Topic: Progress on my '75 CB400F Garage Find

[Master_Moose]

I bought all new floats and it has new needles and seats, but carb #1 still overfills and leaks. 

Did you check the fuel level in the floats with the clear tube method?   Maybe the float level in that carb set set too high?

Yeah, I hooked up clear tubes and I watch all of the others stop just short of the gasket level, while #1 just continues going up.

I don't see any cracks in the overflow tube, but also it doesn't start leaking until the level in the tube goes well past the gasket level, so I do believe it's overfilling and not a cracked overflow tube (also, it's definitely coming from inside the tube and not around it, if that matters).

There are clear, disposable little dessert cup things you can get. I think I'm going to rig one up as a makeshift fuel bowl that I can see through and maybe get an idea of what's going on.

It's frustrating, because this is the only thing left that is keeping me from riding this bike, but summer is almost over!

[jas67]

Yeah, I hooked up clear tubes and I watch all of the others stop just short of the gasket level, while #1 just continues going up.

I don't see any cracks in the overflow tube, but also it doesn't start leaking until the level in the tube goes well past the gasket level, so I do believe it's overfilling and not a cracked overflow tube (also, it's definitely coming from inside the tube and not around it, if that matters).

There are clear, disposable little dessert cup things you can get. I think I'm going to rig one up as a makeshift fuel bowl that I can see through and maybe get an idea of what's going on.

It's frustrating, because this is the only thing left that is keeping me from riding this bike, but summer is almost over!

Your float valve is not stopping hte flow of fuel for some reason.    Try swapping the float and valve between the one that is over flowing and another to rule out a defective float or valve.   Also, could the float be getting hung up on the bowl gasket?   This can be a problem with some carbs.

Try giving the float bowl a good wrap with a small hammer on a wooden dowel, or even the plastic end of a screwdriver handle.

Sometime my CB400F will have a float hang open when I open the petcock at the beginning of the season after having drained the float bowels the previous winter.   The hammer on the wooden dowel thing tends to shake things and get the float unstuck from the bottom of the bowel.

The '73 CB350F I'm currently working on does the same thing.

[jas67]

I don't see any cracks in the overflow tube, but also it doesn't start leaking until the level in the tube goes well past the gasket level, so I do believe it's overfilling and not a cracked overflow tube (also, it's definitely coming from inside the tube and not around it, if that matters).

I couldn't see the cracks in the overflow tubes in my carb bowls either but they were there.
Just a suggestion that you double check to be sure. 

Should be a simple matter of taking the bowl off and filling it level full of fuel, and see if any comes out the over flow.

You could also switch bowls between two of the carbs and see if the problem follows.

[carnivorous chicken]

Seconding Jas 67 if it's only overflowing when it gets above the o-ring level. 400F carbs have o-rings for the bowls not gaskets (like twins) so floats don't get caught up on them, but it's either a bad float valve or bent or damaged float. One thing that can cause this is the tang on the float being bent improperly, causing the valve not to close. Can you rap on the carb with a rubber mallet (firmly, but not too hard and not to gently) to make it stop? If so it's likely float (tang or getting caught up on something). One thing to try is switching out components with a neighboring carb.

[Master_Moose]

Seconding Jas 67 if it's only overflowing when it gets above the o-ring level. 400F carbs have o-rings for the bowls not gaskets (like twins) so floats don't get caught up on them, but it's either a bad float valve or bent or damaged float. One thing that can cause this is the tang on the float being bent improperly, causing the valve not to close. Can you rap on the carb with a rubber mallet (firmly, but not too hard and not to gently) to make it stop? If so it's likely float (tang or getting caught up on something). One thing to try is switching out components with a neighboring carb.

It can't be a problem with the floats, because I replaced all 4 of them and carb 1 is still overflowing like before. Tomorrow I'm going to try swapping carb 1 and 2's needles and seats to see if the problem follows them or stays on carb 1. I'm also going to fill the fuel bowl off the carb and see if overflows. I'll update tomorrow evening.

[carnivorous chicken]

Can't always assume that since it's a replacement it's good, but check that tang too.

[Master_Moose]

I removed the bowls for carbs 1 & 2 and filled them up. No leaks at all.

Next I swapped the needles and seats between 1 & 2. Then I used painters tape to hold the floats up and turned the fuel on without the bowls on. Carb 2 stayed dry, but carb 1 dripped steadily, with about 2 drips per second. When I untaped the floats, carb 2 poured fuel (as I believe it should), but carb 1 continued to drip the same as when the float was up.

So the problem stayed with carb 1 and is not caused by a cracked overlow, bad float, needle, or seat.

[jas67]

I removed the bowls for carbs 1 & 2 and filled them up. No leaks at all.

Next I swapped the needles and seats between 1 & 2. Then I used painters tape to hold the floats up and turned the fuel on without the bowls on. Carb 2 stayed dry, but carb 1 dripped steadily, with about 2 drips per second. When I untaped the floats, carb 2 poured fuel (as I believe it should), but carb 1 continued to drip the same as when the float was up.

So the problem stayed with carb 1 and is not caused by a cracked overlow, bad float, needle, or seat.

It sounds like fuel is leaking between the carb body and the seat.
Is there a lot corrosion there?

[Master_Moose]

I may have been wrong. I just swapped only the needles and the leak issue followed the needle to carb 2.

On carb 1 I can stop the flow with the needle remove and my thumb over the seat, so it might just be a defective needle after all. I'm guessing when I first swapped the needles and seats I fumbled them or something and got them mixed up so they never actually switched places.

[jas67]

I may have been wrong. I just swapped only the needles and the leak issue followed the needle to carb 2.

On carb 1 I can stop the flow with the needle remove and my thumb over the seat, so it might just be a defective needle after all. I'm guessing when I first swapped the needles and seats I fumbled them or something and got them mixed up so they never actually switched places.

Frustrating, but, now at least you have an answer, and can proceed forward!

The CB400F has a sweet motor, you'll really enjoy it once you get it sorted out.

[Master_Moose]

It was pretty sweet the one time I got to ride it up the street to get gas, but then I had to pull the carbs back off.

Definitely frustrating, but I'm hoping the new needle and seat will arrive tomorrow so I'll be riding it again by Sunday. I'm especially looking forward to riding it up to the office where the original owner works. I'm sure she'll be excited to see it looking good and running again after it's been sitting in her garage for the last 35 years!

[jas67]

It was pretty sweet the one time I got to ride it up the street to get gas, but then I had to pull the carbs back off.

Definitely frustrating, but I'm hoping the new needle and seat will arrive tomorrow so I'll be riding it again by Sunday. I'm especially looking forward to riding it up to the office where the original owner works. I'm sure she'll be excited to see it looking good and running again after it's been sitting in her garage for the last 35 years!

That'll be a cool moment.    I'm almost done with the green '73 CB350F that I've been working on for the last 8 months.    I bought it from the original owner's daughter (was her mother's) after they had to move her mother into a nursing home.    It had been off the road for 14 years.   Daughter has fond memories of riding with her mom.   I'm going to ride it over to her place (they live not far from me) to show her when I'm done with it.   Unfortunately, I need to pull the head to replace the leaking oil orifice seals.   The rest of the parts came yesterday.    I found two OEM head gaskets on eBay (hopefully actually real -- the second one is for my next project).   There are a couple threads on here about CB350F/CB400F oil leaks between the head and cylinder.    Most, if not all, the aftermarket head gaskets and oil seals that go between the head and cylinder end up leaking, so, if you ever have your head off, use all OEM parts (getting harder to find, might want to get 'em while you can).

[Master_Moose]

That is awesome. When I can tell someone really cares about the vehicle I'm buying from them I like them to know that I'm taking care of it.

And, yeah, I definitely have learned not to go with cheaper aftermarket parts on this bike. The carb rebuild kits are leaving much to be desired. The gaskets in the kits were good, but I ended up using all of the original brass and now replacing this needle and seat as well.

[Master_Moose]

Alright, good news and bad news.

The good news is that the fuel is no longer overflowing!

The bad news is that I tried to bench sync the carbs by matching all of the sliders. I noticed that two of them had very large gaps and two had almost no gap at all (see pics on page 2). I figured I'd split the difference and set them all so they had a small amount of gap (~2.5mm).

Well, now the bike won't stay running. Before, when it was still leaking fuel, the bike fired up easily. Now I have to give it throttle and even then it's almost impossible to keep it from dying again. One second the RPMs are over 4k and the next second it's died.

Is there any way to get a good base setting without having to remove the carbs again?

[innovativems]

You can bench sync them with a drill bit while they are on the bike.  Get them the same and you can use the idle adjuster to get it to the point that it runs for you


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[Master_Moose]

You can bench sync them with a drill bit while they are on the bike.  Get them the same and you can use the idle adjuster to get it to the point that it runs for you

I'd have to remove the air filter assembly to do that. I think that's more work than pulling the carbs.

I think maybe I just opened them up way too much when I synced them off the bike. It idles at like 3k with the choke on, but when I take the choke off the idle goes over 5k (maybe higher, but I never let it go that long), even with the idle screw turned all the way out.

[innovativems]

Get a vacuum sync done after you get the idle down.  You will have to lower the slides to get the idle down.


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[Master_Moose]

I'm trying to vacuum sync the carbs and I can't get it right at all. Carbs 1 & 2 have almost 0 cmHg with the screws turned all the way in. If I turn them out, the idle goes way up and I have to turn the idle screw all the way out, then the bike dies. It feels like there's just no in between.

[MoMo]

You can bench sync them with a drill bit while they are on the bike.  Get them the same and you can use the idle adjuster to get it to the point that it runs for you


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It sounds like you need to do this....Larry

[innovativems]

How far off is 1/2 vs 3/4 with the vacuum sync?  The two sets could be off enough to do that


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[Master_Moose]

I haven't had a chance to work on the carbs anymore yet, but today I figured I'd take a few minutes and swap out the muffler. It didn't seem like it would be that difficult and it probably wouldn't have been, except that the "replica" muffler has a slightly smaller inside diameter than the original and won't go over the new graphite gasket. It's like the inside diameter of the new muffler needs to be about 5mm larger. So much for anything being quick and easy on this bike!

[Master_Moose]

I finally got my muffler on. I had to create my own gasket using hi-temp foil tape used for furnace pipes, to build up the diameter that the muffler was going on, and then I used RTV red hi-temp silicone to seal it. It seems like it worked pretty well.

Now I can get back to dealing with the carbs!