Author Topic: K75 engine flattens out during throttle - new exhaust  (Read 137 times)

Offline Vlad

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K75 engine flattens out during throttle - new exhaust
« on: January 12, 2018, 03:47:37 PM »
Hello, I've have my '93 KT RT for about a year and decided to do some modifications to it. Mostly visual stuff, but after installing a new exhaust I noticed that there is a lot of popping during deceleration and that on hills or under slightly heavier throttle, the engine just flattens out and the exhaust just sounds like its farting until you 'reset' the throttle and give it gas again.

I'm wondering what this could be. I've done no other mechanical modifications other than the new exhaust. I also chose one that had the same diameter outlet pipe as K75's exhaust.

Any idea what is causing this?

Thanks!

  • Victoria, BC
  • 1993 K75RT

Offline Martin

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #1 on: January 12, 2018, 04:34:54 PM »
Even with a standard exhaust some K75's can back fire on overrun. The following can reduce or eliminate the backfiring sometimes, but not all the time on some bikes. Check for exhausts leaks at the joints, adjust the TPS, correctly set the mixture, check the injector spray patterns, check for inlet leaks. After twenty years of chasing this problem my K75 does not backfire. The final cure was getting rid of an exhaust leak and getting the correct injector spray pattern. A modified exhaust may contribuate to the problem, and therfore it might not be possible to eliminate the popping entirely.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s, Lefaux, Vespa V twin

Offline Laitch

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #2 on: January 12, 2018, 05:35:06 PM »
Hello, I've have my '93 KT RT for about a year and decided to do some modifications to it. Mostly visual stuff, but after installing a new exhaust I noticed that there is a lot of popping during deceleration and that on hills or under slightly heavier throttle, the engine just flattens out and the exhaust just sounds like its farting until you 'reset' the throttle and give it gas again.
Your bike might backfire less if you close the throttle during deceleration rather than leave it partly open. The engine should not "flatten on acceleration". If you haven't done a full tuneup including a throttle body balance, consider putting one on your schedule.
  • Along the Ridley in Vermont.
  • 1995 K75T 60,000 miles
I wept because I had no radials until I met a man who had no splines.

Offline Vlad

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #3 on: January 12, 2018, 05:46:44 PM »
Thanks for the replies guys. I think I will have to take it in to a shop to have a look at it. I don't have the tools or the knowledge necessary to check the fuel/air mixture and make any adjustments unfortunately. Just seemed an odd thing to happen from switching out the exhaust. I've read lots of posts about café racers getting modified and none of them have mentioned the flattening out during acceleration problem. It's quite noticeable and also happens when I go up any steep hills.

Here's a couple photos of the exhaust itself just if anyone is curious or wants to avoid getting this one.


  • Victoria, BC
  • 1993 K75RT

Offline Laitch

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #4 on: January 12, 2018, 05:57:47 PM »
Just seemed an odd thing to happen from switching out the exhaust.
It is odd. Did you look down the pipe for something that might be restrict the flow, like a mouse nest or a small potato?  Did you put the fuel tank cap on backwards or did it come that way?
  • Along the Ridley in Vermont.
  • 1995 K75T 60,000 miles
I wept because I had no radials until I met a man who had no splines.

Offline The Mighty Gryphon

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #5 on: January 12, 2018, 06:13:53 PM »
Back in my mispent younger days which occurred during the late '60's and early 70's I modified several bikes for higher performance.  One of the modifications I always did was to swap exhaust systems.

One thing that replacing an exhaust system always required was adjustment of the carburetion often requiring jet changes to compensate for the different flow characteristics.  It wouldn't surprise me at all if the exhaust you have installed has messed up the tune of the engine requiring timing and/or mixture adjustment. 

As Laitch has mentioned, get the valves adjusted, balance the throttle bodies, install new spark plugs and double check the ignition timing.  Then you can start doing test runs to do plug readings.  Based on what the plugs look like you can make the required adjustments.

Did you get any suggested tuning adjustments with the new pipe?
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '92K100RS White, '94 K75S Dakar Yellow
"Dohn f wit me, honkey, cuz I dindu nuffin'"

Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'94 K75S "Cheetos"
'92 K100RS "Moby Brick"

Past:
'82 Honda FT500, '80 Honda XR185, '78 Honda XL125, '76 Honda XL125, '74 Honda XL125, '71 Ossa Pioneer, '68 Kawasaki 175

Offline Vlad

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #6 on: January 12, 2018, 06:19:17 PM »
I have not come across any tuning required for this exhaust, the website at coneengineering.com, where I got it, doesn't provide any. I will likely just have to take it down to the bmw shop to have it fixed professionally I think. Hopefully it's just a quick adjustment.

Thanks guys.
  • Victoria, BC
  • 1993 K75RT

Offline rbm

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #7 on: January 12, 2018, 08:02:11 PM »
Maybe check the operation of the barn door on the MAS for correct movement throughout its range.  It could be coincidental and not directly related to your exhaust change. Or maybe the new exhaust is exacerbating a latent problem with the intake system.
  • Regards, Robert
Toronto, Ontario

1987 K75 - Build Blog @http://k75retro.blogspot.ca/
2011 Moto Guzzi V7 Racer

Offline Chaos

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Re: K75 engine flattens out during throttle - new exhaust
« Reply #8 on: January 12, 2018, 10:09:59 PM »
I've had several different exhausts, and a free flow air filter, and never had the symptoms you mention.  The exhaust may be a red herring, check fuel filter, air filter, collapsed fuel lines, etc.  The only time my engine went really flat was when my Luftmeister pipe coughed out it's baffle on I-75, leaving an open cone that sounded more like a T-34 than a motorcycle. 
1987 K75S    VIN 0231
Original litter & owner
185,000 miles and 5 paint jobs
2012 Ural Gear-Up
(BMW's bastard step child)