One cylinder

[RogerTheGray]

My $500 CM400 only idles on the left cylinder. I have adjusted valves, cleaned the carburetor 4 times, and changed the coil. It has good spark and fires when I rev it up, it just doesn’t fire at idle.
I’ve even swapped all the jets and parts from the 2 carbs. The right side carb did look like it had a biscuit in it when I opened it up. What’s my next play?

 

[twinfan]

I would imagine that your carbs are due for a full-blown cleanup, followed by a rebuild and a sync job?

 

[esmith250624]

you could try turning the idle speed screw to increase idle speed. (Black knob at the bottom of the carburetor.) if your sync is off one of the throttle plates could be choking off the air to one of your cylinders and not firing turn it so the idle is higher and and see if it holds idle there, like 1500-2000 and see if that other side is firing.

 

[LongDistanceRider]

Check your plug end cap, resistance should be 5K ohms.

 

[RogerTheGray]

you could try turning the idle speed screw to increase idle speed. (Black knob at the bottom of the carburetor.) if your sync is off one of the throttle plates could be choking off the air to one of your cylinders and not firing turn it so the idle is higher and and see if it holds idle there, like 1500-2000 and see if that other side is firing.

It fires when I rev it up and that makes me think there’s a problem with the idle jet

 

[J-T]

If it's running normally above idle I'd be looking at the sync. Sounds like your right throttle plate is closing slightly ahead of the left. You'll need a vacuum gauge (preferably 2).

 

[esmith250624]

It fires when I rev it up and that makes me think there’s a problem with the idle jet

I would not rule it out the idle jet yet, but you have to start eliminating possible issues. you say you cleaned the carbs 4 times. did you fully extract the idle jet and clean it or just poke a wire through it and shoot some carb cleaner down there? if you had the carbs off 4 times did you do a bench sync? (setting the throttle plates as close to even as possible) There is plenty of good info on this site how to do all of this stuff, so you just have to spend some time and start eliminating possible issues, but we have to assume you have read and followed the carb cleaning thread on this site.

https://www.vintagehondatwins.com/forums/threads/how-to-rebuild-the-vb-carburetors.24/