Topic: 1973 CB350F Float Height

[GreenHorn]

I recently acquired two CB350F's, one in nice condition (ran 4 years ago when put into storage, according to PO) and one a parts donor.  Opened the carbs up on nicer bike and all jets were clear and appeared to be new.  Float needles and seat also appeared to be new- no ring around the conical portion of the float needle.  The PO said the carbs were rebuilt shortly before storage 4 years ago, and I believe him (family friend).

My Haynes manual and the Honda Service Manual say float height is 21mm.  I checked the float height and it was measuring around 24mm.  Floats are foam.  My method of checking float height is as follows: turn carb rack sideways so as to allow the float to hinge on pin freely, tilt slightly until tang of float TOUCHES the spring loaded tip of the float needle, but DOES NOT depress tip, measure highest point of float relative to the flat flange that is the mating surface for the bowl.  I am using digital micrometers, so there is no estimating or guessing.

Hooked the fuel up and it is now all over floor- running out the over flow tubes and in the carb bodies.  The situation is consistent in all four carbs.  Checked the float height with a float from the donor bike and this is also measuring 24mm by the above method of checking.  They are also foam.  Both the float from the nicer bike and a float from the donor seem to float at the same level in a container of gas, so I assume they are not "water logged".

Now, my question: what gives?  Is the 21mm spec for brass floats?  Is my method of measuring float height incorrect?  This is the same method I used to do my 750's carbs and that bike runs wonderfully.  I even checked to make sure that the needles were the same out of the donor bike's carbs and the nicer bike.  I am at a loss.

In the mean time, I'm going to bump the height up to 24mm and see if that does it. 

[HondanutRider]

I believe if you did a search of these forums for "CB350F Float Height" you'd find a few topics already posted for this question.  Your float heights seem to be within the correct range, as you measured them.  I would suspect that your float valves are not sealing initially when dry and when the gas flow is initially opened.  That, or your floats are sticking on their hinges.  As several have reported, this leakage often happens under these conditions, and several light taps on the bowls will often correct the problem.  Otherwise, carefully clean or replace the valves.  Both the seats and the pin valve come as a set.

[PhilipB]

Hey Greenhorn, 21mm is what the book says but I put mine at 23mm just to be safe. I had to replace my overflow tubes. I set the overflow tubes as close as possible to factory but it's hard to tell. All 4 of my overflow tubes had very, very small cracks that you couldn't see unless you were looking for them and right at them. I also used the same method and a caliper to set the float height. I also rigged my gas tank up on the bench so I could test the carbs ability to hold gas and let them drain into a 5 gallon bucket. (I figured that out after about the 6th time of taking them off/on the bike) back when I was chasing down that same gas overflow problem during my restoration. Check for cracks and just fill the bowl with water, alcohol or gas before you put it together to make sure they hold fluid. Then check the needle valve springs and for any debris that might prevent the needle from stopping the flow of fuel. It's a really small hole that the needle goes into the valve. also check the float ear travel across the needle for smothness of movement. My bet is cracked overflow tubes. Keep us informed of your progress.

[GreenHorn]

Since the donor bike had the floats set around 24mm per the method described above, I was kind of curious.  I installed one of the floats from the parts bike (with the floats that were around 24mm in height), gave the float a flick, and let it come to a rest on the spring loaded tip.  In this situation, the full weight of the float was partly depressing the spring loaded tip.  I measured the highest point on the float... around 21mm using this method.  Coincidence?

I don't know, but I set all the floats to 24mm (21mm when the the full weight of the float was on the spring loaded tip), did a check to see if the float needles were sealing (they were), and buttoned the carbs up.  No more overflow.  I am beginning to wonder if the procedure for these carbs is different than that of a 750...

Can anyone weigh in on this?  Can anyone provide an written procedure for checking float height other than "float height should be set at 21mm."

[wdhewson]

While it is nice that Honda has tight specifications on float height, the fuel level in the bowl is often not too upsetting to the mixture until you get to extremes. 

The operational evidence for this statement is the act of running out of gas, where things either go dead quickly or you get into a lean surge, then onto the reserve petcock lever.  Probably the fuel level has changed a lot.

Also, consider that many less sophisticated engines like, Briggs & Stratton, the fuel tank is the carb bowl, and operates pretty well from full to empty.  Just my thoughts.  Other opinions are most welcome.  Don

[fmctm1sw]

There are a number of thoughts I see on this lately.  Mine are set to factory spec while others lately seem to be moving up to 24 or so.  I would say it is imperative to make sure that the floats operate smoothly.  Just push up on them when you have the bowls off.  I used a small brass brush on my rotary tool (and how did I ever live without that rotary tool?) on the needles including the sides.  The needles can snag any number of ways when the push into the float valve.  I've also seen the needles snag on the tab that you use to set the height.  I wish I took a picture of it.  Hope that helps, sorry I've been drinking...

[GreenHorn]

Between my personal experiences and what I have read on this forum, I believe anything is possible when it comes to problems.  However, I have a hard time believing that ALL FOUR float needles got hung up in the seats or that ALL FOUR floats were stuck.  When I used the 21mm height setting, all four overflow tubes were running like garden hoses.

I got the bike running, timing adjusted, and carbs sync'd this afternoon.  Between all that, and a ride down the road, I never once saw the carbs leak.  I'm leaving them at 24mm.

I'm chasing new problems now: I've got a little hesitation when I goose the throttle and there seems to be ever so slight of a power surge (speed up, slow down, speed up, slow down) when I have a steady load applied to the motor.

Still would be interested if anyone can tell me about the float level.

[Kevin400F]

The Honda spec is 21mm. 

Is the top of your overflow tubes approximately 4mm higher than the top surface of the float bowl?  Did you try filling the bowls with fuel, off the bike, to verify the integrity of the overflow tubes? 

Do you have any pictures?  If you have OEM floats, the overflow tubes are intact and the float valves are good, there's no way it should leak at a 21mm setting.

OEM components, properly set at a 21mm float height, ought to look like these pics:

[GreenHorn]

Thanks Keven- the pics are great and you bring up some interesting points.  I filled a bowl with fuel when it was off the bike and no leaks.  I did not take note off the overflow tube relative to the bowl flange (but I do believe the overflow tube is taller than the flange).  As for OEM parts, I assume that more than one carb rebuild kit has been used on the bikes in the past 35+ years.  I did question the float- I thought brass floats were OEM, but I don't know for sure.  I'm sure I'll be corrected.

I just find it very odd that two 350F's had the same incorrect (according to the manual) float height.  Furthermore, I've found some other posts tonight where folks said that the float height should be 24mm on a 350F.  I'm not saying 21mm is wrong, I just want to know what is going on.

Kevin- are you checking the float height with same method as I described in my first post?  I thought I was going nuts when the carbs started to leak and had another Honda guy check the height, he came up with 21mm also.

l

[HondanutRider]

I did question the float- I thought brass floats were OEM, but I don't know for sure. 

The OEM floats originally on these models were foam.  I have two sets, and both are this construction.

[Bill/BentON Racing]

These came with foam floats.Early model 70's bikes,69 750 etc. came with brass floats.Honda saw that foam was better and less expensive to produce.Until you learned the pin hole leaks in them and overflow tubes could drive you nuts! ,Bill.......21mm should work fine.Kevin spot on.