Carburetor refinishing

[TheNeedForZ]

Hi,

 

I've done some modification on the primary side of my spare Q-jet(mainly streamlining the air path and modifying venturies). Now a lot of bare metal are exposed, which I assume to be aluminium?

 

The carb originally has a yellow-green finish for protection, I wonder if I can get the carb refinished myself by buying the right chemicals?

 

Any idea how to refinish the carb?

 

Thanks

[DaEvil]

Check Eastwood they have a ton of that sort of stuff.

[Guest tony78_280z]

I've done some modification on the primary side of my spare Q-jet(mainly streamlining the air path and modifying venturies).

I don't know much about Q-jets, but modifying the carb? What exactly do you mean by streamlining the air path? And how did you modify the venturies? I'm not sure I'd recomend modifying venturies. Any modification would change air flow, air speed, and air pressure; thus the fuel pick up will be off. Unless you are talking about the venturie boosters, which from my understanding aren't a good idea to mess with either.

 

I'm glad this is a spare.

[blueovalz]

He may be writing about the typical smoothing of the bore, cast flashing, slight mismatch of mated parts, thinning of the throttle rods, and smoothing of the sharp corners above the venturies. It's said to increase the CFM without losing any of the signal. As far as a finish on the carb, you'll never see any difference months down the road. I've done this to several carbs, and the exposed metal (it's not aluminum, but instead some kind of zink alloy I believe) will develop a patina the makes it unnoticable after a while. It will not corrode either unless it's constantly exposed to moisture, which it shouldn't be with all the heat and filtering.

[Guest tony78_280z]

He may be writing about the typical smoothing of the bore, cast flashing, slight mismatch of mated parts, thinning of the throttle rods, and smoothing of the sharp corners above the venturies. It's said to increase the CFM without losing any of the signal.

Interesting, I keep trying to work this out in my mind, but I can't see how that could work. How could the CFM be changed without loosing any signal? He may be able to smooth the bore, and make it easier to draw air upto the venturies (like those shiney demon carbs with no choke/air horn) but that doesn't increase CFM as it passes the venturies. Better to say it allows smoother less restricted air flow up to the venturies. Or am I wrong? (Does happen from time to time )

[blueovalz]

Not an expert on this, but I understand it as like this. Take the example of: A flat plate with a 2" hole will pass a certain amount of air through that hole at a specific pressure drop across the plate. Now place an air horn around the hole, and even though you've not increase the area of the hole, the amount of air passing through the hole will increase for the same pressure drop due to reduced losses. So a factory carb, rated at 500 cfm, may now flow 530 cfm without losing any vacuum signal on the low end of the air flow scale (low rpm range). But to do this you've got to stay away from opening the ventury or reducing the size of the booster. In other words, keep it to carefull cleaning up of casting flashing, and mismatch of parts (base to body, etc).

[Guest tony78_280z]

I keep trying to see it the way that you describe, but the diamater of the hole doesn't change regardless of how smooth you make the bore leading into that hole.

 

A flat plate with a 2" hole or a 2" hole with a funnel shape above it, either way only 2" of air can pass through at any time unless the air pressure inside of the 2" hole was changed. The only benifit of the funnel is that the air would be directed towards the hole, making the air more accessable.

 

I'm not saying your wrong, I'm just saying I can't see it. I must be missing a piece to the equation. Maybe I just need to sleep on it.

 

Regardless, if the 500cfm becomes a 530cfm, you are gonna need to change the jetting because you just let in more air, but not more fuel, and thus created a lean condition. Or does this boring and smoothing actualy increase air speed or air pressure in such a way that it pick up more fuel? Wouldn't it just be more effective to purchase a bigger carb?

 

(We just hijacked this thread and turned it into a technical discussion on "how does this work")

[blueovalz]

In a nut-shell, yes. But then the low rpm response would not be as good as the smaller carb's response. I see examples of this in testing air flow characteristics of intake ports. Clay is used to round off the edges of the intake port opening when no manifold is attached. A 2" round hole in a plate flows differently than a 2" round tube, and the longer the tube, the greater the change, so obviously drag, is envolved in this issue. Turbulance around the sharp edges, the thicker throttle plates and rods, and flashing as well. I suppose you could compare this to intake port changes that allow more air simply by cleaning up the ports without any porting. I also suppose that removal of any material would constitute "porting", but configuration of the port can have as much to do with air flow as the actual diameter of the port. Many examples are known where opening specific areas of a port hinder flow, while other locations help it.

I'm sure someone more knowledgable will chime in later today to offer a more scientific approach to explaining this.

[jt1]

Need, you can buy the stuff to touch up the finish from Eastwood as mentioned above, or send the carb to Pro Max carbs and they will beautifully refinish the whole thing.

 

Cutting off the choke tower and smoothing and radiusing the top of the carb will increase the CFM of the carb. How much varies depending on what's done, but can be 10 to 30 cfm. The reason the airflow is increased is because the air flows smoothly into the carb without sharp transitions creating turbulence. The "entry loss coefficient" quantifies how much of the max flow of a fluid in a passage is lost during the entry into the passage. A tube extending into the body of fluid pretty much is the worst, at about 0.45, and a radiused bell with the radius 2x the dia of the passage is as good as it gets, close to 1. Smoothing the top of the carb may or may not require adjusting the mixture, many variables are involved.

 

My efforts:

 

I mainly did this because I'm using a drop base air cleaner with the stock hood and the choke horn was less than 1" from the top of the air cleaner, which I felt like had to be a restriction.

 

John

[Tony D]

A L O D I N E

 

That is the trade name of the stuff, and just about any place that does any sort of small aircraft engine work will be able to give you a small vial to touch up what you need to do.

It is the stuff that gives aircraft aluminum that off-color tan gold look.

 

Alodine. Aircraft Spruce Co in Corona sells it by the gallon... But you only need a bit to touch up what you did.

 

FYI, I redid the entire upper throat area including the fuel distribution "trees" in the throats of my Corvair almost 15 years ago now (hacked up NOS Carbs, believe it or not!) and to this day I have not gone in and retreated with Alodine. I have no corrosion yet. But this is SoCal... and the car has sat for extended periods, so I dont' thing the corrosion will be a big deal. But if you "want it to look sotck and untouched" then alodine is the way to go! You can stick the whole thing in there and boil the whole carb so it all looks like new...

 

You went this far, why not! LOL

 

Oh, and as for a 2" hole only being a 2" hole, to visualize it look crossways at the plate: The turbulence caused by the SHARP EDGE of the hole will make the air at the upper edge tumble towards the center of the orifice, with fluid flow only happening across about 1.750" of the orifice.

 

When you add the velocity stack, you get linear flow through the WHOLE orifice since there is no tumbling of the air around the outer edges.

 

In reality, a 2" hole in a flat plate, is actually quite a bit smaller due to the turbulence. One with a velocity stack is actually a full 2" of useable, unturbulated space.

[Guest tony78_280z]

In reality, a 2" hole in a flat plate, is actually quite a bit smaller due to the turbulence. One with a velocity stack is actually a full 2" of useable, unturbulated space.

Ahh... I see. Now that makes sense. I supose with only 1-30cfm increase one would not need to re jet the carb, but I still think more than that would definatly need more gas.

[TheNeedForZ]

Thanks to all replies. True, I was only modifying the booster clusters and butterfly shaft. The main venturies are untouched. I even went so far as removing the "wings" that connect boosters to the wall. So the metal is zinc....I heard the original finish is "zinc dichromate". Well, dichromate solutions can cause cancer so I'll just paint it.

 

Tony78_280Z,

 

I wouldn't dare to modify my carb if it weren't recommended by David Vizard. He said modding the primaries like so not only increases cfm but also increases booster signal.

 

Here's what I think : Increased cfm can be had in two ways---

 

1. Increase size of passage---all else being equal, this reduces air speed, therefore could hurt the low end. This happens when too big a carb is used.

 

2. Remove protrusion in the passage---all else being equal, this increases air speed which improves booster signal. Here you have some cfm increase as well as booster signal increase. It's a win-win situation.

 

You are prolly right, it is likely something will need to be changed after that kind of mod. "Changes one thing changes everything", especially in a carb where everything is hanging on a balance.

[Guest tony78_280z]

2. Remove protrusion in the passage---all else being equal' date=' this increases air speed which improves booster signal. Here you have some cfm increase as well as booster signal increase. It's a win-win situation.

[/quote']Very interesting where did mister Vizard say that? I'd like to read that article. I'm fascinated with the wizardry of Vizard. Vizard auto Wizard. Hee hee I'm a poet and didn't know it.

[TheNeedForZ]

You can find it in "how to build horsepower vol.2" p83, bottom right corner :

 

"The bottom line here is that careful work on the primary side alone can

net in excess of 30 cfm and the good news is that all this comes with

an increase in booster signal."

 

plus some photos on p84. The Q-jet needs some mod because it has a lot of unwanted stuff in the primaries, like the "support wing". Your Holley should not need any mod but will still benefit from a thinned down butterfly shaft.