Topic: Nighthawk 550...more electrical problems

[Laminar]

I finally got around to working on the Nighthawk and I can't get it to start. I'm pretty sure it's an electrical problem.

It's to the point where it will occasionally pop and sound like it's running until I let off the starter button, at which point it ceases to do anything.

The battery's good, I have good power straight to the coils, and the carbs and fuel lines have been recently cleaned out. There is compression on all four cylinders and the timing is set correctly. I see spark on all four cylinders, but it's a tad weak on cylinders 1 and 4 (same coil).

The biggest thing I noticed is that with the engine off I see the tach needle jump with the turn signal. I can hear a hiss or buzz from the coils when the turn signal blinks.

I took a Dremel and sanding drum to all of the grounding surfaces at the coil to make sure the ground is good. I have the yellow wire going to cylinders 2 and 3 and the blue wire goes to cylinders 1 and 4. Color on top, black and white on bottom. I took a light piece of sandpaper to clean off the pickups on the crank sensors.

All of this started after about a 45 minute ride in the rain led to stuttering at about 5000rpm. I'm not sure what to do next. Grab a new set of coils?

[Alan F.]

Maybe you could run a new positive lead directly from the battery to the coils?  Maybe they're not getting enough voltage to produce an adequate spark?

Could you measure the voltage the coils are getting now?

You should be able to measure your coils with an ohmmeter, across the primaries of each coil you should get a similar reading, and across the output leads of each coil you should also get similar readings,  if there is infinite resistance across any of these points...you've found the fault you're looking for.

-Alan

[Laminar]

I already performed the relay mod so I have 12V straight to the coils from the battery turned on via a relay. I measure very little drop between the battery and the coils.

Just to clarify - you're suggesting that for each coil, I measure resistance between the terminals where the colored and black/white wires connect, and then measure resistance between where each of the two plug wires connect?

[pikeymick]

Were you able to get it running again?

[Laminar]

Nope, not yet. I bought a set of replacement coils on eBay that turned out to be DOA, so I sent them back. I bought a replacement CDI box but that didn't change anything.

I'm open to suggestions.

[pikeymick]

Hmmmmm, have you run a compression test?

[Laminar]

Hmmmmm, have you run a compression test?

Yep. Equal compression on all 1, 3, and 4, and 2 was about 10 pounds below them.

[750goes]

check for adequate fuel flow..
is there fuel in all bowls?
after cranking are the plugs wet ?
is the off/run/off switch positioned correctly?

[pikeymick]

You have spark so it's not the kill switch, you have fuel, so it's not the carbs... Try this. Open the vacuum ports on the intake rubber mounts and spray some carb cleaner or starting fluid in there, crank over.

What about flopping your coils and spark plug wiring so that 1&3 and 2&4 are on the opposite setup that they are on now.

[Laminar]

check for adequate fuel flow..
is there fuel in all bowls?
after cranking are the plugs wet ?
is the off/run/off switch positioned correctly?

1. Petcock cleaned, fuel line cleaned, fuel seen flowing through filter
2. Equal fuel in all four bowls
3. Don't remember
4. Yes.

[Laminar]

I just redid the compression test and got this:

1: 60psi
2: 120psi
3: 150psi
4: 120psi

The fact that the compression numbers changed this drastically between my first test and this one tells me that something funky is going on. Maybe I had the choke on before or maybe I screwed the tester in tighter this time. Either way, this current test doesn't look good.

The plugs were mildly damp after cranking, I wouldn't call them wet. They were more sooty than anything.

I need to put a little oil in #1 to see if that improves compression. If not, I'm assuming this means I have to pull the top end apart for the third time and make absolutely certain that the tappets are bled correctly. PITA.

[pikeymick]

How many miles on it?

[pikeymick]

But I don't think one low cylinder would keep it from running... I was low on two and they were the only two that worked, and I could still ride it haha.

If it won't start there must be some other complication going on...

[Laminar]

How many miles on it?

25,000. I put 5,000 of those on this past summer.

[Laminar]

I realized last night that the low cylinder (1) was the one I was squirting carb cleaner into to see if it would fire (it didn't), so the carb cleaner probably just washed the cylinder walls of oil, causing low compression in that cylinder. I put a little oil in it and saw a 30psi jump.

My money's still on spark. Maybe I'll try swapping the coils from my '76 onto it.

[pikeymick]

Phew! That's good to hear! Good luck with the coil swap!

[kirkn]

Hmmm......  all this started with a ride in the rain leading to a stutter at/above 5000 rpm?

That sounds like water contamination on a wire somewhere.  Pretty straightforward.

And now it won't start at all.  But you never really said - do you have spark at all four plugs?  The simple test - laying the plugs against the head and seeing a nice fat blue spark when you crank it over?

Swapping coils?  I'm not sure that's going to work, nor tell you much.  A '76 is a point-type ignition and that Nighthawk will be an electronic ignition.  Do the coils interchange?  Point-type ignitions function differently than electronic type.

Carbs?  Compression?  That all doesn't seem to follow from a simple rain-induced stutter....

Seems like you're running all over on this one.  Time to track down one item at a time...   

Kirk

[Laminar]

Hmmm......  all this started with a ride in the rain leading to a stutter at/above 5000 rpm?

That sounds like water contamination on a wire somewhere.  Pretty straightforward.

You'd think so.

And now it won't start at all.  But you never really said - do you have spark at all four plugs?  The simple test - laying the plugs against the head and seeing a nice fat blue spark when you crank it over?

Yes, spark at all four wires.

Swapping coils?  I'm not sure that's going to work, nor tell you much.  A '76 is a point-type ignition and that Nighthawk will be an electronic ignition.  Do the coils interchange?  Point-type ignitions function differently than electronic type.

The coils appear the same, all the way down to the wire colors coming in. I figured it was worth a shot.

Carbs?  Compression?  That all doesn't seem to follow from a simple rain-induced stutter....

Seems like you're running all over on this one.  Time to track down one item at a time...   

I'm open to suggestions for which one item to check, as I've checked every one item at a time I can think of.

[w1sa]

It sounds like you've got a coil ground short (similar to points that don't open properly).

I haven't got a circuit for your bike, but looking at a '84 750 N'Hawk, the 2-3 coil ground (yellow) completes the Tacho circuit via each 2-3 pulse generator/coil firing. That yellow wire also connects to the tach thru a common connector that includes the common harness ground for instrument and blinker lights.
So, I'm thinking along the lines, that during the 45 minutes of riding in the rain, water has got into that connector (or other) and created a short between the yellow wire pin (2-3 coil ground) and the harness ground pin. It would also explain why your tach pulses as the blinker operates i.e. blinker ground acts as ground for tach.

That could (I think) cause your coil firing to be weak on those two plugs, as you indicate.

Probably worth checking anyway!              

[Laminar]

Very interesting. I'll take a look at this this weekend!

[Alan F.]

Just to clarify - you're suggesting that for each coil, I measure resistance between the terminals where the colored and black/white wires connect, and then measure resistance between where each of the two plug wires connect?

Sorry man haven't been back to your thread in a bit. Here's some reading to clerify measuring coil windings... http://forums.sohc4.net/index.php?topic=59101.0
Best of luck.
-Alan

[Laminar]

Going nuts with the multimeter.

Coil/Primary/Secondary

2-3/2.8ohm/14,250ohm

1-4/2.8ohm/13,850ohm

The Clymer's manual says specs for the coils are 2.8ohm primary and 13,600-15,500ohm secondary. I saw roughly 5,000ohm in each of the spark plug wires, which is right on.

I pulled each spark plug out and confirmed that it was either wet or damp after cranking, and that if laid against the engine it produced a spark when cranking.

I have compression.

I have spark.

I have fuel.

I have air.

That's all it takes, right? So WTF am I missing?

[w1sa]

Are the sparks bluish or yellowish?

Also, a faulty spark unit/connection may affect ign. timing. Is it possible for you to check the timing with a strobe while cranking?

[Laminar]

Are the sparks bluish or yellowish?

Also, a faulty spark unit/connection may affect ign. timing. Is it possible for you to check the timing with a strobe while cranking?

I tried a replacement CDI box but that didn't change anything.

Sparks are blue/purple.

BUT I'M SO CLOSE. IT FIRED.

It managed to sustain itself once I let go of the starter button, but only for a few seconds. Doing that a bunch of times let me on to the fact that exhaust pipes 1 and 4 were too hot to touch and 2 and 3 were ice cold.

I put the multimeter to the 2/3 coil and it tested identical to the 1/4 coil, so I know the coil is good. The wires also tested good. I tested each spark plug in its respective wire and saw a spark.

The battery's charging now (5 minutes of cranking will wear it down pretty quickly) but once it's up again I'm going to swap the coils to see if 2 and 3 suddenly start firing.

It wants to run but it won't keep going on only two cylinders like that. I'm considering a new set of spark plugs as that'd be a cheap and easy fix that would help anyway.

[750goes]

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

[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....