Topic: Float height experiences

[deeveebee]

This is probably totally obvious to the very experienced among us, but for those of us newer to 70s-era SOHCs, perhaps my experiences (and conclusions) will be helpful.  I've gotten so much useful info from this community I feel compelled to give a little back, however modest.  So here goes:

I have a bone-stock 78 CB550K, with PD46C carbs.  I thoroughly cleaned and reconditioned the carbs, including rebuilding the choke mechanism (which is a whole potential thread in and of itself).  All went brilliantly, aside from my experience with the floats.

My advice:  Ignore the specs on "float height," unless you're using absolutely new-condition, OEM floats!!!  And maybe even then, too.  I very diligently set mine to 12.5mm, using an accurate digital caliper.  Clearly too rich.  Pulled the carbs, and then tried 14.5mm, which is the spec for the '77 model with the PD46A carbs.  Also no good.  In the end I finally figured out (due to bike symptoms and -- ultimately -- the clear-tube test) that these specs, with my aftermarket floats, just allowed WAY too much fuel into the float bowls.  With the aftermarket floats I bought, after multiple attempts and bench-checks using the clear tubes, I determined that the correct measurement in order to get the desired fuel level (3-4mm below the gasket seam) was about 17.5mm.  In my case this is measured at the approximate center point of the float, because at this setting the surface of the float is actually on an upward angle from the pivot. 

I'm actually quite annoyed with myself that I didn't go to the clear-tube test sooner.  Should have reasoned that it was unlikely that the aftermarket floats I got would be exact replicas of the Honda OEMs, both in terms of size, and -- probably as importantly -- buoyancy.  Anyway a smart man learns from his own mistakes, but a wise man learns from others'.  Be wise!

[stocky]

i also share your frustrations with the pd41a carb. granted it was my first experience working and rebuilding carbs, so it was a bit of a steep learning curve. luckily there were several detailed tutorials and videos that helped out a lot.

that being said, the float adjustment was also one of the more frustrating parts of the rebuild, the factory measurements just didn't work out for me either as others have also complained about. long story short it also took a lot of effort to setup correctly with the clear tube method. ultimately im happy i went through with the full rebuild, learned a lot and made sure it was dialed in, so i wouldnt have to deal with more headaches down the line.

rebuilding my 77cb750k i learned that OEM makes a huge difference on this bike. lot of the replica stuff is junk and just doesn't perform properly.

[MJL]

I think I'm going to do the clear tube test on my 750. as far as I know the float heights are set correct, but the bike runs rich and gets 60-70 miles per tank.

[willbird]

I think I'm going to do the clear tube test on my 750. as far as I know the float heights are set correct, but the bike runs rich and gets 60-70 miles per tank.

My K2 got 50 MPG using an action fours header and the stock air box. Like most users I used the trip odometer as a fuel gauge. Memory tells me that I would buy gas at 100 miles on the trip. I know most hate E10 gasoline but one attribute of 70's and 80's E0 pump gas was that many fuel tanks had some water in them. In a 70's Honda flipping to reserve was like pulling the pin out of a grenade a lot of times. Had we been more on the ball (some were probably) we would have taken the sediment bowl off once a month and flipped to reserve to drain any "stuff" out of the lower part of the tank maybe.

[Don R]

 My 76F had drilled jets and the PO had "adjusted the floats" I relied on a local shop back then. He had an F in the shed and borrowed the jets from it to test mine. I got lousy range too, I'm sure it didn't do the cylinder walls any favors.

[Nurse Julie]

That's similar with any aftermarket parts, floats in this case. You can only reliably use factory settings if you have OEM factory parts fitted. Anything other than OEM is hit and miss, as you've found out.

[MauiK3]

Clear tube tells the story.

[PeWe]

Even if floats are set correctly, the result can be floading carbs. Clear tube, adjustements and cleartube again until good is the way to go. See the float height in mm as a starting point.

I struggled with my Mikuni Smoothbores some years ago. Ordered clear tube tools and fixed them.
I had CB750 K6 carbs that also floaded. Made a cleartube check setup with extra bowls.
Later  adjusted 2 overfloading carbs when the overflow hoses leaked.
Adjusted a little until it stopped leak.

Maybe different fuels affect the levels?

[madmtnmotors]

The float needles make a difference too. The little spring inside many aftermarket float needles are weak and often require clear tube verification. Stock float needle springs (inside the needle, under the little "button") are firmer in the Honda branded float needles. I've had good results so far with K&L branded float needles too (made in Japan).

[HondaMan]

The float needles make a difference too. The little spring inside many aftermarket float needles are weak and often require clear tube verification. Stock float needle springs (inside the needle, under the little "button") are firmer in the Honda branded float needles. I've had good results so far with K&L branded float needles too (made in Japan).

I've had the opposite luck: all of the aftermarket float valves I can find (and the OEM versions, when I can find those) now have stronger springs in them than the old OEM ones, BUT - the K&L springs ARE softer than the ones in the Keyster kits (and I like those better, too). I had the OEM valves in my own K2 until 2018, then switched to aftermarket in it: on trips 'up the hill' into the mountains it would run the bowls low from opening too late and my speed would drop to 55-60 tops in 4th (wouldn't pull 5th) on the steep ones, riding 2-up. I put the OEM ones back in and the problems vanished. I've always run the staggered floats (657b carbs, first edition) with +1mm on one side compared to the other.

When I built the bike on the cover of my book (K3) I used the K&L float valves in it and set those at 25mm both [plastic] floats (instead of the spec 26mm) to get it to stay 'wet' when climbing the steep ones here.

A tip that works well in the 750's roundtop carbs: stagger the floats by 1mm: 25mm on the sidestand side of the carb and 26mm on the other. Regardless of the type of float valve used, this seems to make them happier overall, and drier when parked on the sidestand in the summertime with the petcock ON. Honda staggered the floats just this way in the carbs up to the 657a series (with the brass floats), purportedly to prevent overflows when parked.

[PeWe]

I had to adjust my Mikuni carbs on 1-2 side to not overflow when parked on sidestand.
No 2 was worst, fuel went over to 1 if I remember correctly. Off with bowls and adjusted the tangs a little.

[grcamna2]

The float needles make a difference too. The little spring inside many aftermarket float needles are weak and often require clear tube verification. Stock float needle springs (inside the needle, under the little "button") are firmer in the Honda branded float needles. I've had good results so far with K&L branded float needles too (made in Japan).

I've also had good results with K&L Made in Japan float needles;with some of them,the K&L float needle springs seemed stiffer than OEM and seemed to make a dimple on the float 'tang' sooner than OEM.
The OEM float needles seem the best imo;I always hunt online for OEM floats which work the best,imo.

[dave500]

one thing ive said before a few times and ill say it again,you can use all the precision in setting those floats,but in the real world once the bike is riding and rolling its a #$%*ing washing machine of fuel inside those bowls!picture sudden acceleration all the fuel sloshes rearwards,hard braking its all forwards,hard corners its fully gravity up n down left n right,up a steep long hill and down the same?picture a dirt bike smashing over rough ground etc?nothing seems to alter the motors running capacity?dont sweat the absolute precision,youll only lose sleeep!

[PeWe]

One little detail about overfloading carbs.
I got fuel leak when parked on sidestand when engine was hot. Fuel must have swelled in bowls.

Same when filling tank really full in the end of a long ride. Back home to garage 2km away. Hot engine.
Fuel swelled in tank and overfloaded thru vent hole.

[MauiK3]

I try never to really top off my tank for that reason. I leave a little head space.

[grcamna2]

one thing ive said before a few times and ill say it again,you can use all the precision in setting those floats,but in the real world once the bike is riding and rolling its a #$%*ing washing machine of fuel inside those bowls!picture sudden acceleration all the fuel sloshes rearwards,hard braking its all forwards,hard corners its fully gravity up n down left n right,up a steep long hill and down the same?picture a dirt bike smashing over rough ground etc?nothing seems to alter the motors running capacity?dont sweat the absolute precision,youll only lose sleeep!

The floats can always be reset easy enough if the setting is not desirable.
I find it's good when the pilot screw settings are set to respond well:optimum.
I've carefully smoothed-off the 'dimple'/indent on float 'tangs' to give used,known good floats more miles use.

[Deltarider]

one thing ive said before a few times and ill say it again,you can use all the precision in setting those floats,but in the real world once the bike is riding and rolling its a #$%*ing washing machine of fuel inside those bowls!picture sudden acceleration all the fuel sloshes rearwards,hard braking its all forwards,hard corners its fully gravity up n down left n right,up a steep long hill and down the same?picture a dirt bike smashing over rough ground etc?nothing seems to alter the motors running capacity?dont sweat the absolute precision,youll only lose sleeep!

What you seem to have forgotten all the times you said this ^, is that the main jet's entrance is right in the middle of the float chamber, so it doesn't matter how hard you accelerate or decelerate or how steep the curve is, the MJ will always see the right level of fuel.

[dave500]

i dont forget anything except my wifes birthday and anniversaries,a shaken up carb has the floats all over the place,the fuel level is all over the place,so long as your in the ball park itll be ok,dont sweat the fine precision,youll only lose sleep.

[HondaMan]

  the K&L float needle springs seemed stiffer than OEM and seemed to make a dimple on the float 'tang' sooner than OEM.

Yep, they do that, too. Some of the K&L valves I've used had squared-off pins in them instead of rounded ones, and they marked the float tang immediately. That was quite a while ago, though...lately they have been nicely rounded.

[grcamna2]

  the K&L float needle springs seemed stiffer than OEM and seemed to make a dimple on the float 'tang' sooner than OEM.

Yep, they do that, too. Some of the K&L valves I've used had squared-off pins in them instead of rounded ones, and they marked the float tang immediately. That was quite a while ago, though...lately they have been nicely rounded.

Mark,are you saying they've improved the Made in Japan float valves compared to the ones from 20 years ago?
I hope I can find better contoured/quality float(spring loaded pins)valves Made in Japan that will not 'dimple' the float 'tangs' when compared with  OEM ?
 The question I'm asking is: can I order a certain 'New stock' (rather than NOS) K&L part#/package description float valve set that will be gentle to the floats w/o causing a 'dimple' as described above??