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SOHC.co.uk Forums => CB350/400 => Topic started by: MikeSB6 on May 22, 2017, 02:08:35 PM

Title: CB400F carb. leakage
Post by: MikeSB6 on May 22, 2017, 02:08:35 PM

Hello Folks,

    I'm restoring the carbs. on the seventies CB400F project and have a slow leak in them. The problem doesn't seem to be with the floats/bowls as such, but the petrol leaks over the outside of the bowls and then drips off the drain screw heads. The cast in (blank?)holes at the outer face of the outer carbs have petrol in them (the ones just above the float bowls in line with the fuel connectors between the carbs.) but I can't see if there's a leak in the holes or whether they're filling up from a leak elsewhere. Also, the carb. bores also seem to have some petrol in them, which wets the choke flaps. I've checked the floats, level pipes and float needles.

       any advice gratefully received, Cheers, MikeJ

Title: Re: CB400F carb. leakage
Post by: Bryanj on May 22, 2017, 05:16:32 PM

If the fuel leaks that high it suggests for a start that the drain tubes in the float bowls are blocked and the only reason for fuel to do that is incorrect float heights and or leaky float valves

Title: Re: CB400F carb. leakage
Post by: MikeSB6 on May 23, 2017, 11:20:09 AM

Hello,

  thanks for the reply, but the overflow pipes are clear and will drain if the carbs are turned at an angle, so I don't think it's that. I've checked the float heights and the float needles are fitting o.k., so it's still a bit of a mystery,

       Cheers, MikeJ

Title: Re: CB400F carb. leakage
Post by: Clem2112 on May 23, 2017, 11:47:31 AM

Have you checked the central fuel inlet T-pipe and the two plastic feed tubes to see if the O rings are ok?

Title: Re: CB400F carb. leakage
Post by: MikeSB6 on May 25, 2017, 08:31:29 PM

Hello Clem,

        thanks for the reply. I don't know how to separate the carbs as they seem to be connected by a cross bar which is pinned to the carb bodies, from what I remember. Is there a way to separate the carbs without damage?

              Cheers, MikeJ

Title: Re: CB400F carb. leakage
Post by: Nurse Julie on May 25, 2017, 09:19:52 PM

As long as the carb plate is removed, you can just pull the carbs apart at the 'T' piece.

Title: Re: CB400F carb. leakage
Post by: MikeSB6 on May 26, 2017, 12:54:14 PM

Hello,

  thanks Julie, I'll give it a whizz,

       Cheers, MikeJ

Title: Re: CB400F carb. leakage
Post by: mike the bike on June 02, 2017, 10:40:32 AM

You need to undo the 8 countersunk M5 screws that secure to carbs to the plate like wot Mike J said

Title: Re: CB400F carb. leakage
Post by: 01thomas on June 03, 2017, 03:22:48 PM

Blocked carb float bowl vents (in the roof of the float chamber) will also cause a high fuel level in the float bowls and for that fuel to leak out everywhere.

Title: Re: CB400F carb. leakage
Post by: MikeSB6 on June 12, 2017, 09:58:35 AM

Hello,

   thanks for all the replies, I've now separated the carbs from the frame and replaced the O-rings but they're still leaking - maybe I've used the wrong size? Are they imperial? I'll blow through the upper vents to check them, too,

          Cheers, MikeJ