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updraft carburetor adjustment question

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I've installed a newly made factory-spec Detroit Lubricator updraft carburetor on my '32 Packard.


While fiddling with the adjustments, I notice that often the car will idle fine and have plenty of smooth power when accelerating or pulling, but in a low-power cruising mode on a flat road it seems to be hesitating or has an irregular miss.  Also seems to be OK when cool but gets worse as the engine gets hot.


Any ideas on what would cause that?


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Yes it is. 

David Frear said it will need to be adjusted once installed. But he was not able to provide instructions.

I am wondering if the float is too low. So I’d like to know if there’s a spec I can reference. .... or what?  

I could just blindly fiddle but if someone knows so much the better. 


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Just a thought since it's a Detroit knockoff.

The idle screw MAY be an air adjustment rather than a fuel adjustment.

Turning the screw in may increase the fuel.

Try to make the the exhaust blow smoke by turning the screw both ways.

It's probably the easiest way to determine which it is.

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Looking for the "idle screw" I see TWO screws which regulate idle. 

In the photo attached I show these two screws side-by-side.  One is larger, the other on the right is smaller.

The left larger one seems to be a stop-set-screw with a blunt end that holds the throat butterfly open.  But the left one has a needle-like end to it and is longer.  I can't tell what it does without removing the carburetor (which I'd rather not do).


Could someone explain what these both are?  What does the left one do, and what does the right one do?  Obviously not the same thing since otherwise why have two screws?

Thanks -- Scott


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I found this page from a vintage Chilton's posted some time ago.  In it they call the smaller screw a "kicker screw". (What does that mean?) And say that to adjust it, make it "0.007 to 0.015 from being flush with the flat on the pump housing".

I don't know what that means.


Does that mean to leave a gap of 7-15 thousandths before it  contacts something?  If so, what good does it do then?? Especially if you adjust the idle with the bigger screw.  I see no other screw to adjust idle.

Detroit Lubricator 51.jpg

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"Flush" means level with, as in a countersunk screw in wood is usually flush with the wood surface. Its not clear to me from that diagram what its meant to be flush with (or near to flush). While it gives a spec for fuel level in the bowl, I cant see what controls the fuel level - usually some sort of needle and seat controlled by the float. I have seen cars surge on part throttle cruise due to a slight over advance of the sparks.


jp 26 Rover 9

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I haven’t worked with this model carburetor. However, I agree with TerryB: According to the diagram, the “kicker screw” controls the point at which the accelerator pump begins to function. If you adjust it such that the clearance is .007 in, the accelerator pump will function at a lower accelerator pedal position than when adjusted to 0.015.


The “T” on the diagram appears to be used to adjust your car’s idle speed by controlling the minimum throttle plate opening. I'm not sure what the specification is, but I would guess an idle of  around 800 rpm would be about right.


The idle mixture is adjusted with “R”. Once I have set the idle speed, I would adjust the idle mixture. Usually, this is done by turning the idle mixture screw counter-clockwise until the car develops a lumpy idle, then turning the screw clockwise the minimum amount to where it runs smoothly. Some people also change the idle screw adjustment until the point of maximum rpm is reached, then go to a slightly richer mixture. 


The low-power hesitation would indicate to me that your current idle mixture setting is a bit too lean. 


If you can’t eliminate the flat spot in acceleration from cruising by enriching the idle mixture –i.e; if enriching causes the car to idle roughly, try setting the kicker screw clearance close to the 0.007 limit. That will cause the accelerator pump to engage earlier upon accelerator pedal depression. If that doesn’t correct the problem, I would go back to R to enrich the idle mixture. Those are my thoughts. I look forward to learning what others think – and, of course, your results. 



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I posted this question to the Packardinfo website pre-war forum and received a reply from "hotrodgss1" who has a similar car and is looking into the same thing.

He referred me to the 1931 owners manual which is much more detailed than the 1932 owner's manual that I have. The carburetor is the same, for practical purposes.


Basically all mixture adjustment is with the T-handle adjustment "R".  Adjust at idle and by some miracle it's good for all speeds. 

Oddly, whereas the instructions which I posted above does not say much about non-idle (cruising) speeds, the 1931 owners manual says nothing at all about the Kicker Screw.  Doesn't even label it ! Perhaps I need to remove the carburetor to see exactly what's going on there and set it for 7-15 thousandths. It's not obvious.


Strange that there's so little information on these Detroit Lubricator 51's.  No complete and unambiguous set of instructions.

When it finally warms up around here I'll get the car out and try some of these adjustments.

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1. There used to be a website www.Old-Carburetors.com That had a bunch of info. The chap that ran that site didn't maintain it, but I think he still has the files in .pdf format. I believe he used Dykes Auto Encyclopedia series as a source. Look for the 19th edition, published circa 1943. There may be an earlier edition to cover the '32 Packard. The 15th edition is 1927 which is too early for you.

2. Possibly contact AACA Library and maybe they could scan the relevant pages.

3. Contact other folks who purchased this carb and compare notes with them.

4. Verify float level.

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