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Motorcycle Repair/CB750K3 Plugs - Running Rich - Rough Idle


I am working on my CB750K3 & have a few questions about spark plugs.  My owners manual says standard plug is D8ES(L)& Honda Shop manual says D-8ES My question is what is the L for?  I can only find the DR8ES L plugs.  Is it OK to use them with new NGK resistor caps?  Also looks like I am running rich as all four plugs are covered in black soot (doesn't burn oil).  I read in an old magazine the Iridium plugs run cleaner in the CB750K's  Are they worth the extra expense?  FYI new air cleaner, 160 PSI all four, Battery voltage 13v-14.3v running, timing spot on & carbs just rebuilt & synced,  runs great through all throttle positions & never fouls out, but very black plugs & idles very rough.  Any ideas would be helpful,  Thanks in advance John

John,  Did you completely disassemble the carbs? Black plugs and poor idle could indicated reversed carb slides, high float levels, too large idle jets or main jets.

26mm float level is standard for those years, with a #40 idle jet. Needle clip in the middle and main jet sizes around 105-110, depending upon your intake and exhaust systems.

The "L" on the spark plugs is for extended tip plugs, which get the ignition source more closely to the middle of the combustion chamber. Standard NGK plugs and caps are fine. They bikes came without resistor caps, I believe. If the ignition system is strong it shouldn't make any difference, especially at idle.

Make sure that your spark advance timing is correct at idle and full advance.

Bill Silver

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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