CB750K1 crank no start, no spark

[devoncote98]

I picked up a CB750K1 that has not run in at least 18 years. After performing preliminary checks and maintenance (changed oil and checked other important things) I replaced the ignition coils because I was informed that they had gone bad before it was put in storage. Time to start! Right?... Nope. I cranked it over with the starter, to find out, no spark.

The tests and adjustments that I have performed:
-Gapped the contact breaker points
-Cleaned spark plugs
-Checked for continuity in wires-- No broken wires
-Tested condensers-- good

Information found:
-very little power to left point/ no power to right point
-no continuity through silicon rectifier bridge.
-Cylinder 1+4 coil warms up when key on
-Cylinder 2+3 coil does not warm up

At this point, I am convinced that the rectifier is the problem, I have tested the rectifier in both directions to eliminated the possibility that the diode is tainting readings.

I need help, the ignition system is my last step to making the engine run.

 

[dirtdigger]

coils get their power from the kill switch on the handle bar which gets power from the key switch, they are not connected the charging system in any way. both coils share a common power source so you need to start looking at why you arent getting good power to both coils. from the coils its a direct path to the points....start looking for loose connections. Its a very simple system you should be able to track it down by following the wires. If you dont have one already I suggest getting a wiring diagram so you can see how the power is fed.

 

[shaunbe]

Not a permanent fix, but you can run direct 12v from battery to positive side of the coils, that will tell you if they are good, I mean I would just do it to make sure it runs but fix it properly once you find out it's a good runner.

 

[chuter]

Maybe I didn"t see it but did you replace the points? I am sure that after 18 years the points are bad. If you did replace them then make sure they are clean with some contact cleaner. New points have a film of rust preventive on the contact surfaces. Chuter

 

[amc49]

A rectifier has zero to do with crank and start unless shorting out. And if you simply unplug it the cranking issue is gone.

If you are not reading the diodes with a diode setting on your meter you will get only error readings saying they are no good. Normal meters unless on that setting do not have enough amp in the leads to 'switch on' the diode to then block power flowing one way.

Bad points? Not on my block. If the pivots are not dead or the ends melted off they can be 'rebuilt' and made to run and quite well in most cases. I NEVER change them until utterly forced.

 

[chuter]

You don't replace points unless your forced too. WOW!!!!! I'll bet it runs really well Huh?

 

[amc49]

Yessir. If it doesn't run at 9500 rpm and all day long I know I've done something wrong that cannot be blamed on parts.

You have no idea of how much OEM stuff I rework to often run better than off the shelf parts, my life has been built upon it. I don't hesitate to discard junk but most stuff people pull off cars and bikes is not, only needing a little work on it to come back again to give good service for years.

Just added a 69 cent screw to my '11 Nissan today to make a $75 part go back to working fine. At 30 minutes on the fix that figures out to about $150/hr. pay rate. I keep records and the most at one time is around $2500/hr. but those are understandably rare. That one was an 'ATX rebuild' using a small washer to shore up a servo that allowed 1-2 and 2-3 to slip heavily, fixed and drove that car car 10 more years and it still ran fine when I sold it. I of course was told by 3 shops I needed a new transmission, pretty funny.

I rebuild $200 alternators as well for as low as 30 cents for some solder to solder the diodes back up to working again. Many other things like that.

When I buy a vehicle the dealers will never see that one again, I have never paid for car or bike repair ever in 45+ years and they all run perfectly, but I am somewhat eccentric.