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  1. #1

    Default Ugh. My new Shovelhead not firing on rear cylinder?

    I just picked this 79 Shovelhead up 2 days ago. Owner said carb needed to be cleaned (mikuni). Got it home and started it with starter fluid (bad mistake). The next day I actually cleaned the carbs and put fresh gas in it. I rode it for about 30 mins straight with no issues. Parked it for the night. I tried to ride it to work and got half way there (15 mins) and it started to misfire. Sounded like it was bogging down, so me, thinking it was the carb and tank and hoping it wasn’t missing, took it home. I cleaned the carb, tank and got new plugs. Started it up, and it sounded the same. I felt the exhausts and the exhaust to the front cylinder was hot, the rear was not.

    This bike Is not stock
    93 ci
    Big bore jugs
    SS piston rings
    Duel plug SS heads
    Single fire ignition
    Andrews cam
    Solid lifters
    Roller rockers


    Things I’ve done so far
    Cleaned carb and tank
    Put known working front plug wires on rear and rear plug wires on front. Same deal. Front cylinder exhaust hot, rear is not.

    Any suggestions?

    Edit: Forgot to mention that as it was bogging on the way to work, it was intermittently run fine again for at most 2 seconds. It this a few times before never running on both cylinders since then. The carb also pops a bit as well.
    Last edited by Superman27e; 04-03-2020 at 5:38 PM.

  2. #2
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    Sounds electrical. is it points or electronic ignition ? Kickstarter still on it. Just asking before giving you some general troubleshooting .
    Last edited by 47str8leg; 04-03-2020 at 6:05 PM.

  3. #3

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    Your symptoms are of an ignition module going bad.
    But, one thought: when replacing the plugs, are you replacing all four?

    Jim

  4. #4

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    Electronic ignition and yes, Kickstarter still on it.

  5. #5

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    Quote Originally Posted by JBinNC View Post
    Your symptoms are of an ignition module going bad.
    But, one thought: when replacing the plugs, are you replacing all four?

    Jim
    Yep. I replaced all four.

  6. #6

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    I just found out that one of the coils has no fire on both coil wires, but the other coil has fire on both coil wires.

  7. #7

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    Good first step would be a compression test (throttle fully open).

    Jim

  8. #8

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    Quote Originally Posted by Superman27e View Post
    I just found out that one of the coils has no fire on both coil wires, but the other coil has fire on both coil wires.
    Does each coil serve one cylinder, or is it set up as a wasted spark system with wires from each coil going to each cylinder?

    Jim

  9. #9

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    Quote Originally Posted by JBinNC View Post
    Does each coil serve one cylinder, or is it set up as a wasted spark system with wires from each coil going to each cylinder?

    Jim
    They both share a cylinder. Which makes me think it would still run on both cylinders, but maybe I’m wrong?

  10. #10

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    Quote Originally Posted by Superman27e View Post
    They both share a cylinder. Which makes me think it would still run on both cylinders, but maybe I’m wrong?
    If each coil has a wire to a front and a rear plug, it should run just fine on one coil. What brand and type of ignition is it, can you post some pics? Channeling Tattooo here.

    Jim

  11. #11

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    I’m not sure if the brand, but it’s a single fire ignition.
    I’ll have to get the pics tomorrow when I take the ignition cover off.

  12. #12
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    Click image for larger version. 

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    Not a dual plug guy but it doesn't sound right to me that each coil is serving both cylinders. Obviously it worked ( at least in the short term ) but when you bought it did he say how long the ignition system was on there ? If not , it could be fucked up from the get go and burned out a coil. Why only one cylinder would be affected on both coils I honestly don't know unless both coils are now burned out in the same fashion .If you can borrow or have another coil I would disconnect the dual coil setup temporarily and run it off one coil.

  13. #13

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    Using a wasted spark system for a dual plug bike is the most efficient way, but rarely done with the single fire systems that are popular today. There are reasons for this that involve a lot of physics. Suffice it to say that fuel bikes, where ignition is PARAMOUNT, used dual, waste spark systems for many years, and many still do.

    Jim

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    Quote Originally Posted by JBinNC View Post
    Using a wasted spark system for a dual plug bike is the most efficient way, but rarely done with the single fire systems that are popular today. There are reasons for this that involve a lot of physics. Suffice it to say that fuel bikes, where ignition is PARAMOUNT, used dual, waste spark systems for many years, and many still do.

    Jim
    I thought that's what I might here and it makes perfect sense in drag racing . To isolate a possible coil problem couldn't he just then hook the ignition wires up the way they are in the diagram and see if either cylinder is cutting out ? If it still is ( which I suspect) he can then reverse them and see if the opposite cylinder cuts out ?

    Edit . If one of the coils is not firing it tells me the problem is electrical . It might still be able to run on one but isn't because a wire might be getting shorted to the coils themselves or loose jumper between the two coils depending which coil got fed first.
    Being that Im a points guy I guess I'd check battery voltage for the fuck of it. Don't know how that effects an electronic system .
    Last edited by 47str8leg; 04-04-2020 at 7:45 AM.

  15. #15

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    Quote Originally Posted by 47str8leg View Post
    I thought that's what I might here and it makes perfect sense in drag racing . To isolate a possible coil problem couldn't he just then hook the ignition wires up the way they are in the diagram and see if either cylinder is cutting out ? If it still is ( which I suspect) he can then reverse them and see if the opposite cylinder cuts out ?

    Edit . If one of the coils is not firing it tells me the problem is electrical . It might still be able to run on one but isn't because a wire might be getting shorted to the coils themselves or loose jumper between the two coils depending which coil got fed first.
    Being that Im a points guy I guess I'd check battery voltage for the fuck of it. Don't know how that effects an electronic system .

    Hmm, I didn’t think about the battery. Could a weak battery cause a weak spark in only one coil?

    I talked to the PO last night and I believe he said he has had it on there for 5 years. He also said he still had the old points system along with a bunch of other stuff if I wanted to come pick them up. I forgot to get it when I purchased the bike a few days ago. Only problem is that it’s 2 hours away.
    Last edited by Superman27e; 04-04-2020 at 9:45 AM.

  16. #16
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    [QUOTE=Superman27e;842920]Hmm, I didn’t think about the battery. Could a weak battery cause a weak spark in only one coil?

    Don't know because I don't know electronic systems. The diagram shows one lead to each coil but , the way the plug leads are coming off like Jim said , it should still run off one coil at least on a dual point system .

    Just thinking off the top of my head , if the lead that would traditionally feed the coil for the rear set of of plugs is timed off the large lobe of the timer and the front cylinder is timed of the small lobe of the timer then if the rear lead fails the front coil and cylinder will still work on but the module won't fire that coil the second time around so your rear cylinder still won't fire because the front coil lead is not being energized by the module in an electronic system.
    Last edited by 47str8leg; 04-04-2020 at 11:27 AM.

  17. #17
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    Do you have any means of getting a few pictures of your setup?
    Taking of the cover and getting a picture of the ignition module. And then taking a couple pics of the coils, showing as much of the wiring for it could get things rolling here.

    I've ran Dyna S with dual coils, with great success. On that setup each coil serves it's respective cylinder. That way it would be easy to track it down to a faulty coil for instance.
    But since the guy and then you rode it the way it is setu up that seems to work fine. I guess it is a matter of tracing it back one step at the time. so if you can post those pictures, more guys could chip in.

  18. #18
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    Since the bike runs you can put a test light on the rear coil and see if it lights up. If it doesn't you have a bad module if it does , a bad coil.
    To double check you could swap the front and rear coil leads and see if it now only fires on the rear cylinder.

  19. #19
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    Ya could check the ohm values of the primary & secondary windings of the coils....
    Last edited by Dragstews; 04-04-2020 at 12:02 PM.

  20. #20

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    Here some some pictures


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    Attached Thumbnails Attached Thumbnails 639DA08D-67FB-41F5-BC75-E428926AC6D9.jpg  
    Last edited by Superman27e; 04-04-2020 at 12:17 PM.

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