'78 CB400TII "Lucy" (NEW PIPES!!!)

[Maritime]

Actualy you are kind of right and I am a bit wrong, I was forgetting the 78-79 are different from the 80-81, Mine have a booster pump in them that these may not. I was speaking from foggy memory from 3 years ago last time I was in them.  I will be re-learing them again when I have time to tune them again after adding the baffles.

[Maritime]

They are diffferent  I think the difference makes tuning mine a bit easier than his.

see pics:

[Maritime]

I guess they have what looks like a slide that is vacuum operated and the butterflys open and close to change the vacuum? I am used to calling "slide carbs" the ones with the cable attached directly to the slide and it being pulled up by the cable and pushed back by the spring.  I call these type butterfly because when you look into the mouth of the intake and atuate the mechanism the butterfly pivots and lets more air in.

[yayuppp]

So running lean (more air than fuel?) or having an air leak would cause it to rev up high and die?

[Maritime]

yep, that is one symptom of lean/airleak.

[MotorbikeBruno]

And if you have already gone up a set of pilots and still can't get that idle down, you will have to shim your main needle with little washers or move it UP (towards the sky so-to-speak) one or two notches if it has them.  Although, this is only needed provided you already took care of your air leaks if there was any.

[yayuppp]

I went from a 110 to a 120.  So, that's 78(or 72)/120.  I bench synced the carbs.  Mixtures at 1.5 out. 

Now it's idling on one(drive side) cyclinder.  Give it throttle and the other one fires up; let off and the whole thing dies.  So I backed out the second mixture out another .5, nothing. 

I noticed I have a float the droops down more than the other, when the carbs are upside down.  Big problem? 

[MotorbikeBruno]

So you went from 78 pilot to 120?  Or you went from 78 mains to 120?  (sorry, just want to verify)

Have you pulled the air screws and made sure there is an o-ring (in good condition) around it.  I had a bike that was driving me nuts, then I pulled the air screw and found a washer in there....missing the O-rings under them.  Ran much better after that!

One more thing, did you open your fuel tank filler lid to see if it was venting well enough?

As for the float, when upside down you are putting pressure on the little spring in the bottom of the float needle.  If one droops because the spring is being loaded, and the other doesn't droop because the spring isn't working, that could be a problem too.

[yayuppp]

110 to 120.  Which jet is which? 

I think the float is bad, like it's not floating or too heavy.  I was letting it idle for a bit and noticed what I presume is fuel, coming out the pipe.

[MotorbikeBruno]

Well the easiest way to find out the float is to get a cup of gas, put it in there and weigh it down with something, walk away for10 min, let the weight off of it, and if it floats your are generally safe, I say this because if it floats but when you pick it up and it's much heavier...there's fuel in there 

Theres a pilot jet (idle jet) middle jet (secondary jet) and main jet on yours.  If I remember right.  The main jet should be the 110 that you changed to 120.  Thats only really in use above 1/4 throttle or so.  The secondary jet is short and fat, screws into the top of one of the spots, should be 78 or something close to that.  And your pilot jet is UNDER a plug of some sort, might be black rubber plug, or white hard plastic plug with o-ring, again I can't remember off the top of my head. When unscrewed, it will be longer and very skinny compared to the others. If you don't have a plug over your pilot jet circuit (the only tube that doesn't have threads in it, so the plug will fit nicely) it will run like crap.  Take a picture of the insides of  your carbs and we'll get to the bottom of this.

[Independent]

I have been playing with the carbs on my 78 Cb400A, which should be the same and this is where I am as of right now. With the stock air box and the same Emgo mufflers as you have I am currently running a 112 main jet and a 92  secondary jet. I have the Idle screws turned out the factory 1 1/2 turns and it seems to run pretty good. I am getting 85's for the secondarys tomorrow and will install them and report back as I think its a little fat in the mid range. I tried a 125 in the mains but with the mid being fat it was just too much and would bog real bad at high rpm. I figured its easy to go overkill and work down from there rather than try and sneak up on it. Good luck and nice progress.

[yayuppp]

Thanks for all the info and encouragement! 

I'll be cracking the carbs apart again over the weekend and double checking everything. I probably won't be able to fire anything up until Tuesday. I'll report back then. Maybe with a video. 

[yayuppp]

Well I'm a little late on delivery. 

Made a little video of what's going on. Mains are 120 and pilots are 78. Mixtures at 2 turns out. Start with full choke and only left idles. Ease off the choke and then only the right idles.

[MotorbikeBruno]

Use the "stumble" method with your air screws for now. In until it stumbles, out until it stumbles, find the number of turns that is in the middle of each stumble.  Get it idling the best you can (even if one side), turn the air screws in or out. I'd think you'd want to turn them the direction that makes them more rich at this point.  I bet you aren't getting enough fuel to the one side that isn't firing at idle.

[yayuppp]

Well I was working on the carbs and trying to get the oil/neutral light to work on my gauges at the same time.  I was using a little leader/jumper wire to test connections.  Went to grab a tool in the garage, the wire had come off and grounded to the frame.

Hooray for fuses!  Check out the little jet of (guessing) melted glass with the little white orb coming from the middle fuse.   But, that ended my testing of the carbs.   

So, I moved on to the front brake.  Do these look like they can be saved?