Topic: CB400F Weird Idle Issue

[William Cochran]

Okay, you all have been great at helping to resolve a few issues with my Honda.  Here is the final thing going on:

It starts easily and after warming up, idles and runs great.  But on occasion, when I am coming to a stop, I pull in the clutch and the engine just stops.  It will start right back up and idles fine afterward. 

It has a brand new Dyna S electronic ignition, new plugs and the carburetors were recently professionally rebuilt and synchronized.  New air filter in stock box and compression is excellent in all cylinders. 

This is very intermittent but would sure like to resolve it.  Any suggestions would be greatly appreciated.

[flybox1]

its possible its starving for fuel, and it needs a second to get back up to a good level to feed the idle/pilot circuit.
use the clear tube method to verify proper fuel level in the bowls.  3+/-mm below bowl gasket seam
second, you might have a fuel delivery issue, where fuel is slow to fill the bowls.  check for any uphill fuel line paths, clogged filters in-tank or inline...

[William Cochran]

Thanks!  I will check the fuel delivery and proceed accordingly.  Will you explain the clear tube method in a little more detail?

[DaveBarbier]

If you search the site there's a lot of info that comes up.

I had the same issue when I would stop downhill or come to an abrupt stop. What seemed to fix it was lowering the fuel level from about 2.5/3mm below gasket seam to 4mm below seam. Maybe in my case it was flooding the pilot jets.

You also might want to raise your idle if it's a bit low. But before that I'd do what Flybox says.

[flybox1]

you'll need some 3/16" fuel line, or similar, taper the end, and screw it directly into drain screw hole in the bottom of your carb bowl
Best to do this off the bike for ease of work, making float height adjustments if needed, etc.  just hook up an aux fuel supply.
do one bowl at a time.

[Redline it]

you can either use a separate float bowl drain plug to drill out to insert a nipple to attach a clear plastic tube to a attach each bowl, turn the fuel on and holding the tube in u shape with the open end above the gasket, hopefully by far, the longer the tube the easier to hold and drain. Or you can get a plastic tube and push into the bowl's drain plug threads and twist a little, it'll seat and you don't have to drill nothing, if the tube is a fraction to big, heat it over a flame and as it starts to melt, pull and the tube will get tapered. Hold it until it cools. Cut it with scissors in the narrow point. You'll now  have 2 tapered ends. I use 4 at one time and have all 4 going into a single line that drains them all at once. It's just a matter of setting up the 4 wedged into the gap between carbs, and use drip sprinkler connectors to tie in from 4's to 2's to 1. The final tube you can hold in your hand above the tank, you don't have to worry about spilling gas. One at a time test only increases the time it takes, the 4 at a time you can mark all 4 carbs in one shot, but just leave them connected while going through the line of carbs.