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Old 10-05-2009, 04:33 PM   #11 (permalink)
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Carbs are interesting beasts

What i was getting at is that with the existing jets, fitting a WAI would cause a richer burn (less O2), however a CAI would net a leaner burn (more O2).

If power can be sacrifised, the question then becomes, should try and take advantage of a wider throttle opening for reduced pumping losses????? by using a smaller main jet and WAI? (less manifold vacuum at cruise)

or is it more beneficial to obtain the desired power with the lease throttle opening? (more manifold vacuum at cruise). keeping factory standard jets (or one bigger) and utilising a CAI and/or RAM air? In essence, forced aspiration seeks to do exactly this (more oxygen in the intake charge) ...... often yielding a net gain in low down torque.

Would this latter scenario also compliment a higher load scenario (i.e. taller gearing), moreso than a WAI?

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Old 10-06-2009, 08:42 AM   #12 (permalink)
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I dont know much about all this, but I have carbie cars and motorbikes.

I always wanted to fit an adjustable main jet.

The ones you find on the net are limited to specific carb models and are only adjustable by hand beneath the bowl. I was talking to the local stihl agent/mechanic and he showed me a self propelled mower with a main jet adjutment on the handle right by the users hand. He also mentioned many small airplanes commonly use an adjustable mixture knob in the cockpit.

Imagine being able to turn a knob on the dash and go into lean burn. Awesome! You could monitor with an O2 sensor or vacuum gauge I imagine.

Not many Kingswoods left over here!
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Old 10-06-2009, 10:11 AM   #13 (permalink)
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I spent a lot of time around carbs.

My 1937 Ford had a passageway right below the carb in the intake manifold, that allowed hot exhaust gas to heat the intake manifold and help with fuel atomization.

The Nissan Z cars from 1970 to 1972 used the SU type carbs. I really like their simplicity and the cars got good mileage. The air cleaner had a flap that you used to allow heated (from the exhaust manifold) into the air cleaner to help with atomization, because the variable venturi carbs were very prone to icing. They also had coolant passageways in the manifold, and it they were not connected the car ran like crap.

Icing is when the air velocity across the venturi area lowers the effective temperature to the point where the fuel will no properly atomise. The plugs will be black. Flip the lever and allow heated air in and the plugs turn light brown (unleaded fuel) to white (leaded fuel).

You will get your best economy with warm to hot air going in the carburetor. The higher density of colder air will pull more fuel than the same VOLUME of less dense air at higher temperatures.

Volume and density are two different factors. Carbs are rated in cubic feet per minute irrelevant of density, but since density affects the vacuum applied to the jets the fuel entering the intake will change based on density. It will also change according to volume.

I like the idea of using oxygen sensors to measure actual fuel mixture, for precise tuning of the carb.

I used a digital thermometer to adjust the manual cable operated brakes in the 37 Ford, by measuring the temperatures of each brake drum after driving and tightening the cables to equal the drum temps.

In the SU carbs we could adjust the float levels without any dis assembly. You could increase the float level by taking the small bolt off the top of the float chamber and using a special tool lift the float up and bend the tab that contacted the needle valve. This raised the float level and allowed more fuel in the chamber.

Mixture adjustments were accomplished by using different needles in the carburetor.

With only the single needle and seat arrangement and no throttle pump the throttle response was amazing. The first time I drove a Z car I could not keep it from bucking when I released the clutch, until I used the heel of my foot to stabilize my clutch release method. The throttle response made my clutch engagement oscillate until I change my engagement method.

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Old 10-06-2009, 10:48 AM   #14 (permalink)
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Quote:
Originally Posted by blueflame View Post
I dont know much about all this, but I have carbie cars and motorbikes.

I always wanted to fit an adjustable main jet.

The ones you find on the net are limited to specific carb models and are only adjustable by hand beneath the bowl. I was talking to the local stihl agent/mechanic and he showed me a self propelled mower with a main jet adjutment on the handle right by the users hand. He also mentioned many small airplanes commonly use an adjustable mixture knob in the cockpit.

Imagine being able to turn a knob on the dash and go into lean burn. Awesome! You could monitor with an O2 sensor or vacuum gauge I imagine.

Not many Kingswoods left over here!
Jam a screw into a piece of fuel line, run it to where ever is convenient for you, and jam the other end of the fuel line onto the adjustment screw in the bowl. You'll have adjustability from wherever you happen to run it.

Lawnmower fuel line... or automotive vacuum line.
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Old 10-06-2009, 10:58 AM   #15 (permalink)
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As far as heated intake for carbs go they are a good thing. Not necessarily for changing mixture but for preventing carburetor icing. The shape of the carb inside creates a vacuum (to get the fuel to squirt in in the proper proportion) and in temps up to 70 degrees it is enough to reduce the temp inside the carb to below zero and cause moisture in the air to adhere in the form of ice to the inside walls of the carb causing it to run very very poorly. Only way to fix it is to stop the vehicle let the engine heat warm the carb enough to melt the ice and then continue on. Or use heated air (typically from a shroud on the exhaust manifold) to heat the incoming air enough that the temperature drop at the venturi is not enough to create icing.

Using a carb from a small plane is one of my ideas for a future mod for my Samurai. the mixture would be used in conjunction with an EGT (exhaust gas temperature) gauge. For it to work well you would need to flow match your head and create an intake system with as close to equal as possible fuel/air distribution to the cylinders. Once you are cruising (you would use rich for speeding up to insure you stay well clear of the Peak and a bit richer EGT) then up to cruse pull the knob out until the engine runs rough (means one calender has gone too lean to support combustion) push back in just a bit until the engine is smooth and that is the lowest fuel burn you can get for that throttle position and engine load.

Last edited by bestclimb; 10-06-2009 at 11:13 AM.. Reason: cause I spell funny
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Old 10-06-2009, 05:49 PM   #16 (permalink)
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I also have dreamed about adjustable fuel jet.
I have placed an cut-off solenoid in my idle circuit and also put a switch in the dashboard to shutdown the idle circuit on highways.
Idle circuit is only partly involving at higher speeds, so cutting off the idle will lean the mixture a little. So I might say i'm half-way there
Then I build an pulse width controller to adjust the idle circuit open time at lower speeds, but sadly I haven't tested it yet.

An oxygen sensor in the exhaust will help seeing whether its lean or rich. And the other thing is the feedback what comes from pressing gas pedal. If not much power then it's propably lean.
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Old 10-06-2009, 07:05 PM   #17 (permalink)
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Ford had a stepper motor adjusted venturi carb for years. total POS.
The motors were mega bucks and failed a lot due to heat, jeep bought them
for a few years as well.

Read any sprint car book , most have deep discussion on how air density affects power. You have to mentally flip some logic because your working for efficiency not ultimate power.

Dave

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