Bike running poorly - suspect vacuum diaphragm may be leaking

BurnsBikes

BurnsBikes

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Joined
Mar 4, 2026
Total Posts
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Location
Florence, AL, USA
Hi,

I had the 1978 CB400A for my customer running nicely, then after just a few miles he reported that it had no power. Checked into it and the right cylinder wasn't firing. I swapped the plugs and the symptoms stayed with the right side, so replaced the coil with a new one. This helped some, but the bike still mostly runs only on the left cylinder. In neutral, the bike revs out just fine but is sluggish to get up to what I'd guess is around 4k RPM (no tach so going by ear). Under load though, it has barely enough power to get out of it's own way. Once it hits about 30-35mph (and you're holding the throttle wide open) it will buck a couple times then take off like a bat out of hell and will run great until you let off the throttle.

Compression is decent on both cylinders (160 left, 155 right). Both carbs have been gone through with new OEM brass. I didn't replace the slide diaphragms though as I just couldn't find replacements for them.

In the opinion of the experts does this sound like a wonky slide diaphragm to you guys or am I barking up the wrong tree?
 
I am not a qualified 400A person, but is there a chance that there is a problem with the float level or float needle/seat on that right side?

If the diaphragm were faulty, I would have a hard time understanding how the bike could run well at WOT.
 
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It's definitely getting plenty of fuel, the spark plug is wet when I pull it and I can smell gas coming out of the exhaust, so I don't /think/ it's a fueling problem. My guess with the diaphragm was that once the RPMs are up it's generating enough vacuum to overcome the small leak and pull the slide up.
 
I was thinking too much fuel. That would jive with the bike running okay at WOT where it might be able to swallow a lot of fuel. Have you tried running it for a short time with the petcock closed?

I know you haven't sourced a new diaphragm, but have you inspected the ones in use?
 
Hmm... I hadn't thought about it being over-fueled. Would be weird but I guess anything's possible. I haven't tried running it with the petcock in the off position. I'll try that in the morning and see what happens.

I did inspect them, the slides moved relatively normally (maybe a hair faster on the return than I'm used to, but I'm used to the Keihin Harley carb, not sure how similar these are) and the rubber /looked/ to be in decent shape. It wasn't dry or cracked. I know on my personal Yamaha XS1100 I had some weirdness with the carbs until I found a guy that made new production diaphragms and replaced them, so that's where most of my suspicion has come from.
 
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