Exhaust Functioning Correctly or Needing Service -- 1994 K75 R/T

davidacaldwell · May 19, 2022

Hi, I purchased a 94 K75 R/T last week, and have put about 500 miles on it.

What's the best way to determine if the exhaust is functioning correctly, or if it needs servicing? The exhaust fumes are really heavy when in traffic, and I'm unsure if this typical; additionally, the bike backfires a decent amount when working through the gears toward a stop.

Any insight would be appreciated.

slowpokepete · May 19, 2022

Sounds like the issue is not the exhaust.

regardless, you should not be getting back firing or excessive fumes.

SPP

98lee · May 19, 2022

David,

Welcome to the Forum!

This sounds like the silica gasket between the header and the muffler needs replacing. It can cause both the issues you mention.

18 12 1 457 142 GASKET $16.51

:dance

dieselyoda · May 19, 2022

98lee said:
David,

Welcome to the Forum!

This sounds like the silica gasket between the header and the muffler needs replacing. It can cause both the issues you mention.

18 12 1 457 142 GASKET $16.51

Interesting comment.

My first thought wasn't that but it is an expensive try.

I would have gone with spark plugs/leaking fuel injectors or idle too high.

98lee · May 19, 2022

Yoda,

There are two things that cause backfire on a K75.

One is the gasket between the header and the muffler. The leakage of air into the exhaust tends to ignite unburned fuel in the exhaust system. This is usually during deceleration. You can sometimes visually see if the the gasket is bad. It is a sleeve that goes over the header and inside the muffler. Obviously, a leak before the muffler can cause fumes. I’ve replaced many of these on other peoples bikes, especially if they have removed the muffler a time or two. They get damaged VERY easily during that procedure.

The other cause of backfire (but usually not fumes) is a misadjusted throttle position switch. It tells the computer to stop injecting fuel if the RPMS are above 1800 RPM AND the throttle is fully closed. This causes backfire because of all the excess unburned fuel that gets into the exhaust.

Obviously a long time BMW owner. Thinks $16 is expensive maintenance. :stick

pglaves · May 19, 2022

98lee said:
The other cause of backfire (but usually not fumes) is a misadjusted throttle position switch. It tells the computer to stop injecting fuel if the RPMS are above 1800 RPM AND the throttle is fully closed. This causes backfire because of all the excess unburned fuel that gets into the exhaust.

To be sure this is clear, if the rider rolls off but does not completely close the throttle then the injectors keep injecting and excess fuel gets in the exhaust, ignites, and backfires. Provided the switch is correctly adjusted, rolling the throttle completely off causes the injectors not to inject so the excess fuel does not collect in the exhaust.

davidacaldwell · May 20, 2022

Thanks very much for the group's responses.

I noticed, when riding today, that the bike backfired consistently, when in 2nd gear, tooling along @ 2500 - 3000 RPMs (city driving...block to block, for example); I don't know if this adds any additional helpful detail toward diagnosing the situation.

A few follow up questions,

--Can a lay person test the throttle position switch? or, is it something that one just replaces?
--What's the inspection/service interval for the fuel injectors?

[The bike's done 44 k miles in total, and was fully-serviced (fluid refresh) in Jan 2022.]

pglaves · May 20, 2022

Usually it is possible to hear an audible "click" from the switch just as the throttle is closed. The switch is adjustable. It is mounted with hot-dog shaped holes so when loosened it can be rotated so it engages just as the throttle plates are fully closed. The switch is the square shaped black box at the back end of the array of throttle bodies.

davidacaldwell · Jun 24, 2022

Leak before the muffler

98lee said:
Yoda,

There are two things that cause backfire on a K75.

One is the gasket between the header and the muffler. The leakage of air into the exhaust tends to ignite unburned fuel in the exhaust system. This is usually during deceleration. You can sometimes visually see if the the gasket is bad. It is a sleeve that goes over the header and inside the muffler. Obviously, a leak before the muffler can cause fumes. I’ve replaced many of these on other peoples bikes, especially if they have removed the muffler a time or two. They get damaged VERY easily during that procedure.

The other cause of backfire (but usually not fumes) is a misadjusted throttle position switch. It tells the computer to stop injecting fuel if the RPMS are above 1800 RPM AND the throttle is fully closed. This causes backfire because of all the excess unburned fuel that gets into the exhaust.

Obviously a long time BMW owner. Thinks $16 is expensive maintenance.

Just a follow up note to see what part specifically you've replaced that gets damaged easily...before the muffler; I didn't see a description of the part indicated...maybe I missed it.

As always, thanks for your help.

98lee · Jun 24, 2022

davidacaldwell said:
Just a follow up note to see what part specifically you've replaced that gets damaged easily...before the muffler; I didn't see a description of the part indicated...maybe I missed it.

As always, thanks for your help.

David,

Reread post #3 and the second paragraph of post #5.

:dance

davidacaldwell · Jun 25, 2022

PGlaves said:
Usually it is possible to hear an audible "click" from the switch just as the throttle is closed. The switch is adjustable. It is mounted with hot-dog shaped holes so when loosened it can be rotated so it engages just as the throttle plates are fully closed. The switch is the square shaped black box at the back end of the array of throttle bodies.

Hi, Is this the throttle position switch? If so, I'm definitely not hearing a click from this part when closing the throttle.

Apologies in advance for being so dense.

98lee · Jun 25, 2022

Yes it is. The click is very quiet. Depending on your hearing, you might not be able to hear it. The engine should be off.

Take off the plug to it and clip an ohm meter to the two terminals on the switch closest to you. You should be able to see the switch make at fully closed throttle and break just off fully closed throttle. The adjustment is made by loosening the two Phillips screws that hold the switch to the throttle body (you must remove the coil cover to access the rear screw).

:dance

davidacaldwell · Jul 6, 2022

Just a note to say thanks to the forum--I replaced the component between the header and exhaust, and that resolved all of the problems...exhaust fumes leaking, and backfiring.

Separately, I was finally able to hear the clicking at the throttle position sensor.

Really appreciate everyone's help.

98lee · Jul 6, 2022

David,

Any time.

Exhaust Functioning Correctly or Needing Service -- 1994 K75 R/T

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