Exhaust Pipe Discoloration Around Baffles '79 CB750K

nebnampach

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Hello everyone, I just bought a 1979 CB750K with roughly 30k miles on it and I am brand new to the Honda CB scene. The stock exhaust was rusted out and the previous owner swapped it to a 4 into 2. The carbs were adjusted and cleaned to work with the new exhaust and he put maybe 10k miles on it with the new exhaust. The pipes are discolored near the baffles and I am wondering if this is normal or if this indicates an issue with it running too lean.

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Is it quiet? I ask because you have no volume in that muffler to handle that much flow, the muffler is sized for something half that engine size.

It is VERY unusual to get bluing that far back, I myself would be dumping that exhaust as it is likely severely restricted and why it would do it that far back. That severe can damage the engine by burning exhaust valves slowly, it won't be apparent until the damage is already done.

That far back should be running much cooler even if lean, that is likely not your problem. When lean they will discolor up at the pipe beginning at the head like your first pic seems to show just starting to happen.

If both sides are not doing it then maybe an internal baffle has come loose to stop the system up.
 
Is it quiet? I ask because you have no volume in that muffler to handle that much flow, the muffler is sized for something half that engine size.

It is VERY unusual to get bluing that far back, I myself would be dumping that exhaust as it is likely severely restricted and why it would do it that far back. That severe can damage the engine by burning exhaust valves slowly, it won't be apparent until the damage is already done.

That far back should be running much cooler even if lean, that is likely not your problem. When lean they will discolor up at the pipe beginning at the head like your first pic seems to show just starting to happen.

If both sides are not doing it then maybe an internal baffle has come loose to stop the system up.

Here is a short video of the bike idling for reference.

 
If that rear can size is not around 2 1/4" then you may be out of luck. Look around in Supertrapp stuff to see but it will be expensive. The exhaust options for these have really disappeared in the last couple years. Maybe MAC makes something. All depends on your OD of pipe there.
 
Wow look at this, I went to go look at Supertrapp for exhaust and they are missing options to select Honda models between 1979-1984.

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