Author Topic: K75 starting issue  (Read 5074 times)

Offline blindman

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K75 starting issue
« on: September 26, 2024, 11:43:10 PM »
Hi all.  New member here in New Zealand.  Had a k75s 20 years ago and loved it so much I thought I'd get another.  I've had it for nearly 2 years but have only put about 1000km on it.  It's done 101,000km.  Don't ride it much but it has always started first pop, even after sitting for a few weeks.  I went to start it 4 weeks ago as I was going to be away for a couple of weeks and and it cranked fine but just wouldn't fire.  I thought I'd flooded it and left it until I came back.  Exactly the same when I came back 2 weeks ago.  It doesn't even splutter.  Plugs out after an attempt and they were wet with petrol.  Checked spark and there's a good strong spark to all plugs.  Dried and cleaned plugs (near new - only 1000km on them).  Still didn't start.  Cranking motor with plugs out and spray of petrol out the plug-holes looked normal.  Fuel pump doing what it should.  Battery near new.  Haven't had the air filter out but can't think it's that.  To go from perfect starting to no starting suggests the problem has a single cause.  Does anyone have a suggestion?  Thanks in advance.
  • NZ
  • '89 K75S

Offline Laitch

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Re: K75 starting issue
« Reply #1 on: September 27, 2024, 12:04:48 AM »
Welcome, blindman!

I hope you're disconnecting that battery when you're neglecting that moto, or keeping the unit on a tender after it has been charged up. You're liable to fuse the starter relay when that battery is low and then you'll be on a romp.  :laughing4-giggles:

Dump the fuel out of the tank and use it in your lawn mower. The fuel filter could possibly be as old as the Brick. Check the fuel filter for clogs by blowing through it in the direction of flow. Rinse out your mouth with ginger beer. Check the air filter; a mouse could have found a home in there; it's cozy in the air box and there's plenty of room for food and bedding which clog air flow. If the fuel filter is clear, add fresh fuel and give it a try. If the inside of the tank has a golden hue, you should clean it.

  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles

Offline blindman

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Re: K75 starting issue
« Reply #2 on: September 27, 2024, 12:13:39 AM »
Thanks for that.  I'm convinced it's not fuel age (only 5-6 weeks old) or fuel filter.  It's getting fuel through so should make some sort of effort to start even if flow was low, I would have thought.   If I have spark the that leaves an air problem.  It looks like the tank has to come off to access the air-box - is this right?  Cheers.
  • NZ
  • '89 K75S

Offline frankenduck

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Re: K75 starting issue
« Reply #3 on: September 27, 2024, 12:14:53 AM »
Pull the plugs and try "drying out" the cylinders with a hair dryer or heat gun. That's worked for me before on a flooded K75.

Another suspect is the temp sensor. If that's disconnected or failing then it can cause the L-Jetronic to think that the engine is REALLY cold and pump way too much fuel.
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
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Offline blindman

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Re: K75 starting issue
« Reply #4 on: September 27, 2024, 12:21:00 AM »
I like the temp sensor suggestion.  I'll get my nose into the manual to see how to test that.  Many thanks.
  • NZ
  • '89 K75S

Offline frankenduck

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Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
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Offline blindman

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Re: K75 starting issue
« Reply #6 on: September 27, 2024, 12:29:02 AM »
Great.  Thanks.  I'll investigate that tomorrow.
  • NZ
  • '89 K75S

Offline blindman

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Re: K75 starting issue
« Reply #7 on: September 29, 2024, 09:15:20 PM »
Have replaced one problem with another.  Finally got the bike started.  I had left the plugs out overnight to dry the cylinders and topped up the battery.  It gave signs of starting this morning and it finally caught.  After a couple of minutes it settled down and sounded fine but from the off it was bellowing out white smoke.  It didn't used to do blow any.  Let it run for 5 minutes and still blowing smoke.  Not sure if I should take it out for a blast.  Any ideas?  Thanks in advance.
  • NZ
  • '89 K75S

Offline frankenduck

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Re: K75 starting issue
« Reply #8 on: September 29, 2024, 10:10:03 PM »
Do not let K75s sit and idle for an extended period. It fouls the plugs and can flood it again. When cold they put out a lot of fuel. (For some reason K75s are more prone to this than K100s.)

From the rider manual:



Don't worry about the smoke. Just ride it.

Smoky starts: If a K bike is left on the side stand or sits for an extended period then the exhaust may be really, really smoky for a few minutes when you first start it. This is due to the horizontal cylinder design allowing oil to seep past the rings into the combustion chamber when the bike is at rest.  It is normal and not a cause for concern.

This is especially true for the first few years of Ks production before BMW upgraded the piston rings. (6/88)  If you have one of these earlier bikes then you can mitigate smoky starts in a couple of ways:

1) Use the center stand if the bike is going to sit for more than a few minutes.

2) If you're going to use the side stand, then before putting the bike on the side stand, turn the bike off and tilt it a little to the right for 10-15 seconds.  This allows the warm oil behind the pistons to drain back into the sump, leaving less oil in the cylinders to seep past the rings into the combustion chamber.

If the bike has been sitting for months or years then it's not unusual to have a VERY smoky start.  Just ride it and the smoke will go away
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
K Bike Maintenance & Mods: Click here.
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Offline blindman

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Re: K75 starting issue
« Reply #9 on: September 29, 2024, 11:03:04 PM »
Thanks for that.  I've just got back from a 40km ride and it's running better than ever.  I added a bit of fuel conditioner before going but I've no idea if that was a factor.  It didn't actually smoke at all right from the start.  It had been very smokey for the 5 minutes or so I'd let it idle a couple of hours before.  Looks like I shouldn't have done that from your comments.  I also didn't know about not using the choke at position 2 when ambient temp is above 0C.  Good info.
I only ever use the side stand for a few seconds before putting on the centre-stand.  The bike's an '89 so should have the upgraded piston rings.
Looking forward to some decent rides this summer (S hemisphere).  NZ is certainly a biker's paradise.
I appreciate your input frankenduck.
  • NZ
  • '89 K75S

Offline frankenduck

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Re: K75 starting issue
« Reply #10 on: September 29, 2024, 11:10:29 PM »
If you want to know when your bike was built you can input the last 7 digits of the VIN here:
https://www.realoem.com/bmw/select.do
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
K Bike Maintenance & Mods: Click here.
Buy parts here.

Offline blindman

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Re: K75 starting issue
« Reply #11 on: September 30, 2024, 12:22:33 AM »
I wish you hadn't told me that.  Mine's a Jan '88 apparently.  Must have the old rings.  Bugger!
  • NZ
  • '89 K75S

Offline frankenduck

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Re: K75 starting issue
« Reply #12 on: September 30, 2024, 07:47:33 AM »
Meh. It's not a big deal which rings a K75 has. It's just something to be aware of with regard to smoky startups.
Once I had a Collie pup. Dug a hole and covered him up. Now I sit there by the hour. Waiting for a Collie-flower.
New to K bikes? Click here.
K Bike Maintenance & Mods: Click here.
Buy parts here.

Offline Laitch

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Re: K75 starting issue
« Reply #13 on: September 30, 2024, 08:47:26 AM »
I wish you hadn't told me that.  Mine's a Jan '88 apparently.  Must have the old rings.  Bugger!
There's a guy here who had an '87 since it was new and put around 200,000 miles on it. The more you ride it the better it'll be for both of you.
  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles

Offline blindman

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Re: K75 starting issue
« Reply #14 on: September 30, 2024, 04:38:16 PM »
I believe it.  My last K75 was a 1985 and had about 160,000km (100,000 miles) on it.  Motor ran like a sewing machine.  Mind you, there was little resistance to the motor when the final drive splines gave up the ghost.  Haha.  I know all about molybdenum now...
  • NZ
  • '89 K75S

Offline Martin

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Re: K75 starting issue
« Reply #15 on: September 30, 2024, 05:33:05 PM »
Open the link below to find about how far a K75 can go.
* !.2 Million BMW K75 V1.pdf (288.16 kB - downloaded 227 times)
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline blindman

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Re: K75 starting issue
« Reply #16 on: September 30, 2024, 06:34:26 PM »
Wow.  I'm assuming the motor hadn't been apart in that time.  That's impressive.  I see you're from Aussie.  I spent most of my life in WA (Perth and Albany).  Jeez - the roads there are at the same time boring and terrifying with the 'roos jumping out at you at inconvenient moments!
  • NZ
  • '89 K75S

Offline Martin

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Re: K75 starting issue
« Reply #17 on: September 30, 2024, 07:30:16 PM »
Skippies can be scary, I actually ran over one's tail while trying to avoid a mob crossing the road in the dark. I was wondering why the reflectors on the white posts were flashing. When I realised it was the Skippies blocking them intermittently it was too late to stop. And luckily something flashed in my head that they can't jump backwards. One bright Skippy had stopped in the middle of the road and was starring at me so I ran over it's tail.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline blindman

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Re: K75 starting issue
« Reply #18 on: September 30, 2024, 07:45:51 PM »
Yep.  Dumb buggers.
Just saw K75S hybrid.  What's that all about?
  • NZ
  • '89 K75S

Offline Martin

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Re: K75 starting issue
« Reply #19 on: September 30, 2024, 07:56:43 PM »
It's got bits and pieces from other things. Chinese master cylinder, a $6.00 bucket, card table hinges, various homemade bits but still looks pretty stock. I think at last count 16 modifications.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline blindman

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Re: K75 starting issue
« Reply #20 on: September 30, 2024, 08:29:23 PM »
Hah!  #6 wire and gaffer tape.  My type of mod.
  • NZ
  • '89 K75S

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