Leaking fuel from overflow line

[mansito]

New to CBs and to the forum. I bought a 79 CB750K about a month ago. It was leaking fuel from the last carburetor on the right side. I thoroughly cleaned the carburetors and the needles seats. I replaced the old floats and needles with 4 brand new floats and 4 new needles. It is still leaking from the overflow lines.  tomorrow I will figure out if it is leaking from the same carburetor or from a different one.  The bike runs and idles nicely. Any suggestions on what I should do next?.
Regards,
Guido in NJ

[Maz]

Hi Guido,
check the little brass overflow pipes inside the float bowls. It's not unknown for them to crack if they've been accidentally bent. Also check that they are firmly seated in the alloy of the float bowl as they can work loose and leak fuel around the base.

Maz

[mansito]

Thanks for the suggestion.  I will test each bowl tonight when I get home and post a reply on the outcome.

[Re-run]

You should have a vacuum operated petcock on the tank. Those sometimes can leak when the engine is off. I would check for that, just in case. And then by extension, the floats and needles if the overflow tubes are ok.

Also, the drain screws can corrode and leaks can develop there. Make sure those all seat well.

[mansito]

By petcock I assume you mean the gas valve on the tank (bottom of tank on the left side for 79 CB750K).  The gas valve does not leak when in close (horizontal position).  I only have fuel leaking from the carburetor overflow line when the petcock is vertical.   I believe the leaks are happening from the overflow lines.  I will confirm tonight and post a reply tonight or tomorrow.   Thank you all for the suggestions.  Keep them coming.  I am on the learning phase.  Hopefully I will be able to contribute to the forum as I learn more about the CB750s.

[shinyribs]

There aren't any overflow tubes in your model carbs. The nipples on the bottom of the bowls are there for draining the bowls only. That's why there are no hoses hooked to them like the earlier bikes had.

Since there are no overflow tubes, if your float valves/ needle & seat were bad it would just flood the engine or the air box.

You just have a case of a leaky drain screw. Which is good since you won't have to pull the carbs, or most likely, even the bowl that is leaking. Like Rerun said, probably just a corroded screw, but it's possible there's something keeping it from seating. Clean the screw and that drain port and you should be ready to rock. No  o-rings on those screws either. Self sealing with a tapered tip. Good luck, and let us know what you find.

[mansito]

Shinyribs, see pics of the type of carbs I have.


I have hoses hooked to the carburetors, and I have the brass pipe inside the bowls.  Now I am confused as to whether my bowls have overflow tubes or just nipples to drain the carbs.  Anyway, the first thing I will do is to check the drain screws when I get home, before I do anything else.  Hopefully that is the culprit.

Thank you all for the suggestions.
Guido in NJ.

[TOOLS1]

You have to excuse Shiny, he has probably had 30+/- sets of carbs on his bike in the last few months, and has forgotten what the original carbs were.  Anyway those brass tubes are overflows, and could have a crack, however it is most likely that your needle valves are not seating. This could be because the seat is dirty, you can clean them with a Q-tip and chrome/brass polish. It could also be that the rubber tips on the needle valves are dried out and not sealing. Another thing that happens is that the plastic floats deteriorate over time, and then do not float as they should.
TOOLS

[shinyribs]

I have three sets of stock CV's none of them have brass tubes in the bowls. Maybe they've all been removed by po's? My bad.

[mansito]

NP man.  Thank you for the advice.  It got me thinking.  I am going to go home in a few minutes to check the drain screws.

[TOOLS1]

shinyribs wrote

I have three sets of stock CV's none of them have brass tubes in the bowls. Maybe they've all been removed by po's? My bad.

Maybe there not stock.
TOOLS

[mansito]

Tools1 wrote:  Anyway those brass tubes are overflows, and could have a crack, however it is most likely that your needle valves are not seating. This could be because the seat is dirty, you can clean them with a Q-tip and chrome/brass polish. It could also be that the rubber tips on the needle valves are dried out and not sealing. Another thing that happens is that the plastic floats deteriorate over time, and then do not float as they should. 

I cleaned the seats, but I will do it again just to be sure.   The needles and floats are brand new.

I will slightly bend the piece that connects the float to the needle to create a stronger needle valves seal with the brass seats.

[Maz]

Bending the 'Tang' (as it's called!) will adjust the float height. This may be your problem, but make sure you set the float heights correctly, as in the manual. If the float height measurement is too large, you will get fuel starvation and if it's too small you'll get flooding. The float height should be 15.5mm (from memory, but the manual will give the correct setting) measured from the base of the float to the edge of carb body with the 'tang' just resting on the end of the sprung end of the needle (spring should not be compressed).


The carbs in the picture may not be the same as yours but the method of measurement is the same.

Maz

[mansito]

Maz,
Thank you very informative.

An update from last night:
After getting a late start (was on the phone with my son who goes to school in Denver for about 1hr) I go to the garage, open the fuel petcock on the tank, get the bike started, let it run for ~5 minutes, and not a single drop of fuel from the overflow lines.  Go figure.   I proceeded to change the oil on the bike.  I will run it again tonight when I get home to see what happens.

Thanks to all for your help.  Great group of people on this forum.

[Maz]

That's what we call "Murphy's law" or "Sod's law" in the UK - When you want to fix something and it works perfectly for the first time in ages.

Just keep an eye on it and if it happens again, at least you've got a few ideas to run through.

Maz

[mansito]

Yeap.   Are you in the UK?  I go to Marlow and Maidenhead frequently on business.

[Maz]

Yeah, I'm in South London/North Kent (depends how posh I'm trying to be!).

I don't really know Maidenhead but Marlow is a quaint little place. That's a long commute from where you are!

Maz

[mansito]

Let the bike idle for about 10 minutes and no gas dripped from the overflow lines.  Oh well.  I'll keep checking in case fuel overflows again.

Maz,
Yeah, long commute from New Jersey, USA.   I spend a lot of time in the air.  I just returned from a two weeks trip to Belgium, Netherlands, and Ireland.   It is a small world, who knows maybe our paths will cross while I am in the UK.
Cheers,
Guido

[Maz]

Entirely possible, Guido.
I just love the fact that mileage/nationality is no barrier to bikers helping each other out. I take part in a little event called "The International West Kent Run", every year. We get 350 riders attending from all over Europe for a weekends riding through the Kent countryside and every single one of them would stop to help a fellow rider with a problem, regardless of nationality or type of bike.

This is an awesome forum. So much knowledge and a genuine bunch of guys.
I'd better stop now as I've previously been told that us Brits are too polite on here!

Maz

[icerigger]

Could be worse, Maz, you could be mistaken for Canadian.