Topic: CB350F carb float level challenge

[Vfrman]

I’m rebuilding my carbs and at the point where I need to check the levels of the floats. I was happy to find they all measured 21 mm… But that was standing up. As I lay them down on the side to take pressure off the very lightly sprung needle valve, they are all high and I have to adjust every one of them. I guess my question is, is what I’m reading in a few places accurate in that you don’t want to measure them with them straight up and down (upside down) so the weight of the float is affecting the height by compressing that needle valve spring?
Taking them off & trial & error measuring with them layin on their sides is gonna take forever

[Oddjob]

Always done them with the carb sitting on its top. You measure to where the float tang is just touching the sprung pin on the float valve, weight doesn’t come into it.

Never had one leak doing it this way in 50 years.

[Vfrman]

Always done them with the carb sitting on its top. You measure to where the float tang is just touching the sprung pin on the float valve, weight doesn’t come into it.

Never had one leak doing it this way in 50 years.

So you are applying some lift on the float to keep it from compressing that spring, right?  You have to hold the float or even its light weight compresses the spring 2mm

[scottly]

I guess my question is, is what I’m reading in a few places accurate in that you don’t want to measure them with them straight up and down (upside down) so the weight of the float is affecting the height by compressing that needle valve spring?

I recommend setting the levels with the carbs upside down, with the weight of the float pressing down on the needle valve spring. It takes a certain amount of pressure on the needle to stop the fuel, and weak springs in the needles will drastically alter the fuel level in the float bowls. The "just touching" method can allow fuel to run out of the overflows if the springs are weak.

[Oddjob]

Very true Scottly, I do test mine by running fuel through them right side up and using the float level gauge set to the right setting to ensure the fuel is shutting off, just to be sure. The main problem with leakage is caused by aftermarket parts being used, sometimes the needle point on the needle valve isn't even straight, it's to one side meaning it would struggle to shut off the fuel. I always recommend using the original brass whenever possible. I also polish the float rod to make sure it doesn't stick in the float, seems to work for me.

[Vfrman]

I guess my question is, is what I’m reading in a few places accurate in that you don’t want to measure them with them straight up and down (upside down) so the weight of the float is affecting the height by compressing that needle valve spring?

I recommend setting the levels with the carbs upside down, with the weight of the float pressing down on the needle valve spring. It takes a certain amount of pressure on the needle to stop the fuel, and weak springs in the needles will drastically alter the fuel level in the float bowls. The "just touching" method can allow fuel to run out of the overflows if the springs are weak.

So if you set the level of the float compressing that spring, do you make an allowance to the recommended height of 21 mm?

[Oddjob]

Try setting it to 21mm without compressing the spring first, see if it seals at that. If it does, then good. If not then compensate or replace the float valve.

[Vfrman]

More background: the bike ran pretty good when I got it, and all the floats were right on at 21mm height (measured standing upside down, compressing spring) after I reassembled after ultrasonic cleaning.

Two reasons why I committed to complete tear down cleaning: it would start cutting out at 70-75mph: that was its top speed. And it started leaking after I rode it the last time when I forgot to turn the petcock off. Yeah I know its important to turn the fuel off—but still- something was wrong for it to leak out 1/4 tank overnight. 

Things I discovered after disassembly/inspection:
-missing 1 of 4 screws on #3 bowl, which seems to be the one that was leaking (the most)
-gasket goo sloppily applied around the bowl gasket o-rings was disintegrating and there were tiny specs of it in each bowl
-slides & needles had coating of varnish

Im reassembling using NurseJulie’s gasket kit (fitting great so far and bowl o-rings lay down & stay perfectly in the bowl covers’ recesses). Got some felt washers from oddjob (Ken) which I havent gotten to point of using yet but they look great.  Ordered the little metal dog bone synchronization links off eBay for 12 bucks that are a perfect replica. Otherwise, everything else that came out of the car is going back in, it was all in great shape after ultrasonic cleaning.

[carnivorous chicken]

It hasn't been mentioned yet, but IMHO the best way to verify float height is using a clear tube connected to the drain plug. There are how-to posts elsewhere on the forum.

[Nurse Julie]

I always stand my carb up horizontal (sorry, I meant vertical, not laying down horizontal) to set fliat heights. You can see what you're doing and the weight stays off the tang so it doesn't compress the needle, the tang just rests on the needle which give you the perfect measurement.

[Oddjob]

And yet you say the exact opposite on your how to set float height thread.??

1. Stand the carbs up fully vertical or lay the carbs down horizontal with the floats dangling down and not compressing the float valve needle. By standing them up you are allowing the floats to dangle with no downward pressure pushing the float valve needle closed. The float valve needle must be in the fully extended open position at all times throughout the process to get the final correct level of the float. I always have the carbs standing up vertical, it works well for me.

[Nurse Julie]

And yet you say the exact opposite on your how to set float height thread.??

1. Stand the carbs up fully vertical or lay the carbs down horizontal with the floats dangling down and not compressing the float valve needle. By standing them up you are allowing the floats to dangle with no downward pressure pushing the float valve needle closed. The float valve needle must be in the fully extended open position at all times throughout the process to get the final correct level of the float. I always have the carbs standing up vertical, it works well for me.

Indeed I did make a typo there Ken, I've now amended it.
And here's the photo 😁😁😁

[Deltarider]

I’m rebuilding my carbs and at the point where I need to check the levels of the floats. I was happy to find they all measured 21 mm… But that was standing up. As I lay them down on the side to take pressure off the very lightly sprung needle valve, they are all high and I have to adjust every one of them. [...]

To all. Just ask yourself: how likely is it a PO messed with all 4 floats?! You should have left them alone. Your issue lies elsewhere. When my friend parted from her CB350F, it had 70.000 km on the odometer. Never had to adjust the floats. Same with my CB500 (over 140.000km). Adjusting floats runs like an epidemic in this here forum and only in this forum. Maybe this Clymer manual you have in the US, suggests it's a regular maintenance thing. Well, it's not. How many more times do I have to repeat this?

[Vfrman]

I’m rebuilding my carbs and at the point where I need to check the levels of the floats. I was happy to find they all measured 21 mm… But that was standing up. As I lay them down on the side to take pressure off the very lightly sprung needle valve, they are all high and I have to adjust every one of them. [...]

To all. Just ask yourself: how likely is it a PO messed with all 4 floats?! You should have left them alone. Your issue lies elsewhere. When my friend parted from her CB350F, it had 70.000 km on the odometer. Never had to adjust the floats. Same with my CB500 (over 140.000km). Adjusting floats runs like an epidemic in this here forum and only in this forum. Maybe this Clymer manual you have in the US, suggests it's a regular maintenance thing. Well, it's not. How many more times do I have to repeat this?

Man I sure hope your right: its been madening trying to get one carb perfect, took float on & off 10X already at least. I just hope to get it back where it was & leave the others alone.

[Bodi]

I don't see the spring force as an issue. just for fun, calculate the pressure on the valve needle at the fuel pressure at the head pressure from the fuel level in the tank that's maybe 8 inches head. Answer: #$%* all pressure, acting on a one or two mm² needle. Does not need a powerful spring to balance that.
I set height not vertical or horizontal. Whatever angle has the float tang just touching the pin without compressing the spring.
Anyway, the 350 Four and 400F carbs are not friendly to the "clear tube" approach. The drain screws are drilled and cross drilled to just drain through the screws, not through the overflow spigots. It is far from trivial to make tiny screw in fittings to pass gasoline to a clear tube.
Except for removing and reinstalling the rack, setting height either to spec or adjusted a bit to deal with lean/rich zones using the method above has never been difficult for me.

[Redline it]

this is a true story (if not the best puntuations and grammar you don't like, then better if you didn't read this.)

10yrs ago, on the bench, a rack of 400f carbs from not running in 10 yrs, i was trying to get a float level stable without leaks. for 30 days straight, i'd get one carb pouring out fuel at the bowl joint or over flow coming out the venturies, so i'd take it apart the next day, and get it fixed from doing some adjustments, put it together and test. fuel would be pouring out one of bowls that wasn't gettin gas and there'd be one other bowl not getting gas. 30 days, i lost hope. and disassebled all the jets, floats, pins and bowls and put them in a pile, and mixed them up. put it back together, no leaks and the fuel levels all were right on. ran for 4 or 5 yrs, that's with a fine mesh garlic bag used for a tank filter crazy glued on. they do strange things.

[scottly]

I don't see the spring force as an issue. just for fun, calculate the pressure on the valve needle at the fuel pressure at the head pressure from the fuel level in the tank that's maybe 8 inches head. Answer: #$%* all pressure, acting on a one or two mm² needle. Does not need a powerful spring to balance that.

I went to buy a K7 750 40 years ago, and when the seller started it up, it didn't sound right to me. I did the spit sizzle test on the exhaust pipes, and the 2 and 3 cylinders were only warm, not hot, and then I looked back towards the carbs and saw gas dripping out of those carbs. This lead to a much lower purchase price.
I messed with them for a bit, and it seemed that while the measured setting looked ok, the carbs still overflowed. I thought that the plastic floats had lost their buoyancy somehow, and swapped a different set of carbs for the time being. Some years later, I had the rack sitting up-side down without the float bowls, and the 2 and 3 floats were markedly off from 1 and 4, from what I found to be weak springs in the needles.
The spring force is definitely an issue, even though it is very small. Granted, supporting the full weight of the floats may be more pressure than required to shut off the fuel, but it does still take some pressure, and the tang just touching provides no pressure.   

[Oddjob]

I've always looked on the springs as a safety device to prevent damage to the float valve surfaces, not only on the needle but on the seat surface as well. They are very well engineered to make them seal with little to no pressure and the floats bouncing up and down when the bikes in motion would mean them constantly slamming into each other without the springs there to absorb most of that force.

The official wording on setting the float height in the Honda WM is "Position the float so that the float arm BARELY touches the tip of the float valve" It doesn't matter if the carb is vertical or horizontal so long as you get that part right.

As for the don't touch them argument, parts wear, they are only brass and brass isn't exactly a hard metal, so eventually they'll go out of adjustment or just break. Plus the bikes are almost 50 years old, more in some cases, that an awful long time to assume everything is still ok. Doesn't hurt to check and may just be the thing that saves you sitting by the road side waiting for some help to come along. If it's not broke don't fix it doesn't really apply to motors, just because it doesn't look broke doesn't mean it isn't.

Just as an observation, I'm currently restoring a Honda VT1100C, it has carbs but it also has a fuel pump. The fuel pump had stopped working when I bought the bike so I decided to replace it and bought another, I thought I'd check and see why it had failed, tinkered with it a bit and was pleased when I connected it to a battery it was making some noise internally. I filled it full of water as I didn't fancy petrol and a spark as the electrical connection was being made, especially as I was holding it. When the connection was made it shot the water out with such force it shot 10 feet across the garage in a solid stream, the force really surprised me, far more than I expected TBH, that made me realise just how good those float valves are at shutting off fuel, if they can stop that kind of force a simple gravity feed should be no problem.

[carnivorous chicken]

Anyway, the 350 Four and 400F carbs are not friendly to the "clear tube" approach. The drain screws are drilled and cross drilled to just drain through the screws, not through the overflow spigots. It is far from trivial to make tiny screw in fittings to pass gasoline to a clear tube.
Except for removing and reinstalling the rack, setting height either to spec or adjusted a bit to deal with lean/rich zones using the method above has never been difficult for me.

I'm gonna disagree -- I've been using them on 350F and 400F carbs for years without issue and with accurate results. I just drilled out a spare drain screw and fitted it with a little copper tube. It's the easiest way to actually see fuel level in a sealed up float bowl, and it takes about 10 minutes to do all four carbs. Of course I measure float height manually as a first step but it's an easy way to verify.

[Rayzerman]

My carb float level story, if it helps..... first how to measure float height as doing it with no pressure on the needle spring has the floats on an angle, so I just took my best guess at a 24mm float height measured in the middle of the float.  All were uniform, I didn't have to adjust anything.... OEM seats and needles.  First go at clear tube showed some differences, only #3 was correct, and #1 wouldn't even flow fuel into the bowl.  Needle stuck.  Cleaning again didn't seem to help as it would still stick.  Another one showed a little resistance to going in and out freely although clean.

Second go, clear tube method, and #1 is pissing fluid from the overflow.... stuck open.

Note the OEM seats have a flat bottom and while visually OK, I dunno... seemed needle to housing clearance was a bit on the snug side, purely speculation on my part.  I pulled them all out and replaced them with ones that came with the 4-in-1 kits.  These have a chamfered bottom, which I decided was better.  Needles moved freely in/out, just seemed more "new".  Rechecked all float levels to my 24mm... all uniform, no tweaking required.  Did the clear tube method, and voila, all were uniform and correct.

Have not been able to test, waiting for new carb boots, one of mine is distorted and cracked.  I'm guessing somebody before me didn't know there are two different part numbers for the boots and got one in the wrong place....

[HondaMan]

Have not been able to test, waiting for new carb boots, one of mine is distorted and cracked.  I'm guessing somebody before me didn't know there are two different part numbers for the boots and got one in the wrong place....

Wow! And he got the carbs back on? Must be a pro wrestler, or something...? 

[Rayzerman]

Of course I didn't make note of which boot came off which cylinder..... only when I was cleaning them I noticed the outlet didn't line up with the inlet on this one boot.  I imagine it took quite a bit of leverage... mighta been on an outboard cylinder, dunno.  Looked like it was forced over a good 1/8" or so, and I guess with engine heat, it offset the thing... the cracks were deep in the bottom of the bore (where the carb seats in), I don't think they would leak.  No matter now, new set on the way.

Someone was asking what the thread size was on the drain screws..... M6 x 0.75.  I did make a set of clear tube adapters by drilling a 1/8" hole right through the drain screw (go slow, drill bit can catch in the brass and break).  I had some plastic "straws" from some Chinese applicator pads that were 1/8" OD and they were a snug fit in the hole.... also had some silicone tubing likely 2.5mm ID and it easily stretched onto the straws and will stay put.

Previous to this I had some 1/4" OD clear vinyl tubing that I took to the grinding wheel and put a taper on.. then just threaded/shoved them into the drain holes.  It worked but the new ones are much slicker...