1975 cb500t intermittent ignition/firing on left cyl.
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1975 cb500t intermittent ignition/firing on left cyl.

This is a discussion on 1975 cb500t intermittent ignition/firing on left cyl. within the Technical forums, part of the Caferacer.net Forums category; I'm working on a cb500t and am having a few issues with the left cyl only firing when the throttle is applied, or the bike ...

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  1. #1
    Member photoputz83's Avatar
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    1975 cb500t intermittent ignition/firing on left cyl.

    I'm working on a cb500t and am having a few issues with the left cyl only firing when the throttle is applied, or the bike is above 2500 rpm. Picked the bike up with 12k on the clock. Was an okay runner but had an issue with firing on the left cyl. I picked it up pretty cheap so I figured I'd be fine spending a bit to get things like I want them. Swapped the carbs with mikuni vm 34s with 180 mains. The carbs were built specially for the Cb450t and 500t. That being said...I know the carbs aren't the issue. It has a new battery, new rectifier, and the harness is completely intact. At idle the engine not only sounds like it is only firing the right side, but it also vibrates more, and the left header is cold. At around quarter throttle and above the right starts to fire like it should. Should I suspect the coil, the plugs, or the points? Any ideas are helpful.

  2. #2
    Senior Member 8ball's Avatar
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    Have you checked valve clearance?
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    Member photoputz83's Avatar
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    Doing it now. Just picked up a set of feeler guages from my matco rep. I'll post what I find.

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    Senior Member hahnda's Avatar
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    I wouldn't rule out the carbs.

    If you pull the plugs and turn the engine over do you have spark on both sides?

    You may have to adjust the idle mixture screw. Just because they are set up for the 450/500 doesn't mean they are ready to run.
    Idle speed might need adjustment. Are the slides synched as in when you open the throttle are they opening at the same time or is one lagging behind the other?
    Last edited by hahnda; 08-22-2015 at 06:11 PM.
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    Member photoputz83's Avatar
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    The original carbs did the same thing. Went through an ultrasonic bath, cleaned all orifices, and synchronized.

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    Senior Member thechief86's Avatar
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    My 350 was doing the same thing, and it turned out to be a loose connection on the small wires going to the coils.
    Squeezed the connector with pliers and plugged it back up, and it runs like a top.
    It was intermittent because it was touching most of the time, but just barely.
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    Senior Member 44dwarf's Avatar
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    Often times the wire on the coil / spark plug cap will corrode and cause this. If they are NGK hard plastic type unscrew the cap cut the wire back 1/4 of an inch and screw the cap back on.making sure the point of the screw goes in to the wire.
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    Senior Member Diamondj's Avatar
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    Quote Originally Posted by 44dwarf View Post
    Often times the wire on the coil / spark plug cap will corrode and cause this. If they are NGK hard plastic type unscrew the cap cut the wire back 1/4 of an inch and screw the cap back on.making sure the point of the screw goes in to the wire.
    My CB500T would not run at all. Turned out the plug caps were no good under load. A pair of new NGK caps later and it was running great.

  10. #9
    Senior Member parkwood60's Avatar
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    Another issue with the 450/500T is that the points can not be timed individually, except by adjusting the point gap. Not sure how many other twins are like that. I would check the static timing on both sets of points (after of course making sure the plug on the cold side actually had any spark at all at idle). Then check the point gap on both sets. If I am remembering points correctly, a smaller gap will open later. I would adjust the gap on which ever was smaller (make it bigger) till the timing was synced, since the gap will get smaller with use, then time it by rotating the plate till it is at the F mark
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  11. #10
    Senior Member knappyfeet's Avatar
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    Quote Originally Posted by 44dwarf View Post
    Often times the wire on the coil / spark plug cap will corrode and cause this. If they are NGK hard plastic type unscrew the cap cut the wire back 1/4 of an inch and screw the cap back on.making sure the point of the screw goes in to the wire.
    44s correct. I would nearly guarantee this is the culprit. Every honda I've owned that was older had this issue at on point or another...........usually caps or wires but also like the Chief said.......connectors, coils, etc. Also just for good measure..........make sure those carbs are getting fuel. They may be being starved on the left.

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