Topic: CB750K float adjustment "never seen this before!"

[lhemrick]

I am in the process of adjusting the floats in my 77 CB750K. I'm using the clear tube off the bowl drain screw method to set them. I have all of them done except the bowl with the pump in it and it's being a pain. I've never seen this before but I can just take the tension off the three screws holding the bowl on and it fills up and stops just fine. I can sung them up to seal the bowl then check it again and it starts overflowing out of control. Loosen them back up and its fine. I've tried everything I know so what gives?

Thanks!

[martin99]

Is the bowl gasket seated properly? The only thing I can think of is that it is out of position and when you tighten the screws it is compressing out and into the float chamber, catching up on the float and causing it to not rise sufficiently to close the needle valve. I'm only guessing, not even sure if this would be possible.

IIWY, I would set the float heights with the carbs on the bench. At least then you will be able to have a clearer view of what is happening. I would imagine the clear tube is a good way of checking fuel levels and even float height adjustment with clip-on bowls (never done it myself), but if you are continually removing and tightening the bowl screws on PD carbs it will end in tears - it takes very little to strip the threads.

[lhemrick]

Thanks for the input! I don't believe the bowl gasket is the problem but I WILL double check. I have them off and on the bench and as for the screws yep your right I dare not tighten them to much, been down that road before too. This is the craziest thing I've ever seen!

Thanks!

[DaveBarbier]

I'm using the clear tube off the bowl drain screw method to set them.

Question off topic, do you have a scribe mark already on the bowls that you're using to set the float? Because, as far as I've been reading, the clear tube method is only for verification of the float height once you already have had it set and scribed. Not for setting the height alone.

But since you said you've never seen this before leads me to believe that you've done this kind of thing previously and know what you're doing. Just making sure!


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1978 Honda CB550K

[lucky]

I'm using the clear tube off the bowl drain screw method to set them.

Question off topic, do you have a scribe mark already on the bowls that you're using to set the float? Because, as far as I've been reading, the clear tube method is only for verification of the float height once you already have had it set and scribed. Not for setting the height alone.

But since you said you've never seen this before leads me to believe that you've done this kind of thing previously and know what you're doing. Just making sure!


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1978 Honda CB550K

Absolutely correct.
The clear tube method is only AFTER you have set them by measuring the float height.

[lhemrick]

Guys you have me here because that's how the manual says to set the float heights in the CB350 Twins I own. I set these first at the 13-15mm the manual told me to but they were overflowing. I decided to try that method of setting the fuel level just below the gasket level. Maybe I just need to start over. Does anyone know the correct float height and drop measurement. The reason I ask for the drop is I just took off all the bowls and the drop is not the same on any of these carbs. I guess it matters or there wouldn't be a tab to adjust.

[DaveBarbier]

I don't really know a lot of about this stuff but what do you mean by "drop"? How far the floats hang down with the bowls off?

I've read that the float height for the 350 vary on year but it looks like it's all around 19-21mm. Not sure where you got 13-15mm That higher number would also help bring the fuel level in the bowls down to minimize or eliminate over flowing.


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1978 Honda CB550K

[lhemrick]

Yes what I'm asking is how far the float should hang with the bowl off. Oddly enough I did a search and found a posting by yours truly about this same issue I had forgotten about. Bobby R (A really old timer) suggested then that I use the clear tube method to set the float height to just under the gasket on the bowl. Now again I ask why can't I stop this one carb from overflowing.

"You have coffin topped carbs and the correct setting is 14mm. If you want a quick and easy test, put a piece of clear tubing in the bottom drain on each carb, hold the tube next to the carb, open the drain screw and you can see the gas level in the float bowl. It should be just below the bowl gasket. Do that for each carb."

[DaveBarbier]

I don't see what it matters how low the floats hang when the bowls are off. I have never seen any spec for that. The floats will never hang that low inside the blows because there should always be fuel in there to keep it afloat. Maybe someone knows better than I do..?

Where is the fuel overflowing from? Drain plug? Overflow  tube?

It's possible that:
Your floats have holes and fuel is getting in keeping them from floating.
The brass tube that sticks up in the bowls are cracked letting fuel out.
Your float valve isn't sealing so fuel keeps pouring in the bowls.
Oooooor, your float height is actually too high and it needs to come down.

You can test pretty easily if the float valve is functioning...take the bowl off and open the petcock and very delicately raise the float with your finger and see if the fuel stops flowing.

And you can take off the float in question and submerge it in water for a bit to see if still floats.


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1978 Honda CB550K

[lhemrick]

There are two brass tubes just inside the throat when I release the fuel to the carb it fills up and begins running out of those and it shouldn't be. As for the floats leaking these coffin top carbs have the foam floats not the hollow brass ones.

[Airborne 82nd]

There is only a few things that can cause this and you have to work your way through each one until you fix it.
First you have the plastic floats they can not take in fuel so forget that.
 Now if you have the preformed bowl gasket they rarely get in the way but look at the bowl and you will see 2 or 3 little tits that the oring gasket slides under to keep it in the groove or channel on the bowl nice so you don't have to glue them down like some other bikes.
 Now the float adjustment it is 14.5mm on my K8 the 77 I don't know could be 12.5 so check your manual for that.
 The cracked tube has been mentioned by DaveBardier so check that Knowing where the leak presents its self will help is it at the bowl gasket or the drain.
I can just take the tension off the three screws holding the bowl on and it fills up and stops just fine.

This means there is an interference with the float and something.
I have seen a few things that cause this.
The float pin could have a burr on it or the float hinge or a dimple on the hinge where it makes contact with the float valve.
 Make sure the float is not twisted one side higher than the other.
 How far down the float drops does not matter.
 It is true that the clear tube fuel level is to verify that the float height is correct. But it is a great way to see the fuel level inside the bowl. If you study the outside of the bowl at the gasket area you will see a flange or lip it measures out to be close to 3mm deep. this is where I set mine and it works fine and when I check the float it comes out to be 14.5mm.
 Is the float in #2 upside down?
 In the #2 bowl there is a little oring for the accelerator pump this would show up as a leak at the bowl gasket area.
If I think of something else I will post back

[lhemrick]

Thanks Airborne! I'm going to play with it some more tomorrow.

[750K]

What is the model number of your pd's? It will be stamped into the side of the carb body, does your carb rack have an atmospheric vent hose? You will see one just in front of the fuel inlet nipple. I had the same issue with a set of PD41a's, I never got the root issue solved. I rebuilt everything and one carb would never hold a steady level, no matter how many tumes I cleaned them or set the float levels one would always overflow out the throat. I had to swap a set of 78 PD42b's in which have a different atmospheric vent system then the 77 41a's, the 77's I have seem to vent through the overflow tubes and not through the rubber hoses like the 78's I have on the bike now.

[lhemrick]

That's the only thing that makes sense to me. If it will fill up properly with the bowl just cracked loose but not when sealed it must be a vent issue. There is no vent in the upper part of the carb body. They are PD41A's. How are they supposed to vent?

[750K]

The 41a's I have do not have an external vent hose like the 42b's or pd's on my wife's cb650 for that matter, from what I can tell they vent through the overflow tube in the float bowl. Try setting the float level with a caliper and don't check it with the clear tube, see if it overflows or not without the tube attached. Possible that with the clear tube attached to the drain of the bowl it's not allowing the atmospheric pressure to equalize in the system. That's the only theory I could come up with. If you find that stops the overflow, you'll just have to trust the float level is correct with the caliper and not worry that you can't clear tube check the #2 body. Every other set of PDs I've looked at have a vent hose externally, my 41a's don't. Interesting that you have the same issue with the same set of pd's.

I completely cleaned my carbs and I mean clean... New oem gaskets & orings, new jets, new floats & rubber tipped float needles. Manually set floats to spec, double checked levels via clear tube. I manually checked my floats and needles shut off fuel flow with the bowls off, I never could figure out why it was overflowing. If I swapped floats and needles from carbs 1,3,4 the issues stayed with the #2 body. Oddly enough the issue stopped when I switched to the 42b bodies, which have an external vent tube as part of the venting system. I cleaned the 42b bodies and put all of the 41a internals in them. No more overflow,

[750K]

Also check the metal tab on the float, the part you bend. If they are old you might see a dimple worn into it, file the dimple out. It can allow the pin of the float needle to catch and not let things seat completely, just a thought...

Pics of my carbs with and without vent nipples




Here's the 78 carbs

[lhemrick]

Oddly enough I have found in a manual from a Kawasaki I also own that you must use a tube that is a least 1/4" ID on the vent end or it will create a venturi effect and continue to flow. Flybox1 has sent me a video from youtube this morning about setting the float level in these so with all the info I have I'm going out there to START OVER from the beginning.

[harisuluv]

it's very very tiny and gets clogged easy.

[Airborne 82nd]

@ Harisuluv.
Is that on all four carbs or is that the only one and it feeds all four?
Thanks for clearing that up.

[harisuluv]

on all four. 

[lhemrick]

Yep its on all four! Never seen that before either and its no wonder damn thats a SMALL hole!

[harisuluv]

Yup, it is very small.  Easy to get clogged, that's why they moved to the rubber tube between carbs in 78.

[lhemrick]

I got the overflowing problem stopped! I don't know how? Started back at square one and cleaned everything again reset the floats according to the video link I was given and made sure those small little vent holes were open. NO OVERFLOWING!

But I back to trying to fix the problem I originally opened them up for. I was sure I float problem but there must be something else wrong. I'm running pod filters and an exhaust that has about half the baffle in it. When I first built the bike I went up several sizes on the jets and it ran decently but lately it has begun something I would really like to get fixed. It cranks and idles fine and as long has your under throttle it has no problems. The issue is when I try to hold a steady speed in the 40 to 60 mph range the bike what I call for a lack of a better word, hickupping. It's in and out of power. I don't know if its bogging or running out of fuel. I'm going to take it out and make it start doing it and kill the engine and pull the plugs to see what it is doing but I'm not sure that will even work if its in and out on the power. If anyone has a suggestion I would love to hear it.

Thanks!

[Vinhead1957]

Make sure your filter in the petcock is clean

[lhemrick]

Thanks but already have. I have thought that I might also cut the fuel off as it begins skipping and drain each bowl to see if any are low of fuel.

[flybox1]

You need to get a set of clean plugs, pack some tools, and go replicate the hi cupping you are experiencing.  Hold and keep the bike in this state for good deposits to form on the plugs. 2-3 minutes will suffice.  make a mental note of your throttle position.  1/2, 3/4, wide open...etc.  When you have the space, and it's safe, kill the bike and pull the  clutch at the same time.  Coast to the side, and pull all your plugs.  Take a picture of their tips, and post it here.

[lhemrick]

Thats the plan. Black-rich, white-lean, light brown tan-just right! But what I don't understand is if it's a jet problem why will it take off and go under a hard throttle but hold constant speed trying to just cruise?

[seanbarney41]

you gotta raise or lower your needles

[flybox1]

Yes...it sounds like a needle height issue, .but all of your jettting overlap so you can't just change it as it will effect mixture at other throttle positions, too.

You cant just assume your main jet is right, you need to do a plug chop to see, then, once jetting is  correct(ed), proceed with a plug chop for mid throttle....which will tell you lean or rich through the needle jet.
 a plug chop for up to 1/4 throttle is performed to get your pilot jet right.
Then a check for idle mixture....

Don't assume the jets in there are right just on your butt dyno alone...

[lhemrick]

Thanks what I was afraid of! These coffin top carbs are going to be a pain in the a$$ to make that change, aren't they?

[750K]

Not really, I have never had a problem accessing the slide and removing the needle with the carbs on the bike. Once you do it once its fairly simple, you just need patience.

[flybox1]

Not really, I have never had a problem accessing the slide and removing the needle with the carbs on the bike. Once you do it once its fairly simple, you just need patience.

+1
There are some small parts.  Just work on a towel so isht doesn't go flying or rolling into the abyss.
Chill, and have a beer while you're doing it.  No need to make it harder than it is. 
Do one at a time.

[750K]

I've always found that there is an order to which of the two screws you remove first on the arm that holds the needle down. It's spring loaded, so once you start loosening them you can't get the screw driver on one of the heads due to the plate springing upwards. The right size of Phillips or jis driver is important, the last thing you want to do is strip one of those screws. I had a #$%* of a time getting one out a PO had almost totally stripped.

[lhemrick]

Thanks Guys! I've in the shop playing with a spare set of carbs I have and I see it shouldn't be to difficult. If it will quit raining long enough for me to do some plug chops to see what it needs and I can get this 350 twin engine back together and out of the way maybe I can get this thing running right again.