, and saw that it wasn't anything to be really concerned about other than it made the bike sound bad when pulling away from a stop.  If you noise is in a fairly small rpm band I would suspect the balancer and wouldn't worry about it.

[Dubuque]

Thanks for the input guys. 

I did try different gears when on the road and no difference.  Just checked while in the garage and I did pick up the knock when in neutral and on the center stand once it hit around 4K rpm.

[The Dude]

Thanks for the input guys. 

I did try different gears when on the road and no difference.  Just checked while in the garage and I did pick up the knock when in neutral and on the center stand once it hit around 4K rpm.

Probably the ballancer shaft/transmission shock absorber resonating because your preload spring is broken.No worries,just keep on riding unless it gets worse!

It seems to me like a wheel was out of balance and the vibration was acting on some other part to create a sound. It might even have been a loose front axle or loose rear wheel bolts. The act of changing the tires seems to have corrected it all. Enjoy it while you can! :giggles

Laitch,you seem to have had some insight beyond my powers of detection.Again.
As it happens I have had loose rear wheel bolts on two occasions,once where I had to conclude sabotage and once when I could only blame myself for being an idiot.Fortunately on both occasions the only symptom to get my notice was what I thought was a rear tyre(tire)puncture...Phew! :falldown: :riding:

[Martin]

Try using a stethoscope, long handle screwdriver, or piece of garden hose to determine where the noise is coming from. Either get somebody to hold the throttle or set it to 4000 rpm. Also when you say you tried different gears and there was no difference,did you try pulling the clutch in and  was there any change at all between road speed, rpm or road surface.
Regards Martin.

[The Mighty Gryphon]

I did try different gears when on the road and no difference.  Just checked while in the garage and I did pick up the knock when in neutral and on the center stand once it hit around 4K rpm.

So the knock is RPM dependent and occurs even when not in gear and turning the rear wheel.

How about when in neutral and the clutch is pulled in?

What is the nature of the noise?  Soft/muffled?  Clanky?

[Dubuque]

The noise is clank type knock.  Not muffled as far as I can tell with the engine noise.  I'll try pulling in the clutch but it was in neutral already so no load on the transmission. 

Could it be the monkey nuts in the alternator? Do they cause a noticeable knock?

Tom

[Laitch]

The noise is clank type knock.

Record it and post it here.

[The Mighty Gryphon]

If that clank is coming from somewhere around the rear internals of the engine and can't be easily pinpointed I would guess that it is the looseness in the coupling between the output gear and the balance shaft.  Being deep in the engine the sound will appear to be coming from everywhere.   This is what caused it on my K75, and the design of the parts almost guarantees that it will eventually occur on almost every K75 motor.   

The good news is that while it can be really annoying to hear, I can't see where it will cause any problems in the engine.  If you want to correct it, the fix is fairly easy, but requires removing the clutch carrier.  With the carrier out, clean the face of the clutch hub and the mating surface of the balance thoroughly with brake cleaner, immediately reassemble and torque the clutch nut to the high end of it's specification.  This will firmly attach the balance shaft to the clutch and prevent the relative motion between the two that results in the clanking.

BTW, you still haven't told us what the RPM is where the clank/knock occurs. 

[Dubuque]

The knock is starting around 4K rpm.

[Laitch]

The knock is starting around 4K rpm.

Here's a thread you should consider reading through, Dubuque.

[The Dude]

The knock is starting around 4K rpm.

Definately a resonant frequency function then,not big or little ends.ie the oil filter hasn't come off in its past.I start it cold and killswitch (on/off) to stop engine rotating and count the seconds for the oil pressure light to come on.indicates how quick any leakage occures out of various bearing clearences.I do it regularly as part of getting to know what is normal.
 Could be your gibbon nuts,switch lights on/off to change the alternator loading and see if that affects it.Probably the spring gone in the ballancr/ shock absorber shaft,too deep in the entrails to be actually bothered with.Slip the clutch to change that loading at knocking speed to see if that might be loose.
A series of recordings under standard conditions would be usefull if only to objectively tell if it is getting worse....I'd just thrash it regardless.They are bomb proof....especially at a genuine 44k.

[The Dude]

Here's a thread you should consider reading through, Dubuque.

Spark plug swap!Id never have thought.Default rearranging the leads as a result?These are truly mysterious beasts.
I've got a little model.I might stick pins in it...I'll try the tyres first. :yow

[Laitch]

Spark plug swap!Id never have thought.Default rearranging the leads as a result?These are truly mysterious beasts.
I've got a little model.I might stick pins in it...I'll try the tyres first. :yow

I think draping shop rags over the rearview mirrors and then randomly banging around with a wrench as though changing sparks might be effective, too. Don't forget about wafting sage smoke into the air box for good measure!

[The Dude]

I think draping shop rags over the rearview mirrors.... Don't forget about wafting sage smoke into the air box for good measure!

Ahhh,the mirrors are the portal into it's "internet of things".The things they must have seen!
Sage smoke wafting,implies acquisition of spiritual wisdoms.Must try that sometime then I'll let the bike have some,if it works.My bike tyres are still up.Its just the model I've got to fix....
Now,about this knocking noise?The late wisdom of Gurru Sage Johnny would suggest "earplugs and a good thrashing."Im prety sure that's good advice but administered with all care and no responsibility,as always.

[Dubuque]

Well I went out for a good thrashing this morning and to let a couple of other people have a go at what they think it could be...... Of course, there was absolutely no knock today!  Went out on the freeway fist thing and ran up to 80 mph and nothing.  Did several stops and ran around most of the day and nothing.  Did about 100 miles overall at various speeds and it was smooth as the pavement would allow.  The other folks I consulted could not get anything out of it either. 

Could the monkey nuts in the alternator be intermittent?

I will change the spark plugs and wires as suggested in the one thread just for good measure.

Must be all the discussion of sage smoke wafting into the airbox.....

Thanks to all for the discussion.  Will see if it happens again...

Tom

[Laitch]

Must be all the discussion of sage smoke wafting into the airbox.....

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[Brad-Man]

Maybe it overheard you say you'll have to work on it again and changed its' behavior to avoid same... :hehehe