CB750C - #1 and #4 cylinders fire intermittently

[DoubleMal]

Greetings all,

this is a topic that I've seen discussed before but was hoping to get some input from the boards (problem's driving me crazy).  Here goes (with apologies for the length):

Bike: 1982 CB750C (DOHC)
Problem: Cylinders #1 and 4 fire intermittently at idle or under load.  Bike was working previously although I was in the process of dialling in my carb mixtures (ie. riding and checking how the plugs were burning.  Had put maybe 200 miles on the bike since the carb rebuild).  Initially thought the problem was specific to cylinder #1 (and consequently blamed my carb-cleaning) but subsequently realise that the issue is affecting both cylinders #1 and #4.

Observations:
- If I put a regular timing-light on #1 or #4 I can visually see that there are missed sparks (the strobe is not steady; there are variable gaps where there is a missed pulse.  The amount of time it fires/doesn't-fire doesn't appear to be a consistent cycle);
- If I ride the bike it pops and farts consistently and performs poorly;
- Last time I was working on the bike was after dark and I note that when the bike misfires (both audibly and when the timing-light stops flashing) I can see both turn-signals flash faintly in tandem (wouldn't have noticed this if it wasn't dark out).

Things done/replaced so far (to no avail):
- Check compression (all four cylinders are ~130 lbs);
- Install new coils;
- Install new plugs;
- Install new leads;
- Install two different used CDIs in turn for the #1/#4 unit (I'm swapping the CDI nearest the front of the bike) and tried swapping with functioning #2/#3 unit;
- Validate that I've got +12 volts at the blue/yellow connector of either coil;
- Set air-gaps on pulse generator per Honda manual;
- Perform Honda manual tests of the pulse generator (ie. screwdriver to pulser/core with plug out and grounded). Both pulsers appear to fire properly;
- Perform Honda manual tests of the CDIs (voltage of ~12 volts at the blue/yellow dropping to ~2 volts when pulse connection grounded while ignition is on);
- Ignition switch is relatively new (has maybe 100 Kms on it);
- Kill switch is relatively new (was updated at same time as ignition switch);
- Cleaned the master ground at the coil bracket and cleaned the coil brackets;
- Cleaned the connections to the CDIs, Pulse generator and coil.

Questions:
1) can the pulse generators pass the test listed in the shop manual (530 ohms +/- 15 ohms) and still be faulty/intermittant (I've got another used unit en route from eBay in case);
2) can a regular automotive timing light be used on these bikes and is the visible lack of consistent strobe pretty much a reliable indication that spark is intermittent?  Note that I've not been able to check the timing itself (beyond a static check) as the strobe cuts out so often that it's hard to read the setting;
3) does the aluminum case of the CDI units need to be grounded to the chassis?  I painted the battery box while doing the carbs and was wondering if I've caused a problem by not having the screw mounts clear of paint (would have thought that I would have the problem on all cylinders in this case);
4) the black/white cable to the coil is attached to the negative on each coil (blue and yellow wires on the positive of the coil).  Is this correct (PO was a real monkey so I consider everything questionable)?

Thanks in advance...

[Rodgil]

Hi DoubleMal, My 1982 was hard to start, and ran on two cylinders until it warmed up, then ran normally. Everything tested ok until the pulse generator failed completely on two cylinders, then it tested open circuit. A new pulse generator fixed it. Re your other questions, any timing light will do. There is not a motorcycle-only timing light. Not sure, but grounding the CDI units can't hurt. The coil b/w wire goes to negative on the coil, but I have also had it the other way around, and it made no difference.

[TOOLS1]

It sounds like a ground problem, or short to the turn signal wire.
TOOLS

[TOOLS1]

The black/white wire goes to the (+) side of the coil.
TOOLS

[DoubleMal]

Thanks for the info Rodgil!  I was pretty sure about the timing light being universal but would rather ask than make further assumptions built on faulty intel.  I guess I'll just sit and drum my fingers until my pulse generator turns up (unfortunately a used unit; hopefully I'll get lucky and this will magically fix the problem....

[DoubleMal]

Thanks for the information Tools!  Your advise is appreciated - I'll swap the polarity of the coil connections and see if it makes a difference and will start hunting down potential ground issues...

[DoubleMal]

So, after obtaining a new pulse generator the problem persisted <sigh>.  At this point, amidst the despair, I thought I'd better re-evaluate just *why* I thought that the problem was *definitely* electrical; primarily because the ignition visibly "misses" badly when watching the strobe coming from the timing-light (it's a good light but I've had it for a looong time).  I put the timing light on my car (which isn't having any issues) and the same "missing" was visible!  @#$$%!@#$%@.  With this new insight I pulled the carbs out (again) and fussed with the slow-idle circuit some more.  Re-assembled and everything works great now.

One item that all my review of the electrics *did* uncover was the polarity of the leads going to the coils was backwards.  My CB750c has been very hard to start from the day I bought it despite various attempts to improve things (Iridium plugs, new coils, etc., etc.).  Tools responded earlier in this thread that the White/Black lead should go to the +ve terminal on the coils; others commented that the polarity didn't matter (which seems to be an opinion I've found all over the web for all sorts of bikes).  I connected the coils per Tools' comment (thanks Tools!) and it made an *immediate* and very noticeable difference to how well the bike starts.  I'm starting to think that if I'd swapped the coil leads on the day I bought the bike from PO that the starting issues would have been far less notable.  For those of you who have a CB750 that doesn't start well do me a favour: check the polarity of the leads to your coils.  If you've got the leads backwards try changing the polarity and see if it makes a difference.  Given that there seem to be a lot of folks who maintain that the polarity "doesn't matter" plus given that the Honda wiring diagram doesn't indicate which lead goes to which polarity on the coil I wonder if there are a whole whack of bikes which the PO-army have mis-assembled.

[LukeM]

One more reason why TOOLS is the man. I'm glad he's here.

Luke M

[cblover]

Hows the carbs? Have you cleaned  and syncro it properly? It could also be a electrical issue.

[DoubleMal]

Hi cblover...  The problem has been solved; it was indeed the carbs when all was said and done.  The point was to mention that while investigating the problem (which wasn't electrical at all, in the end) I found that the coil polarity made a big difference to how my bike started...

[efra]

hello its seems that i have tehe same issue in my bike but with the number 2 cylinder but just when its warming up once the bike engine its warm up the problem disapear What can i check? (Please help im new in the mechanics motorcycle world)