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Afternoon All, Recently rescued a '89 k75C from a neighbor after it has been sitting for years in his garage. After fixing some electrical issues, got it running this weekend. starts, idles and runs like a champ, but seems to be running rich. Occasionally when letting off throttle i get backfires at the exhaust. pulled and replaced the plugs, which were black with soot. Again, the bike has been stored for years, so vacuum leaks, fuel injectors gummed up, etc are certainly real possibilities.

So, where to start? Anything sound familiar?

Thanks in advance for your help.

B
 

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Could be the fuel pressure regulator, measure fuel pressure...should be 2,5 bar.

Next, check the water temp sensor.... measure the resistance between contact 10 and 13 at the FI ECU connector.
You should measure ~2,5 Kohm at 20 degree Celsius.

Inge K.
 

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FlyingDuck said:
"They all do that." K75s backfire under moderate to heavy engine braking so its likely there's nothing wrong it.
Yup, sounds like mine. Runs great but will get a backfire under heavy engine braking.
 

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My K75 burbles and afterfires under engine braking as well, however it will give a good bang if I quickly roll off the throttle to engine brake from highway speeds. I collected this information from various sources in my efforts to cure this behaviour:

1) check that the throttle position switch on the rear of the throttle bodies is working properly.

2) there is an air by pass that adjusts the fuel mixture at low air speeds .. under the fuel tank inside the air box. This is normally set with a CO2 meter..

3) Another cause of afterfiring could be leakage in the first 20" or so of the exhaust system. I have read that the gasket between the headers and muffler can deteriorate, causing the leakage. Consider using Permatex Ultra-Copper high temp RTV silicone gasket maker #101BR to seal that joint. It might also be an idea to replace the copper gaskets at the engine exhaust ports to ensure no leakage at those points either.
 
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