MTFCA Forum

My car idles reasonabley well, hot or cold. However, I have noticed that it will idle a little smoother even when hot if I richen the mixture slightly from where it runs well at highway speed and general driving. I've noticed that when I come home and get out to open the barn door, then step on the right running board to climb back in, the engine idle picks up a little and smooths out a little. Getting off the running board and pushing on the car to make it lean toward the driver's side makes the idle slow down slightly and get slightly rougher. I can do this with the hood open and watch the idle screw on the carburetor throttle lever and it does not move at all. Does this indicate the float level needs adjusting? The amount of fuel in the tank makes no difference. The car is a '27 with an NH carburetor. The timer does not move when I step on the running board, either.

If you continue to stand on the running board, does the idle stay the same (higher rpm) or go back to where it was originally while you stand there? On an NH, the fuel inlet for the idle circuit is on the engine block side of the needle valve chamber. Stepping on the right running board would effectively lower the fuel level on the left side of the carb. But you said it idles better with a richer mixture. Now I'm confused.

Not speaking to your tilting the car, but may be related to the following.

The float setting, as shown in the service manual is an INITIAL setting. I discovered thru a deep dive in service essentials. The fuel level must be fine tuned so that @ hi speed & idle, the mixture needle is in its optimal position. Once set correctly, your carb will not need continual tweeking or any touching. I find no need to even richen my mixture for cold starting.

As described, your fuel level is a bit low for idle. Raise it a small amount & retest hi speed & idle running. Tweek again if necessary until you would not want to make mixture adjustments for the range of motor demand/speeds.

That is all about fuel level in the bowl

at idle and speeds below about 15 MPH, the majority of your running is through the idle circuit, where richness is acutely affected by height of fuel in the bowl.

the fact that adjusting richness for idle vs fast running is an indicator that either the idle passages are somewhat compromised or more likely in your case, float level is a bit off

Holley's early NH literature indicated that the nut on the top of the carb was to be TIGHTENED and LOCKED DOWN once ideal running setting had been achieved. They obviously abandoned that advice (even to the point of having a friction device on richness adjustment on later NH versions) and "T" owners fiddled incessantly whether necessary by temperature conditions or not. Importantly, though, it is this advice which shows that it was never intended to be adjusted for various speeds as both the idle and fast-running settings were identical in a properly adjusted carb by design.

I am going to bet that your carb does not have the "B" float which was larger than standard, having been changed due to extra head pressure supplied by the cowl tank's height...rough roads and extra fuel pressure led to problems which were to an extent, alleviated by the larger, more buoyant float

Does the B float have any identifying marks?

The slight improvement in idle seems to remain as long as I stand on the running board. Before I do anything else, I'm going to run the car for half an hour or so, then experiment with stopping it where it will lean left, then try leaning right, then level. The area where I stop to open the barn door is a very slight upward incline and the whole apron has a slight slope to the left. or driver's side. It's possible that stepping on the right running board actually brings the car closer to dead level, which would seem to make more sense. The car will idle for some time without "loading up" unless the weather is very hot. Under those conditions, most any carbureted engine will lose idle quality. I usually start this car cold with the carburetor adjustment open 1 turn. After a short time, I move it to about 7/8 turn. When the engine it warm, it will run well at most speeds up to about 45 MPH with the adjustment at 3/4 turn, but that seems to lean out the hot idle slightly. When I adjust the knob, I turn it gently until the slack is out of the linkage, then make the adjustment. The carburetor does not leak or flood. It has an oiled foam type air filter on it. No manifold heat pipe.

Here you go, Pat

they are not marked, but the difference is stark

I'll look into float and the float needle as soon as I get a gasket set. Some further observations on the idle question: 1. Stepping on the running board DOES move the carb linkage very slightly. Parking the car such that it leans to the driver's side will allow a good slow idle with the carb needle open 1 turn. So will parking it so that it leans to the right. The amount of lean is about what you'd get parked along the curb on a typical city street. Since I now know that stepping on the running board affects the throttle slightly, I think I can ignore that. The fact remains that the car can idle quite well with the carb needle open 1 turn (hot), and yet it idles poorly or may even die with the carb needle open 3/4 turn. The car runs well otherwise with the carb needle open 3/4 turn or just slightly more. With the needle open 3/4 turn and going 30 MPH on a level stretch, you can open the throttle abruptly and the car surges ahead and continues to accellerate up past 40-45 MPH. This car does not pop, backfire, or surge when running. The throttle shaft seems to fit tightly. The throttle and spark linkages are free of excess slack or binding. I'm thinking that the float level may be slightly lower than it ought to be. I was able to get a smooth, slow, hot idle with the car leaning toward the driver's side with the throttle shut and the spark all the way up and the carb needle open 1 full turn. After letting it idle about a minute, I tried closing the carb needle a little, and the idle immediatley deteriorated. Weather, 88 F and humid. Elevation about 1200 ft. Non-ethanol fuel.

If your carb does not leak while parked, dont screw with the float needle & seat. When resetting the float/fuel level, DO NOT put any pressure on the needle/seat. Use a screw driver to bend the float tab. Do not just pull down on the float.

If it ain't broke don't fix it Had a friend with dual carb setup, had the car purring after adjusting them at the service station where I worked. At the last moment he thought it needed one more tweak. I drove him home.

it's the altitude! When you stand on the running board, the carburetor is at a lower altitude!
Actually, if the engine idles smoothly, I wouldn't worry about it unless it is an extreme change in speed.
I do agree I think it is due to change in float position. when you step on the right running board, the float level will drop just a little and let a bit more gas into the bowl.
Norm

I'll have to wait for gaskets before doing anything to it. I do need the larger float, if it doesn't already have one.

Norman, I was thinking maybe stepping on the running board caused the coil box to tip toward the spark plugs and pour a little more electricity into them....

It appears that the carburetor leans out at idle, so I need to consider the float level and look for any possible obstruction in the idle circuit. I believe the throttle shaft is in good shape, but I can tell more about that with it off the car. This car has a conversion throttle linkage that goes over the cylinder head. It all seems to work very well. I think the slight throttle movement when stepping on the running board is due to normal Model T flexibility. Most all transverse spring Fords can get some feedback from the chassis through the throttle linkage when running slow on rough surfaces, which can cause jerking at very low speeds. Keeping everything tight and lubricated minimizes it. A throttle cable would eliminate it, but where's the fun in that?

I might have a couple of the "fat" ones left Pat - I'll check.