Topic: 78' CB750K Plugs with pictures

[zeus87]

Well, dropped my poor running bike off to a mechanic , had him go over everything that i had already done (valves,carbs,timing,cam chain) (previous post about poor low rpm throttle response)  Now after getting it back, it ran much better...After feeling some backfiring in cylinder four i decided to check the plugs...this is what i found.  NOW before i dropped it off i rode it around town say 25 miles, checked the plugs..they were lean but looked nothing like plugs 1+4.  What is happening here? i know he said he adjusted the timing with a ohm meter, it must be off correct?  i have a timing gun, but before i fool with anything i wanted some input.   AS you can see, the metal has broken off the tips of the plugs.  Please let me know what is happening, and what i can do about it.. THANKS

[05c50]

Looking closely at the negative electrodes indicates that 1 and 4 are running way too lean/hot. If you look real close, can you see little flecks of aluminum? If so,detonation would be indicated. Detonation is caused by timing being way too advanced or improper fuel mixture. My guess is you have both. Contiuing to ride it like that will surely cause damage. If you have a timing light and know how to use it-check the timing,especially on 1 and 4. Then rip into those carbs. Something just aint right.

..........Paul

[zeus87]

wtf...Checked timing, both idle and advanced is spot on. 

[zeus87]

NEWS:  I called the mechanic who went through this bike for me.  He's an old school triumph and honda racer.  He ground down the negative electrode to look like this, he said that back in the day, the 1.5 hp gain was key in winning races.  He didnt get cylinders 2
+3 because he didnt have a socket that would get in there. Now im sure this is out there for you guys, but just crazy to me.  Anyway, as I look closely you can see the grinder marks.  Let me know what you guys think.

[TwoTired]

I guess in theory, the reduced ground strap exposes more of the mixture to the spark for a more complete burn.

Odd that those are the darkest and most sooty of the bunch.  I think I would swap 1&4 with 2&3 and see if its cylinder related or plug related.

[andy750]

Unless you plan on racing Id get a new set of plugs and install them. The rest of us seem to be doing fine without grinding up our spark plugs.

Good luck
Andy

[05c50]

Ahhh,Yeah,back in the day......when we looking for anything that would make an engine run better,we tried that. Good in theory,but was never able to justify it....kinda like indexing plugs. Maybe your mechanic is 1/2 old school,or....just half....
Keep us posted on your findings.

.......Paul