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1938 D8 air/fuel mix


johnny38

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hi all,

just wondering is they any good way to find the rite air/fuel mix on my stromburg carb or is it that you just have to play with it to you think its rite any help would be helpful I'm sure all have diff ways carb was rebuilt timing good plugs,wires cap rotor points all new

thanks johnny

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Yes the only way to set the air bleed screw is with a vacuum gauge. Disconnect the wiper hose and put the gauge hose on that nipple. Run a warmed up engine at a fast idle ,no more, and turn the air bleed screw for highest vacuum. If the needle jumps around a lot it may be time for a valve job. Just a grinding. But it could be spark plugs or points or condensor (capacitor) as well. You should get 15 psi or better on a "good" engine. The reason you have to use a vacuum gauge is "your" engine is unique. The best vacuum it will produce has to do with the present condition it's in. Those settings in the books of 1 1/2 turns out etc. etc. is for a brand new tight engine. Everything is downhill from there. Tom McCahill used to use a vacuum gauge to set his timing. He also said it was because "his" engine was unique and the factory settings weren't valid anymore. He would twist the distributor as well as turning the air bleed screw for the highest vacuum reading. This,he said, was the best you would ever get out of any engine at that point is it's life.

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Use a vacuum gauge, seek highest reading. Also while you have the gauge hooked up look at the online vacuum gauge chart to see that everything else is OK. Vacuum gauge will tell lots about an engine's health.

All good advice from the above for setting up the idle mixture; assuming you still have the original carb fitted, there is not a lot you can do about affecting the higher speed mixtures, as they are set by the size of the fixed orifice jets within the carb.

A good indicator of normal running mixtures is to take a look at the color of the spark plugs, after a lengthy run, with the engine at normal operating temps. If all is well the ceramic insulator under the plug electrode should be a nice light tan/brown color, if it tends too a whitish color you are lean, sooty black is too rich.

Often overlooked with mixture issues at all speeds is the correct height setting of the needle/seat and float mechanism, these have a significant impact as a too high setting wil cause rich running and too low float level will cause lean running; as these parts wear in service its always wise to check for the correct float level, if you have the carb apart, or you are dealing with mixture problems.

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I disagree with the float bowl level thing. I used to set my needles and floats exactly by factory specs as well. But I had a problem with a carb leaking fuel from the separation of the top and bottom of a carb and tried everything to stop it. Finally I set the float/needle cutoff so that the fuel was below the factory setting by a 1/4" of an inch or so and this stopped the fuel from sloshing out the gasket . (Believe me I tried eveything to stop it. I believe a lot of these old cars leaked at this joint almost from the start.) The lower fuel level in the float chamber made NO difference to the running of the car! Initially the only way I had been able to clean up the spark plugs was to put in parts from later carbs which didn't change the authenticity from the outside. Ball and Ball/Carter carbs on CPDD engines were basically the same for many years,just getting better as the 30s went on. I eventually came to the conclusion that the height of the fuel in the float bowl had more to do with keeping the gas flowing at high speed ,should the driver decide to push his car. At the speeds I drive,around 50mph/80 kph, I experience no problems and the carb has been this way for many years now. I assume if I decided to push it up to 70 mph or so, the lower gas level would work as a governor and hold the engine from obtaining max factory available speed. The manufacturer would have known this and allowed for this in the setting of the level so the car never "ran out of fuel" with the pedal to the metal.

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I disagree with the float bowl level thing. I used to set my needles and floats exactly by factory specs as well. But I had a problem with a carb leaking fuel from the separation of the top and bottom of a carb and tried everything to stop it. Finally I set the float/needle cutoff so that the fuel was below the factory setting by a 1/4" of an inch or so and this stopped the fuel from sloshing out the gasket . (Believe me I tried eveything to stop it. I believe a lot of these old cars leaked at this joint almost from the start.) The lower fuel level in the float chamber made NO difference to the running of the car! Initially the only way I had been able to clean up the spark plugs was to put in parts from later carbs which didn't change the authenticity from the outside. Ball and Ball/Carter carbs on CPDD engines were basically the same for many years,just getting better as the 30s went on. I eventually came to the conclusion that the height of the fuel in the float bowl had more to do with keeping the gas flowing at high speed ,should the driver decide to push his car. At the speeds I drive,around 50mph/80 kph, I experience no problems and the carb has been this way for many years now. I assume if I decided to push it up to 70 mph or so, the lower gas level would work as a governor and hold the engine from obtaining max factory available speed. The manufacturer would have known this and allowed for this in the setting of the level so the car never "ran out of fuel" with the pedal to the metal.

Disagree as you wish but the priniple of setting the float level has to do with the law of physics, remember a u tube from school days, when you could balance the level of water and then affect the level by blowing or sucking at one end of the tube.

Carburettors work on the same process; without getting too technical, the level of fuel in the float bowl and the amount of suction through the venturi action is what makes the fuel flow through the jets and past the throttle plate, in fact the venturi action actually has to lift the fuel out of the fuel bowl then out into the discharge nozzle/s. With this in mind, if you lower the float level the venturi action for a given throttle setting lifts less fuel out of the bowl resulting in a leaner mixture, and if the float is set high the opposite occurs and you finish up with a rich mixture; thats why manufacturers specify a given float level setting.

Having said all that, you may not notice too much running difference at town speeds if the float level is off a 1/16" or so, but if you were to go to the trouble of putting an exhaust analyser on the tailpipe you would certainly see the effect; most engines will run on a fairly wide variation of mixture settings but they will run better or more efficiently if the mixture is set correctly.

So in the bigger scheme of things your observations are understandable and you may choose to disregard manufacturers specs, where it may impact on your longterm usage is burnt exhaust valves using leaner mixtures, town usage may take a long time for this to show but the cumulative affect of some long hard trips might make a difference.

Solving the leakage issues would probably have been addressed by facing the float cover and float bowl joint surfaces, given the material quality and joint screw tension it is inevitable that they will warp in time, but then thats one of the meany frustrations we put up with with our older vewhicles.

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