Knock, knock... Who's there?.... Catastrophic bottom end failure!

[pommie john]

When I said the oil light would come on at idle, I meant when the oil was good and hot. You'll always get high oil pressure in a cold engine.

[ME 109]

I know that knock!
I don't know what causes it, but I know it.
About 5-6 times a second.
The noise seems to come from the bottom of the motor, listened to from both sides.

My 81 will sometimes do it, if it idles low enough.
Don't worry about it........


Crank end float has been thought of as a culprit, but no conclusion.

[Bill Harris]

I feel my stress levels dropping:

Sequence: check:

clutch
oil filter and sump
transmission
timing chain
big end bearings

So what I'm going to do is carefully and methodically eliminate each and every possible cause of the problem...

No, no--I was only semi-serious about the rod bearings. Don't bother with those unless you have the cylinders off for other repairs.

Go ride. Enjoy the bike. Don't overthink.

[motog]

Originally Posted by Bill Harris

No, no--I was only semi-serious about the rod bearings. Don't bother with those unless you have the cylinders off for other repairs.

Go ride. Enjoy the bike. Don't overthink.

One of my mates recently said to me "there are two types of motorcyclists, Jim, those who like riding their bikes and those who like playing with them". I like riding my bike but I love playing with it. One of the reasons I bought a 40 year old BMW was because I can do just about all the work on it myself (I live in a fourth floor apartment with a small car space in the basement so this can be a bit challenging), without the need for any tools more complicated than a torque wrench. I'm not saying that I look forward to things breaking, but I can't say that I get upset.

There's a certain type of satisfaction and happiness that you get from pulling apart something like a carb and putting it back together again so it works as good or better than it came from the factory.

But I take your point. I was intending to do new pistons and a cylinder bore next year (when I've saved a bit of cash) so I'll probably wait till then.

When I said the oil light would come on at idle, I meant when the oil was good and hot

Point also taken. I'm thinking back to my recent rides. I can't remember the oil light coming on at all. Except once when the oil actually was low ( topped it up within 10 mins and the light went off and stayed off).

Don't worry about it.....

Yeah. Sure. Thanks for that. Started all over again now.....

I might have got a bit enthusiastic when I said I was going to start her up and check for clutch noise when I got home tonight. I might have, sort of, maybe, forgotten that I've got no carbs on the bike at the moment (waiting on the parts to finish the carb rebuild).

Again, thanks for all the responses. First question I've posted to the board and the response has been overwhelming.

[noman]

wishful thinking, but your "knock" may go away when the carbs are balanced (equal parts experience/voodoo), and idle's raised a tad. good luck!

[mattcfish]

Worst case senario happened to my friend's /6. His oil pick-up (aftermarket deep sump) came loose (bolts not locktited properly) while on a long ride. The big end bearings lost lubrication and one of them spun. He started hearing a knock when the piston started tapping the head. He then turned around and road 90 miles home (tough engine!). Amazingly the main bearings were uneffected, the rods and crank were in bad shape however. I rebuilt it for him 11 years later and she's running great today. He should have checked the OP light, but maybe it wasn't working properly. That's why an oil pressure guage is a must in my book.
If you have good oil pressure it probably isn't the big end bearings. Make sure your oil sender is working properly though. Big end bearing are easy to change on these bikes. Not a bad idea to change them when you have the heads off for service. I gained 10-15psi at hot idle when I did mine recently. The cranks are overbuilt, very tough and rarely need attention if they aren't scored.

[motog]

Yeah, thanks for that mattcfish, just what I needed - more paranoia.

So I put the rebuilt carbs back on this weekend. Still got a noise below 1000rpms. Put the mechanics stethoscope onto the engine to check if it's noise from the timing chain - nope.

While I had the stethoscope out I had a listen around the bottom end. Pulling the clutch in doesn't make the noise go away but I did notice that the noise did disappear as soon as the revs went up a bit and not just because other engine noise drowned it out.

So what did I do? I set the idle at 1000rpm and decided I could learn to live with it until I do the piston rebuild next year. I'll listen out for any change or increase in the noise but I think I'm accepting the collected wisdom of those who replied - this is probably just one of those weird noises that old BMs make.

Thanks all.

P.S. If one day you see me stopped at the side of the road with a rod poking out of the side of the engine casing, please give me a lift, after you stop laughing.

[ME 109]

A good idle speed is 1050-1100 in my opinion.
The noise is disconcerting, so don't let it happen. Lift that idle.

[Morro]

Could be your flywheel.

My r80g/s (84) had a tapping noise at idle and I put it down to timing chain. Put on new timing chain & gears and it was still there. A close listen and it sounded like it was coming from the clutch area so thought it was just a wearing out clutch. A couple of weeks later out riding I pulled up and the bike was making an almighty rattle. I thought the starter gear may have jammed. Started OK but with a horrible noise. Got home Ok but that was it, horrible noise when I hit the starter. I took the starter cover off and notice the flywheel was loose. Took gearbox off and the flywheel rivets (connecting pressed steel clutch housing to the starter ring gear) had come loose and some missing. The slow speed tapping was the loose flywheel ring gear hitting the engine case which disappeared whith revs due to centrifugal action. I found a used one and put it on the bike and it is still going strong 5 year on.

Good luck
Morro

[ME 109]

That must be a NSW specific problem Morro. My RS had the same deal.
Haven't heard of it happening to too many folk tho'.

[Morro]

I just re read the OP and my flywheel comments in my previous post would not be on a 75/5 as they have a solid flywheel. I have not deleted them as they may help someone.

Morro

[naginalf]

I had already gathered the "just ride it" mentality from previous reading, but it's very comforting to read it so adamantly acknowledged here. Mine does the same thing. It idles quite smoothly at 900RPM but makes a rattle and clank just as you described. Carb balancing and valve rocker endplay adjustment helped the noise a bit, but still has that knock, just idle it up if you care. Otherwise, just wait for someone to ask, "is it supposed to make that noise?"
"Yup" PPPTTTTTBBBBBBttttttbbbbbpppp..............