Topic: Can I use water (or other ideas) to test my carbs before remounting to the bike?

[nzdreamer55]

I have a 1978 CB750F and have just spent about 10 hours pulling my carbs after having them leak fuel through the overflow tubes.  I cleaned them with carb cleaner, set the floats (I think I did it correctly) and when I put them back on.  Started her up and she ran with out any leaking.  I then took her on the road (the first time in about 8 years).  Stopped at my local motorcycle shop and noted that there was a lot of fuel running out the tubes (I think just 1).

I am going to pull them again and check everything again and wanted to know if there is a way to test them off the bike for leaking so I don't have to put them back on only to find out they leak.

Also could I use water instead of gas so I don't have to keep losing brain cells doing this?  Could I use another type of fluid (not sure what I would use)

Lastly, what size if tube is the overflow line and has anyone swapped it was a clear hose?  What did you use and where did you get it?

Thanks again
-S

[Gordon]

I don't think water would be a good replacement for gasoline.  Not only would you have to flush it all out with gasoline (and/or alcohol) before you could use the carbs again, but it has a much higher surface tension than gas, so it won't leak out of small openings as readily as gas will.   

[Stev-o]

+1 I would never put water in there. If you "think" you did it right, set the height again so your sure its correct. 
A float can stick after putting the bowl on and sometimes be freed by a gentle tap with a screwdriver handle.

[78 k550]

replace the tube or pull it off and make sure it isn't clogged. I had 2 do that to me before.

Paul

[oldschoolcarbs]

Assuming that you've got your float heights correct it's likely that you merely got a little bit of crud down the fuel line and it got hung up at one of the float needles where it rests against the seat.

Pull the needles out and flush gas through them, then attach an inline filter and NEW fuel line to make sure that nothing new gets introduced once you know they're sealing.

BTW, you don't want water in your accelerator pump circuit so best to use gas.

We have a remote fuel source with a pump so we test with that but you can also hang a bottle from the ceiling to get some oomph behind the gas to make sure that you get a good reading.

That being said I've also used a funnel and chunks of wood to prop them upright to fill.

You definitely want an upright test so you can operate the throttle to ensure that the accel pump is working.

OSC
http://www.oldschoolcarbs.com

[KJ790]

I'm assuming you have the stock carbs on that bike (the ones with an accelerator pump)? Is it coming out of the line that runs out of the bottom of the float bowl? If so, make sure that the drain screw in the bottom of the float bowl is in all the way and that it is not leaking at all through that.

[nzdreamer55]

Thanks everyone for the help.

KJ790 - So the black line (one at the bottom of each bowl) is where the fuel is leaking from.  This I think is where the fuel runs when there is too much (overfill problem) within the bowl.  The drain screws are not leaking that I can tell.  The leaking seems to happen when I come to a stop.  I guess it could be leaking all the time while I was riding, but I suspect that it wasn't as when I started it, there wasn't any leaking, but I'll recheck the drain screws.  You saw that my carbs are the ones that came with the '77-78 CB750's?  They are stock.

oldschoolcarbs - So on these carbs it lists the float height in a haynes manual as 14.5mm.  Do you put the gauge at the middle of the float, toward the tip of the float or back by the pivot point?  Do you turn the bank of carbs on their side (i.e. starting from the normal position on the bike if you could run a rod from the head light between carb #2 and #3 to the tail light and then turn the bank of carbs on this line 90 degrees in their plane so now instead of the plane being at 9 to 3 o'clock, it is now 12 - 6 o' clock. C=carburetor body

From this                C1---C2-+-C3---C4
to

This

C1
|
|
|
C2
|
+
|
C3
|
|
|
C4

Or do you set the floats with the carbs upside down?

Steve-o - i'll try the tapping trick before pulling it all apart.  Thanks

Thanks again everyone

-S

[KJ790]

Here's what I would do. Put a little gas in just the float bowl to make sure that the drain screw is not leaking. If that is good then hook your carb up to a gas supply and hold the float up to see if the needle and seat are sealing properly. If gas leaks while you are holding the float up then it is an issue with your needle and seat.

[nzdreamer55]

Thanks KJ790.  I'll give that a try too 

[Stev-o]

Here's what I would do. Put a little gas in just the float bowl to make sure that the drain screw is not leaking....

....and or your float bowl is not leaking. One of mine had a crack

[KRONUS0100]

i use rubbing alcohol.  99cents bottle at the dollar store

[matt mattison]

Thanks everyone for the help.

KJ790 - So the black line (one at the bottom of each bowl) is where the fuel is leaking from.  This I think is where the fuel runs when there is too much (overfill problem) within the bowl.  The drain screws are not leaking that I can tell.  The leaking seems to happen when I come to a stop.  I guess it could be leaking all the time while I was riding, but I suspect that it wasn't as when I started it, there wasn't any leaking, but I'll recheck the drain screws.  You saw that my carbs are the ones that came with the '77-78 CB750's?  They are stock.

oldschoolcarbs - So on these carbs it lists the float height in a haynes manual as 14.5mm.  Do you put the gauge at the middle of the float, toward the tip of the float or back by the pivot point?  Do you turn the bank of carbs on their side (i.e. starting from the normal position on the bike if you could run a rod from the head light between carb #2 and #3 to the tail light and then turn the bank of carbs on this line 90 degrees in their plane so now instead of the plane being at 9 to 3 o'clock, it is now 12 - 6 o' clock. C=carburetor body

From this                C1---C2-+-C3---C4
to

This

C1
|
|
|
C2
|
+
|
C3
|
|
|
C4

Or do you set the floats with the carbs upside down?

Steve-o - i'll try the tapping trick before pulling it all apart.  Thanks

Thanks again everyone

-S

You really would want to hold the carbs on a 45 degree angle, not 90 degrees. The needles will stick and not fall to the float tang keeping them at a 90. You can even hang a carb (if separated) or the whole rack in their natural up/down position to check the height. You almost need to be a monkey to do this , but you will get it. Just make sure your needles aren't hanging up whilst checking the height. And you really want to use the float tool against the gasket base and check the tallest point of the float circle. If your pretty confident you did the float adjustment right, and don't want to pull the rack, do the clear tube fuel height check method. It's covered how to do it in several threads here. All you need is about 6" of clear fuel line, and the plastic ball point pen end to do it. Fit the fuel line onto the end of the pen end, and thread the plastic pen end into the carb drain hole. Hold the fuel line up 90 degrees, without kinking it and turn on the fuel. You will "see" where the actual fuel height is.

[Scott S]

 The local carb guy near me uses a brake bleeder type system that puts a vacuum on the carbs. This will test the O-rings between the carb bodies where the fuel flows as well as checking to see if the needle valves/seats are sealing.
 He applies a vacuum and watches the gauge. I'll see if I can get some pics of his set-up some time.

[Scott S]

 Here ya go...laying on the bench in this pic is one of the one-man speed bleeder type set-ups, for bleeding brakes by yourself. I've seen this used to apply a vacuum to the fuel system and check for leaks around the O-rings in the fuel T's, etc. and to make sure the needles/seats are working.

[Bailgang]

All you need is about 6" of clear fuel line, and the plastic ball point pen end to do it. Fit the fuel line onto the end of the pen end, and thread the plastic pen end into the carb drain hole. Hold the fuel line up 90 degrees, without kinking it and turn on the fuel. You will "see" where the actual fuel height is.

If the carbs on his 78 F are like the ones on my 77 F then you wont need a ball point pen end because the fuel bowl drain is different than earlier carbs. Rather than having a large brass screw that you have to completely remove to drain the bowls the later carbs had a small screw that you just loosen up and the fuel in the bowl drains from the overflow tube. What that means is that all you have to do is attach some clear line to the bottom of the overflow tube, hold the clear fuel line up 90 degrees and simply loosen up that small screw, not completely remove the screw, just loosen it up. Also when checking float height with the carbs off using a float height gauge, check it from both sides of the float to make sure the float itself isn't slightly twisted.

And forget using water to set your float height, it's specific gravity is denser than gasoline. The float in the carb will be much more buoyant in water so if you set the height using water then the the height will probably be sky high once you remove the water and put gas in it. Basically if you're going to set the height using the clear tube method then use what ever fuel that you're actually going to be using.

[lucky]

Just one drop of water in the carbs will cause them to not work. especially if that drop of water gets stuck in the idle jet.


You "think" you got the float height right?
You need to go do it again, and use a measuring device like the book says FIRST.

No wonder it leaks.


IF you took your bike to a professional  Honda mechanic and he said, "I think the floats are right"

You better get your money back.

[TwoTired]

Just one drop of water in the carbs will cause them to not work. especially if that drop of water gets stuck in the idle jet.

What a hair brain idea.  Good thing physics ignore human rationale.

The carbs will still work.  They'll just atomize water, which the cylinders won't burn.  Once the water is sucked dry, the carbs will atomize gas the cylinders can ignite.

Water bottles are cheap.  Drill a .010" hole (pinhole) in the bottom of them and see if they leak.  Idle jets have a larger hole than that.

[tlbranth]

I spent a couple of days, once, with the carbs on the bench, trying to find stuck float problems and getting the levels even. After a while I was smelling pretty heavily of gasoline and decided I didn't want to become a living Ronson ad so I did the rest of my testing with kerosene. I figured it'd be better than using water but that was just a guess.

[nzdreamer55]

Thanks Lucky for the help.  I have a float gauge and used this to set them.  So when you say "the book says", do you mean the BIBLE?!?  I don't remember anything in there about motorcycles, but I'm not one to go to church either.  I have a haynes manual and a shop manual.  The only reference I have found for the float hight on a stock 1977-1978 CB750 (A,F, and K) is 14.5 mm.  If someone can cite where it is in the service manual that would be great.  Did you know that these floats on these years of CB750 are .......NOT round?!?!  This makes it even more of a mystery as to how to set them when taking the reading.  There seems to not be a specific way to place the carbs when setting the 14.5mm.  Some posts on this forum say on their back, some say at 90 degrees some say 45 degrees.  I even included a ASCII diagram picture and got an answer that was helpful, but was still hard to figure out (there are 2 directions to put a rack of carbs in from the answer and they would both be at a 45 degree angle so which one do I pick).

Why the heck don't any of the "Books" have a picture of how to do this procedure?  Is this a honda secret or do you have to be born into a motorcycle family and have it passed down from father to son.  The secrete of where the set the carb body when setting the float height. 

So I'll watch out for the honda mechanic who tells me "I think I set the floats right" and demand my money back, but here you are comparing my skill set to a honda mechanic.  I am not a honda mechanic, just some dude with a bike trying to get it on the street so hence the quote I think I set the floats right.

If I have not seemed sincere, I am and I want to thank you and to everyone who is offering their advice and time to respond.  I know what you do is free and I appreciate your insight and have a lot of things to try to get my bike on the road again.

-S

Just one drop of water in the carbs will cause them to not work. especially if that drop of water gets stuck in the idle jet.


You "think" you got the float height right?
You need to go do it again, and use a measuring device like the book says FIRST.

No wonder it leaks.


IF you took your bike to a professional  Honda mechanic and he said, "I think the floats are right"

You better get your money back.

[nzdreamer55]

SUPER COOL!  Thanks for the picture.  This is a great idea that I will have to try.

-S

Here ya go...laying on the bench in this pic is one of the one-man speed bleeder type set-ups, for bleeding brakes by yourself. I've seen this used to apply a vacuum to the fuel system and check for leaks around the O-rings in the fuel T's, etc. and to make sure the needles/seats are working.

[Bailgang]

Thanks Lucky for the help.  I have a float gauge and used this to set them.  So when you say "the book says", do you mean the BIBLE?!? 

Aw come on nzdreamer, where you been all your life boy? It's the St James version of the Holy Honda manual. We don't allow slackers around here so get with the program will ya?!! 

[nzdreamer55]

Just a follow up on this situation in case anyone is listening.  I opened the petcock and waited for fuel to start to leak.  The leaking was coming from 1 carb (#1) and so I started first by backing out bowl drain screw.  I noticed that the o-ring was not in the right place and so I move it to where it should be and reinstalled the drain screw.  Opened the petcock again and wait.....


NO LEAKING!

I finished with a small dance and then took her for a spin.

Thanks for the help

-S

[Stev-o]

Good news, I love easy fixes.

[lucky]

Just one drop of water in the carbs will cause them to not work. especially if that drop of water gets stuck in the idle jet.

What a hair brain idea.  Good thing physics ignore human rationale.

The carbs will still work.  They'll just atomize water, which the cylinders won't burn.  Once the water is sucked dry, the carbs will atomize gas the cylinders can ignite.

Water bottles are cheap.  Drill a .010" hole (pinhole) in the bottom of them and see if they leak.  Idle jets have a larger hole than that.

Yes the stock idle jets are .013 thousandths.
Water and gas do not mix.
When a drop of water gets in a jet it usually stays in the jet.
Some water CAN go through an engine (carbs) but after that happens, then the gas will not go through the water. Then it quits running.

But the specific gravity of water and gas are different so testing the float height with water could give a different result than with gas.

Do you want to take the chance that NO drop of water is left in any of the passages of the carb?


Think about the ethanol in our gasoline now and the havoc it has caused by attracting water in the gas. We put in Seafoam to keep the moisture in suspension so that the water will not cause more problems.

[Black 750K8]

Just one drop of water in the carbs will cause them to not work. especially if that drop of water gets stuck in the idle jet.


You "think" you got the float height right?
You need to go do it again, and use a measuring device like the book says FIRST.

No wonder it leaks.


IF you took your bike to a professional  Honda mechanic and he said, "I think the floats are right"

You better get your money back.

What size is this drop? I'm glad my carbs don't know about this or they would never work with the gas around here. Some how my bike will keep running and pretty damn good I might add. I don't know where you come up with this crap? Do you ever read some of the stuff you spout out?