I'm having terrible trouble with the carb, I've got a new seal on it but still petrol is leaking like crazy from the top of the large union cup type nut. It's as tight as I dare. There is a tiny hole just above the thread on the bottom of the carb - could it somehow be coming from here?

email (option): Adh325@hotmail.com

It's probably coming from there but the reason is likely to be that it's overflowing. Normally, the float needle should be shutting off the fuel supply before this happens.

If the carb has been OK in the past then it's not likely to be caused by mismatched parts. If you've replaced the fibre washer in the mixing chamber union nut with a thinner one, that could raise the float level slightly.

The first thing that I'd check would be the float chamber and needle sitting. Assuming that you've got a bottom feed float chamber, you can remove the top and check if it continues to fill with the float up.

So is the little hole an overflow? It didn't leak before restoration. Perhaps I went a bit overboard with the tickler pin? Will check the float chamber isn't sticking too. Thanks very much.

email (option): Adh325@hotmail.com

Ok so I've checked everything again, float isn't sticking. I can actually hear the float click when I open the petrol tap. The hole doesn't weap immediately, it will do it after a few minutes - then stop, then do it again! I've replaced all the fibre washers - could this have messed up the levels? Is it really that precise?

email (option): Adh325@hotmail.com

Hi Andy..the washer thickness isn't super critical...Don't be afraid to really swing on the lower fitting on the carb body, they have to be really tight. Also it may be worth a look (before doing that) to make sure the base of the jetblock is flat and undamaged where the washer sits...Ian

email (option): ian@wright52.plus.com

Hi, Andy,

I agree with everyone, and have another small advice:

As the float rises up, there is a drill in the top float cover, intended to accept the top of the needle and keep the float on a horizontal axle and parallel to the chamber body.

It might be worth checking if you get the same fuel spill when chamber to is not fitted.
If all is ok without this cover, you probably need to clean it. It might prevent the needle to go all the way up.

By the way,
I looked allot, and finally found on the forum a thread with picture from Ron Pier:

http://pub37.bravenet.com/forum/static/show.php?usernum=3155626639&frmid=16&msgid=1158358&cmd=show

So, as you can see-the drilling should be there.

Another thing:

I do remember that the newer float needles, the one sold now by “Amal” has 2 groves.
If you have this type of needle, check if you are on the correct one..


Good luck !

Noam.

email (option): Zehavi

i might be teaching you how to suck eggs but have you checked to make sure the float needle is seating properly and not bent just check it with the top off the float bowl to see if it just keeps filling up

email (option): roger.beck@node6.com

Thanks for the usual great replies - I'd be up a certain creek without a paddle if it wasn't for you guys!

I've checked with the lid off that it's not still filling up, it's definitely working as it should - with the cap on too, but after a while it looks like petrol is still getting past the needle. Any clever ideas to make sure the bottom tapered part of the needle is properly sealing the little hole in the bottom of the float chamber?

In a way I'm glad I had this problem - I understand the carb much better now!!

email (option): Adh325@hotmail.com

Andy, if you have just done the bike up you could have a tiny bit of bead from bead blasting, rust flake, minute part of washer/gasket around the base of the needle. If it was me I would take the bowl off and take needle and float out and use compressed air to blow through the base. If it was bad it would pour out just above the big nut as you describe. If its a new needle with the two grooves put float bow in top groove which you must have done anyway other wise it would pour out. The needle may just need seating in by a bit of use or can you toothpaste or Brasso to lap the needle in.John in Australia

email (option): ukcarbs@hotmail.com

I've had the float chamber off again, it's definately clean. It's an old needle so only one notch to choose from. The carb was pretty gummed up when I started, I wonder if I inadvertently stuck something too sharp in the bottom of the bowl to clean it out and ended up damaging the part that the tapered needle goes inside. I have a long thin pointed circular file - should I gently try to ream the part that the tapered needle sits in? And also try the idea of some grinding compound to make sure it's nicely seated?

Failing the above - would an additional fibre washer be the answer to change the level?

I suppose I could try filling the bowl with thinners while it's off the bike and see if pulling the needle up stops the flow.

I'm so close!

email (option): Adh325@hotmail.com

I wouldn't take a file anywhere near the float needle seat. Unfortunately, there is no easy way to overhaul the seats and they are cast-in.

The only thing that you can really do is a few twists back and forth with a little 'Brasso' and check how the needle is looking. Have you looked at the needle under a magnifying glass to see if it's ridged ?

If the needle is leaking then dropping the chamber isn't going to help much. It's certainly worthwhile checking it off the bike. It should stay completely dry underneath, simply with the float doing its job.

Martyn Bratby now provides a service where he replaces the float needle seat..Tel 01543 572583...Ian

email (option): ian@wright52.plus.com

I have had porous fibre washers from Amal.

I tested the chamber off the bike with some thinners - it seems to hold fine. As soon as it's on the bike though - after a while it starts to overfill.

I'm thinking two things:

When I take the chamber off, there seems to be some fine debris in the fuel that runs out. My tank is sealed and filtered - so I'm wondering if it's some rubbish that wasn't quite cleaned out of the copper fuel pipe. This debris could be getting lodged on the sealing seat.

The other possibility could be the length of the needle - it only has one notch, is it possible the the needle is 'topping out' inside the guide tube in the float chamber cover, therefore meaning the needle can't lift high enough to seal?

email (option): Adh325@hotmail.com

The 'fine dust' will be enough to stop it seating properly, the brass seats are not very tolerant of dirt. In use, with the motor running, this is not likely to be a problem and if the fuel is turned off, there should be no overflowing.

However, it's not a good thing and indicates that the whole system needs flushing through again. The gauze filters only stop the big lumps. Are you in a region with ethanol in the fuel ? If so then even new tank liner could be breaking up and allowing or causing contamination.

Original needles have only one notch. Current production has two simply beacuse they use a different system to locate the plastic float and still machine the original groove to allow both types of float to be used.

I have had a cap where I found that it seemed to impede the free movement of the needle and there also seem to be various lengths of tickler plunger - you could try removing that to see if it has been holding the float down.

There have been several types of float chamber cover over the years and we're all having to sort out mix-and-match components.

Will check the needle length, might be a case of grinding a tiny bit off the top. I'm preying the debris is just coming from the copper pipe - it was badly blocked when I started - inside of tank is very clean.

email (option): Adh325@hotmail.com

By assembling the whole float bowl and turning it up side down and blowing through the fuel inlet by mouth, will give a good indication that the needle is seating. You can also test the tickler at the same time. Ron

email (option): ronpier@talk21.com

Same problem exactly with mine. Andy, did you ever sort it out? Mine is a carburettor that was on it for years and never played up, so I am thinking that it must be the needle valve. Before finding this thread I thought it was a problem with the washer under this big union. Like Andy I have cranked this up as much as I feel comfortable. One has to wonder what the point of having a washer under here is if its not going to stop fuel flowing out! Ummh, annoying. any more thoughts guys?

email (option): andrew.honychurch@btinternet.com

If the fuel level is too high then the fuel in the mixing chamber would have to go somewhere - either out of the overflow or else through the needle jet into the cylinder and out of the bell-mouth.

It is the float needle which should be stopping the fuel getting this high in the first place. Most likely is dirt under the seat, followed by a leaky float.

I have checked the needle by blowing through and its a perfect lock out. However, after tightening up the bottom union as much as I dare it still leaked. On taking it apart most of the thread off the main body casting fell out! I cannot go any further with this one until I get a new body for the carb. Bit disappointing to lose my original carburettor body, but I guess these things happen. I think the problem came when I dismantled it, I reckon it was so corroded together it ruined the thread. An well, another problem to sort out!

email (option): andrew.honychurch@btinternet.com

ok so I have kindly been donated a second hand carburettor body by one of our members, thank you again Ron, and fitted this to my carb, or rather built my bits into this body. The good news is that the bike fired straight up after 25 years of being inactive and my subsequent rebuild and sounded superb, but I had to turn it off straight away as I had this blessed leak again from the bottom union nut. To summarise the problem to save you re reading the whole thread, the large bright chrome like nut on the bottom of the carb body is leaking fuel up past its threads. Or at least it looks like thats where its coming from . It could be coming out of the small pin hole above the threads though. I have checked this is tight, having ruined one body by overtightening this I am not going to wreck another one. It should not have to be this tight, its not been designed to be tightened to an inch of its life I feel sure, the thread will not hold out. The needle valve is shutting off the fuel supply ok, with the lid off the bowl, and the leak doesnt seem to happen unless I tickle the primer at which point it flows through and wont stop. Any more thoughts from anyone that may help me solve this most irratating problem?

email (option): andrew.honychurch@btinternet.com

Fuel will come out of the holes above the large nut if the fuel level is too high and you will get a bit of fuel round it as you tickle it, it still sounds to me that your float needle is still not seating as you say when you tickle (prime) the float it does not stop coming out of the hole in the top of the float bowl. If it comes out and won't stop flowing when you depress the tickler you should try re- pressing it and it may stop as you depress it a second time, another thing is to tap the float bowl with a screwdriver handle and this may stop it. Most carbs at some point will do this as you tickle them, the unfortunate thing about these types of float is that the muck/debris tries to sit on the very part that seals it, unlike the monoblocs which come in at the top and the debris tends to fall away from the seat.

email (option): davmax@ntlworld.com

hi Dave, so what you are saying is that there is no leakage problem here. To be clear, all carbs will leak from this area if there is too much fuel? If thats the case then I can leave tightening the bottom union any more and concentrate on the float. I will take another look at the float valve but on sucking or blowing through its totally air tight. Why I am so paranoid about this is I had a fire when I first tried to start the bike a while back and nearly lost the bike and the shed it was in. She spit back and ignitied the excess fuel around and for a good few seconds it was a most worrying situation.

email (option): andrew.honychurch@btinternet.com

Basically yes! Just try to concentrate on getting the float valve to seal and the other problem will be fixed too. Other things to check too are the filler cap, is the vent hole clear? (I know the cap isn't a perfect seal!) if the vent is blocked it may pressurize the tank enough to make the valve leak. Usually when a bike has been rebuilt or a tank sprayed there will be a bit of leakage from filler dust and rust that has been loosened during the time the tank was empty and dry. If it only happens occasionally it is no problem, just undo the petrol pipe union, blow the pipe out, uncrew the carb top, pull out the float, wipe it's seating area clean and replace it and reassemble. It only takes a couple of minutes at the roadside with the BSA combination spanner. It is always a good idea to turn off the fuel as the heat of the engine when you shut off can pressurise the tank, leading to the needle being forced off it's seat. You may notice this if you watch it, turn off the hot engine, leave the fuel on and wait for a minute or so and it may leak out, then open the filler cap and it will probably stop leaking.
Needless to say that your bike is at risk from fire if it lands on the hot exhaust or if someone passing by drops a cigarette, so it always pays to turn the fuel off when you stop.

email (option): davmax@ntlworld.com

Andy when fettling the carb try to use the main bike stand rather than field stand as the field stand is not very friendly towards the pre mono carb, Ideally bike needs to be upright vertical for float needle to work at its best.

Hi fellas.

I haven't been on here for some time so sorry for the delayed response.

Haven't looked at the carb again for a while but have a pretty good idea what the problem is.

The fuel tank is almost clinically clean internally after restoration, however when I took the leaky carb to bits there was some debris in the bottom of the banjo nut. I'm convinced it's coming from the fuel pipe which was totally blocked before I started. I cleaned it best I could, but having being sat in fuel for a while the residue must be starting to come away - and getting lodged on the seat. And that's when petrol starts to run over.

I'll also keep working some fine grinding paste - or brasso on the needle and seat to ensure a good seal.

If that doesn't work - I'm stumped!

email (option): Adh325@hotmail.com

Hi Guys,

So, sorry to have to raise this one again - but i'm struggling big time, and I just cant work out what the problem is.

I really dont want to roll down the road in a fire-ball on my maiden voyage!

Things i've tried so far:

-ensured the float pin is seated correctly, used grinding paste then brasso. filled the chamber with petrol while the bottom nut was off the chamber - no leaks.

-bike is toally level

-no debris in the petrol

-ive added another fibre washer to the connection between chamber and main body of carb. leaks a little less, but still leaks.

the weap from the hole is very slight now since adding the further washer - is this just normal when the taps a left open for a few minutes? i'm assuming while the bike is running this shouldnt happen.

are the any other adjustments i've missed?

note that the bike hasnt been properly run yet, and card is un-tuned, just trying to get it all leak free before I start!