MTFCA Forum

I have 1926 model T roadster. Cannot get NH Carb to quit leaking gas from the throat.
New viton tip needle valve and seat and new float, all gaskets replaced. Adjusted float level to 15/64ths, works on the bench but as soon as it is installed it leaks a stream. Leaking stops when air screw is screwed all way in.
Anything I’m missing here? Replace needle valve and seat again is next step?

Are kids running this site? We don’t need no stinking icons. How about instructions to post a photo instead?

Click on Attachments below.

Click on Add files.



Use the resulting window to find the file from your computer, select it, and click Open.


Click Place inline and the file number will appear in your Reply box.

Type anything you want to add, and proceed as usual.

So it passes the suck test off the car?! Procedure in the service manual. That eliminates the needle & seat.
Improved T's used a larger float due to more head pressure.
Try lowering the float level another 1/16" as a test. A full tank of gas may be overwhelming the float @ the current setting..
Air screw?? do you mean mixture needle? A flooding carb will leak out the input side first, then the air intake port under the name tag.

A leak that profuse is very often a plugged bowl vent - either rust or blasting media

A carb will pass the "suck test" just fine, but still leak like a sieve, as the float will not rise enough to close off the float valve

Often times, the fuel will "pulse" out as the trapped air is compressed and subsequently expels fuel, repeating the cycle until something is done to mitigate the fuel flow to the carb.

Note: the air passageway under the Mfg tag is NOT the vent; it is the air inlet to the low-speed circuit. The vent is a tiny hole in the structure that the bowl fits to and is cross-drilled to the outside - that hole adjacent to the choke shaft is the vent to the outside world.

A friend has the same problem. 27 roadster. Passes the bench test. Leaks like a sieve when installed.
Will check & confirm the bowl vent is open. Thanks Scott.
This friend has looked at all? possible problems, but hasnt figured it out.
The one thing that "fixed" the problem. Sounds screwey!, was to put a hose clamp around the base of the main jet casting to limit the full drop/opening of the float. No more leak. Carb functions. Could it be that a sloppy hinge allows the float to push against the casting & stick open?

Check the sealing surface of the carburetor casting where the needle seat gasket mates to. Sometimes the surface can have some pitting and the gas can leak past it and make it look like the needle and seat are leaking.

Also, make sure there is not an old gasket still down in the bore where the needle seat goes. Sometimes it may look fine but there can be an old gasket still down there.

Ed Fuller wrote: ↑

Sun Jul 13, 2025 5:34 pm

Check the sealing surface of the carburetor casting where the needle seat gasket mates to. Sometimes the surface can have some pitting and the gas can leak past it and make it look like the needle and seat are leaking.

Also, make sure there is not an old gasket still down in the bore where the needle seat goes. Sometimes it may look fine but there can be an old gasket still down there.

If this was the case.
1) it wouldnt pass the suck test.
2) The leak would be an annoying drip, not a flood.

Scott Conger it seems you no longer sell the full throat valve? Funny story: I put in a viton tip needle valve which did not fix the leak. I noticed that the valve I had removed was a full throat of yours!
Thank you all for replies. I’m pretty sure the air vent hole to the bowl is plugged. Waiting for a manual.

Not an ad, but a correcting a misconception: Valves remain in production and are in stock

JuanitoOgrady..... I agree with Ed. Fuller's recommendation to seal inlet valve mounting into carburetor. If you can find an original NH float valve and seat, both tapers can be dressed , put into a drill and then you can apply finger pressure on a fine abrasive medium to take out burrs where slight depression circles in the needle valve itself. Ford supplied these valves for many years and they always work. Improved runabout gas tank is in the cowl which does give a little bit more gravity push for the gas to flow down to the carburetor. Those carburetors came through with a little bit larger float assembly to compensate for the gravity pressure from that raised tank. ..... Please keep us posted on your progressive work on the car. Thank you.

speedytinc wrote: ↑

Sun Jul 13, 2025 4:20 pm

A friend has the same problem. 27 roadster. Passes the bench test. Leaks like a sieve when installed.
Will check & confirm the bowl vent is open. Thanks Scott.
This friend has looked at all? possible problems, but hasnt figured it out.
The one thing that "fixed" the problem. Sounds screwey!, was to put a hose clamp around the base of the main jet casting to limit the full drop/opening of the float. No more leak. Carb functions. (Didnt flood) But hasnt been driven yet.

Scott C. you nailed it.

My friends carb, upon further, closer inspection, had the bowl vent hole plugged intentionally during a "rebuild".
Hole drilled back out & hose clamp removed. No more leak.

OP. Check that bowl vent passage. Gotta be it.

The T runs. I’m running it without front floorboards because the bands need tightened, but with no boards the pedals go down further and apply better! Next project, tighten or replace bands.
Back to the Holly MH; After reading the forum postings that passages in the carb might be plugged, and may need drilling out. I bought the 1/8 inch bronze rod to rivet closed after drilling. Meanwhile I dropped it into the carb cleaner gunk. Couple hours later I cleaned squirted WD40, brake clean, carb spray through the passages. (Hoping to avoid drilling).
As previous stated, I replaced the full float valve and seat with the new viton tip valve, I feared I had damaged the full float cleaning out the seat. With the .100/in. orifice, installed carb and it still leaked! I hit the starter and it fired right up and quit leaking. Blocked passages were the problem, not the needle valve.

Good news!..