K75 engine flattens out during throttle - new exhaust

[Vlad]

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!

[Martin]

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.

[Laitch]

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.

[Vlad]

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.

[Laitch]

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?

[The Mighty Gryphon]

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?

[Vlad]

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.

[rbm]

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.

[Chaos]

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.