Bike running poorly - suspect vacuum diaphragm may be leaking

BurnsBikes

BurnsBikes

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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.
 
LongDistanceRider said:
AH, there is no diaphragm on the VB carbs. So the question is? what carbs are you running?
Click to expand...
You're right, I have so many bikes I'm doing carb work on I forgot these don't have diaphragms - so there goes my theory unless there's somewhere else for vacuum leaks to happen.

Running the bike without the petcock on resulted in it running on 1 cylinder until it ran out of gas in the bowl.
 
This bike uses a single ignition coil with dual output and it's a wasted spark system. So, you should get spark at both plugs every time it fires. It's hard to imagine how a faulty coil could cause it to run on one cylinder unless there was an issue with the high tension cable, the spark plug cap, or the spark plug.

Have you verified spark on both sides since replacing the coil? Did you check the cap resistance to make sure it's not an open circuit?

If you've got good spark now, it comes down to fuel and compression.
 
Yep, it's sparking MUCH better now with the new coil. The old one had some cracks in the wire (if you ran your fingers along it while the bike was running it'd zap you). For giggles I swapped leads on the coil and the left cylinder fires just as well on both sides of the coil.

The right side has 155psi of compression, which I'd think should be fine. I guess I can pop the valve cover off and adjust the valves to get the number up some and see if that makes a difference.
 
Hmm. The jet on that side was an absolute BEAR to get out (I replaced it with a new one as it was too corroded to use). I'll blast some more carb cleaner through that circuit and see if that helps.
 
Yep, once you get "over the hump" it takes off like a bat out of hell and both cylinders are firing (confirmed with an infrared thermometer on the exhaust).
 
Sounds like the idle circuit is plugged somewhere. The brass ports under the piston are air bleed passages that sometimes are blocked so check those. Compare the side drillings of the old vs. new idle jet to verify sizing and placement, critical. Here's a picture of the idle circuit
Carbcut.jpg
 
Yep, confirmed it's the idle circuit. I pulled the vacuum sync screw out of the right side and hooked up a little squeeze bottle of gas to that port. Bottle feeding it a little bit smooths the idle out and makes it run on both cylinders.

Ugh, I guess I'm pulling these carbs again and giving that right hand side one a longer bath in the chem dip.
 
That's the guide I followed the first time, but I guess I didn't clean out that idle passage well enough. I got some guitar E string to stick up in there and hopefully knock and crud loose this time. May blast the air compressor through there as well.
 
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