General => Douglas Motorcycles - General Discussion => Topic started by: derek morgan on 07 Jan 2019 at 13:56

Title: 1931 t6 carb
Post by: derek morgan on 07 Jan 2019 at 13:56
good afternoon, would be very grateful if any members could give me any info on my t6 carb. the prpblem I am having is the float chamber does not shut off and petrol keeps running out of bottom of carb, this happens when fuel tap is switched on without using tickler. have taken carb off and needle appears to shut off when blowing air into reservoir and turning it upside down while blowing, grateful for any advice Derek morgan
Title: Re: 1931 t6 carb
Post by: yosemite on 07 Jan 2019 at 17:27
could be that the float is leaking, and no longer floating at the height required to stop flow before the carb leaks, to check strip carb  and shake float, you may hear the fuel sloshing about orimmerse float in hot water, if it bubbles then the float has a pin hole
If the float has a pin hole you may be able to seal it with solder (after you dry it out )
Title: Re: 1931 t6 carb
Post by: derek morgan on 08 Jan 2019 at 12:31
thanks very much for advice paul, have the carb out as I was going to take it down the shepton show beginning of feb. will try your advice at weekend and let you know results regards derek
Title: Re: 1931 t6 carb
Post by: derek morgan on 10 Jan 2019 at 14:22
good afternoon all, have checked carb float for leaks as per advice from paul and there does not appear to be any leaks from float, would be grateful for any advice as to what else could be causing the problemof carb flooding. it is only the carb that is flooding as spark plugs remain dry, many thanks regards derek
Title: Re: 1931 t6 carb
Post by: douglas1947 on 10 Jan 2019 at 16:39

I would check again the needle and the seat for it.
Are the needle lenght the right lenght?
Is it in the right position to close?
( for a test:  you can try to set the bowl a bit more down on the needle to have it closed eaarlier)
If it is not worn too bad, you can try to grind it in.

Title: Re: 1931 t6 carb
Post by: derek morgan on 11 Jan 2019 at 09:30
thank you very much for your advice Michael, I think you are spot on and that the problem is coming from the needle and bowl, will keep you updated, regards derek
Title: Re: 1931 t6 carb
Post by: DSIM on 11 Jan 2019 at 11:00
I take it that the carb fitted is the 5/116/S the one with the slide horizontal,   if so the level in the float chamber is critical to starting and running on the pilot jet  The level needs to be JUST BELOW the main jet, too high and it leaks, too low and there are problems pulling fuel through the main jet to start. The pilot jet will only come into play once the engine is running and the throttle is closed. It is possible over the years that various attempts to control leakage by grinding in the needle and seat have reduced the length of the needle and hence the float has to be higher to shut off the petrol which means the petrol level is above the main jet orifice and results in the leak.
Most parts for the carb are readily available with the exception of the needle which is a special because it is shorter but  according to amal drawings can be made by modifying a 5/065 needle. 
Title: Re: 1931 t6 carb
Post by: graeme on 12 Jan 2019 at 06:51
Absolutely spot on - I had to go through the exact same procedure to stop fuel leaking on my S6. I ended up putting a washer on top of the float under the spring clip to lower it.
Title: Re: 1931 t6 carb
Post by: derek morgan on 12 Jan 2019 at 11:00
many thanks for your reply dsim( sorry I do do not know your name but have you as a buddy now) this is exactly the knowledge I was hoping to receive and will show your reply to mate who should be able to help me out. I rode the bike a few times in the spring and carb seemed fine but the last couple of times the carb leaked without using tickler and she takes ages to start after. as you say over the years carb has been played around with as there are two washers on top of float already . have contacted dick to see if club has any spares for t6 carb and am awaiting reply, again many thanks for your time and interest regards derek
Title: Re: 1931 t6 carb
Post by: DSIM on 12 Jan 2019 at 19:52
Apologies for an omission in my last message I should have been clearer that the needle I referred to in the last sentence is the needle jet needle. also that I had the same problem with leakage.

The two washers on top of the float make it heavier, which means the petrol level will need to be higher to lift the float. You should start by taking the washers off the float but also make sure the float is very clean, and see if that cures the problem.

I worked out the level in the float chamber by floating the float and needle assembly in petrol, in a suitable glass jar. Then noting the petrol level on the float and marking it.  With the float and needle installed in the float chamber top, in the valve closed position, I measured from the mark on the float to the flange on the float chamber top. This measurement transferred to the float chamber/ carb body gives the petrol level. You can then confirm that this level is correct or make some adjustments to get it right.

The other thing worth noting is that the SG of petrol in the 1930's was probably higher than petrol today (0.72 - 0 .775  for BP Unleaded petrol which includes the ethanol) I would guess that, despite the ethanol, modern fuel has more light ends as the refining process is much more controlled,  however the principle of the float having to displace its own weight to float hasn't altered. That means today the petrol will probably be higher up the float before it lifts it and closes the valve making the possibility of leaking through the main jet higher..
Title: Re: 1931 t6 carb
Post by: cardan on 12 Jan 2019 at 22:02
Sorry, but I don't agree. The float floats and that's it, and how high it floats is determined by the point at which the needle is pulled (bottom fuel feed) or pushed (top feed) firmly on its seat. At this point the float still has lots of buoyancy and would float higher were it not for the constraint of the needle.

So washers on top are NOT to do with weighting the float down. Washers can, however, be located between the top of the float and the clip on the float needle. This will cause the needle to hit the seat (and shut off the fuel supply) at a lower fuel level - changing the number can help you to find the correct fuel level. Little red fibre washers are good because they won't weight the float down. If the fuel doesn't shut off with a few washers in place (between the top of the float and the needle clip), you'll need to work on your needle and seat. A light lap with Brasso will work unless it is badly worn.

Pity you're not around the corner as I have nothing planned for this afternoon...


Title: Re: 1931 t6 carb
Post by: DSIM on 12 Jan 2019 at 22:47
Sorry you disagree Leon.
Once the float pushes the needle into the seat and if it shuts off the petrol there can be no more increase in force pushing the float up. The float will not lift until it displaces enough liquid to float it. Once it floats as the petrol level rises the float rises until it stops the flow of incoming petrol. Yes lowering the float will shut off the petrol earlier but the float only weighs a few grams and the addition of washers increases its weight  to which I see you agree.  Dependent on the weight of the washers it may be counterproductive. I did say take the washers out and try first, but setting the level will require knowing where the level is. Amal agree that the level in this type of carb is critical.
Title: Re: 1931 t6 carb
Post by: graeme on 13 Jan 2019 at 01:54
I agree with Leon
The float will actually float a lot higher in the fuel if the needle didn't shut off the supply; it needs to, to be able to shut it off with any weight on the needle. Like Leon also suggested, I used a fibre washer under the clip to set the level so that the fuel was at the correct level just below the needle jet
Title: Re: 1931 t6 carb
Post by: derek morgan on 13 Jan 2019 at 11:22
thanks very much to all for all your interest and knowledge, very much appreciated, will keep you updated. regards derek
Title: Re: 1931 t6 carb
Post by: DSIM on 13 Jan 2019 at 11:29
Ok I guess I'm wrong and Archimedes got it wrongs as well. The float floats by virtue of displacing an amount of petrol that is equal to its mass. That is a fixed level on the float which can be altered by differences in density of the fuel or the mass of the float, (also by the volume of the float a dent perhaps). If the needle valve leaks the petrol level will of course go higher. But the float does not and cannot float higher in the chamber because it has seated into the valve. The large seat on the needle will if in good order give a good shut off with little force. If it was made of a flexible material increasing the force may well improve the sealing but these are brass to brass.  The reason for a needle valve is that the needle presents a small area to the incoming fuel so the level in the petrol tank on a motorcycle has little effect on the level in the float chamber.
 I don't find washers for adjusting fuel level in the Amal list of spares.  Its a reasonable bodge though as long as they don't increase the mass of the float by much.