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Discussion Starter #1 (Edited)
Well, on my way home yesterday on my 78 CB400A, everything was going fine until I was almost home and at a stop light I noticed I was bogging a little almost like I was out of fuel. But that didn't seem likely, as I only had 40 miles on the tach.

When I got a chance to pull over, I saw a fair amount of gas dripping from the tubes in front of the back tire. I closed the petcock and the dripping stopped pretty quickly. I managed to limp it home on reserve and a prayer.

Because the petcock seems to be functioning appropriately, this seems like a pretty certain case of a carb issue, right? That's what my research here indicates, anyway.

I really wasn't having any running issues at all before this (even just before I stopped at that particular light--actually, it's possible the only running issue I had was low gas because of the leak), so I was wondering if there are any high points I should check before I get elbows deep in a carb rebuild. I guess I'd plan to do that this fall anyway, but while I'm somewhat mechanically inclined, I'm not experienced in engine work and I'm reluctant to engage in something that may leave the bike out of order for a month... not because I think it will take that long, but with two young kids, free time comes it small doses.

In any case, I'm picking up an ultrasonic cleaner, gaskets, and a couple other odds and ends that I think I'll need (if not now, then soon). I've also saved LDR's sticky on carb rebuilding, for which I'm extremely grateful. Here, for anyone else who comes across this: https://www.hondatwins.net/forums/63-fuel-supply-carburation/24191-rebuilding-vbulletin-carbs.html
 

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LDR carb rebuild is invaluable but an A carb is a little different from the 400s . A cb 400 a FSM will give you the settings for your carbs. I used his sticky to rebuild my carbs then the manual to fine tune them.

If you havent worked on these carbs before give us a shout maybe we can give you some tips on removal , build and reinstall.

Bill
 

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Discussion Starter #3
Thanks Bill! Hope you're recovering well from the surgery. I've read some tips on getting them out, so I think I'll start there and report back when I run into any issues. I do have a manual saved somewhere. Good point on that.
 

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Check the bowl overflow tubes first...
They crack and then can dump enough fuel that the carb jets barely get (sometimes don't get) enough to run the engine.....
Fortunately, there's an easy fix....Solder them......There is a topic on this.....
 

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Discussion Starter #6
I'm having a heck of a time finding these screws: Stainless Steel Allen Head Screws: 8: 4x12mm, 8: 4x16mm, 4: 5x12mm, 5: 5x16mm, 4: 6x14mm. I'm trying to find them online after striking out at local hardware stores.

Sorry if this is a dumb question, but does anyone know the thread pitch of these screws?
 

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I think LDR is on the road. You might shoot dtsmjr8dan a PM and see if he can help with the thread pitch. Fastenal on line will probably have them but searching thru their web site is a pain.

Leg is doing fine. Ahead of the PT but have been told to back off a little as the muscles need some more time. Getting way behind on the yard work etc.

Bill
 

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Discussion Starter #8 (Edited)
Bill, great tip on Fastenal. I think I've located them. They're only offering "coarse" threads for these allen/socket heads of any size, so I think that must be it. I know from personal experience PT is more marathon than sprint. Glad to hear you're doing well.
 

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Discussion Starter #9
Can you guys see how one float is sitting higher than the other? Would this have caused the carb overflow issue? I checked the bowl overflow pipes and they both look intact. The bowls were actually very clean.
 

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It looks like the right side float is higher. Is it the one leaking?

Follow the CV carb rebuilt to the letter and you will have perfect carbs. Did the same with mine and soak them in the ultrasonic cleaner as well. It looks daunting to do the carbs, but actually very doable. Good luck!
 

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Discussion Starter #11
It was leaking pretty significantly and I couldn't tell if it was coming from one tube or the other unfortunately. I plan to do that, I was just curious if there is a spring or something here that is out of whack. When I tilt them, they both tilt to the same spot, although the one that is sitting higher moves a little slower. I'm going with the theory right now that there is a bit of dirt or grime impeding the movement and preventing it from seating properly.
 

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If it was running fine before it started leaking I would think the problem would be in the float valve . Maybe some dirt or rust under the seat. i would pull the float and seat and see whats going on. There has to be a reason the float isnt setting right. If you clean, replace the seat and the float resets I would take a hose on the fuel intake and with the float in the closed position try to blow thru it. If you can find out why . If you cant then I would put them back on and start it up and see what happens. If you take the carbs apart mark the syinc adjuster so you can put it back in the same place. Just some thoughts.

Bill
 

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If you take them apart, use 3 different thickness materials, I use different sizes of lock wire, but zip ties would do the job as well, and they will be pretty close to be synch. Set them with the smallest one, then the second and third.
 

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Yes, Bill I was on the road again. Went off on the El Diablo Run again, 3 days of Tecate, Taco's and Tequilla in San Felipe, Baja with 500+ close friends.
The float level settings can only be checked with the carbs face down (pivot pin up), not upside down. This sounds like a float needle is binding in the bore. I polish the 3-4 ribs of each needle as well as the bore so they won't stick.
 

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Discussion Starter #15
Thanks LDR! Sounds like a fun trip. I realize that with them being upside down you're not really getting a picture of them as they operate, it just looked odd to me that one would sit differently than the other. I should have mentioned that the one that appears to be out of place was also not moving quite as freely when I tilted them to see the movement. I definitely appreciate the feedback. It gives me something specific to look into while I make sure that "everything" is pristine.

Unfortunately, as I suspected, free time to work at them has not been as forthcoming as I'd hoped, so this project is on hold until at least this weekend, but I will update as soon as I run into anything, or, ideally, when she's running normally again..
 

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Discussion Starter #16
Well, I got everything cleaned in an ultrasonic bath and put back together. The bike is running again although I need to make some adjustments to the idle and the ******* choke cable disconnected after I spent 30 minutes forcing it in there. So I'll need to take care of that but for now we are riding again! I also changed the oil in case any gas happened to make its way down there during this ordeal. And got a fuel filter installed. Thanks again, LDR, for the guide, and Bill for the words of advice.

I really didn't find any major points of interest except that one float needle was slightly longer than the other. I suspect this was PO's handiwork.



I replaced the long one with one that fit. The weird float issue was actually caused by a bent clip that holds the needle to the float. I bent it back and now everything looks good in there.
 

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Discussion Starter #17 (Edited)
Minor issue... I have been having a heck of a time getting the left side boot connecting the carbs to the air box to seat all the way over the ridges on the carb. As a result I think I have a slight vaccuum leak on that side. That rubber is so stiff and it seems reluctant to return to shape after being crushed a bit while I wiggled the carbs out and back in. I was thinking about hitting it with a heat gun (very carefully... there is gas nearby after all) while I manipulate it a bit. Has anyone experienced this and found a better solution?
 

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Some people here have soaked their carb boots in a mixture of something (acetone?) and some type of mint oil (again, I think) and it softened them... wish I could recall what the mixture was, but someone will remember... or you could buy a new pair, but they're a little pricey I'm sure
 

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I use silicon dielectric compound on rubber parts to lube them and soften them. I also use RuGlyde, which is a rubber lubricant that softens too. The RuGlyde is a maitenance type product, meaning you can use it often. Like once a week or monthly, it was for tire changing, ease of on off mounting tires.

Sent from my SM-N920R4 using Tapatalk
 

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Discussion Starter #20 (Edited)
Thanks, Charlie. I'm going to pick up one or both of those today and see if it helps.

Ancientdad, I'll look into that as a fallback. I'm hoping to avoid taking the carbs all the way back off as it really is a pain on this bike. Maybe I'm just being a baby. I definitely want to avoid buying new parts if possible. Everything is there and whole. Just needs a little love. I hope.
 
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