K Bike"rattle"?

[tricyclerob]

Hi All, To begin this saga, I had a 1992 K75S for a few years and it semed smooth as silk. Tiny bit of very slight noise under acceleration, which I had always figured was the K bike "rattle". Well, I sold the "S" to my neighbor, at 24,000 miles to add to his stable, and bought a 1992 K75RT, with 37,000 off e-bay. The RT looks great but has an issue which worrys me. Beginning at about 1200-1300 rpm, a very noticable vibration type noise comes from the left rear of the engine. [somewhat loud to me] It is initially slightly intermitant, but with a gradual increase in engine speed, 100 to 200 rpm, it becomes constant. As speed increases it stops abruply at about 1400-1500 rpm. It is very noticable and can be slightly felt in the grips. I've ck'ed all the easy things, like any parts of the fairing rubbing and vibrating, no luck. I thought it could be the rubber drive bushings for the alternator, and replaced them, no change in the noise. [even though they were hard and worn and needed replacing]. I hear what might be the same noise when letting off the gas in any gear but 5th, and I think if I were going faster, I would hear it then as well. After listening w/a stethoscope it seems like it could be coming from the area of the output shaft[looks like a balance shaft to me] I was looking at that section of the manual and it mentions "tension gear shimming". Could this be it? Has anyone had a problem like this? I also looked at the "auxiliary shaft w/freewheel" which seems to be located in the same general area. Has anyone ever had problems with that? I'm hoping that after 5 e-bay bikes,[2 sight unseen] I did not get a turkey. I'm hoping the group can help! thanks, Rob Jones note:just remembered, when running at a rpm to make the noise constant, pulling the clutch in and out makes no change in the sound.

[dbrick]

Hi Rob,

I had an '88 K75S from new, and it developed this noise somewhere around 40K miles. I sold the bike at 66K in 1997, and it was still making the noise, burning no oil, and perfectly happy. I remember having this discussion with the kid I sold it to.

I'm pretty sure that the shadetree diagnosis at time (no disassembly required!) was that it was coming from a (or the) big gear on what you call the "output shaft." The gear is made of two thin segments, and tension is kept between them by several springs inserted in the periphery. The design is that springs try to rotate the two segments away from each other, thus reducing the "slop" between this (composite) gear and the next gear in line. The failure theory is that the springs get soft, and thus allow the gears to rattle at low RPMs.

I remember folks saying that this wasn't unheard of in K75 motors.

[tricyclerob]

Thanks for the reply, did you ever get an estimate for repair?

[tricyclerob]

I'm looking at the exploded view of the "output shaft" and item #10 is a "spacer ring" listed in 4 diff.sizes, and #8 is a "spring " that looks like a snap ring. I guess the spring is what you are refering to? In the manual there is also mention of shimming the tensioning gear. I guess thats what the diff size "spacer rings" are for. Has anyone done this? it looks like the engine would need to come out? [if this is what the noise is.]rj

[Burnszilla]

I asked my mechanic Ted Porter this exact same thing. I thought it was the cam chain tensioner. Mine rattling chain noise occurs around 1800 rpm. He said it is normal and inherent to the K engine. Don't worry about it, there is nothing you can do. Live with it.

[dbrick]

Thanks for the reply, did you ever get an estimate for repair?
I think I asked someone once, but as it required complete disassembly of the engine block, I didn't consider actually having it done for more than...oh, say a second or two.

Looking at the diagram, spring #8 loads gear #7 against gear #9 (the two gears mesh with another gear on another shaft). I don't think the problem has to do with different thicknessess of #10, but rather with the springiness of #9. But I may be talking through my hat here.

[tricyclerob]

I hate the thought of fixing a mechanical problem with better earplugs. Looking at the manual, I'm not sure the engine would have to be disassembled. does anyone know for sure? rj

[dbrick]

Well, just rev it. Then you can't hear it.

[tricyclerob]

Dave, was the noise on your's noticable on deceleration as well? rj

[wmubrown]

The RT looks great but has an issue which worrys me. Beginning at about 1200-1300 rpm, a very noticable vibration type noise comes from the left rear of the engine. <<Snip>> I thought it could be the rubber drive bushings for the alternator, and replaced them, no change in the noise. <<Snip>> After listening w/a stethoscope it seems like it could be coming from the area of the output shaft[looks like a balance shaft to me] I was looking at that section of the manual and it mentions "tension gear shimming".
You started by saying the noise was coming from the left side of the engine (assuming from the riders point of view), why are you looking at the right side? Several things can cause this kind of noise - the cam chain, as mentioned by Burnszilla which is on the left side. The other two possibilities are on the right side: alternator bearing, and output shaft. Two failure modes are indicated for the output shaft: the rivets are known to loosen and/or break, and the tension spring wears out. I've done this procedure, though it was some time ago, and the destructions are given here:
http://www.ibmwr.org/ktech/output-shaft-noise.shtml
and another diagnosis method is given here:
http://www.ibmwr.org/ktech/output-shaft-diagnosis.shtml

[wmubrown]

I sold the "S" to my neighbor, at 24,000 miles to add to his stable, and bought a 1992 K75RT, with 37,000 off e-bay. The RT looks great but has an issue which worrys me.
OH... I also wanted to point out: the output shaft problem, as far as rivets, was not an issue with the K75's since they used the euro design which had more rivets, and was not an issue in any K-bike (that I am aware of) manufactured after 1990. So that can be eliminated, but you may have a soft tension spring.

[tricyclerob]

John, thanks for the links, although it was not pleasent reading from a repair standpoint. Initally the noise was noticed from both sides and sounded as if it was coming from the center. I ck'ed the alt.rubber drive bushings which were worn, with a small pile of rubber dust, so I thought I had found the problem. I think at first I was hearing a combination of 2 noises. Now it is def. coming from the left side, aft of the cylinders, most noticable where the intermediate housing bolts to the engine. Does anyone know of a link to an exploded view of the entire engine? With the small photos in the shop manual, and the isolated views in the parts diagrams, I'm having a little trouble envisioning where everything is located. Looks like a bit of a challenge. rj

[Burnszilla]

source: http://www.bmbikes.co.uk/mechanicalpages/kseriesmechanical.htm
http://www.bmbikes.co.uk/photos/mechanicalphotos/K75%20eng%20cutaway%202.jpg

[jdiaz]

Rob, did you check the lower fairing brackets on the left side? They are thin sheet metal and do fail over time, especially if the bike was dropped once upon a time.

[wmubrown]

Rob, did you check the lower fairing brackets on the left side? They are thin sheet metal and do fail over time, especially if the bike was dropped once upon a time.
GOOD point Jon! I'd forgotten about that... and it DOES produce a very annoying buzz...

[wmubrown]

OH... I also wanted to point out: the output shaft problem, as far as rivets, was not an issue with the K75's since they used the euro design which had more rivets, and was not an issue in any K-bike (that I am aware of) manufactured after 1990. So that can be eliminated, but you may have a soft tension spring.
Yes... it's definitely NOT a weekend job. I learned a lot in the process of doing the job, but I am comfortable with delving into areas previously unexplored. I was loaded up with manuals and a LOT of good advice and pointers from the likes of Don E., Brian Curry, and Joe Katz. Without those resources, I never would have tried it.

[tricyclerob]

I did look at the fairing mounts, and on the left side I could see where it had been touching, and when I reassembled after ck'ing valves, I loosened the brkt and moved it out a bit for some clearence. I normally try for the easy things first, learned only after getting myself in trouble over the years. I have only a 7-8mi ride to work and many times I skip earplugs, as the helmet "sliks" is ofter enough. I did wear the plugs this am and it did help. But, still not a fix. I think I may ride it a while as the repair looks like a "snow on the ground" job. I'm guessing if the "spring" and or shims are the culprits, I could expect the same repair in another 40,000? Seems like the general opinion is the spring just gets tired? rj

[dbrick]

Dave, was the noise on your's noticable on deceleration as well? rj
Noticeable in all states (acc, decel, steady state) below ~1400 rpm, inaudible or absent above that.

[mrich12000]

:beer Totally excelent.
My K has a slight knock when at idle, pull the clutch and it disapears.
Thank you for the web on the bike blowups. :bikes