CB400T has fuel, spark, and compression. Why no boom?

Joshwthomas2002

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Aug 24, 2024
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Hello all, I own a 1979 CB400T. The bike has been nothing but trouble. When I got the bike, it had a no spark condition. I troubleshot as required and determined the problem to be the stator. I swapped in a stator from a known to be running engine and the bike ran after the swap about 5 or 10 times. Shortly after the swap, it stopped running completely. Will not even sputter on start up. Absolutely zero combustion is happening. I’ve done a top end rebuild, replaced the coil, plugs, leads, cdi. The bike has great compression (180 on both cylinders), produces a bright blue spark when turned over, and is most definitely pulling fuel through the carburetors. So I’m very inclined to believe it is an ignition timing issue. The bike won’t even spit or sputter on straight ether. I’ve performed troubleshooting of the stator IAW posts on several forums, as well as the CB400T shop manual. I received good results on all readings expect for the low speed source winding (green wire to white wire). Spec is 400-500 ohms, my reading was 310 ohms. Does this indicate a failed stator? Is the low speed source winding a power generation circuit or a trigger/signal circuit? Is it possible for a bad stator to still produce a spark? Do we think I might have a bad CDI? Can spark be produced with a bad CDI? Possible that I’m missing something? If she’s sparking, why no boom? Any input is appreciated. Thx gents, GOBBLESS!
 
Is it possible the idle circuit in your carbs is plugged?

Will the bike start with starter fluid?

Are you using choke to try and start it?
 
MrPepper said:
Is it possible the idle circuit in your carbs is plugged?

Will the bike start with starter fluid?

Are you using choke to try and start it?
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No sir I don’t think so, I can see atomized fuel coming out of the exhaust valves when I crank it. I have tried to run it on starter fluid with and without the carbs installed. I do choke it when I try to start. Highly confident that it’s an ignition problem.
 
Have you tried starting fluid?

Can be ignition. Double check the primary. If you put an aftermarket coil on there chances are it's greater than 1.0 ohm which is the limit. It may spark but I've seen these coils at 5.0 ohms for aftermarket and they just won't start.

Only way to diagnose the stator properly is with a peak voltage adapter while cranking. Resistance tests will only tell you if it's obviously open or obviously shorted on a winding. The PVA tool is about $30 and you set your multimeter for AC Volts.
 
Maraakate said:
Have you tried starting fluid?

Can be ignition. Double check the primary. If you put an aftermarket coil on there chances are it's greater than 1.0 ohm which is the limit. It may spark but I've seen these coils at 5.0 ohms for aftermarket and they just won't start.

Only way to diagnose the stator properly is with a peak voltage adapter while cranking. Resistance tests will only tell you if it's obviously open or obviously shorted on a winding. The PVA tool is about $30 and you set your multimeter for AC Volts.
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Understood. I’ll look into that and report back. Thanks!
 
I just ran the coil test and found the primary to have a reading of 4 ohms and the secondary to be 23,700 ohms. Both wayyy out of spec. Will be installing new coil shortly. Can a faulty coil cause the bike to not run even though it still produces a strong spark?
 
Last edited:
Joshwthomas2002 said:
I just ran the coil test and found the primary to have a reading of 4 ohms and the secondary to be 23,700 ohms. Both wayyy out of spec. Will be installing new coil shortly. Can a faulty coil cause the bike to not run even though it still produces a strong spark?
Click to expand...
Yes
 
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