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CB750 F1 - possible coil problem

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by nich_s, Apr 30, 2007.

  1. hi there, any assistance with the following problem would be great.

    Bike - 1976 CB750 F1



    Problem - Started her up yesterday and she seem to only be running on one piston. I diagnosed this by firstly the sound...pretty lumpy sort of note, secondly i felt the exhaust headers, only one was hot, i pulled the lead out of that one and she stopped.

    The bike is setup with electronic ignition which is reasonably new, the carbs are in good working order so there should be no fuel issue - should i now assume that maybe i have a problem with my coils and if so, what is the best way to diagnose a faulty coil?
     
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  2. i've had a quick scout about re: coils.

    It appears that DYNA produce replacement coils for the CB750's amongst many other bikes.

    Although more expensive can anyone recommend these DYNA coils...are they worth the extra cash?
     
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  3. It would be best to be certain that it is indeed the coil at fault before racing off and buying some others.

    You'll need to measure the coil resistance, and also need the original manufacturer's tolerances. It might be easier to take it to a sparky if unsure.

    Regarding Dyna, I'd be more than happy with any coils as long as they're reliable and provide a spark, dunno if they're worth the extra.
     
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  4. Highly likely NOT coils. If I remember correctly (runs hand through greying hair) those new fangled Honda thingies run 2 coils ONLY, one of 'em goes to 1 and 3 the other to 2 and 4. Two sets of points (one for each coil). Now the new electronic ignition would replace the points only I expect so check fuel supply and plugs themselves before going for coils.
     
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  5. +1

    Also unless you're kitting high-performance coils up against a worked engine, you may as well stick with standard ones. Are the Dyna ones just 'normal' replacement ones? Sounds like they are.
     
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  6. thanks very much for the input guys.

    I checked the spark plug leads as i was turning over last night and they appeared to be producing spark...i couldn't actually see spark but could hear the sound of a spark arc. ..is this to be expected or should i SEE SPARk within the lead plug end.

    Another observation was that my one of my coils (after trying to start her) was hot compared to the other which was room temp. I should also mention that the coils are different, one looks original (or original replacement) the other looks like after market bolt on replacement (exactly like the $99.95 replacement coils available on z-power). its the after market one that is running very hot.

    i did also read somewhere that the DYNA ignition i have installed prefer coils with primary resistance of approx 3ohms. the old contact points liked coils with 4-5ohms. can issues occur with my combination..new ignition, old coils?

    but in saying that, this bike has been started in my garage once a week and run for approx 15min for the past year or so now with this setup..until two nights ago.

    ok. so assuming i have adequate spark, i must now look at fuel. what is the best way to tackle this. FYI, i am running the stock KeiHin carbs and have an inline fuel filter installed.

    thanks again in advance
     
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  7. Dyna ignitions are good. A few things to check. First off, it won't be running on one cylinder, as said above, these older inline fours (and my GTR!) use a "wasted spark" setup, where teh coil fires two plugs every revolution. It doesn't matter about teh second plug firing, because it happens at TDC on exhaust stroke.
    Now, back to teh Dyna, they do like teh coils you mentioned. I had Dyna coils on my Z along with a Dyna ignition, and it was fantastic. IF you have a coil problem, I'd recommend spending up for these coils.
    Teh Dyna system uses two pickups (if it is like the Z's system, I am assuming it is as it is a Japanese inline four of the same era) that are 180 degrees apart and have two magnets that move past them using Hall effect to trigger teh control box to spark a coil. I would check that both your pickups have solid wiring to them with a multimeter, that the air gap between pickups and reluctor (thing that goes past pickups) is correct.
    The next thing would be the coils themselves,check them properly by holding a spark plug against the head with insulated pliers and watching for a strong blue spark. Anything less than a strong blue spark is unacceptable in a Dyna system.
    Also, the Dyna system likes a solid 12v, if your battery is a little low, and you crank teh engine over with starter, you may not get a full 12v.
    Another issue is that the Dyna system MUST HAVE resistor plugs and leads. It will freak out if subject to EMF from non resistor equipment.

    Regards, Andrew.
     
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  8. thanks andrew. your feedback was excellent, cheers.

    I think i shall save my pennies and purchase some Dyna Coils. It makes sense to me Dyna ignition talking to dyna coils.

    I think i had a breakthough last night though. I have a second donor bike, cb750 (F2 version- in pieces). i borrowed the coils off it and installed. I check resistance using multimeter and they both gave me 4ohms which was expected.

    I then tested the spark in the leads - all good. i then placed the lead on the removed spark plugs (because i had the plugs out for inspection) and turned her over to see plugs were functioning - to my horror...NO SPARK on plug one. i tested remaining three to have pretty much the same result.

    I then removed the D8EA spark plug from my trusty little cb100 (both use same plugs) and tested the same way. exciting result..Heaps of blue spark.

    So, this morning i purchased myself four new spark plugs and i shall test tonight after work. fingers crossed she will kick over.

    i still find it odd that she has been turning over and idling nicely each week for ages now and all of a sudden the four plugs are dead.
     
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  9. Sounds very similar to my CB750 K7.problems or where it started.The Honda shop replaced my coils.Went fine for a 1,000 kls then started to fade on power.They then replaced my alternater with a second hand one.This does not seem to work all the best as it just runs the battery flat.I;m now on to my 3rd battery.The Honda shop has finally admitted they do not know what the problem is. I think the problem is in the rectifer.Check to see that your battery is fully charged.
     
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  11. its terrible when the mechanics throw their arms up in an unknowing fashion.

    I have actually got two batteries that i have been swapping whilst working on the bike. Constant attempts at turning over etc eventually runs them flat. so as one charges on the battery charger i use the other. so i should have a good 12v working for me.

    thanks for your input greg and all the best with ya K7, i hope you have her running 100% real soon.
     
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  13. Ducfreak, you were absolutely right, the spark plugs on upon their removal were black like you described. I had to work back late last night so no chance to install new plugs and test but its likely to sort my problem.

    i'm kinda kicking myself i didn't scrutinize the plugs earlier in the piece...but i guess the silver lining is that i now know a hell of a lot more about my motorcycles electrical system.

    As i am in the process of working on my bike does anyone have any suggestions on the way i should maintain my engine since it won't be taken for a run for at least the next 6 months.

    all the help has been excellent. thanks again.
     
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  14. new plugs installed. started first go. bloody incredible.

    all i can say is, if you think you got electrical issues...check and test ya spark plugs thoroughly early in the piece and don't go jumping to crazy theories before doing so - like i did.

    'sigh'...back to normal life.
     
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  15. Sorry, just jumping in late here, but you're very close.

    The ignition is paired 1-4 and 2-3. I think my hair may not be *quite* as grey as yours :wink:

    As to the original problem-I would have suggested to check the condition of your battery, 'coz these bikes really like the full 12 volts. Next step would have been to check your plugs :oops:

    Just for fun, you could swap them for a set of Iridiums. That'll get you going in the mornings.

    cheers
    ian
     
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  16. As a post script.
    Guess whose 29 year old Ducati 900 ran like a 450 last Wednesday :oops: ?????
    2nd question. Guess what the cause was :oops: :oops: :oops: ????




    2 new plugs and starts and runs proper like :LOL: .

    Bloody unleaded makes plugs and pipes all black.
     
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  17. I had this problem with my CB750F once when I was redoing the loom. Had everything working perfect....except the starter button. Turned out I could bridge the master solinoid and the engine would kick, but the starter button had no life in it at all - turned out to be the "Silicon Rectifier" that was rooted. So I nicked the one off my parts bike and BAM! it worked perfect then.

    Also man check your coils for cracks. you may not be able to spot the cracks, but when it rains, you WILL notice how shit the performance gets.
     
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  18. I had this problem with my CB750F once when I was redoing the loom. Had everything working perfect....except the starter button. Turned out I could bridge the master solinoid and the engine would kick, but the starter button had no life in it at all - turned out to be the "Silicon Rectifier" that was rooted. So I nicked the one off my parts bike and BAM! it worked perfect then.

    Also man check your coils for cracks. you may not be able to spot the cracks, but when it rains, you WILL notice how shit the performance gets.

    I always use Premium Unleaded in my ride. 91RON fuel is crap. Nothing below 98 for me.
     
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