K75 Engine Knock @ 3700rpm

[TimTyler]

For the last six thousand miles I've experience an audible engine knock when increasing past 3700rpm when the bike is under load. It happens regardless of the fuel octane rating or gear, although i don't notice it in 1st gear.

I first noticed it after a valve adjustment with fresh spark plugs and wires. There's no noticeable loss of power, just a knock. After I started hearing and feeling the knock I had the bike's timing and CO adjusted on a dyno.

It's been 6k miles and it has not become better or worse.

Any ideas?

[johnny]

greetings timtiorler...

3700 rpm... are you kidding me... you need to tach that thing up and wear ear plugs...

j o

[billday]

Now I know why I've never gotten around to adjusting my valves. It causes knocking.

Thanks for the reminder, Tim!

[TimTyler]

Right, Bill  :eek:

How to describe the knock? It's like, when two spinning gears are meshed tightly, and then at 3700rpm they are both meshed loosely for a split second (cogs contacting at the same speed)... and then as the rev's increase beyond 3700rpm there's a "konk" as the cogs engage tightly again.

Other than this the bike runs better now than any other time I've owned it.

[pallum]

I've had loose spark plug wires do that, same RPM range too.

[Chaos]

I've had loose spark plug wires do that, same RPM range too.

+1

[TimTyler]

Thanks, guys. I've tried re-seating the newish ignition wires before without luck.

Today I swapped out the NGK plugs for some Champion RA6HC Autolite Xtreme Sport Iridium Spark Plug - XS4164 plugs. Available at my local O'Reilly's in an aisle with small engine plugs.

Test run started fast and ran very smooth.

[TimTyler]

The knock is gone  :clap:

Just got back from a 75 mile country ride. I'm liking these new spark plugs. The engine is running more smoothly, especially at higher RPM's. I found myself shifting-up at higher RPM's than usual. 40mph in 2nd gear didn't sound like it was straining the engine. Kind of cool. I suspect my fuel mileage might improve. We'll see.

These new plugs are "suppressor" plugs not "resistor" plugs, and from what I've read that shouldn't be a problem. I'm using Magnecor ignition wires that do not have resistance built-in like the OEM ignition wires.

[Chaos]

The original wires had some weird air gap resistor that BMW charges dearly for now.  Good to know your found a replacement combination that seems to work.  Glad it was a simple fix.

[TimTyler]

I've ridden about 1000 miles on the new plugs. I'm happy but the engine seems burdened above 75mph. Tried to get it up near 100mph this weekend (loaded with camping gear) and it was having a tough time.

I warmed up the engine and checked the timing this morning and found that @ 4000 rpm my adjustable timing light was showing above 40 degrees.  :mbird  So I adjusted the HES to bring it back to 33 degrees. (attached photo)

Now at idle I get about 0 degrees on the timing light and flicking the throttle at idle almost stalls the engine. Hesitation.

Any ideas?

[TimTyler]

Reset the HES to it's default position.

Throttle response is improved. Engine sounds great but according to the timing gun the engine is timing at 50° @4000rpm.

Just read the Clymers bit on ignition and it seems only the idle timing can be set since timing above 1350rpm is advanced dynamically by the ignition system, and that system uses two different timing curves. I feel like I should have already known this.  :dunno2:

But then the IBMWR timing FAQ suggests that timing should max out at 33° and mine clearly reaches beyond that at very high RMP's.