Interesting results 72 CB350
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Thread: Interesting results 72 CB350

  1. #1
    Member Alewiska's Avatar
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    Interesting results 72 CB350

    So a little back story, went to fire up the bike today, hooked up the fuel tank, didn’t realize the drain plug on the right side was open until it was a little late. Points are on, power is checking out, have spark, good compression but no ignition. Could that have just been the plugs getting wet by chance? Another question, I tried to hit the starter button just because I haven’t yet, the entire bike shorted out. I checked everything and had good continuity and no problems except, the entire main ground wire is now dead. I’m going up to a 6ga from an 8ga but I’m just trying to see if anyone can help with these two issues. I chose electrical discussion because I think it’s the spark plugs and the smaller gauge wire.

  2. #2
    Sensei 66Sprint's Avatar
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    Probably NOT the wire....8 ga. can easily handle 200 amps @ 12v in the short loop from battery to starter motor and back......
    My guess would be a "dead short" in the starter motor itself....
    Usually caused by using only one wrench to tighten and loosen the positive terminal connection nut instead of holding the lower jamb nut with one wrench and turning only the top one with another......

    For the engine to run you need:
    An appropriate air/fuel mixture to compress...
    Compression (preferably about 145+ PSI on a 350)....
    Spark at the correct time to ignite the compressed mixture.....

    Can't even guess how many time I've found the sparks to occur exactly at the index alignments, BUT on the exhaust instead of compression strokes....
    (360 crank or 180 cam degrees out)....
    Intake valve closure is the positive sign you are beginning the compression stroke for that cylinder........
    Last edited by 66Sprint; 05-26-2019 at 11:37 PM.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  3. #3
    Member Alewiska's Avatar
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    I think you’re spot on with the dead short. I’ll look into that more.


    As for the timing, I made sure to set the points on the compression stroke. My method to test it is to put my finger over the spark plug hole I’m working on and get the suction noise.

    Quote Originally Posted by 66Sprint View Post
    Probably NOT the wire....8 ga. can easily handle 200 amps @ 12v in the short loop from battery to starter motor and back......
    My guess would be a "dead short" in the starter motor itself....
    Usually caused by using only one wrench to tighten and loosen the positive terminal connection nut instead of holding the lower jamb nut with one wrench and turning only the top one with another......

    For the engine to run you need:
    An appropriate air/fuel mixture to compress...
    Compression (preferably about 145+ PSI on a 350)....
    Spark at the correct time to ignite the compressed mixture.....

    Can't even guess how many time I've found the sparks to occur exactly at the index alignments, BUT on the exhaust instead of compression strokes....
    (360 crank or 180 cam degrees out)....
    Intake valve closure is the positive sign you are beginning the compression stroke for that cylinder........

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  5. #4
    Sensei 66Sprint's Avatar
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    I've also seen the coils mounted facing the wrong way, or "swapped" on the bracket and even though the points wire colors and connections were correct, the actual high-tension wires ended up on the "wrong-side" plugs... (Again 180 cam degrees out)
    chaindrivecharlie likes this.
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  6. #5
    Member Alewiska's Avatar
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    I hadn’t switched anything around on the coils from when it last ran. I ran a new ground today, fixed that issue. Changed out the plugs in case they were fouled. Drained the carbs and I’ve been kicking for 20 minutes with no luck. I even (probably ill advised) sprayed a little starter fluid in one of the filters. It’s still not firing. I am noticing a small amount of fuel back spray into the filter. Battery is at 13v. I’m a little lost right now.


    Quote Originally Posted by 66Sprint View Post
    I've also seen the coils mounted facing the wrong way, or "swapped" on the bracket and even though the points wire colors and connections were correct, the actual high-tension wires ended up on the "wrong-side" plugs... (Again 180 cam degrees out)

  7. #6
    Senior Member doode's Avatar
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    Can I get a timeline recap from when it last ran?
    What work was done, what was touched?
    Did you take the carbs apart? Etc?

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    Sensei 66Sprint's Avatar
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    Kill switch allowing power to coil Black/white stripe wires?
    Engine turned CCW when setting points?
    Breaker cam/advance unit rotating? (wouldn't be the first time the key pin fell out or wasn't in slot)

    At this point, it's going to be something dumb we are overlooking......
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  9. #8
    Member Alewiska's Avatar
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    Went down to the head gasket to replace. Had the cam and lifters resurfaced. Replaced all gaskets. Cam was originally off by two teeth, yet still able to run. Carbs were rebuilt and cleaned out. (As per our last thread, my slide tube on one carb is 180° off and I will be fixing that this weekend).

    Quote Originally Posted by doode View Post
    Can I get a timeline recap from when it last ran?
    What work was done, what was touched?
    Did you take the carbs apart? Etc?

  10. #9
    Member Alewiska's Avatar
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    I did rotate CCW while setting points. I’ll have to double check the coils getting power. Both are registering at around 4 and change on resistance. I’ll also have to check the advance unit spinning this weekend. I sent you a message, I may need to give you a call if possible this weekend when I’m in front of the bike. If you’re available and don’t mind talking a rookie through it!

    Quote Originally Posted by 66Sprint View Post
    Kill switch allowing power to coil Black/white stripe wires?
    Engine turned CCW when setting points?
    Breaker cam/advance unit rotating? (wouldn't be the first time the key pin fell out or wasn't in slot)

    At this point, it's going to be something dumb we are overlooking......

  11. #10
    Senior Member doode's Avatar
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    Oh, I didn't realize it hasn't run since the rebuild.

    Did the spark advance mech come apart at all?
    When you set points to open on compression stroke, how did you know it's the right stroke?
    You set the points to open on F and LF and not T/LT, correct?
    You set the valve on T and LT, yes?
    Follow the wires from the points to the coils. What color wire goes to which coil? Post a photo of the points so e can see the wires connected.
    Somewhat separate, but remove the left plug and rotate to LT. Can you see the piston? If not, rotate 360. Can you see the piston? Make sure LT is the top of travel for the piston, and tht it isn't off.
    Confirm both sides spark.

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