Topic: Is this a carburetor float problem?

[Kurt V]

CB400F. I got some crud in the carbs and tore them down for a rebuild. Prior to the problem the bike ran great. I took the carbs off, cleaned and rebuilt them. Set the floats at 21mm and did a bench and then on the bike vacuum synchronization. Starts and idles great. In neutral will easily rev right up to redline. On flat ground will quickly run up to redline in the first four gears, but once I get up around 75mph it feels like it is cutting out. Going uphill the same thing, only much, much worse.

Are my floats set too low leading to gas starvation? When I set the floats I have the float just touching the needle and then measure from the bowl edge to the bottom of the float closest to the hinge end of the float. Thanks for any and all help!

[Black 750K8]

CB400F. I got some crud in the carbs and tore them down for a rebuild. Prior to the problem the bike ran great. I took the carbs off, cleaned and rebuilt them. Set the floats at 21mm and did a bench and then on the bike vacuum synchronization. Starts and idles great. In neutral will easily rev right up to redline. On flat ground will quickly run up to redline in the first four gears, but once I get up around 75mph it feels like it is cutting out. Going uphill the same thing, only much, much worse.

Are my floats set too low leading to gas starvation? When I set the floats I have the float just touching the needle and then measure from the bowl edge to the bottom of the float closest to the hinge end of the float. Thanks for any and all help!

I think the part in red is your problem. It should be the highest part of the float the way you did it the fuel level would be low. You should search for the clear tube method to check this. The level should be about 3mm below the bowl gasket.

[flybox1]

with the float tang just touching the float valve, you should be measuring the float height at the highest part of the float, which is usually the furthest away from the hinge.
the clear tube method is usually good if you are absolutely sure you've set your floats correctly, but have a suspicion the fuel level inside the bowls is incorrect.

tell us about your intake and exhaust system, and what jetting you have in your carbs. 
knowing this info, and what is stock for the 400f might give us a hint as to whats going...

[Deltarider]

Set the floats at 21mm

Were they wrong then?

[mono]

+1 to what flybox and black said regarding adjusting the floats.

also, i agree with your suspicion that it's getting fuel-starved when you are going uphill or running at high speeds.  sounds like you're on-track

[Kurt V]

with the float tang just touching the float valve, you should be measuring the float height at the highest part of the float, which is usually the furthest away from the hinge.
the clear tube method is usually good if you are absolutely sure you've set your floats correctly, but have a suspicion the fuel level inside the bowls is incorrect.

tell us about your intake and exhaust system, and what jetting you have in your carbs. 
knowing this info, and what is stock for the 400f might give us a hint as to whats going...

Well you guys have certainly answered my question! I was measuring from the wrong end, which explains the fuel starvation!

As for my 400F. It is, with the exception of the tires and bar end mirrors, bone stock. 12,000 miles and still sporting all the OEM stickers.



I've had the bike a couple of years. Prior to my ownership it spent 12 years as someone's living room art:

[flybox1]

Nice looking scoot    but unless you are the original owner, you still need to verify the PO didnt swap out jet components for something leaner, and measuring the wrong end might not account for ALL of the leanness.
tell us your jet sizes.

[Kurt V]

Thought I would follow up on this thread and let you all know the problem is solved. I reset the floats to the proper height and double checked that the jets were the stock size. Bike ran a bit better, but still fuel starvation at high speeds. So I get home and am sitting there looking at the bike, willing it to fix itself. Then I heard a small voice cry out from the horn "check the fuel filter you moron." To which I responded "stupid bike, I put that fuel filter on right after I cleaned out the tank and carbs." But to shut up the voices in my head I pulled the fuel filter and put a brand new one on. Problem solved. Bike is back to running great. Thanks for all of the help!

[lucky]

CB400F. I got some crud in the carbs and tore them down for a rebuild. Prior to the problem the bike ran great. I took the carbs off, cleaned and rebuilt them. Set the floats at 21mm and did a bench and then on the bike vacuum synchronization. Starts and idles great. In neutral will easily rev right up to redline. On flat ground will quickly run up to redline in the first four gears, but once I get up around 75mph it feels like it is cutting out. Going uphill the same thing, only much, much worse.

Are my floats set too low leading to gas starvation? When I set the floats I have the float just touching the needle and then measure from the bowl edge to the bottom of the float closest to the hinge end of the float. Thanks for any and all help!

I guess you did not have a workshop manual?

[Kurt V]

I guess you did not have a workshop manual?

I have both the Clymer and the Haynes. The section regarding setting float height sucks in that the pictures do not show a 400F float and and the instructions obviously leave a lot to be desired. Probably should have asked here before setting them, but all is well now!

[Deltarider]

If you don't mind me asking why did you think you had to 'adjust' them?
Untouched, they're always right.

[flybox1]

If you don't mind me asking why did you think you had to 'adjust' them?
Untouched, they're always right.

..assuming the PO did not ufck with them, and float valves had not been replaced, yes, they'd not need adjusting.

[PeWe]

I checked my carbs floats with this forum in my mind. Floats are OK unless somebody had tangled with them. I checked my VM29's and saw that all were the same and I know that I have not touched them

During my long restoration project I have complained about last repair/tuner. I have no PO to claim, only myself when my bike was untouched when bought.
How could I do that bad thing? I remember I was very eager to get it together and change things during the first half of the 80's. Maybe not all parts together just the most important ones.

[Kurt V]

If you don't mind me asking why did you think you had to 'adjust' them?
Untouched, they're always right.

As I stated in my original post the carbs got some rust particles in them. So I got the rebuild kits for them which include new float needles and seats. Turns out they were not the same height as the original seats. Hence the need to reset the floats height. In hind sight (which is always 20-20) I should have just reused the old parts and just replaced all of the o-rings.