1989 K75 engine rattle/knock.

[Laitch]

It was 10.31.2022 1500hrs at the entrance to the Wrightsville Reservoir picnic area on a solo ride, stopped on my idling Brick as I sought the address on my phone of a vehicle inspection station likely to approve my motorcycle's continued use without my waiting a week for the event, and while I was wearing a 12-year old Fox Creek Leather racing jacket, Scorpion Seattle overpants, EMS polyester cargo pants, a long-sleeve polyester jersey, an azure blue ExOfficio boxer brief with 4" inch inseam, a green Buff imprinted with the logo of the 2021 Montpelier virtual Corporate Cup Challenge and State Agency Race, Darn Tough hiking socks, TCX Explorer 4 vegan riding boots, a Diamond Silver Arai Signet-Q helmet and Firstgear insulated gloves, I heard the unmistakable faint rattle that might be the one plaguing @uldy. Once I realized it had returned, I greeted it happily like an old friend and the two of us roared up through the sweepers on Route 12, landing at the Worcester country store where I bought half a cup of coffee and possible future rights to simultaneous tax burden and tax relief. At Mazer Performance in Bolton, VT, my Brick passed its inspection with "flying colors." I flew home over the scenic pothole fields that are my town's roads.

Separation from the knock was brief and our reunion was ecstatic.   

[daveson]

A cupala days ago I was reading this thread and noticed a disagree comment was registered on the first post. I thought, how can you disagree with a first post, who would disagree with a first post? I clicked on it and it was me. I tried to undo it but couldn't. Not the first time I've done that either.

Musta been half asleep at the time, dunno.

Oops.

@uldy, I'm thinking your problem is likely to be a worn bearing more so than the tabs. There are past cases similar to yours; Rhoads rash, bgcameron, and MLR over at the K100-forum, to name a few. If the bearing is at the earliest stages of wear, then tightening the clutch nut could help, if very worn, the rattle would continue, regardless of how much torque is applied to the clutch nut.

I'm happy to be corrected, but I can't see how the tabs theory fits.

This problem has been solved in the past, buy replacing worn parts or torquing the clutch nut, but I don't think it's ever been fixed by altering the tabs.

Some people have reported no tab wear marks, I haven't found any comments that there was wear.

Cameron described an irregular clatter, sometimes just once while on level ground, but more when going slightly uphill. I don't think that should alter if it's caused by tabs banging around.

Let's say for a minute it is caused by power pulses banging the tabs around. At 3500rpm, it wouldn't knock just once out of thousands of pulses. Output shaft endfloat could match those symptoms though. With a three cylinder engine having one and a half power pulses per revolution, at 3500 rpm that's 5250 pulses. That's 87 clacks per second. I'm pretty sure he's not getting that.

I haven't found an example yet of an early model K75 with a high rev rattle from the output shaft, including yours. Don't know why they're always late models, but somewhere along the line the idle gear spring was replaced with a shim, but I've only read about a small number of the total cases.

In the design stage I think the K75 had an output shaft similar to the K100 but the absorber dampers just got knocked around too much due to the unbalance of three cylinders. So a balance shaft was chosen, good idea. One side affect of this afterthought is that it has lost the benefit of the extra diaphragm spring inside the absorber housing.

I think the only reason for the tabs is so that that balance weights are phased correctly. The tabs are offset so that the balance shaft can only be installed one way.

I don't think there's a BMW service bulletin about solving an output shaft noise problem by altering the tabs, but there is a service bulletin about replacing worn parts to solve this problem.

[stokester]

I haven't found an example yet of an early model K75 with a high rev rattle from the output shaft, including yours. Don't know why they're always late models, but somewhere along the line the idle gear spring was replaced with a shim, but I've only read about a small number of the total cases.

I revisited this thread after rides on my 1993 K75S (11/92 - 81886 miles) and 1994 K75S (4/94 - 12768 miles).  The '93 is smooth and quiet, the '94 has that annoying rattle at low RPMs.  Beside the difference in fuel pumps there must have been other internal changes.  Changed the monkey nuts and found the old ones in decent condition and cleaned the starter while I was in there.  I knew it was probably not an easy fix from reading this topic some time ago.


The plan right now is just to leave as is and checking the output shaft when I do a full spline lube.

[daveson]

Since that post I have seen at least one thread where an early model has had these symptoms, not surprising on second thoughts.

Since you have two bricks, basically the same, you're in a good position to do some simple tests. In one thread, a rattle problem was solved due to a loose spark plug lead. I spose you won't be that lucky, but a simple test would be to swap the plugs and leads around from one brick to the other to see if the problem transfers to the other brick.

I still only have a limited sample number to go by, but there have been cases where the rattle suddenly disappears at about 1,800rpm, and other cases where it suddenly disappears at about 3,700rpm. Some of the cases at 1,800 were solved, with work related to the auxillary shaft. Some of the cases at 3,700rpm were solved, with work related to the output shaft.