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Discussion Starter · #1 ·
DH8A spark plugs show white insulator with some tiny brown specks - specks reportedly indicate detonation so thinking lean mix... However...

insulator also shows a smoke ring at the base so main jet and plug heat range seem compatible. Also it gets a higher top speed with a leaner main jet. Richer jet causes the engine to fall off noticeably at full throttle. Ground strap is tan to brown, threaded portion is dark gray-black. I don't hear pinging.

Is it possible to have both too rich a main jet and get detonation???
(BTW removing the main jet had the engine running way too rich by 1/2 throttle - so probably not a lean needle jet/needle combo)

CB160. Timing is retarded 3-4 degrees from stock (attempt to cool the plugs and prevent detonation). Running 92 octane premium. Previous owner installed new Mikuni VM20 273s. These are running 140 mains, 15 pilots and needles are all the way up. Previous owner installed overbore pistons (2mm?). I installed Emgo exhaust of roughly stock length.

Thanks for you help!
Shawn
 

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Shawn, sounds like you are on top of the problem. Don't think that richness will cause detonation. Suspect that spark retardation may be more likely the suspect. White specks point to leanness though. I'm running stock ignition parts at the factory settings.

My D8-HA's show carbon around the barrel ring but tan electrode and core. Idle mixture is tough to get just right with the stock carbs. Am running the stock factory jetting and mid needle position. Don't have experience with the Mikuni's or their jetting but someone with one of the newer/larger bikes that run them would be helpful.

I'm running the first overbore size pistons. Pod air filtration on the intakes. Exhaust is through standard pipes into megaphones (like Emgo's) with fiberglass wrapped wire "glass-pak quieting" inserts. Fuel is 92 octane premium like you are using.

I"m running around 300 to 320 degrees F on my Trail Tech Vapor's temp sensor under the right plug base.
 

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Discussion Starter · #3 ·
Thanks Curmudgeon. Woke up in the middle of the night to check timing again (Nothing like making your problems public to focus the mind) - found I'd turned the points plate the WRONG WAY. Set the timing to stock and did my normal test run. Performance was good, but still getting the specks that suggest detonation. So now I really will test with retarded timing.

This bike also has pod air filters.

I like the temperature sensor data - now I need to get one of those. Is it a ring that sits under the spark plug?
 

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SED: The temp sensor is indeed a copper ring that sits under the spark plug and wired back to the display head. Other model Trail Tech Vapors are set up for the newer water cooled engines and are supplied with a hose insert that carries the sensor. I know the 704 model works for air cooled bikes as that is what I'm using.
 
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