DragBikeMike wrote on 09/25/26 at 16:06:12:


This is a photo I took when I took it apart to fix the oil leak a the head gasket (aftermarket head gasket without sealant between the plies for those folks not familiar with that fiasco).  The engine had about 500 easy miles on it - the carbon wiped right off the Cerakote Thermal Coating.

Mike,
I cannot understand how I could miss this thread as it progressed over the summer. Thanks for the valve timing (advance/retard) chart, it is a very helpfull reference.
Since you are investigating detonation, do you think it could help to install a second spark plug, like in the LS650 and the XF650 Freewind? It would not be too much trouble to  drill and tap that extra hole. It would take an extra ignition coil and I don't know if the ignitor can handle the extra current.
The output transistor is an FUJI D1071, capable of handling 6A, so perhaps, otherwise it could be replaced with a beefier type.
Or use the Ignitec, which can handle 21A, has programmable ignition curve, rev. Limiter and can take input from throttle position to, for example, retard ignition (also fully programmable) during WOT.

I think the second spark plug would help if it were on the right-hand side of the engine (like the Harley M-8), but our timing chain drive precludes that location.  Locating the second plug on the left-hand side would weaken the structure too much.  Could possibly do a weld build-up first and then do the machining for the second plug but I don't feel that the benefit would be worth all that invasive work.  The two plugs would be too close together to achieve the desired results.  

Regarding a coil, the stock coil primary windings have about 4.2 - 4.4 ohms resistance.  I believe a Harley 5 ohm dual-fire coil could be used without fear of frying the igniter.

Regarding the skirt of the piston cooling the top, I had this detonation problem with both the pop-top and flat-top pistons.  The pop-top has a partial skirt, and the flat-top has a full skirt.  The pop-top had the clean area on the intake side of the piston, and the flat-top seems to have clean areas all the way around the periphery.  Both pop-top & flat-top detonation damage was confined to the intake side of the piston.  That mystifies me because that should be the cooler side, but....it's also the side that should have more fuel.  

Currently, the engine performs very well at 9.8:1.  It has all the power I could ever want, and good manners too.  I find it surprising that such a small increase in bore size initiates this detonation issue.  I think Zevenenergie has an excellent point about the surface area increasing exponentially as the bore size is increased.  The additional heated surfaces would tend to light off the trapped fuel (end gas).  Bringing the CR down to 9.8, retarding cam timing a bit, and retarding ignition timing a bit seem to work well.  The engine runs great.

Thanks for the comments.  I will keep at it.  As the miles roll up I will do inspections and post the results.

What is unclear to me is why exhaust valve 1 has a different carbon footprint than exhaust valve 2.
Is the valve seal leaking more at one of the intake valves than at the other?

I have also been thinking about the shape of the exhaust port. I don't think this was done so much to achieve anti-reversion. I think it was designed this way as a rev limiter. The higher the RPM, the more the exhaust flows from the exhaust channels block each other?