Topic: Got my cb700 running after 3 years, then BAM

[Hondawggie]

I spent all day bringing my 1986 cb700 Nighthawk back from slumber and the bike was purring after several hours of flushing the gas tank, new fuel line and filter, etc. etc.

Purring like a panther then the engine just shut off instantly like someone had hit the 'Off' button  (the 'Off/Run' button next to the throttle).

I checked and there is no longer any spark.  I checked the 'Off/Run' button, nothing wrong from what I see.

I checked all the fuses, including the big one next to the starter relay and all fuses are good.

The bike was standing still and running a fair amount, I was enjoying hearing it run, I kept starting it and revving it up, and day dreaming about which canyon in the hills here I'd ride first.  The bike was getting hot, it's an air cooled engine.

Okay, that's the way things went today, very sad just now thinking I may have a major issue that I have no clue where to start looking.

Is this a known problem with the 1984-86  CB700 Nighthawk?

[Hondawggie]

Part of the "bringing back to life" work I did yesterday was installing new oil for the engine.  So it definitely has oil.

I admittedly got lost in a haze as the bike sounded so dang good after 4 years of slumber:

1)  "All the other bikes I've brought back to life -- only Hondas seem to start right up and run without needing to pull and clean carbs!  First my 1980 CB900c, then my CB750f, and now my cb700."

2)  "Soichiro Honda -- just like George Washington -- incredible.  What a great man he was"

3)  "Think I'm going to take the Mt. Hamilton ride up in the east foothills tomorrow"

Freaking haze dudes, I was mesmerized the bike was up and running and sounded like new.  Honda, what a great company.



Then I thought "I wonder if this model has an overtemp shutoff that protects the engine?"

Could that be what it is?

[Hondawggie]

All the other electric functions are fine:

- starter motor cranks the engine
- running lights and headlights work
- dashboard lights work
- oil lamp on the dashboard goes out after pushing the starter button and the engine cranks over, then when I let go of the starter button, the oil lamp comes back on
- battery is brand new (that was part of the 'bringing back to life' work too)

So last evening, after I allowed it to run while on the centerstand, blipping the throttle, it was fine, if a little hot from lack of air flowing around the engine, then BAM it just stopped running; I checked and no spark.

[Hondawggie]

Easy enough to check, will do -- a couple questions though:

- does this model have an 'overtemp sensor' that will shut off spark?  I don't have a manual for the bike and searching the web turned up nothing about a temp sensor for it

- not knowing much about ignitors at all, but if the ignitor failed would that shut down spark?   A sudden failure of an electronic component would be consistent with the sudden shutdown of the engine that I experienced yesterday, so I'm wondering if the ignitor could be the culprit?

[70CB750]

I know you dont have kick start but are you able to turn it over easily enough? 

Just to eliminate bad things like stuck engine from overheating.

[Hondawggie]

No, the starter motor cranks fine after the shutdown incident occurred.   If the engine suddenly froze up, I don't think the starter motor would sound normal or crank, and it sounds completely normal.   Yesterday when I had the #1 spark plug out, checking for spark after the shutoff incident happened, I could hear the #1 piston creating suction, I was close enough to hear it, because I was looking at the spark plug and its gap, as the plug laid against the cylinder.


I have heard, but not personally experienced,  a coil failure that can occur when the heat from the engine -- which is rising up and heating the coils -- that a coil can intermittently stop working.  If true, that might mean after cooling off now for about 18 hours since last night, the bike might fire up again (might).  I'll check this out.

I'll have to check for voltage at the coils too.

I'm familiar with the "pull the clutch in" lockout that's on most bikes since the late 70s' and onward -- the functionality was to not allow the *starter* motor to crank unless the clutch was pulled in.

On 1970s bikes, they did not have that "pull in the clutch to allow the starter motor to turn" feature  -- if the bike was in gear, and you pushed the 'start' button, the bike would leap forward.    I have personally experienced that; started riding in 1976.   I'm pretty sure even my 1977 Kz1000 had that issue.  Then the bike companies began putting the clutch-lever-pulled-in-or-no-start interlock switch to prevent that.

There are various fail points like that: clutch lock out, sidestand lockout, Start/Run button.  This bike has all of those.

  Because of the fact that the bike was

- on the center stand
- in neutral
- and was running fine

yesterday,  since the sidestand/clutch/Start-Run lockouts are just switches, and the bike was already running,  I just figured the chance that a mechanical switch like those three will suddenly fail is very remote.  Possible, but unlikely.

The way it shut off like instantly, it was more like an electrical component failed.

I know nothing about ignitors and wondered if an ignitor -- which is an electronic component, not a mechanical switch -- might explain the sudden shutoff.

[Hondawggie]

I got real lucky on this one.

When I checked the bike one hour ago, when I pushed the starter button, the exhaust popped and I knew that the plugs were firing. 

But then nothing again.


So I pulled plug #1 and checked for spark (grounding the plug on the head with the plug wire connected).  No freaking spark.

I thought "okay that's a dirty connection somewhere -- otherwise it wouldn't have fired at all just now."

I pulled the top off the 'Stop/Run' switch on the right handlebar.  Using my $6 Harbor Freight multimeter I found that when the switch was set to "Run", the switch was only very intermittently showing a connection.

So I disassembled the top of the switch.  In the process I dropped the spring that's behind the copper contact plate on the switch.  Got real lucky and found the little basterd, dang thing is only 1/8 of an inch long. Dang man, got lucky.

Well the copper contact plate and the two connection points were oily and dirty, so I cleaned them, reassembled and the meter now showed continuity across the switch when it was on 'Run.'

I checked for spark on the #1 plug -- YEE HAW.

Reattched the switch pod to the right handlebar, put the #1 plug back in,  purrs like a panther.

So dang relieved it's not the ignitor or pulse generator, etc.

Thanks for everyone's suggestions.

[70CB750]

Excellent!

I have a soft spot for these, since I had CBX750 over in Europe - thats a European version of your bike.

Loved that thing:)

[ekpent]

Good call Cal !!

[mrbreeze]

That's cool that you stuck with it and found the simple problem on your own. A shop could have easily clipped you for a couple hours of labor to find that problem. I had an intermittent cylinder 2-3 drop out couple of years ago that was driving me nuts. It was a simple bad connection to the power wire of that coil but I didn't just happen onto the cause of the problem right away.

[Hondawggie]

Maybe its 'sympathy karma' from the almighty -- I spent 3 solid days on my 2002 Bandit 600, which exhibits exactly the same problem -- turns over fine, but no spark.  I traced all possible points of failure on the 600 -- stop switch, clutch cutout, sidestand cutout, fuses.  Only thing left is the pulse generator or ignitor.  *I think.*   

So when my Nighthawk popped a bit this morning I knew it must be a real simple issue.  But the 600 Bandit,  I have never even heard the damn thing fire at all so it might be time to part it out.

The thing about the 700 Nighthawk is -- the bike has sat so long that the hydraulic clutch fluid in the master cylinder dried up into a brownish cake-like texture. 

And yet the 4 carbs work fine!   This is not the first Honda I've experienced that.  My 1980 900c had sat for 5 years and was put away wet and still the carbs were fine.

The Nighthawk only has 18,000 miles and it still feels tight like a new bike does -- first ride coming up.