Just installed a vacuum gauge.

Figure I can get better mileage if I can keep the carb out of the power circuit by having the metering rod down in the main jet.

But, I don't know how much manifold vacuum is required to keep the metering rod down in the main jet.

Knowing my luck, it would be just going rich when I think acceleration is ok..........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

To estimate the vacuum for enrichment, I used a US nickel - very close to 5.0 g - as a weight atop the spring to find the spring constant. If you know that and the area of the plunger, you can tell how far it'll move for a given vacuum. If you also know how far the rod extends into the jet at 0" Hg, you'll know at what vacuum the taper starts leaving the jet. Only the last is hard to measure, but it'd be close to assume that the enrichment end of the rod just barely extends through the hole in the jet with no vacuum. (You could actual measure it by removing the rod assembly, measuring down from the top of the carb to the jet, measuring the rod, then putting the rod back in and measuring its position, and use that to find the distance between the taper and the jet.)

Now here is something I don't know how to do! Can you tell us more about your method and take us through the maths for finding the spring constant and calculating the movement for a given vacuum?

Also, what year US nickel? Have they stayed the same for long enough that it doesn't matter? I know pennies and quarters have been through a few different constructions with attendant different weights...

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Cool...........

BTW, with the old style tall BBS jets (BBD???), the metering rod is either in the hole or out of the hole. The top part of the jet is an alignment hole.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Not necessarily. Starting in the late '60s, a two-stage (BBS) three stage (BBD) metering rod was used with dual springs on the step-up piston. Same tall-style jets.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Ok......

How about the rod is out of the jet hole when on the power circuit with the tall jets.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Some time ago while teaching I had the students weigh US nickels, and they did come out quiet close to 5.0 grams. A web search puts the weight of all US nickels since 1946 at 5.00g when minted. The US Mint says that the Monticello nickel is 5.000 g.

All you need is something convenient that you know the weight; washers would work too. Weight a number of them together on a postage scale.

I simply used a caliper to measure the spring just sitting upright w/ and w/o a nickel atop it; the spring constant k= F/x, where x is the change in height and F is the weight added. Try to compress the spring 1/4" or so.

I measured the plunger's diameter D and got the area from A=pi*D*D/4.

The vacuum * area/k = motion of the rod. The only trick is to get the units right.

Let me try to find my notes tonight and then I can give you the numbers for my AFB's springs.

I found my notes on measuring the stepup spring in my Carter AFB.

[For simplicity, I'm pretending that grams are a unit of weight, not mass, but in measuring vacuum in "inch Hg" the acceleration
of gravity is implicit. The acceleration due to gravity is 9.8m/s2]

Piston diameter= 0.457"

I removed the step piston cover and used a very lightweight tube to pile nickels atop the piston. To get the piston down to where the cover held it took 8 nickels
(40g), to about the middle 55 g, to the bottom required ~75g.

density of mercury = 13.546 g/cm3 = 221.98 g/in3

area of piston = pi * d*d/4. = 0.164 in2

so 1 in.Hg => 36.41 g

To drop all the way takes

75g * (1 in.Hg/36.41g)= 2.1 in.Hg

Edelbrock says their weakest spring is 3 in.Hg.

The vacuum actually underneath the piston is probably a little less that the vacuum in the intake manifold - the piston must leak some.

I didn't record the travel X [cm], it would have been easy, but if I had the spring constant k in N/cm
could have been determined:

k= (.075 [kg] * 9.8 [m/s2]) / X[cm] = [N/cm]

or informally if X [in]

k= 75 [g] / X [in] = [g/in]

You can then find the motion/vacuum

x= F/k = (vacuum*area)/k = (36.41g/1inHg)/k[g/in] = [in/inHg]

Can you measure the actual size of the AFB setup spring?

And the thread pitch of the main jet?


With nothing better to do today, I took the top off my BBS to see about the spring. Will post results when I have them.


As for the thread pitch, the current jet is a 10-32 thread. I'm thinking there may be a way to use other rods (current edelbrock with some mods) and jets to get some tunability.


Holley jets are evidently 1/4 -32 threads. So any jet from anything that has a thread pitch of 10-32 and an overall diameter of less than 9/32" (maybe even 1/4") should fit.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Spring:
length..............1.450"
diameter..........0.208"
wire diameter...0.014"
coils................16

Stepup piston:
diameter..........0.373"
area................0.10927sqin
weight.............4.75grams
.......................0.16755oz
.......................0.01047lb


Piston installed height (top of piston to top of bowl):
free,no stop/no load...........0.765"
with stop..........................0.305"
lean starts.........................0.295"
full depression...................0.105"


Added 190.7grams to fully compress stepup piston, so it took 195.45 grams to compress the spring 0.66" (.765-.105) or 6.8942oz for .66" or 10.4457 oz/in .65256 lb/in

To compress the spring down to engage the metering rod requires a compression of (.765 - .295) 0.47" or .3067lbs (.47in * .65256lb/in)

To get .3067lbs on the stepup piston requires a pressure differential of .3067lb/.10927sqin or 2.807psi

Change psi to inHg is 2.807 * 2.036 (14.695psi = 29.92 inHg) = 5.715 in Hg manifold vacuum to get lean mixture.....

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes