MTFCA Forum

I just had a similar problem.

Saw that I didn't have the hot air pipe on the carb intake. That fixed it.

I think. It runs better now anyway. Damned if I can figure it out.

If it is an NH:

the idle by-pass circuit may be clogged or restricted, which will stop air from being pulled down into the gasoline well and spewing fuel-rich vapor into the air flow at the "dip" in the venturi, or the float level is too low and the carb is running too lean at the main (only) jet at high-vacuum (idle). Both issues are unique in cause, but are related in the scheme of things in that you are trying to get fuel up into the air stream. With lower vacuum but higher velocity, the idle by-pass is taken out of the loop and the airflow will make up and mask the effect of too-low fuel in the bowl and thus the jet. At idle, all of these things need to be right in order to have the right enrichment at high vacuum/ low flow.

That said, I went to start a freshed up engine this spring and had much the same problem and found that adding a hot air stove solved the idle (never could get the mixture right for more than a few seconds) and stopped all fouling of plugs. It was remarkable the difference between not having and having it installed. My '13 in FL would NOT run EVER without a stove. In WY they are all but useless but I found this motor in cool weather MUST have it installed. Paddy is not wrong in thinking that it solved his problem. He is not the first.

The '13 I'm working on had a similar issue, of 'No Idle'.. I was thinking it was the Severely warn out Throttle Body on the Late Model NH Carb that was on it.. I bushed the Body, and Installed New Throttle body Shaft and Butterfly.. Better, but Still had no Idle i Liked..

I ended up drilling out the Brass Plug that is in the Idle Circuit Next to the Intake manifold Flange of the Carb. The TWO holes that Bypass the Idle Air/Fuel around the Throttle Plate in the Carb Throat were rusted mostly closed. I took a SMALL Drill, and used it as a round file to 'File' out the rust from the Holes..from the Throttle Plate side.. go in at an angle.. Don't open them up, just clear out the rust.

I then used Compressed air to blow all the debris FORWARD from the Vent Hole behind the NH 'Ident Plate' ( there is an Idle Vent hole behind the plate.. Just Blow behind the Plate and the Debris will come out )..

I used a 'Wad' of Solid Core Solder to Re-Plug the Hole I drilled out from the Carb Gasket Side.. You really don't have to Plug the Hole, as it will mate up to the Intake manifold Flange Gasket and seal off.. Your choice..

I can now get the '13 to idle WAY down low in a stable RPM.. and it does not stumble when coming off idle.. I really like it.. I might have to check my '24 TT to see how the Idle Ports on it are doing..

Cap

If the engine is tight from a rebuild, it will not idle at a real slow rpm untill it loosens up a lot.

Here is a shot of a NH T Body at the Idle Circuit.. Plug you drill out is on the Flange, the Holes you get the Crud out of are in the Throat.. DO NOT MAKE THEM BIGGER.

You can also see the Bushing I installed in the T Shaft Hole.. One is a Through Hole, and one is a Blind hole. Cap

If it’s running pretty good as you say it is I think I would run it and let it get broke in some more. The more you run it it will loosen up and may start to idle slower.
Did you have the the same issue before you rebuilt the engine?
If you didn’t the issue may be more of a tight engine than a carburetor issue.

Here's an interesting comparison for you Scott...out of my 3 T's, only one has a hot air pipe. I can't tell any difference between any of them! Go figure THAT one!!

It’s amazing. You’re still finding stuff wrong. By the way I’m with the clogged passage guys.

Tim

I have 4 T's. The '13 MUST have a stove on it or it simply requires such a rich mixture that it will foul plugs within a few miles...in ANY weather in ANY humidity. My '23 with refreshed engine requires a stove below 60F and that's about it. The other two don't shiv-a-git and I don't ever run a stove on them. At any temp. Right now, all are running NH's, though there are some pretty strombergs sitting on the shelf waiting for the right time to install.

Interesting info Scott. Thanks. My '25/26 Fordor has the pipe on its rebuilt engine. The '11 & '13 do not. The '11s plugs run cleaner, it also has (ahem) Tru-fire, I don't know, let alone doubt, that has anything to do with it, and also fwiw even the pistons on that engine look clean as new when I peek inside the plug hole. As for the '13, its plugs run pretty clean, little bita soot on #1 per usual, the others are usually pretty clean. I do pull 'em about once a month and clean em anyway, as I drive the car a lot. And for something to do!

Some threads about cleaning the passages. http://www.mtfca.com/discus/messages/80 ... 1253048587
One suggested to scrub the crud. ( A trick I learned from Stan Howe for cleaning out passages: a brass-wound guitar string. The brass winding is rough enough to dislodge dirt and soft enough to not damage the carburetor.)
http://www.mtfca.com/discus/messages/41 ... 1402151664
Found these thought they may be useful to others
by Royce Peterson
Holley NH rebuild instructions https://modeltfordfix.com/rebuilding-th ... el-t-ford/
Holley G rebuild instructions https://modeltfordfix.com/rebuilding-a- ... arburetor/
Rebuilding the Kingston 5 Ball Carburetor https://modeltfordfix.com/rebuilding-th ... el-t-ford/
Repairing a Kingston L4 Carburetor https://modeltfordfix.com/repairing-a-k ... el-t-ford/
Rebuilding the 1915 and 1916 Kingston Model L Carburetors https://modeltfordfix.com/rebuilding-th ... rburetors/

Hot air pipes are more for carburetor icing than for winter warm up. Here in the south with our humidity they are just about a requirement. I have had cars that would not run without them, but sometimes you find one that runs better without one. Dan

I am going to turn things a bit and ask whether you have checked magneto output? Also if running on magneto, if you put the spark all the way up for idle, it might not idle, but if down a few notched, it will idle smoothly. If the magneto output is too low, it might not put out enough for idle, however, it runs at speed. For a quick check, I would recommend two things. 1. would run on battery and see if you still have the problem at idle and low speed. 2. Check magneto output across a 12 volt light bulb. Should be at least 6 volts AC at idle increasing to almost 30 V AC at higher speed. These checks are easy and if you do them before you tinker with the carburetor, you might find the problem is not fuel related. As others have said, it might just be a tight engine and will improve with running.
Norm

Revisiting the Picture I posted..
It's not the Passageway that plugged up, it's the Throttle Body Ports that caused my Idle Issues.. as the First time I went into the Car, I only did the Bushings, Shafts, and Float Level.. Better but not good.. Then I Drilled out the Plugs and Rodded the Passages.. I was leary of Messing with the Idle Ports.. Put it back together and the Performance was the same..

The Third time I went into it, I 'Reamed' the Idle Ports to get the Rust off the Holes.. That was the Cure for THIS Carb..

Cap

Dan

I agree about the humidity. My '13 supplied me with my first Model T College degree. Florida: 94F and 95% humidity. New "T" purchased out of Virginia. Would not easily start, and would foul plugs in very short order. Would not start at all on Motorcraft plugs, would start with great difficulty with Champion X. After about 2 miles, car was dead with sooted plugs.

Rebuilt good carb that ran in VA
Rebuilt coils that ran in VA
Replaced intake/exhaust manifold seals that ran in VA
Replaced coil box wiring that ran in VA

No difference

Out of desperation and against the advice of many experts of that period, I added a carb stove. Got car running, and was able to lean out the mixture a full turn within seconds...car ran wonderfully for the next few years. So much for arm-chair experts.

Out of sheer curiosity, I took the stove off one spring. The car started fine, as usual, and again, made about 2 miles requiring a richer and richer setting to remain running, before finally conking out with fouled plugs. On-site cleaning of plugs, replacement of stove that was under the back seat and I was on my way. Stove has never been off since. As far as I am concerned the hard and fast rule of "fuel today does not require a stove like the old fuel did" is neither hard nor fast. It is simply a generality which can lead some strict adherents to perpetual failure.

For years I was perplexed by people who insisted that what I experienced was due to a myriad of other things that were simply misdiagnosed and that the specific successes with installation and failures upon removal were mere coincidences and that my insistence that THIS car benefited from a stove even in hot/humid weather was simply because I was full of shite. No, the carb was not "iced up", but based on the copious condensation on the very cold original aluminum intake on the '13, vs the perpetually hot iron intakes on my other cars, it wasn't a difficult leap to say that the larger swept volume of the aluminum intake and the expansion that takes place there, has a marked effect on fuel vapor and it's cooling and re-condensing leading to incomplete combustion in the engine, without the added benefit of heat to minimize the condensing effect of the aluminum intake; the hot air of the stove keeps the aluminum warm, where on my other cars, the engine keeps the iron intake warm.

Beating a Dead Horse:..

I had a Honda Four Cylinder Motorcycle that when I purchased it, the Previous Owner said it wont run in the Rain or Foggy Days..

Took a while, and a lot of Pushing that Damn bike.. Finally I made a Hot Air intake from the Header to the Air Box.. Ran Fine after that..

Carb Ice...

Cap

It's amazing the variations in experience with carb stoves, here's mine.

My 1924 runs a Kingston L4 carb and Edison 14 plugs fired by a Truefire ignition. Plugs are gapped at 0.030. The car came to me with a 6V one-wire alternator that works great, so I've left it on.

I run with an air cleaner and no carb stove most of the year, but late in the fall I remove the air cleaner and install the stock carb stove.
Once the temperatures in the spring get up to 60 degrees or so, I remove the carb stove and re-install my air cleaner.

The car runs fine either way, I don't even have to touch the mixture control. I do get condensation on the outside of the carb body with the air cleaner, but no icing (yet).

Mark

L-4's are very often a set-it-and-forget-it carb. With the flapper massively enriching the starting and idling of the car, many times you can do exactly as you do. Sometimes you don't even need to choke them (and leaning them out to the point of needing to choke them will pretty much totally screw up the mixture at speed). I ran an L-4 for about a year on one car and saw a very steady 12-14 MPG with it. No choking, no adjusting, no nothing. It just ran. Switching to an NH saw the mileage jump to 18-20 MPG and a (probably imaginary) minor improvement in performance. That was enough for me to relegate it to a box which will be opened someday during my estate sale.

If it was mine, and I had a spare carb, or a carb on another good running car, I'd swap it on and see what happened.

RustyFords wrote: ↑

Thu Jul 15, 2021 11:47 am

If it was mine, and I had a spare carb, or a carb on another good running car, I'd swap it on and see what happened.

Agree good idea. we never know. Better to remove that variable.

Fast running and inability to idle smoothly can also be a symptom of a vacuum leak. If you haven't already, repeat your leak test, even though you worked hard to fix it once already.