Topic: Nighthawk 550...more electrical problems

[Laminar]

maybe timimg for 2/3 is out of whack somewhere ??

All electronic, non-adjustable. I replaced the CDI box to see no change.

BUT IT'S RUNNING NOW.

I switched the coils around and 1/4 still fired so I knew it wasn't a problem with the coils or wires.

After some cranking it and trying to keep it running on two cylinders, the other two seemed to "pop" on and a puff of black smoke came out of the exhaust. The revs would jump 2 or 3000 and then it'd be back to two cylinders. After a while all four cylinders started firing reliably and it would idle with the choke on. It seemed to rev okay, but there were still occasional backfires and pops.

I'm starting to wonder if there was some water in the gasoline or something that just needed to be worked out of the system.

It's still not reliable or road-ready, but I'm leaps and bounds ahead of where I was this morning.

I'll get some new plugs for it tomorrow (if I have time; I'm buying a car, getting a dog, and getting a desk for my wife, all after 12, so busy day).

My other main concern now is that there's something loose in the fuse box. Every once in a while nothing will happen when I turn the key, but if I wiggle the fuse box then everything pops back on. Yesterday I pulled the fuse box out, cleaned all of the connections, and hit everything with dielectric grease. Maybe I'll just have to fasten it in a working position and hope for the best.

[750goes]

If your fuse box is anything like mine was, just replace it  - when I took mine off the bike, I was surprised at how crappy the connections were on closer inspection, and how loose they were in the box itself - I thought a $10 investment for some blade fuses and a new 6 bay fuse box holder was worth it... I now have solid connections and can even put in some blade fuses that have inbuilt led's if I want to ......

[Laminar]

Are you familiar with the Nighthawk's fuse box? It already uses blade fuses, and it's not a simple four wires in/four wires out type of deal. It's more of a power distribution box, and trying to wire in something generic would cause way more problems than it would solve.

[Laminar]

You can hear all four cylinders try to kick in around 0:21 and again around 0:34. This was after about four minutes of running on just cylinders 1 and 4.

[w1sa]

Do a carb bowl(s) drain to eliminate the possibility of water contamination. It's hard to imagine enough water getting into a carb while runnng to cause a problem, but that was your suspicion.
If your carbs have bowl drain screws and outlet, also do a fuel level check with a clear tube attached to the drain point (if possible) and verify fuel height comes upto within say 3mm of gasket line.

It might still be the ignition, so don't discount the possibilty that sufficient spark is failing to occur at the correct time, while cranking.      

[Laminar]

Do a carb bowl(s) drain to eliminate the possibility of water contamination. It's hard to imagine enough water getting into a carb while runnng to cause a problem, but that was your suspicion.

I did that right before the video

If your carbs have bowl drain screws and outlet, also do a fuel level check with a clear tube attached to the drain point (if possible) and verify fuel height comes upto within say 3mm of gasket line.

I did this back when I was first having trouble, but I suppose it's worth checking again.

It might still be the ignition, so don't discount the possibilty that sufficient spark is failing to occur at the correct time, while cranking.      

I'll pick up some new plugs when I get a chance and see if that helps.

[w1sa]

You could test the pulse generator sensors (resistance test). I dont know what the spec resistance  is (330 I think on the 750).
Assuming the static timing is good, good clean fuel/height, new plugs, good spark, reasonable compression, (if)pulse sensors are good and it still doesn't fire properly, it would seem to point back to the spark unit.(which does affect ign timing if faulty)

Another thought is, that with all that cranking, if you've got poorly timed/weak ignition (as a result of the spark unit) occuring in some cylinders, with good gas flow, it might be flooding beyond cold start ability.

[Laminar]

You could test the pulse generator sensors (resistance test). I dont know what the spec resistance  is (330 I think on the 750).

I don't recall the Clymer's mentioning this test. What's the process?

[w1sa]

Disconnect the pulse generator connector at the spark unit and measure pulse generator wiring resistance for each sensor, blue to white pair and then yellow to white pair. (or it may be wired blue/blue  and yellow/yellow).
The '84 750 is 330ohm, but they can be quite different on diff models. I have a '81 900 and its 530 ohm (per Clymers manual) They are considered good if within +/-about 10%

Does your Clymers describe a 'screwdriver test' i.e. ignition on, and bridge the gap between the rotor and the sensor pick-up several times to see repeated good blue spark at the plug(s)? To do this you want the rotor tip away from the sensor pick-up.

[Laminar]

Here's the deal.

I managed to measure the resistance across the pickups. I saw 340ohm across the blue wire and its respective white wire (blue is for 2/3, right?) but I saw zero continuity across yellow and its respective white. Oh well.

I drained all of the gas in the tank (put it in the wife's car) and the carbs (dumped it) and got all fresh 87 octane with 1/3 can of Seafoam.

I replaced all four plugs with brand new NGK plugs gapped to 0.034".

I cranked it, and voilà.

I took it for a ride around the block and holy **** I forgot how fast this bike is. The CB750 feels like a lumbering ogre compared to this bike. Also, the stock handlebars are absolutely awful. I pulled the bars that had been on this bike (Stock '76 CB550 bars) to replace the mini ape-hangers that came with my CB750.

But the cam chain still rattles. And the stuttering that started all of this hoopla was still there. Around 5500 rpm it bucked and stuttered. After a mile or so I came back and put my other CDI box on the bike. After doing that, the stuttering almost went away. There's still a tiny bit of lag from 5500 to 7000 and after that it evens out and pulls haaaaard up to 10K.

I suppose the next steps would be to get new pickups and a new air filter, as the current filter is pretty dirty and gross. I also need to pick up some new bars and grips. I love money pits...

[750goes]

congratulations....