Leaky carbs

[tevan]

I presume you did a clear tube float level check... Were they all at the right level?

[Kool_Biker]

So,
the existing needle valves are back. Floats are checked and reset as needed at 15.5 mm (Honda gauge) and the carbs put together, on the bench for a wet test.
Short clear tubes are connected to each drain pipe and the drain screws are shut tight.
Fuel, fed by a little aux tank is connected and all carbs fill up nicely, as previously, and hold fuel well. No leaking whatsoever for 15 minutes or so.

Then I take my heat gun to try & simulate their behaviour at high temperature. To have some form of control, I heat up 1,2 & 3 only, leaving 4,5 & 6 in room temperature (sort of, the are too close to each other).

AND SURE ENOUGH, with a body temp ~ 45'C 1,2 and 3 discharge a bit of fuel, as shown below, in more or less equal quantities and then stop. 4, 5 & 6 remain bone dry.

P1090793.jpg

Come on chaps, carbs experts, all, what is it I am missing here???

The obvious answer would be that my floats are way higher than where they should be, but I use the 79 Honda manual set them at 15.5 mm and have checked them 100 times.

Could it be that heated fuel stored somewhere within the carb bodies is allowed to seep back to ALL float chambers, flooding them? I could conceivably accept this if fuel from say the accelerator pump railing somehow returned back down through a leaky accelerator pump diaphragm, but this would only flood float chamber no 3 ???

And before we go back to leaky needle valves, IT CAN'T HAPPEN TO ALL OF THEM, ALL AT THE SAME TIME.

Very odd but we will get to the bottom of it.
Cheers, Aris

[Kool_Biker]

Unless of course my floats are the wrong type?

P1060529.jpg

[Jeff Bennetts]

Start swapping parts, put a bowl from a non-leaking carb on a leaking carb, same thing with the needles and re-test and re-test and re-test.

Also,

Thinking about what Nils said, when you park a hot bike some fuel will evaporate from the bowls, my guess is most of that evaporated fuel leaves through the vent tees, check the lines and the tees, make sure they aren't kinked or blocked.

[bobber01]

Just a couple of thoughts, tossing out for some discussion.
1-Would it help to bench fill the carbs and then drain each carb into a beaker, measuring the actual cc's in each bowl? I don't know how much each carb bowl should hold but am sure someone must know what a properly float adjusted bowl would contain.
2-Remove/hold open the main metering needles and then warm up the carbs. Instead of dumping fuel thru the overflows the excess fuel should come out the tubes(I would think), maybe help determine a particular carb having an issue.

[bobber01]

"Unfortunately, the new needle valves have different internal spring ratings, you can even tell by pressing with one's hand, an inconsistency which I know will create more problems than I am trying to solve in the first place."

Curious about this observation you made. The differences are amongst the 6 new valves themselves or with the existing valves?

[Jeff Bennetts]

"Unfortunately, the new needle valves have different internal spring ratings, you can even tell by pressing with one's hand, an inconsistency which I know will create more problems than I am trying to solve in the first place."

Curious about this observation you made. The differences are amongst the 6 new valves themselves or with the existing valves?

To be honest with you I think Aris is over thinking that point a bit, as long as the plungers move smoothly and aren't collapsed it shouldn't affect anything, you set the float level with the float tab just touching the plunger, not compressing it and the pressure created by the raising volume of the fuel should move the plungers regardless of slight differences in their is resistance .

[Kool_Biker]

To be honest with you I think Aris is over thinking that point a bit, as long as the plungers move smoothly and aren't collapsed it shouldn't affect anything, you set the float level with the float tab just touching the plunger, not compressing it and the pressure created by the raising volume of the fuel should move the plungers regardless of slight differences in their is resistance .

Jeff, I would not argue this point with you - clearly you are infinitely more experienced

In any case, the old needle valves are back in, and the carbs were once more wet tested on the bench.
With the fuel tap left 'on' all night, thee no leaks from the overflow tubes (or any where else for that matter) what so ever, despite leaving them alone, tapping them, shaking them, what ever. They remained 100% fuel tight.

But put some heat on them, and unburnt fuel (not a lot) flows out of ALL overflow tubes of the heated carbs

[Jeff Bennetts]

Take the leaking bowls off, heat them up and try the compressed air test on them, maybe the stand pipes are not sealed in the bowls or are cracked, fill the bowls with soapy water durning the test and look for bubbles.

[Kool_Biker]

ALL bowls 'leak' Jeff when heated. I think we are missing here something gross.

I wonder:
1. How can I check that my floats are the correct type?
2. What is the exact level fuel should reach wrt the carb body when I open the overflow screw with the transparent test tubes connected?

I have a feeling that my floats are set higher than they should be

BTW, I've done the compressed air test few days back, cold and w/o soapy water, just air and used my ears to detect and leak - there was none.

Thanks, Aris

[bobber01]

Just to end any question about the float bowl stand tubes being possibly cracked would you consider removing the bowls and filling with hot water? If there is an issue there the water should expand any cracks and weep out the drains.
Is it possible the stand tubes are too low, pressed in further than should be thus lower their height relative to the fuel level?
After all you've described and tested I'm tending to agree your floats are set too high. Does anyone know how many cc's of fuel a properly set up carb should hold?
A comment on the vent tubes. If the vent tubes were partially blocked, allowing a vacuum lock when heated, wouldn't this cause the bike to run somewhat lean?

[Kool_Biker]

Just to end any question about the float bowl stand tubes being possibly cracked would you consider removing the bowls and filling with hot water? If there is an issue there the water should expand any cracks and weep out the drains.
A comment on the vent tubes. If the vent tubes were partially blocked, allowing a vacuum lock when heated, wouldn't this cause the bike to run somewhat lean?

Re your points above, of course I would Bobber and my vent tubes (Nils brought this up) I have checked, blowing and sucking into them (great fun) and they are not blocked at all.

On the recommendation of a good friend who PM's me a short while ago, I will now take some careful measurements (and pics) of where the fuel level rests inside the fuel bowl, by way of my test overflow tubes, post the data and see where we go from there

Cheers, Aris

[Jeff Bennetts]

The hot, cold thing is what's got me puzzled. The logical thought would be the floats are set too high.

[Kool_Biker]

Ok, we are getting somewhere ...

Check (referenced to the no 6 carb bowl gasket line), the fuel levels in the bowls of 4, 5 & 6.

P1090807.jpg

Now I KNOW my brass overflow pipes in the bowls do NOT clear the bowl gasket surface. Indeed if memory serves me right, they end up a bit lower than the gasket surface. No wonder I get the overflows ...
I have more than 1000 pics of my Z, and the only picture of a bowl I could find showing the overflow pipe is this - no much use of course ...

P1050872.jpg

I will conclude my testing and then remove a couple of bowls to take accurate readings of the height of the brass tubes.

But guys, the writing's on the wall. One of three:

1. My floats are of a wrong type
2. I set them wrongly
3. The brass pipes in the bowls are too short.

[oldschoolcarbs]

I've seen this rear its head once or twice and, to give credit where it's due, I recall that Mike Nixon once made a very intuitive observation. What you may have there is, in essence, a distillery. Warm fuel, and more specifically the additives therein that have lower vapor pressures, will evaporate and condense in the cooler drain tubes.

My guess is that if you tried some aircraft gas you'd see the problem disappear; and also, that you are less likely to see this phenomenon in warm weather.

I've not seen it with my own eyes because I live in a year-round temperate climate, but that's all the more reason to attribute it to something other than float height or needle integrity (which are of course vitally important for their own reasons) as those variables do not change relative to climate. Try capturing the fluid in a glass vial for a closer inspection.

Hope this helps.

Jack