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78 cb550k 605cc bore misfire...

4K views 32 replies 7 participants last post by  photoputz83 
#1 ·
so i had a little issue with a strong misfire that would lead to stalling out after. it seemed like it would only happen when the engine was hot or when i hit high revs while riding and then let off. there were a few occasions where i would try to start it again and then the battery would almost be completely drained. i ended up going all out replacing the coils, the ignition (dyna s), and eventuallly the entire engine as i wanted to have more spare parts on hand or to rebuild so when one engine messes up or needs a rebuild i would have one to drop in. well after all of this was done it seems like the misfire hasnt got any better. i pretty much chainged everything but the carbs which have been cleaned, the wire harness, and the rectifier. would the rectifier cause a misfire for any reason. im looking for any ideas as to what may cause this to happen. the riding season is almost at its end so any help would be halful at this point.
 
#27 ·
Remember, we are having to guess at your mods and skill level since you provided neither.


Seasoned cafe racer builder <------- there is your problem right there.

Have you checked for things shifting while you ride? I had a coil once that tried to weld my fuel tank to my frame because of a loose bracket. Only happened when braking, and it weirded me out why I would drop a cylinder on deceleration. Maybe something is loose somewhere and shorting out.

We are trying to help, honest.
 
#28 ·
when i say seasoned i mean 15 plus years and has modded cb750s for the last 10 to 12. numerous 836 big bore kits and such. not to mention the fact that he is from essex. so across the pond. i ended up dropping in a new rectifier and it was the key. the only thing electrically that i didnt replace. runs fine from idle to 12k. also upped the octane a bit with a new mix ratio of 30:70 turbo blue, 93 octane no ethanol.
 
#30 ·
always find it funny when guys from england that talk about the honda cafe thing....ace closed before the 750 came out right?....sorry thats just a pet beeve with me.

so back to your bike.....didnt really answer many of the questions posed buy people trying to help.
an answer of "it cant be that because its new" or "its not that because a master did it" is not helping anything.

and really for the life of me i cant figure out what your "issue" is because you keep popping back and forth with the old and new motor.....or maybe im just drinking to much....



What resistors did you install for the coils and how did you install them? this IS a difference...

also dont just look for one component thats wrong broken or just messed up....since there are so many variables that you added its hard

Please write up what ALL the mods and settings are and then talk about the symptoms are again
 
#32 ·
I had same issue. I rebuilt those damn carbs so much I can do it in my sleep. I replaced my rectifier yesterday...and BAM! That was it.
If anyone wants a good price on them go to www.oregonmotorcycleparts.com/rectifiers.html He sells brand new rectifiers for 22.00! Plus he has many other electrical parts. Also thanks to www.dansmc.com/electricaltesting.htm for such great detail on how to test the rectifier, stator, regulator, etc...
 
#33 ·
jag, actually ace cafe re-opened. was there two years ago on a 68 t-120r. and i ran a bunch of tests on the old rectifier and there was zero continuity. the coils are 3 ohm and with the way i have the wiring i have them at 5 ohms now which is the stock spec.
 
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