The International CBX Owners Association

Following a nut and bolt resto of my Z, I am now conducting initial test rides.
All seems quite good with one exception:
Between 1.8K and 2.5K RPM I experience a snatch-like condition, as if ignition comes in and out in some of the cylinders. Misfire like, not sure how else to describe it.
The idle is perfect, and again, anything over 2.5K RPM is also perfect (really ).
Also, the wider the throttle opening, within the above range, the more intense the effect, always disappearing above 2.5K RPM.
What I know:
1. My carbs have just been rebuilt, courtesy of a lot of help from the Forum, including removal of the idle jets etc, etc, etc
2. The acceleration pumps works well on all 6 carbs
3. The bike works well otherwise, plenty of power, really nice over 2.5K
4. I have used the original ignition system, original coils, original plug caps but new HT cables and new iridium tipped plugs
What I DONT know:
1. Which is the point one of the 3 HT circuits is more likely to short?
2. Is the off idle transition under load a situation which would encourage a marginal coil / HT circuit to short or temporarily fail to deliver a spark?
Thanks in advance for any help!
Aris

To me, it sounds more like an electrical problem, but I have no idea where to look for a problem like that. I'm sure someone will know.

Try retarding the timing slightly and see what happens. Won't hurt a thing, and it might give us some more ideas.

I would look into the ignition advance unit not working properly.
The revs are usually higher for the advance symptoms to show up but it is another place to look. The advancer can get "tired".

Thanks everybody, seems there is a consensus towards an electrical problem rather than one related to carburation ...
Will certainly look into this tomorrow (late night over here now ) and report back.

We had a member recently with similar symptoms. Seems the float levels were set very low. This would starve the idle circuits, but not affect the main circuit as much (during normal riding). Anyway, the preliminary report is that raising the float levels to the proper height cured the 'misfire'. All indications were that it was electrical also, but exhaustive evaluation and replacement of all components did not clear it up. Raising the float levels did, or so it seems as of now. Perhaps the member will post a follow-up report on all he tried and how the float level worked.

Aris: Can you describe just how your float levels were set?

Dave

The floats were set as per Mike's booklet and numerous other publications, using the actual Honda service tool.
Frankly, I would be amazed if this were the problem. Of course, it would not be the first time .
But thanks for pointing this Dave. I will begin with the ignition circuits and if there is no luck there will look deeper into the floats possibility.

Got it!
This morning I removed seat and tank, started the bike with a test tank, and before anything else, I took an earthed cable, moving its other end around the 3 coils area.
And presto, the RHS coil (cylinders 2 & 5) had developed a pin hole through which I could get some really nice sparks to earth, killing at the same time cylinder no 5, and may be no 2 as well (not sure, never seen a cross section of a CBX coil before). Looking carefully, the pin hole had developed along a line which I thought was a scratch but obviously was more likely a crack of what must have been a 30+ year old coil ... The ailing coil was replaced with a spare NOS I had, put the bike together and dashed out for a test ride. The 'snatch' was gone, and my Z was as sweet as I thought a CBX should be (never owned one before, though).

But to be completely satisfied, I wanted to answer to myself WHY the 'snatch' was only manifested in the 1.8K to 2.5K RPM range as reported initially.
So before swapping the coils, I did a bit of revving (no engine load of course) with the earthed electrode about 0.8 cm away from the pin hole.
Amazingly, the coil was working perfectly on idle, as well as high revs, shorting only in the approximate RPM range reported . Now I know, and you know, this is far from a true simulation of a bike under load, but this is what I found. Only explanation I could think of is that under the specific circumstances the plugs are at their hardest to fire (why, too rich? too lean?), so the spark chooses the easiest way to ground, through the compromised coil pin hole. As a side, I dread the thought that this little bit of sparking was taking place below and close to the fuel tank

I absolutely look forward to a proper ride tomorrow Sunday, notwithstanding the weather

Thanks for ALL suggestions - this is a terrific Forum.

P.S. Can I suggest we move this thread to Electrical, it might be more useful to others there.

Bingo! It's great when you get to enjoy a Eureka moment with these cool bikes.

Aris:
Great find! I like your very methodical approach to diagnosis. I certainly wouldn't have thought to do what you did. This may indeed help others with similar maladies. Great description and write-up with pics!

Dave

Thanks for your kind words Dave.
However, I was very lucky as the pin hole was in the most convenient point imaginable to find (haven't the Japanese heard of Murphy's law).
I guess if this was happening somewhere BELOW a coil, it would be a different story.

On the same page, I wonder if I should change the remaining two coils with NOS.
30+ years of service and a hostile high voltage / high temperature environment must be tough for them.
Any thoughts? Any other compatible, modern coils using the same amplifiers?

Thanks, Aris

Aris:
Cracking of these coils is very common as they have aged. I really never had a problem with the service from a cracked one, that I knew of anyway, but have replaced those that were severely cracked, usually with newer, school bike coils. The Dyna or Accel aftermarket coils seem to work well and adapt OK, but the stock plug wires are a bit fiddly to use with them, if you want to keep the stock look. They do provide a much hotter spark than the stock ones do. I try to scrounge up as many stock coils as I can find that are good. Hope your ride goes well tomorrow and you get to really feel what a great motorcycle the CBX is.

Dave

daves79x wrote:We had a member recently with similar symptoms. Seems the float levels were set very low. This would starve the idle circuits, but not affect the main circuit as much (during normal riding). Anyway, the preliminary report is that raising the float levels to the proper height cured the 'misfire'. All indications were that it was electrical also, but exhaustive evaluation and replacement of all components did not clear it up. Raising the float levels did, or so it seems as of now. Perhaps the member will post a follow-up report on all he tried and how the float level worked.

Aris: Can you describe just how your float levels were set?

Dave

Dave, I'm getting close to solving my carb problem but not quite there just yet. I have started a write up with pictures that I will post once I am 100% sure I have fixed it.
This coil discovery by Aris is interesting and I will check mine the same way this week.
Ross

Ross do you need a solenoid? I have a nearly new one I'm fairly certain...

Let me know if you need it and I'll make you such a deal .

N.

NilsMenten wrote:Ross do you need a solenoid? I have a nearly new one I'm fairly certain...

Let me know if you need it and I'll make you such a deal .

N.

Thanks for the offer Nils but for reasons that escape me my solenoid has started working agian. See my "Starter won't turn" post. Ross