Author Topic: Stock exhaust woes! 1991 K75s  (Read 183 times)

Offline KappoBrick

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Stock exhaust woes! 1991 K75s
« on: October 12, 2021, 07:47:06 PM »
Hey again, this time I have another problem I'm consulting the grand consortium of Brick wizards. Since I have started my crusade to make my bike stock, since I bought it, I ordered a stock muffler to replace the obnoxious Luftmeister. I replaced the 3 copper crush gaskets, studs, and nuts aswell along with the muffler, headers, and rubber stopper under it. The problem is now, a few things, first the smell. It smells absolutely awful, I smell terrible when I get done with a test ride and it stinks up the garage, my mom hates it and therefore it's first on the list. Second, the bike is slow as hell for the first 5 minutes of riding. It's not until it gets super hot that the throttle wants to be responsive, because when it's cold it's so weak. I assume I need to get fuel trim adjusted? With the Luft, even as it was loud as hell, it was responsive in all ranges and felt great, with the stock muffler I got, the bike is different for the worse. Any help is appreciated as always, and I will try to reply in a timely manner.

NOTE: I have performed the lean drop test and the RPMs do not deviate but maybe 50 rpm in either direction, but no stalling or stumbling. I have indeed tuned up the bike since I got it, Oil, Plugs (I got the right ones with the threaded end and caps), Filter, Fuel Filter, FD and GB Oil, Tires, and of course now, Exhaust.
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Offline K1300S

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Re: Stock exhaust woes! 1991 K75s
« Reply #1 on: October 12, 2021, 08:13:52 PM »
What does it smell like?  Where did you get the muffler? Can you feel/see the exhaust coming out of the muffler? 
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #2 on: October 12, 2021, 08:15:40 PM »
What does it smell like?  Where did you get the muffler? Can you feel/see the exhaust coming out of the muffler?

Got the stock muffler on Ebay, Headers too. No cracks that im aware of, I cant see any leaks or feel any really, but I smell metal/gas. Its strange because Ive never smelled this before so I dont really know what it smells like. It just smells.
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Offline Laitch

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Re: Stock exhaust woes! 1991 K75s
« Reply #3 on: October 12, 2021, 09:54:29 PM »
It seems clogged, like K1300S could be implying. Maybe you're disturbing a mouse septic system. It should be possible to clear it.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #4 on: October 12, 2021, 10:11:59 PM »
It seems clogged, like K1300S could be implying. Maybe you're disturbing a mouse septic system. It should be possible to clear it.

Definitely would have spotted a clog when I put it all together, however some metallic white flakes did come out when I first started the exhaust, but have not returned since. Figured the smell was the metal having heat cycles again after years of sitting but I honestly dont know at this point. Is it worth taking it in to get CO checked?
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #5 on: October 12, 2021, 10:13:56 PM »
Should I try my hand at a lean-drop?
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Offline Chaos

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Re: Stock exhaust woes! 1991 K75s
« Reply #6 on: October 12, 2021, 10:25:59 PM »
usually it's the other way around, bike runs almost the same or worse with an aftermarket exhaust.  I suspect something odd is going on inside your new muffler, though the PO may have optimized the FI for the goofmeister.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #7 on: October 12, 2021, 10:27:57 PM »
usually it's the other way around, bike runs almost the same or worse with an aftermarket exhaust.  I suspect something odd is going on inside your new muffler, though the PO may have optimized the FI for the goofmeister.

Thats what I was thinking, or my bike refuses to warm up because I know the brick hates being cold. But I doubt it. Lean drop is my future?
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Offline Laitch

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Re: Stock exhaust woes! 1991 K75s
« Reply #8 on: October 12, 2021, 10:40:29 PM »

Thats what I was thinking, . . .
It was?  :laughing4-giggles: Air pressure or suction is in your moto's future.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #9 on: October 12, 2021, 10:42:36 PM »
It was?  :laughing4-giggles: Air pressure or suction is in your moto's future.

Im more on the side of optimized for the old Luft.
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Offline Past-my-Prime

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Re: Stock exhaust woes! 1991 K75s
« Reply #10 on: October 12, 2021, 10:57:55 PM »
What are the valves like? I'm asking this really because I just did it to mine (an '89 with 80,000 km or 50,000 miles). And after I put new shims in the exhaust valves (0.05 to 0.1 mm thinner) my bike runs much much better.
Actually, initially after I was working on it, it ran like complete shit, hardly wanted to start and did not want to go at all, and I tracked THAT problem down to an improperly tightened Z-hose that I had also replaced when I was at it. THEN in ran much much MUCH better! The lesson for me here is: when I do a bunch of things, there is no telling which of those things is responsible for me screwing it up.
So anyway, this might not explain the bad smell (preserving oils in the muffler? Although should be burned off by now) maybe valves staying open too long is a problem with the stock muffler but was OK with the other one?
Honestly I don't know and I could be all wet here.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #11 on: October 12, 2021, 11:00:00 PM »
What are the valves like? I'm asking this really because I just did it to mine (an '89 with 80,000 km or 50,000 miles). And after I put new shims in the exhaust valves (0.05 to 0.1 mm thinner) my bike runs much much better.
Actually, initially after I was working on it, it ran like complete shit, hardly wanted to start and did not want to go at all, and I tracked THAT problem down to an improperly tightened Z-hose that I had also replaced when I was at it. THEN in ran much much MUCH better! The lesson for me here is: when I do a bunch of things, there is no telling which of those things is responsible for me screwing it up.
So anyway, this might not explain the bad smell (preserving oils in the muffler? Although should be burned off by now) maybe valves staying open too long is a problem with the stock muffler but was OK with the other one?
Honestly I don't know and I could be all wet here.

I dont know what the valves are looking like, it's only got 11k miles on it and I have no experience with valves. Ill call my local shop (Rep Bombar's!) and see what he can do.
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Offline Laitch

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Re: Stock exhaust woes! 1991 K75s
« Reply #12 on: October 12, 2021, 11:19:58 PM »
Im more on the side of optimized for the old Luft.
Checking for blockage is too simple for you?  :laughing4-giggles:
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Offline The Mighty Gryphon

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Re: Stock exhaust woes! 1991 K75s
« Reply #13 on: October 12, 2021, 11:38:31 PM »
Despite only having 11,000 miles, the bike is 30 years old.  A lot can happen in 30 years of light usage. 

How long have you owned it?  How many miles have you put on it? 

When you say it doesn't run well until it's fully warmed up it tells me that the ECU isn't getting good engine temperature information or that there is a lean mixture issue possibly caused by an air leak. 

The smell could be a problem with oil burning, a blown head gasket, mixture problems, or debris in the muffler.  You mention a metallic smell, that would lead me to suspect a head gasket problem. 

Your list of things you have done does not include the infamous crankcase breather(commonly known as the Z tube).  As mentioned above, the valves have not been checked, and this is part of early dealer maintenance so may need attention. 

It doesn't appear that the coolant has been changed.  What does it look like?  Is it at the proper level? 

Have you checked the ignition timing?  It's possible that the previous owner may have advanced the timing in the pursuit of more power along with the noisy exhaust. 

It's also quite possible that, as Laitch mentioned, that a rodent had taken up residence in your exhaust at some point.  If it entered through the muffler inlet, it's possible that it's furnishings got blown further into the muffler as opposed to being blown out the outlet.

First thing would be to check the Z hose for cracks at the clamps.  Then check the temperature signal at the ECU connector.  Then check the timing, valves and the coolant.  Let us know what you find.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #14 on: October 13, 2021, 12:34:08 AM »
Checking for blockage is too simple for you?  :laughing4-giggles:

Its clear? there is no visible block as I checked it when I installed it.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #15 on: October 13, 2021, 12:43:32 AM »
Despite only having 11,000 miles, the bike is 30 years old.  A lot can happen in 30 years of light usage. 

How long have you owned it?  How many miles have you put on it? 

When you say it doesn't run well until it's fully warmed up it tells me that the ECU isn't getting good engine temperature information or that there is a lean mixture issue possibly caused by an air leak. 

The smell could be a problem with oil burning, a blown head gasket, mixture problems, or debris in the muffler.  You mention a metallic smell, that would lead me to suspect a head gasket problem. 

Your list of things you have done does not include the infamous crankcase breather(commonly known as the Z tube).  As mentioned above, the valves have not been checked, and this is part of early dealer maintenance so may need attention. 

It doesn't appear that the coolant has been changed.  What does it look like?  Is it at the proper level? 

Have you checked the ignition timing?  It's possible that the previous owner may have advanced the timing in the pursuit of more power along with the noisy exhaust. 

It's also quite possible that, as Laitch mentioned, that a rodent had taken up residence in your exhaust at some point.  If it entered through the muffler inlet, it's possible that it's furnishings got blown further into the muffler as opposed to being blown out the outlet.

First thing would be to check the Z hose for cracks at the clamps.  Then check the temperature signal at the ECU connector.  Then check the timing, valves and the coolant.  Let us know what you find.

I replaced the Z-pipe as it had cracked last year. Ive owned it for a year and a some months, and put 1000 miles on it so far. I use it to commute mostly. the coolant is bright green, no smoke or coloured fumes come out from the exhaust, and there is no visible signs of a blown headgasket around the edges (oil blowout, leakage from a corner, etc.). My coolant level has stayed the same since I bought it. The fuel tank is pristine. It sat for years but clearly in a controlled enviroment. the oil when I changed it showed no metal or debris. Upon some more inspection it's not a metallic smell, but it is very close to the smell my dad's R1200RTP shoots out when he starts it after a long while (rich, condensation in the pipe mixing with the gross exhaust). The ignition timing doesnt look like it has been touched as the bolts holding it in show no signs of wear or the usual marks of a allen. Ive got not vacuum leaks from what I can discern, when the bike warmed up with the Luft, it ran great, loud but great. This leads me to a mixture issue.

Also I dont leave it on the sidestand too long if at all due to the oil pooling into the chambers. Learned that the hard way once when I thought I blew up my engine on a cold start and smoke started billowing out of the exhaust.
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Offline frankenduck

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Re: Stock exhaust woes! 1991 K75s
« Reply #16 on: October 13, 2021, 12:54:07 AM »
when I put it all together, however some metallic white flakes did come out when I first started the exhaust, but have not returned since.

The "disco flakes" (my term for the metallic specks that blow out) are not an issue. They're just foil bits from the gasket between the header collector and the exhaust can.  I have this happen about every time I replace a K75 exhaust.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #17 on: October 13, 2021, 01:10:18 AM »
The "disco flakes" (my term for the metallic specks that blow out) are not an issue. They're just foil bits from the gasket between the header collector and the exhaust can.  I have this happen about every time I replace a K75 exhaust.

They stuck to my garage mat but I refuse to clean them they add a nice vibe.
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Offline Chaos

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Re: Stock exhaust woes! 1991 K75s
« Reply #18 on: October 13, 2021, 06:57:09 PM »
Checking the valve clearance on a K75 is crazy simple, just need a socket and feeler gauge.  If one needs adjusting it's still relatively simple compared to many machines out there.
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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #19 on: October 13, 2021, 08:48:31 PM »
Checking the valve clearance on a K75 is crazy simple, just need a socket and feeler gauge.  If one needs adjusting it's still relatively simple compared to many machines out there.

I dont have a valve compressor or shims? Unless im missing something or the 2V is different?
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Offline Laitch

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Re: Stock exhaust woes! 1991 K75s
« Reply #20 on: October 13, 2021, 08:59:37 PM »
Fergawdsake! You're way out in left field, maybe even in the parking lot.  :laughing4-giggles:

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Offline KappoBrick

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Re: Stock exhaust woes! 1991 K75s
« Reply #21 on: October 13, 2021, 09:02:50 PM »
Fergawdsake! You're way out in left field, maybe even in the parking lot.  :laughing4-giggles:

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