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Discussion Starter #1
hi all
Need a little help from someone wiser than myself.
I got an old CB750, 4 cylinder, 4 carbs.
THe bike sat around for a long time.

I got it running, but every time the 3rd carb from the left overfills and starts dripping from the drain hole.

Just to eliminate the obvious, I'm quite sure it's not the float or the needle, as they got swapped around just to check if that was it.

Any suggestions what could be causing it?
 

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Twin A
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did you clean out the needle's seat?
 
K

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The needles could still not be seating properly..maybe your float has a hole in it.. cant be much other then that
 

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The seats would be my guess too since you tried swapping floats and needles around.

And check for a gasket or O ring on the float needle seats as well. If that is missing, damaged or mashed flat with age it can leak as well.

If you clean the seats be sure it's not with something that is abrasive or harder than the brass. You wan't to clean and slightly polish it without in any way altering the shape.

The only thing that will make a carb overflow like that is if the float needle isn't seating properley for whatever reason.
 

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this is my...boomstick!
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Did you check to see if the float height was actually right when they were switched around?

If it is leaking *from the drain hole* on the bottom of the float bowl, it is likely the drain screw is not sealing up.

There isn't much you can do about that, as it is a machined passage in the float bowl. you might try a new drain screw but, usually the body itself is worn, since it's softer than the screw.

Red Green may help here. Disconnect the fuel, dry 'er up and squeeze a blob of the toughest silicone you can find in there. It won't hold forever, as silicone dissolves in contact with fuel but, it'll slow it down enough for the bike to be rideable. If you don't mind draining the carb by removing the bowl, you can cement it with epoxy steel or some sort of permanent glue. Old heaps like this are notorious for this kind of thing, as you might expect.

CR
 

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Discussion Starter #6
CrotchetyRocket said:
Did you check to see if the float height was actually right when they were switched around?

If it is leaking *from the drain hole* on the bottom of the float bowl, it is likely the drain screw is not sealing up.

There isn't much you can do about that, as it is a machined passage in the float bowl. you might try a new drain screw but, usually the body itself is worn, since it's softer than the screw.

Red Green may help here. Disconnect the fuel, dry 'er up and squeeze a blob of the toughest silicone you can find in there. It won't hold forever, as silicone dissolves in contact with fuel but, it'll slow it down enough for the bike to be rideable. If you don't mind draining the carb by removing the bowl, you can cement it with epoxy steel or some sort of permanent glue. Old heaps like this are notorious for this kind of thing, as you might expect.

CR
I don't think it would be the screw, because it doesn't leak from the get-go
it starts leaking after the bike runs for 15-20 seconds.
as if the float needle doesn't stop the fuel flow when it's supposed to.
I think the guys above may be right..

I'll take it apart again, make sure everything is seating properly, perhaps there IS an o-ring missing or damaged...I'll have to check carefully.

I'm not sure sealing the float bowl or the drain hole is the best idea..I mean, it won't solve the problem, it'll just push it off to the side for a bit.
Besides, fuel is not leaking from the bowl gaskets as far as I can tell.I put brand new gaskets on there, since the old ones *magically* stretched after I took the bowls off lol
so afterwards they didn't even fit properly, as they tend to do when they're old :)

If I figure it out, I'll post on here just in case someone in the future has the same problem.
 

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I'm confused.

Are you sure the fuel is leaking out of the float bowl drain screw?

Are you sure the fuel is not coming out of the carb vent?

If its just the drain screw, try and seal the threads with something. Their are sealants that will not be affected by gasoline. Omc M adhesive is one such product.

If gas is overflowing the carb and coming out the vent then it is definately a needle/float/seat problem. The float may be sticking and not closing off the fuel flow via needle/seat. Or the needle and/or seat could be damaged. Or just a sticky needle in seat. You have to make sure it is cleaned properly or replace.
 

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this is my...boomstick!
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Ditto. You say, "drain hole" in your initial post and that is what I assume you mean. The drain is in the bottom of the float bowl and is used to, no surprise, drain the carb. If you mean the vent or some other orifice, my advice won't help you.

If it's overflowing from the vent, it's the fact the float height is set too high or the float needle and seat aren't meshing up and sealing off the flow. It is VERY common for these seats to corrode given the age of your bike. Sometimes they are replaceable, sometimes not. Clean 'er up, replace the needle if you can and hope for the best.

To avoid confusion, the other "needle and seat" often referred to is the needle connected to the diaphragm, sticking into the emulsion tube. This, and the slide, allows the carb to adjust its air/fuel flow for light/median throttle conditions. A worn needle and seat here won't cause fuel leaks, it'll cause lousy running, particularly in the midrange.

The *float* needle and seat are the leakers you are looking at fixing.

CR
 

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Depending on what carbs they are it's very common to have an overflow tube that drains out of the same outlet nipple as the drain screw uses. This overflow prevents the carb filling up so much that it fills the body so high that it floods into the neck and runs down the manifold and fills the cylinder.

There's two options for why it takes some running before it overflows. One is that the vibrations are affecting the float and needle and letting too much gas in. The other is that it's a vacuum operated petcock so no gas flows until the engine is running. And once it starts flowing then it overflows because the float needle seat is damaged or the sealing gasket or O ring on that seat is damaged.

Or the float level is not set right in that carb and in conjunction with a vacuum petcock it overflows one the gas is flowing in again.
 

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Discussion Starter #10
ok, update here.
I've taken the carbs off today again, took them back apart, clean them best I could, put them back together, and NOW...carb#1 overfills, #3 and the others are fine...
Just as before, it doesn't happen right away. only after it's been iddling for a minute or so
TeeTee, it is vacuum operated petcock.
I'm pretty sure the floats are fine, but I'll swap them around some more.
I havea second set of carbs for this thing, junky ones, so I can scavange the floats & such.
 

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It's NOT your floats unless there's gas sloshing around in the float when you take it apart. It's the float needles that have the little rubber tips on them or it's the brass seats that the rubber seats against. the floats generate very little force and all it takes is one tiny little bit of misfitting for the float needle to not be able to shut off the gas flow into the carb.

The fact that you have a vacuum operated petcock tells me that the problem is there all the time. Turn the petcock to PRI or PRIME and watch the gas flow out the overflows if I'm right.

I'll bet that you somehow swapped the rubber tipped float needles around and now the bad one in on the #1 carb.

Another posibility is that your float levels are not right. Did you check that? When you take apart the carbs it's important that the floats and needles all stay together for that one carb since the little metal tang that pushes the float needle closed is very critical to any small differences of the needle.

Take 'em apart again but this time if the float needle seats are not brightly brass colored then lightly polish the opening with a bit of paper towel and some toothpaste. Only do this enough that you see the edge go from dull brown to semi shiny. Don't overdo it. You only want to clean it by removing any buildup and not change its shape. The toothpaste is abrasive so only work it enough to get there and then stop. Clean well to remove the toothpaste residue and put back in.

If they still leak then try the rubber tipped float needles from the extra set.
 

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I have a 1980 KZ440 which will slowly drain one of the bowls when not in use. I have yet to take it apart but I did buy the rebuild kits.

However I have heard that some of the rebuild kit's needles and seats are not of the best quality and I am inclined to use some Brasso (better than toothpaste?) before I replace any of the metal parts. Comments?
 

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Discussion Starter #13 (Edited)
TeeTee said:
It's NOT your floats unless there's gas sloshing around in the float when you take it apart. It's the float needles that have the little rubber tips on them or it's the brass seats that the rubber seats against. the floats generate very little force and all it takes is one tiny little bit of misfitting for the float needle to not be able to shut off the gas flow into the carb.

The fact that you have a vacuum operated petcock tells me that the problem is there all the time. Turn the petcock to PRI or PRIME and watch the gas flow out the overflows if I'm right.

I'll bet that you somehow swapped the rubber tipped float needles around and now the bad one in on the #1 carb.

Another posibility is that your float levels are not right. Did you check that? When you take apart the carbs it's important that the floats and needles all stay together for that one carb since the little metal tang that pushes the float needle closed is very critical to any small differences of the needle.

Take 'em apart again but this time if the float needle seats are not brightly brass colored then lightly polish the opening with a bit of paper towel and some toothpaste. Only do this enough that you see the edge go from dull brown to semi shiny. Don't overdo it. You only want to clean it by removing any buildup and not change its shape. The toothpaste is abrasive so only work it enough to get there and then stop. Clean well to remove the toothpaste residue and put back in.

If they still leak then try the rubber tipped float needles from the extra set.


Ok Bruce, I'll take them apart, clean the seats and try it all over again.
I keep them together, always take them apart one at the time, so I'm pretty sure the needles or floats are not mixed up..
My next guess was that perhaps there is still some residue, some dirt making its way out of the system and getting on the needles, preventing random ones from seatting all the way?
Do you think that's possible?

Also, I have an extra vacuum shut-off. I'll throw that one on too just to see if it does anything.
thanks for all the help so far..
These sunny days are KILLING ME!
 

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Martin

You should pour out all the old gas, flush out the tank, and put in a proper inline gas filter, it's easy and cheap and needed, old bikes do not have proper inline gas filters!
 

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Discussion Starter #15
Chumly said:
Martin

You should pour out all the old gas, flush out the tank, and put in a proper inline gas filter, it's easy and cheap and needed, old bikes do not have proper inline gas filters!
Thanks for the tip.
I have an in-line, cause I've cut the fuel line and put one in :)
however, the bike has been sitting around and I'm thinking there is still some gunk crap and lord knows what making it's way out, I'm just hoping I can get away with running fresh gas+ carb cleaner through it and it'll eventually 'sort itself out'..
Most rubber parts seem ok, so I'm hoping for the best..
THis phantom goblin jumping from one carb to the next though is slowly starting to piss me off.. :)
 

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Jester666 said:
....These sunny days are KILLING ME!
I know what you mean. Just today while I was riding I was thinking about how annoyingly warm I was feelingwhile bending the ol' 'Zook dual sport through a nice set of S's.... :evilgrin

Good on you for keeping all the parts for each carb separated. It's one of those things that just seems like common sense but you'd be surprised at how uncommon common sense really is these days...

Brasso is a good alternative if the seats are brown and in any way seem crusty looking. But like I said, go easy here. You just want to clean and smoothen them with an absolute minimum of metal removall. If you get them out of round then they will leak for sure.

And wipe the rubber tips of the needles as well. There may be a small particle stuck in the rubber. It's a long shot but it doesn't hurt to try while it's apart. Oh, and I should mention that if the rubber tips of your float needles are in any way deformed then you just found your problem. They should be a perfect cone shape. Any grooves or other deformaty is going to cause problems.

Use a super clean jar to drain some gas into from the tank using the PRIME function. If you see particles in the bottom then you likely found your problem.
 

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Discussion Starter #17
ok, update here.
Carbs are LEAK FREE!!!!!
turned out, as suspected (thanks Tee Tee) that there was a tiny piece of some kind of debree lodged on the needle seat.
That taken care of, no more leaking :)

Also, I'm starting to get pretty good at whipping those goddamn carbs on and off the bike...
 

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Nice to hear that our presence here serves a purpose... :D
 

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Discussion Starter #19
hehe, your help is truly appreciated. If we ever meet in person, I owe you a beer.

That said, I still have to figure out why the RPMs go up once the bike worms up a bit..
it iddles more or less ok at first, then the RPMs clime up to 2500-3000 after a minute or two.

first and foremost suggestion seems to be that worn rubbers between the engine and carbs would cause that..
Any suggestions?
 

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Discussion Starter #20
ok, I had some time to work on it today, here's a fresh update. I'll try to iclude as much detail as I can.

The bike starts up with no problems, iddles quite well for a long time.
Usually what happens is I will rev it up for a moment, and THAT's when the RPM issues kick in.
I started it up, let it sit iddling at 1100 rpms, the revs varied slightly, but basically stayed around 1000-1200 range.
I was listening to it run for about 10 minutes like that, it was doing ok.

However, when I kicked the revs up to about 4-5000 range for a moment, they would not drop 2500-3000.

I sprayed carb cleaner as well as water all over the rubber boots between engine and carbs and I'm relatively satisfied there is no leaks, as the RPMs were not affected.

Whenever I plugged the rightmost or leftmost carb with my palm, the revs would drop right away.
If I did the same with the middle 2, the affect wasn't quite as dramatic though.

Now, after plugging the right one, as long as I didn't stall out the bike, the RPMs would hover around what they should be, then work their way back up to 25-3000.

What do you guys think?

there is one more diaphram I haven't changed on this thing.
It's the horizontal one with a long needle on it.If I remember correctly, it's on the second carb from the left.
I'm not 100% what it's called, but I think it controlls the fuel flow.
any and all help is most appreciated.

Martin
 
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