Topic: 76 CB400F Carbs leaking after rebuild

[marsbound]

Howdy y'all,

I've never done my own work on a bike (I just never learned) but when the carbs on my 76 CB400F started leaking I figured it was time to start. I took them out, disassembled them and rebuilt them using a 4into1.com kit (not the one with new floats). Yesterday, I put them back together and reinstalled them on the bike. It runs when I hold the throttle open slightly which I'm very happy about, but whenever the bike is not running and the fuel petcock is open, several of them leak fuel out of the overflow tubes. The floats were all in good condition, and it seemed like the rest of the carbs were as well. I adjusted the float heights correctly (or so I think, I used a float height tool set to the factory spec and made sure the floats were up against when just barely touching the needle). Any ideas? Not really looking forward to taking the carbs back apart...

[jwurbel]

Check to make you have no cracks in the overflow stems.  Happens a lot in older carbs.

[marsbound]

Gotcha. What would be the best way to do that? Remove them, fill them with water/gas and see if it leaks on the bench? Is there any way to repair them, or will I just need to get some new ones from ebay?

[Bodi]

Sure, check the overflow tubes. Not likely to be the problem.
"the one with new floats"? Yet you say the floats are in good shape. So you didn't use the new ones? I've never seen one of the original black floats have a problem unless some brutal PO broke a float tang off. Maybe early 350s had brass floats?
Anyway:
The problem is that aftermarket brass is not good. Aftermarket jets are not equal to original KeiHin, and usually the engine runs like crap with them. The original brass really doesn't wear - the emulsion tube orifice and needle may wear after many many miles but as a rule the original brass is fine and should not be replaced with aftermarket, if jetting changes are needed get KeiHin jets.
The aftermarket float valves are terrible and I think your overflow issue comes down to them if you used them.
This is Honda PN 16011-329-004 and is crazy expensive: four of them can solve your leaks but cost more than that rebuild kit.
To clean and rebuild the carbs you need those (unless you put back the originals, but the valve should stay with the seat it was in so did you keep them together?) and a gasket set (Honda PN 16010-333-305). If you can get Viton O-rings (someone here was selling them) you can use ethanol fuel without the crossover fuel tubes leaking when the original ones break down from the ethanol. I think four gasket sets gives you enough O-rings for the crossover and tee tubes. If you don't take the bodies off the frame (not fun) these O-rings aren't used but keep them in case leaks start from there.

[jaytee-nz]

Definitely check the overflow tubes for cracks - I've had several sets of carbs with this problem.
Easiest way to check them is to remove the float bowls, support them on a flat surface (or put them in a vice) and then fill them to the top with fuel.
If they are cracked, they will leak until the fuel level goes below the bottom of the crack.
You can try and repair them with solder / JB Weld etc, but the best way is to find some good bowls.

[Tim2005]

While you are checking the overflow tubes, set the bowl-less carbs up the bench with an auxiliary fuel tank and manually move the floats to see that the valves are shutting off correctly & there are no other leaks (you did fit the o-rings around the float valve seats?). Be careful doing this, best to set a bench up outdoors due to fumes & fires

[Flyin900]

It is good practice to always check the rebuilt carbs for leaks prior to installing them, as it saves lots of extra work afterwards in removal again. A small bottle with isopropyl alcohol connected like a IV drip is what I use to avoid the gasoline mess and fumes that can be dangerous.
If you have a Mitey Vac then you can test the fuel rail and fuel valve seating by pulling vacuum on the carbs when set upside down with the fuel valves closed on the fuel valve seat. If they won't hold vacuum you are going to have a leak somewhere if you install them. A spray bottle of water and dish detergent can be used to check the fuel rail tubes and the fuel valve seating if you have a vacuum leak. Do this with the float bowls off the carbs in the same upside down position if you have a vacuum leak, to determine the area(s) in question that are leaking.

+1 on not using the 4 into 1 carbs brass in their kits, as they are not anywhere near the quality of the original Keihin brass parts. They will likely cause you issues with running properly once you get the leaking sorted out.

[carnivorous chicken]

"the one with new floats"? Yet you say the floats are in good shape. So you didn't use the new ones? I've never seen one of the original black floats have a problem unless some brutal PO broke a float tang off. Maybe early 350s had brass floats?

(not the one with new floats).

Missed the "not." And 350F's never had brass floats.

But yeah, probably the float valves, especially if it didn't leak before the rebuild. "Several" I'm assuming is 3 (not "two" and not "all"), so it would be quite a coincidence if three (or four, I guess) of the overflow tubes developed cracks in the process of the rebuild. But yeah, if they're off the bike, might as well check.

Julie Seal sells the viton o-rings. I have a set, haven't installed them yet.

[marsbound]

Dang, lots of good tips here.

The carbs were leaking before as well - that's why i took them apart to rebuild them. I inspected the brass parts that were in the carbs, and only one needle had the Keihin star. Despite the leak, the bike had been running fantastically. I'll take the bank back off next weekend and do some leak testing. Any recommendations for where I might find replacement bowls, if I need them? I did some cursory looking and didn't find any.

P.S. - Here's what she looked like before she sat for while!

[newday777]

Welcome aboard the forum marsbound
Your bike looks very clean in that picture. How long have you had it? What is the history of it?
You ask where to find some bowls if you need any but you you haven't said where you are (or have your location in your profile). This is a worldwide platform and some don't want to ship out of country.
I know a friend that might have extra bowls but will have to check with him if he wants to sell them, he has a few 400s, a couple are project bikes. He might have a whole set of carbs too.

[marsbound]

Newday,

Thanks, I've had it since 2022. I'm not sure about much of it's history, but it was restored in 2016 by a gentleman that was retired and had bought out a Honda parts department back in the mid naughts. My father bought it in 2016 from him, and then I bought it in '22 from my father. Neither of us had the know-how to work on it, so the good work by that older gentleman unfortunately fell by the wayside. Hopefully I can bring it back up to his standards, though!

I'm located in North Texas!

[newday777]

Let me know what you find with the bowl vents

[marsbound]

Weird, the bowls are all liquid tight. Sounds like I need to test if the floats are turning off the flow via the needle now?

[marsbound]

I'm even more confused, and I think I'd like to just get new carbs for now so I at least have a working bike! I bench tested the carbs with rubbing alcohol, and there were no leaks. However, when I put them on the bike, the #3 carb just started pouring fuel out of the overflow tube. I went back and readjusted the floats to shut off the flow at 21mm, but still no dice. @Newday, is your friend still willing to part with those extra carbs?

[Flyin900]

Usually when you reinstall a set of refreshed carbs they can leak from one or more float bowls. The fuel needles may hang up in their brass seats from the handling and installation gymnastics. This causes the fuel valve to continue to flow fuel until it overflows the brass standpipe in the bowl.  Normally by tapping on the offending carb it will usually stop the leak.

Did you replace all the brass parts including the fuel valves and their brass seats with the cheap Chinese stuff that 4-1 sells? If so especially the fuel valve and seats, you could have a issue originating there. Usually the rubber bits in these cheap kits can be used, yet not the brass parts if you don't want issues afterwards.

Honda's carb kits just for the rubber bits are around $25-$30 per carb for a perspective. Where most of these aftermarket complete kits are that price for all the brass parts and the rubber bits too.

[carnivorous chicken]

The fuel needles may hang up in their brass seats from the handling and installation gymnastics. This causes the fuel valve to continue to flow fuel until it overflows the brass standpipe in the bowl.  Normally by tapping on the offending carb it will usually stop the leak.

this, I have also seen old carbs where the tang gas developed a little indent that the float valve hangs up on.

[marsbound]

Usually when you reinstall a set of refreshed carbs they can leak from one or more float bowls. The fuel needles may hang up in their brass seats from the handling and installation gymnastics. This causes the fuel valve to continue to flow fuel until it overflows the brass standpipe in the bowl.  Normally by tapping on the offending carb it will usually stop the leak.

Did you replace all the brass parts including the fuel valves and their brass seats with the cheap Chinese stuff that 4-1 sells? If so especially the fuel valve and seats, you could have a issue originating there. Usually the rubber bits in these cheap kits can be used, yet not the brass parts if you don't want issues afterwards.

Honda's carb kits just for the rubber bits are around $25-$30 per carb for a perspective. Where most of these aftermarket complete kits are that price for all the brass parts and the rubber bits too.

Got it. I'll try again when I get home and tap on the carb to see if that fixes it.

I did use the rubber and brass from the kit - but the brass that was in the carbs already wasn't stamped with the K*, so I think it must've been aftermarket brass in there already... where can i find oem jetting and rubber bits? I saw the oem float valve set linked above, but what about the rest of the brass and rubber?

Thanks

[marsbound]

Okay, I think I've found all of the leaks. Now, a different question: my choke plates are not all in line with each other. That is, the plate on the number 1 carb fills the throttle inlet completely, while #3 and 4 both are slightly open. Is that a common problem?

[newday777]

Aren't the choke plates on adjustable links to the one pull spot? They should all close fully at the same point.

[marsbound]

I figured it out, I think. I had just poorly adjusted the linkages.

Now even more things are going wrong though. I feel like I'm going crazy! I simply cannot for the life of me get that stupid airbox (intake manifold? the one between the carbs and the air filter box) on the bike without something popping off of it. Any tips? I tried spraying the boots with Boeshield, no dice. I tried heating it up, but also no dice.

Also, I think I might've broken an electrical connection somewhere in that rats nest of wires around that area as well.

I need a beer and a day off from working on this thing, lest I pull my hair out!

[jonda500]

Still never an easy task, but you need to put the phelum thingy in before the carbs - i.e. the carbs are last to go in
John

[marsbound]

John, thanks for the tip. Any tips on how to get the individual boots onto the carbs? that was my hesitation about putting the air intake thing on first as i just couldn't seem to get those to sit right and be able to tighten down the clamps.

[Mark1976]

John, thanks for the tip. Any tips on how to get the individual boots onto the carbs? that was my hesitation about putting the air intake thing on first as i just couldn't seem to get those to sit right and be able to tighten down the clamps.

   If you're having issues installing the air chamber (black plastic chamber), it goes in last, pull the air filter and then the airduct tube (oval silver tube) with the seal out of the airbox. It's difficult the first time but after that its much easier (put a little lubricant on it when you reinstall), that'll give ya the room the slide the air chamber in without much issue.
   The problem with the boots not sliding over the carbs is they become hard from age (unlike some things in life) and as a result less pliable with age, heat them up (heat gun) they'll become more pliable just long enough to install them (if ya want you could put an ever so small amt of lube on the inside edge of the boots to aide in slipping them on over the carb opening). While the airduct tube is out you can reach in (with the handle end of a screwdriver) and push on the backside of the airchamber onto the carbs.
   Then slip the airduct tube and seal back in, followed by the airfilter and filter retaining clip.

[M 750K6]

On my 750 I leave the rubbers on the head and tighten the carbs into them first. I put the airbox on last, without the air filter section. Gentle heat on the rubbers pays dividends. Initially it was difficult because the air box seemed a fraction too far from the carb intakes, putting some tension on the rubbers. I filed the holes each side on the airbox mounts. This gave a couple of mm and it all fits up easily now.

[newday777]

On my 750 I leave the rubbers on the head and tighten the carbs into them first. I put the airbox on last, without the air filter section. Gentle heat on the rubbers pays dividends. Initially it was difficult because the air box seemed a fraction too far from the carb intakes, putting some tension on the rubbers. I filed the holes each side on the airbox mounts. This gave a couple of mm and it all fits up easily now.

The 750 is a totally different animal.  It is far easier than the 350F, 400F and 500/550 assembly. No comparison.