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Carburetors

Carburetors

Larry Lee, Susanne & James Warren, and Lou D.

Jason Philbrook wrote:

I'm fixing up a 77 XJ6L with Stromberg carburetors. One of them has light oil of some sort in it and makes a slurping liquidy sound when the top (the metal rod) is removed and oil is present at the end of it. The other carb, the one in front, is dry. (Which one is normal may I ask?)

The Haynes manual says to put oil in the carburettors, but doesn't say what kind. Any suggestions?

I am pretty ignorant about carburettors, (this is my first vehicle with carburettors) but the Haynes manual mentioned that that is one of the important things to check.


Larry Lee

The oil is normal--and necessary--for proper operation of these carbs. Here's why. (I run dual SU carbs, but the principle is the same for Strombergs.)

These carbs have a tapered needle that moves up and down in a fixed orifice to vary the fuel supply as air flow varies. This also changes the size of the throat directly over the orifice. The needle is moved by a piston (SU) or diaphragm (Stromberg) that has ported (varies with throttle setting) vacuum in the dome above it. The greater the throttle opening, the higher the needle and the greater the annular opening between it and the fixed orifice--hence more fuel--and the greater the throat opening to accomodate more air.

All this works great so long as the engine is running at a constant speed, but an engine needs a richer mixture momentarily to accelerate without hesitation or even stalling. In SU and Stromberg carbs, this is achieved by retarding how fast the needle can rise as the throttle is opened. Here is where the oil comes into play. It acts as a damper in conjunction with the little piston in the cap you removed. Remember the resistance and "squishy" feeling you got when you pushed the cap down? Well, the diaphragm-needle combination feel that when they try to rise during operation. This slows that rise down. In the process the size of the throat is also restricted momentarily. This causes the air to move across the orifice faster than normal for steady-state operation. As Bernoulli's Theorem states, this lowers the static pressure of the air and causes more fuel to be sucked past the orifice. Without the oil, the needle rises too fast and acceleration suffers. Note that this only works one way--going up. The needle falls without delay as the throttle is closed.

Discussions about the "right" oil to use can be found in the list archives. I have found a straight 30W petroleum oil to work well in my SUs, and I suspect it would do well for you, too. It doesn't take much. I use enough so that I can feel definite resistance for about one-half inch before the threads make contact when I am replacing the cap. This should be checked regularly, but mine rarely need to be topped up more than once a year or so. I had essentially the same experience when I maintained a pair of Strombergs on my mother in law's Sunbeam Alpine GT several years ago.

These carbs really are rather elegant little devices. I hope this helps without being too boring. Feel free to holler if you have any further questions. And good luck!


Susanne & James Warren

Jason, the slurpy one is correct. Without the oil you will run lean on hard acceleration.....I just deleted a long explaination of why and how SU carbs work, but noticed you have Strombergs. Same principle but different nmechanical action: on an SU manifold vacuum lifts a piston withdrawing a tapered fuel metering needle from a hole to allow more gasoline through. The Stromberg has a diaphragm instead of a piston. Although you would think lifting the needle faster would open the fuel oriface and richen the mixture, the slower rise creates lower air pressure right at the fuel oriface (jet) sucking fuel out faster briefly, until the piston or diaphragm lifts.


Lou D

If you really want to understand both the theoretical and practical aspects of these carbs, (and also how to fix the dern things) I would suggest securing a copy of Stromberg CD Carburettors Owners Workshop Manual by Don Peers and published by Haynes. It was published in 1976 so I am not certain if copies are still available from the publisher. It has gotten me through two rebuilds with no trouble.

In answer to your question, the manual says "Fill the hollow air valve guide rod to within 0.25 in (6mm)of the top with thin oil (SAE 20). Fit damper assembly and do not overtighten."

By the way, you are also supposed to lightly oil the guid rod when refitting the cover with the same oil.

If the car has been sitting for any length of time, the carbs may be gummy and use a good cleaning.


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