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Discussion Starter · #1 ·
Looking for a little guidance here. I just finished rebuilding a 1978 CB400T and when I finally got it back together enough to kick it over, the right side cylinder does not appear to be firing. The header on that side does not get hot at idle. When I rev the engine, it backfires like crazy on that side (custom shorty muffler tips on stock headers)

I have rebuilt and replaced a ton of parts along the way. Summary of each system below:

Electrical

New coil, new plug wires, new plug caps, new NGK D8EA plugs. Tested the spark and confirmed it was working at a 20k volts and 30k volt gap size using a spark tester. I swapped the plug wires to opposite cylinders and the right side still would not fire. This leads me to believe it is a fuel issue.

Carburetors

I completely cleaned and rebuilt the original keihin carbs. Soda blasted, disassembled every part. Cleaned and replaced ever single gasket, jet, air diaphragm and o ring in that baby. Soldered cracked overflow tubes (although it still appears to still have an incredibly slow drip from this carb bowl). Fresh, ethanol free gas in the tank with a new inline fuel filter. Carbs were jetted for pod filters exactly the same on both sides. I pulled them after the cylinder wasn't firing to make sure I didn't miss anything and they are completely fine, no blockage. The only thing I didnt check was the air cut diaphragm on the right carb because I replaced it the first time around and didn't want to split the carbs again.

Air

I'm running pod filters. Before you scream about this being the reason, I'm running a 3" long aluminum pipe extension, wrapped in silicon tubing, that doesn't block the air inlets on the back of the carb and has a flow blocking restrictor plate that drastically improved throttle response. Seems to rev fine through the rpm range on the side thats firing.

Compression- 130psi on both sides


What else am I missing? Seems like fuel is the most likely suspect but I literally touched every piece of the carb twice now. Do I buy the mikuni kit I'm eyeing up and hope that solves it? Is there something else I should look at? This is the only thing keeping me from riding it...

Thanks in advance!

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Discussion Starter · #2 ·
Update: Decided to buy the Mikuni VM30 kit anyways. Wanted it even if it isn't the problem.

Also bought a new compression gauge and got the readings up to 150psi on the left cylinder and 155psi on the right. I'm kick only and not sure how many times I should be kicking it for this reading? Until it stops going up?

Anything i should check in the mean time?
 

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Is that right side plug wet or dry? Did you swap sides with the plugs? Spray some gas or carb cleaner into the right hand carb throat while it's running, does that cylinder pick up some revs?
 

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Discussion Starter · #5 ·
Well the mikuni kit fixed it. Still blows my mind that I couldn't figure out the keihin carb issue. All the main carb body was cleaned and clear of clogs. Could it be a sticky slide at idle?
 

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Discussion Starter · #6 ·
Is that right side plug wet or dry? Did you swap sides with the plugs? Spray some gas or carb cleaner into the right hand carb throat while it's running, does that cylinder pick up some revs?
Cylinder did pick up some revs with starting fluid
 
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