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This is a discussion on spark within the Technical forums, part of the Caferacer.net Forums category; I'm trying to get a 71 cb350 to run and I'm having trouble getting it to fire on the left side. The plug sparks when ...

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  1. #1
    Junior Member deanbratcher's Avatar
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    spark

    I'm trying to get a 71 cb350 to run and I'm having trouble getting it to fire on the left side. The plug sparks when out of the engine, and a timing light shows that it's sparking when its running, but the exhaust is cold and when I pull the plug, it's still looks new. There is definitely gas getting to the cylinders. It's got a new points plate and points, and the carbs have been cleaned. I'm sure the carbs need adjusting, but I can't get it to run well enough to so. How much spark is enough? Could it be just that the spark is too weak?

    Dean

  2. #2
    Senior Member spacmanspif300's Avatar
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    quote:Originally posted by deanbratcher

    Could it be just that the spark is too weak?
    dean,

    1. it could be. you COULD know for sure if you swapped the plug and wire from the other side (the one that fires) over and changed nothing else.

    2. or it could be carb related. does it smell wet and gassy in there after giving it a gas and attempting to start? if not, you're probably not getting gas into cylinder, which tells of carburetor issues.

    3. it could also be that you have no compression in the cylinder. you could know for sure by doing a compression test with a compression tester.

    4. if compression is subpar, it could be valve/cam problem or scored jug/rings. but worry about that after checking the first three off the list.

    why in this order, you ask? this is in order of free-->costing money, and easy--->slightly less easy.

  3. #3
    Junior Member deanbratcher's Avatar
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    It is definitely gassy. The wires are attached to the coil so if I swapped it over it would be firing on the intake stroke, right? I'm gonna do compression check tonite. It ran well enough to ride before I tore it down to clean it up. It started right up but ran like crap when put it back together.

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  5. #4
    Senior Member spacmanspif300's Avatar
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    quote:Originally posted by deanbratcher

    It is definitely gassy. The wires are attached to the coil so if I swapped it over it would be firing on the intake stroke, right?
    I don't know. I guess you're saying the 350 coil is a 180* fire pattern, asynchrous? If your manual says so, or if one of the many 350 guys chimes in, then yes. I don't know.

    Maybe you just flooded it. Tried letting it air out? What about a new, properly gapped plug? They're cheaper than a compression tester, but then a compression tester is worth having, anyway.

    Not trying to be insulting, just reminding about the easy stuff.

  6. #5
    Senior Member Judeyramone's Avatar
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    Major symptom of bad battery on cb360 is left cyl stops firing. Don't know how similar these two bikes are in this regard, and you didn't say anything about the condition of your battery. [u]New</u> battery (not the old one with a charge) has solved all the troubles on my 360 before.
    Many a man who falls in love with a dimple makes the mistake of marrying the whole girl. - Evan Esar

  7. #6
    Senior Member bmartin's Avatar
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    I think what above meant by switching is to take the spark plugs from on side to the other and see if the right side goes cold. That would indicate you have a bad spark plug. But Judeyramone may have your solution with a new battery. I personally have not heard of this before but Hey - anything is possible.

    If switching the plugs and a new battery don't work - then you need to start looking at your coils or timing. If the cylinder is getting gas and fire - it should run...
    Bob - Palmyra NY
    2 - 69 CB750, 1 Turbo
    1 - 71 CB750

  8. #7
    Senior Member HackAsaw's Avatar
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    If your copper core plug has been fouled

    Screw in new ones. Cross over to iridium core plugs.

    Todays pump gas has a nasty habit of fouling a plug in a manner so that even bead blasting them will not bring them back to life.

    There have been many various factory tech bulletins on this subkect and iridium plugs are indeed oem standard on many motorcycles.

    This may not address why it fouled, which could be carb or electrical.

    How much spark you ask?

    Perhaps I can share with you how to help yourself answer that question.

    Benchmark your ignition systems output by seeing how large a gap, in open air, your good firing side will jump.

    Test the other side the same way. If it won't do as well, you have a problem in the primary or secondary ignition.

    Come back with results and we'll go further if need be.

    Caveat. ... when testing max gap....

    Once you find the gap it will not jump

    do not repeatedly attempt to make it do so as this non firing secondary condition can result in internal firing inside of the coil and kill it

    This is why often a burnt and wide gapped plug, open resistor plug wire end or open plug wire will also require a coil to complete the repair

    the hurt coil may still fire, but, it won't have all its spunk

    Almost carte blanche.... I replace resistor plug wire ends with non-resistor

    Resistor plug wires with solid copper core

    and iridium plugs are installed

    period
    Accidental inventor of the worlds fastest wearing rocker arms and other edible engine parts! Try my yummy blue cheese camshafts!

  9. #8
    Junior Member deanbratcher's Avatar
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    Thanks for the info. I'm in Nashville and haven't had time to do any thing on the bike do to a little bit of bad weather. I'll get back on it this weekend and hopefully will get it figured out.

    Thanks
    Dean

  10. #9
    Junior Member deanbratcher's Avatar
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    I messed with the bike all afternoon yesterday, and now cannot even get it to start. I re-cleaned the carbs and rechecked the ignition timing and it still will not start. It reminds me of when I used to mess with old cars and would put the distributor in 180 deg. off. Plenty of gas and spark, but no start. It seems like cam timing, so I removed the advance to see if the crank and cam were lined up, and on LT the pin on the cam is just a hair off from straight up. I assume this is from the timing chain stretching some. It just doesn't seem like it's off enough not to at least start and run badly. That's all I'm looking for right now. Just start and run like shit.

    Dean

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