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Discussion Starter · #1 ·
Hey y’all, new here,
I picked up a 78 CB400TII in October as my first bike with no real knowledge of how they work or how to fix them so I’m pretty new to this. I just successfully rebuilt/repaired the carbs, which are from a CB400A. It runs but I think I still need to figure out the mixture screw situation.

The engine has some blown gaskets which didn’t really bother me at first but now the shifter isn’t quite working 100% of the time and that scares me. So now I’m thinking of an engine swap. I think I’ve found one in good condition. My question is what really goes into an engine swap? Is it just time and muscle? Or is there any kind of tuning/tweaking of some sort that would need to be done? Oiling it up is obvious but besides that I’m hoping I can just bolt everything on and start it up.

Also the CB400 II has a tachometer, electric start, and kick start while the CB400 I has no tach and just a kick start. Would I run into any problems if the engine happened to be off a Type I?
 

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You are using vb24's on your 400T? Vb24's have a venturi of only 28mm as opposed to the original VB21's which have a venturi of 32mm. It will run, but using the 400a carbs make your engine severely detuned and under carburated.
 

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Any of the manual transmission SOHC 400/450 engines will work as a replacement if you decide to go that way.

If you're having problems getting it to idle you may have issues with the mixture screw adjustment. If you have issues with performance above idle you have other problems to address.

Post moved to SOHC engine section.
 

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Discussion Starter · #4 ·
Yes VB24s. I didn’t realize that was what was on the bike until I had a rebuild kit for the VB21s and the new jet needle wasn’t the correct size. Thankfully all the other parts were interchangeable.
I am able to get it to idle and run and everything but still have no idea what the proper setting for the screws should be. But thanks for that info on the 400/450. I assume the bolt/mount points are the same for those?
 

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Idle mixture screws will be 1.5 to 2.5 turns out from seated as a starting point. Idle speed screw will be @1 turn in from initial contact with the linkage as a starting point.
Get it started and set the idle and mixtures for best idle at 1200 rpm, not over 1300.
Now sync the carbs.
Now you can set the idle speed to 1200 and pick a carb. Adjust the mixture screw to get the highest idle speed, you may need to readjust the idle speed up/down to hold at 1200. Once done repeat on the other carb and go ride it for a few miles. Recheck the adjustments.
 
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