78 cb750K won't fire up. (1 coil fires other doesn't)

Posted by mitchberry on Sep 06, 2010; 10:00am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/78-cb750K-won-t-fire-up-1-coil-fires-other-doesn-t-tp1425305.html

Hey guys,

There's going to be a lot of questions here.. i really hope that you guys don't mind taking the time to answer most of them.

Not sure how many people are familiar with me at this point, so i'll tell my story again. I am new to cb750's but not new to bikes, fabrication, or working on motors n electrical etc...

so, i just got this 78 cb750K from a dude for 200 bucks and he IS a novice.. basically, he had no idea what the hell he was doing. he had wires all mis-matched in the bucket, was missing a bunch of things, put parts from various years on this bike, and i have no idea what year they were off of so cant buy stuff to fix them etc etc etc..

It's really a clusterfrack...  

 ok, so, i do have a few questions.  

1. Am i safe in assuming that if the oil light is lit on the dashboard, that the bike will not start?

2. If the bike is running down the road and the oil light comes on, will it kill 1 coil / 2 pistons to stall the motor out softly as opposed to completely cutting the power completely and killing it while riding?

basically, i have power to one coil (the yellow wire one)  where as i do not have power to my blue coil.

yes.. i've done all of the obvious stuff..  

A. tested the blue and the yellow wire for juice with a volt meter at various parts of the revolution to make sure the points were either opened or closed.  

B. swapped the blue and yellow wire around to see if the blue wire coil fired with the yellow wire, since the yellow one is the one that has juice.  (it did by the way)...

C. ran a brand new wire in place of the old blue one in anticipation of there being a short somewhere in it..
(it still didn't get power to the coils)

D. printed about 5 wiring diagrams and color coded them / completely re-wired the bike as if starting from scratch.  

E. made sure that all 4 plugs were grounded firmly to something before doing my test... in fact 2 are in the engine still and the other two are firmly "mounted" to a place they will ground.

still no luck getting that damned coil to fire..

the only common denominator that i can find is that the oil light is on now whereas it wasn't before i re-wired it.   not to say i screwed something up, but on the contrary, i don't think it was plugged in correctly before whereas now it is.

before i did this, you didn't have to hold the clutch in to start the bike.. now you do... (took me a while to figure out why the hell it wouldn't engage the starter... i thought i blew the solenoid somehow.

i've done other stuff too, just cant think of it right now..

SO..... given all that i said, is there anything -else- that would cause only 1 coil (2 plugs) to fire and not the others?

another question....


there is a little "thing" that i am completely unable to find on ANY parts diagram that i've looked at including the clymer manual. i have no idea what it is, how it works, or what it does..

 it is mounted under the battery box and has some 1/4" to 1/2" hoses coming off of it.. i think 2 or 3...

is it for an oil cooler that's missing????

Does this bike NEED/REQUIRE an oil cooler? if so, i sure as hell don't have one with the bike.

does it go to the airbox that didn't come with the bike?

is it an oil pump?  

 i really don't know..

i'm NOT trying to advertise another forum here, however, i DID post the pics of this gizmo to another forum and didn't get much of a response out of it aside from "take it apart" and "are you sure it's supposed to do that?"


http://www.motorcycleforum.com/showthread.php?t=103752


Is that part something that will disallow it to run if the tubes aren't plugged into something?

There IS oil in the bike by the way, so there shouldn't be a reason for the oil light to be on that i know of..

 is it for oil LEVEL, or oil PRESSURE?  

 if it's for pressure, then why do i not have pressure?

I guess you're supposed to fill the bag with oil and not the crank case right? I do have 3 vintage bikes..  this included... the other's are 69 suzuki 2 strokes... my other bike is an 06 busa...  so, i'm really not sure what the oil bag does on the bike either...

Basically, i got no crash course on this bike, and the LITTLE things i don't know about it are all things that could be keeping this bike from starting!


well sorry for the long post.. i do hope to hear some answers though..   it's very appreciated!