Carb Leak, Simple Fix, but Long Time Finding Fault!!!

[rbt1548]

 I normally turn of the petrol tap on my bike, (recently acquired CB400/4), if I am not going to be using it but I forgot to do it and came into the garage a couple of days later to find 2 carbs dripping petrol onto the floor, not a lot but enough to let me know there was a leak, they were No. 1&3. I thought starting the bike and letting it run for a few minutes would sort it and it did for No.3, not so No.1, so off with the float bowl, valve seat and float valve, quick clean and polish and back on, still leaking!

As I didn't know the history of the bike, and going by a couple of things I found previously, I thought it may be best to remove the carbs and give them all a good check over and clean, which I did, and re set the float heights to 21mm. rather than fit it all to the bike then discover I still had the leak, I connected them to the tank whilst on the bench, No.1 leaking again!!! Now at this point I decided to order some float valve seats and valves, my thinking being, the ones that have been in, as far as I know, have been there for over 40 years and may be the problem. I then changed the seat and valve in No.1, (can use the others as spares, should I need to), and reset the float height, still leaking!!!!  I don't mind working away at stuff but those of you who have done it will know that on the bench the carbs as a unit are a bugger to drain and move without petrol going everywhere, and each bowl is on with 4 screws unlike my CB750 which had a clip holding it on.

What next to check?, I swapped the float and valve bits from No.4 that wasn't leaking and checked the float height settings, off we go again, petrol on and leaking on No1, no leak on No.4!!!,
This was doing my head in and I am trying to think the big chap upstairs has it in for me, but it's a while since I tried to play a Led Zeppelin album in reverse  , so hopefully I'm ok on that front.

Now where does all the swapping parts, putting new and other parts back on, marking down what I have moved and rebuilding leave me?, exactly where I started. I then thought the only thing I hadn't looked at was the bowl itself , what's in the bowl?, basically a brass overflow tube shrunk into the body of the bowl, so I thought I wonder if there is a hole in it? something in the back of my mind was saying I may have read about that somewhere.
I then connected up the rubber overflow pipe to the tank, put my finger over the hole in the brass tube, turned on the petrol and it all came pouring out of the joint where the tube was shrunk into the float bowl, at last I found the problem!, I tried the other bowls and they were all fine.

I have used Chemical Steel to try and seal it, if it doesn't work at least I know where the problem lies and I can try and source a new bowl.

I am now going for a lie down with the temptation to look out my Led Zeppelin 4, but play it the correct way just in case, before I try and find a good painter as I would like my tank and panels done.

[McCabe-Thiele (Ted)]

That was not an easy fault to find if I understand you correctly the brass overflow tube is leaking where it fits through the base of the bowl.

[rbt1548]

That was not an easy fault to find if I understand you correctly the brass overflow tube is leaking where it fits through the base of the bowl.

Yes, that's exactly where the leak was, by connecting the overflow rubber tube of the float bowl to the petrol  tank and putting my finger over the hole in the overflow brass pipe to stop the petrol running out the top, basically connecting it back to front, I then saw petrol coming out of  the connection between the brass tube and bowl.
 What I imagine, and I stand to be corrected, was happening was, when the bowl was filling and the petrol level got over the joint some would start to dribble out the joint and down the overflow pipe.
I have let it set overnight, hopefully all will be well in the morning.

[taysidedragon]

I had a bowl do that on mine. It's a known problem on these old carbs.
I believe some people have had some success shrinking in new overflow tubes.
I replaced my leaky one with a spare bowl that didn't leak.

[Matt_Harrington]

I'll be interested to see how the chemical steel survives with modern fuel. Well done in finding the issue.
Matt

[McCabe-Thiele (Ted)]

If attempting a re-seal might be worth a trickle ring of Hondabond on the underside. (outside)

[kevski]

I have in the past made complete screw in overflows.

[Davelu88]

I did a test with Araldite 2 part epoxy steel , I put a hole in the bottom of an old tin , fixed it with the araldite and filled with petrol and let it sit for a couple of months . It never leaked or softened 

[rbt1548]

I mentioned chemical steel, what I actually used was 'JB  Weld Steelstick', which I had previously used on a Monster tank hinge where it is a common weakspot, it flexes and over a period of time can start weeping, it seemed to work there so I gave it a go.

I tried it this morning and there was a slight weep still coming from it so I removed the Steelstick patch and tried to run some solder onto the brass at the join, it of course won't adhere to the alloy but seemed to flow ok, I then put some Steelstik around it, let it harden for a couple of hours and that seems to have done the trick hopefully!, we'll see in the morning.

Some Hondabond prior to covering is a good idea, I don't have any but I do have Threebond which is similar, so if it doesn't work this time I may try that. Someone I know suggested Exhaust Sealant/jointing compound, which might not seem as daft as it sounds as basically I am just trying to seal a tiny gap.

Kevski's idea is good and makes sense, but I do not have the stills to do that.

Thanks for all the advice, all gratefully accepted.

[LetsRide]

I am in the process of re-assembling my CB00/4 carbs and the float  bowls were going to be fitted today. I came on to the forum to see if I could establish (beyond doubt) which way the main jets go in and saw this article. Timing could be good for me as three of the four carbs were leaking fuel out of the over flow pipes when I started this project. Assumption was that the floats were stuck in the gloop at the bottom of the float bowl (and there were signs on the floats this could be the case). I will now do a quick check around the overflow pipe/bowl joint.

[rbt1548]

I am in the process of re-assembling my CB00/4 carbs and the float  bowls were going to be fitted today. I came on to the forum to see if I could establish (beyond doubt) which way the main jets go in and saw this article. Timing could be good for me as three of the four carbs were leaking fuel out of the over flow pipes when I started this project. Assumption was that the floats were stuck in the gloop at the bottom of the float bowl (and there were signs on the floats this could be the case). I will now do a quick check around the overflow pipe/bowl joint.

The way I did it , and I'm sure there are other ways, but it seemed easiest for me, was, took the float bowls off and connected the flexible overflow pipe to the petrol tap outlet, I then turned the float upside down so petrol would flow out of the brass overflow pipe, then put my thumb over the hole in the brass pipe and then turned it correct way up, held it for a minute or two to ensure the tube or joint stayed dry. If you put your thumb over the pipe before turning fuel on the fuel won't flow up to the brass tube as you will have created an air pocket and with the float turned upside down initially the petrol will run out the pipe without getting the pipe or joint wet making it easier to see if there is a leak, it was more of a seep on mine but enough to cause an overflow over time.
I may have made it sound more complicated than it is but it is very, very easy to check.