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Newly rebuilt engine by me! New pistons, new rings, new overbore to .25, new valves, new guides, head and jugs decked. Run for about 2000 miles. Had an exhaust stud snap while riding. took the head off to take to the machine shop to have the remnants extracted and found this. What on earth causes that much carbon buildup? It was running great. I was just alarmed at how thick it is! And on both sides! Looking at the plugs one side is running rich, which I will correct when I put it all back together again. is this level of carbon normal on these engines? I run premium gas in this.
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Premiums gas may be your problem.

The bikes were designed to run on what is today regular.

Higher Octane fuels burn slower.
 

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That would explain it. I thought I was using the right fuel based on the original user manual. Seems octane ratings methods changed since that manual was written in 1974! Some digging in prior discussions on this seems to agree with your assessment. I've got it all apart now and am having the time of my life getting these deposits off. lol
 

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i've had good luck with regular gas and lead substitute. But I have to concede it could just be luck... I don't know if lead substitute is really that effective.
 
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