Annoying leak in Zenith 24T carb

NKirov

Member
Cant seem to solve the constant tiny drip from my Zenith 24t carb. Tractor is new to me and previous owner has rebuilt the carb with new parts. I've gone through it again and can't see anything amiss.

Gasket seems to seal, surfaces seem good, float is new and not holed. I've installed a filter and tap downstream of the bowl in case it's getting tiny bits of crud from the tank but I still get this tiny drip from the air inlet after a few hours.

I've been though every drilling and they are clear.

The needle valve is clear and sealing when depressed by hand but maybe it's slightly faulty and the float pressure isn't enough to stop the fuel flow completely?.

After reading a few posts they do mention the possibility that even new valves don't seal. Is this common?

There obviously insn't much pressure on the valve so I guess even a tiny fault could cause it to stop sealing?
 

Pretty common problem , gets so bad sometimes that the entire fuel tank can leak out and fill the air intake , rot the hose and spill out all over the place .
Try tapping the carb float chamber with a screwdriver handle to get the needle to settle (don't hit too hard though). Sometimes the float edges are rough and bind on the chamber walls , smooth these if they are .
Putting a washer under the seat will lift the float level a bit and put more pressure on the needle that might make it seal better .
 
Charles you are a fountain of knowledge on all subjects!

That's a good idea about the extra washer. I also spoke to Burlen here in the UK who make the genuine parts and they said there are Indian and Chinese copy parts floating about that don't seal perfectly.

Mine looks pretty well made but I've bought a genuine needle to at least try. I have read that you can lap in the needle to the seat to get a better seal but I can't see how you can get it apart to get anything in there not at least without damaging it.
 
It doesn't take much to cause that.

But be sure you are really having a flooding problem. It is normal to have a drip of gas following a shut off or a failed start. Gas that was on it's way up the intake, comes back down once the engine vacuum stops.

If it is truly failing to seal, check the clearance between the OD of the needle and the ID of the seat, be sure it is not too tight, look for burs where the screw driver slot crosses. Be sure the needle is tall enough the float tab is not hitting the end of the seat. Be sure the float is not touching the inside of the bowl.

A good test, before putting the bowl on, turn the upper housing upside down, suck on the inlet fitting, put your tongue over the hole. It should hold vacuum. If not, there is a leak somewhere, somehow.
 
That's a good idea.

It's definitely a leak though pretty fair sized puddle there overnight maybe a drip every 5 seconds or so.

When you say check the clearance do you mean the needle in the valve? not sure what you mean by the crosshead? My valve looks like this?
mvphoto80451.jpg
 
No, I was thinking of the 2 piece needle valve, the kind you install with a screw driver.

Does your carb have a screen inside the inlet fitting. Most do, last chance protection from
something getting in the needle valve.

Something else to try, drain the carb, then open the fuel valve. Sometimes that will let the float
drop, fully opening the needle valve, and the inrushing fuel will flush away any trash caught in
the valve. When you remove the carb drain plug, catch the flow in a clean glass, see if there is
any rust making it's way down. The filter should catch it, but you never know. If the tank is
flaking rust it will be an ongoing headache.

Are those aluminum gaskets? Possibly the seat is leaking around the gasket. Might try wrapping the
gasket and threads in teflon tape, be very careful not to leave any exposed, or get any beyond the
threads.
 

The Aluminium ' gaskets ' are height adjusting washers Steve . There are a few different thicknesses so that you can play around with the needle adjustment .
 

cvphoto98119.jpg

Think the mystery may have been solved. I found the original needle valve amongst a pile of stuff the previous owner gave me. Comparing this to the new ones (I have two now) there is a significant difference in
the length on the valve pin, a good 1mm at least. I suspect this is enough to reduce the pressure on the seat and cause the seepage. I'll add another washer as suggested and hopefully this will fix it.
If that doesn't work I may just try the original, which probably hasn't leaked in 70 years!
 

If the original works , stick with it , even if you have to lap it gently . New parts are useful sometimes but sources of grief for the most . I saw a cylinder liner , new straight out of the wrapping paper that bad a pin hole right through the side . Worse still brake shoes that had linings stuck on with glue that let go at the first application . There is little if any quality control on just about anything .
Keep any original parts you can , soon , one day you will also find out just how bad new points and condensers are.
 
The leak has been fixed, the needle valve was sealing ok but as above the pin on the new part is slightly shorter which meant the fuel level was fractionally too high, just enough for it to constantly overflow into the main choke choke. I set up the carb on the bench and filled the hose with petrol and then watched it down the inlet, you could just see it seeping. Adding the thicker 1.5mm washer under valve was enough to lower the level so no dripping any more.

Thanks for the advice from all of you. Just need to fix everything else now...
 

Good stuff . Next thing to do is make sure the fuel tank is clean and that the hidden filter or filters in TED tractors are not there . These are terrible things that cause no end of trouble , they look like a cigarette filter and sit upright on the fuel tap inlet , you can only access them by taking off the fuel tap and withdrawing it.
Often they have disintegrated and filled the tank with bits , if they are there remove them and throw them at your worst enemy .
A simple one inch upstand made from 1/2" copper pipe pressed onto the fuel inlet will stop rubbish entering the carburettor , simple and less likely to restrict flow than an inline filter .
 

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