Some of the most important functions of any car (especially old ones) are that it starts easily, warms up quickly and idles well. Whether it's when we are showing our cars to friends, taking our wives out to dinner, or just enjoying them ourselves, the car needs to start without whining, flooding or running rough.
In last month's DOC, I covered making a choke heat pipe that supplies hot air from the manifold to the carburetor's thermostatic choke (see our Archives at www.southernwheels.com). This month, we will talk about choke adjustment, including the thermostat fast idle and curb idle* adjustments. These will be discussed in general for most carburetors of the late '30's-'50's. Each carburetor manufacturer's specs might be different, so it is important to consult your manual for exact specifications. Note: Some manuals give their adjustment settings using a drill bit for spacing. For example, insert a #41 drill size between the choke valve and air horn. The problem with this measurement is that some drill bits have undersized shanks, giving an inaccurate reading. We use a feeler gauge for an exact measurement. For instance, the decimal equivalent for a #41 drill bit is .096; something to consider before adjusting a choke's thermostat. Sometimes its a case of the manual's specs not working in today's world. Other variables are the thermostat's springs themselves. The original bi-metal springs have gone through decades of extreme hot and cold cycles, and this has changed their tension. If the spring has been replaced with a rebuilder's spring (the type used by today's carburetor rebuilders), that spring will have a different tension than the original spring had. The only way we can follow factory settings is with an OEM, NOS spring.
Before setting our choke adjustments, it is important to review the operation of the choke.
CHOKE OPERATION: COLD ENGINE When the engine and choke are cold, the choke thermostat's spring increases its tension to close the choke valve (flap). The flap is prevented from closing by the throttle stop screw that holds the fast idle came in the slow idle position. This has the choke flap in the partially-open position, and it needs to be closed to start a cold engine. When the accelerator pedal is momentarily depressed, the throttle stop screw is lifted clear of the fast idle cam, and the thermostat then closes the choke valve. After the engine starts running, intake manifold vacuum causes the thermostat housing's piston to partially open the choke valve against the spring tension of the thermostat by admitting sufficient air to give a satisfactory running mixture when the accelerator pedal is released after starting the engine (during warm-up). The throttle stop screw should now be against and near the highest step on the fast idle cam which was rotated to the fast idle position by the closing of the choke. This provides correct throttle opening, preventing stalling during warm-up of the cold engine. If the throttle is partially opened while the running engine is cold, the vacuum piston, along with the increased force of air flow against the offset choke valve, will open the valve against the thermostat's spring tension. The air and spring's opposing forces balance the choke valve to prevent loading,an excessive rich mixture that would foul out spark plugs and produce black smoke from the tailpipe. Note: At wide-open throttle, the vacuum piston does not help to open the choke valve.
CHOKE OPERATION: DURING WARM-UP As the engine and exhaust manifold warms up, air is drawn through the heat pipe into the choke's thermostat housing by manifold vacuum. This warms the thermostat causing it to reduce its spring tension on the choke valve in proportion to the increase in temperature when the throttle is opened and the throttle stop screw is lifted from the fast idle cam. The choke valve now begins opening and the fast idle cam moves to a lower step on the cam. This makes the engine run at a lower speed at closed throttle.
CHOKE OPERATION: HOT When the engine reaches normal operating temperature, the choke thermostat is heated to the point where it no longer exerts any spring tension on the choke valve. The choke now is in the wide-open position and the fast idle cam is in the slow idle position, so that the throttle stop screw is against the lowest step on the fast idle cam at closed throttle.
CHOKE ADJUSTMENT: Before making any adjustments, remove the choke's thermostat cover and check to see that all moving parts are working. Check to make sure the spring isn't broken, the housing cover isn't cracked and there is a spring tab for the spring to hook onto or push against, as some do. These adjustments require a good working carburetor, no vacuum leaks and that the heat pipe is getting enough hot air from the manifold to the chokes thermostat.
With the air cleaner off, engine cold and not running: The first choke adjustment we will make is to ADJUST THE THERMOSTAT by first backing off the spring cover (cap) screws, making sure they have lock washers on them, and rotate the cap back and forth, watching the valve flap open and close. This shows that the spring and cap are hooked upand are working together. Now open the throttle by hand to bypass the fast idle cam. Turn choke cap slowly until it just closes. Push your finger down the air horn (carburetor throat) and feel the tension. Just a little pressure should close the flap. If there is too much tension, requiring more than a little pressure to close the flap, it will result in a rich start, fouling out plugs during warm-up. Now, snug down the cap screws and start and stop the engine several times, checking for a fast start, smooth warm-up and correct curb idle. This is trial and error, and varies a little from car to car. Our '51 Packard 327 straight 8 starts and runs best when the choke valve is set coldat not-quite-closed, but about 1/8" from the valve flap to the air horn.
Next SET THE FAST IDLE CAM ADJUSTMENT Some carburetors do not have curb idle adjustment screws, so the fast idle screw pre-sets the curb idle, as on our '49 Buick Roadmasters Stromberg AAV-267, although our '37 Packard Super 8's Stromberg EE-23 has both. To set the fast idle cam and choke unloader, set the choke to the closed position by depressing the accelerator pedal and releasing it, then back off the fast idle screw where it doesn't touch the fast idle cam. This will allow the choke valve and the fast idle cam to move together manually. To set the fast idle cam with engine cold and choke closed, turn the fast idle screw until it touches the highest step on the fast idle cam, now pull the throttle back to free tension off the screw on the fast idle cam, and with your finger, fully open the choke valve flap. Then release the throttle linkage. The fast idle adjustment screw should now be on the lowest step of the fast idle cam, with the choke flap opened Getting the fast idle cam setting is important because a cold engine during warm-up with too-low RPMs will gas wash the cylinders, resulting in gas in the oil. Generally, fast idle will be around 1500 RPMs. After this adjustment is made, start and stop the engine, checking RPMs at fast and curb idle. Remember, COLD (high step on the fast idle cam) and HOT (low step on the fast idle cam).
Last is the CURB IDLE This also is very important and although we can hear what we think sounds like the right idle, its best to use a dwell tachometer to set to the factorys recommended RPMs. Being off just 200 RPMs over the factorys recommended curb idle can cause an engine to run rich and have hesitation upon acceleration. Conversely, if you can't get your engine to idle down, you probably have another problem. Check for vacuum leaks, compression or timing problems. Curb idle on some carburetors can be set with a curb idle adjustment screw. Some involve bending a linkage to set the curb idle. If there's no curb idle adjustment screw, it is best to consult your manual.
With these adjustments made, the engine should start easily, warm up without fouling out plugs, and idle at the correct RPMs. Just changing to a manual choke won't do all of the automatic choke functions. Fords of the '40's had a manual choke and a hand throttle, so you hand-choked the car to start and held up the engine RPMs with the hand throttle. Chevys of the '40's had manual chokes but not hand throttles, so you had to hand choke it to start, and hold up the RPMs with the accelerator, while pushing in the manual choke as the engine warmed up. The automatic choke, when set correctly, works pretty well and is worth the effort to make it work right. Our old cars are never really finished, and there's always something to do to make them run and look better. Enjoy your cars. See you next month, and keep 'em driving!