I finally got around to tuning my Eddy carb with the kit. I needed the power booster to come on more so I changed from the stock booster springs which were 5 hg, and jumped one step to the stiffest spring set for 8 hg. I liked the power, but noticed that on the freeway at cruise I dropped from 14:1 to 12:1. That made me assume that the springs were too strong and I assumed they were maybe lifting the m.rods up slightly at cruise. So I dropped down to the 7hg springs and that was a disappointment, power-wise. I suppose I need to change the m.rods to ones that are a bit leaner at cruise and go back to the 8 hg springs for power.

 

I think you are on the right track. You fattened it up with the jump 5 hg to 8 hg springs, then you dropped back to 7 hg and noticed a slight leanness. Thinner rods might well cure it.

 

Hey Tbear. Thanks for responding. I like to use the forum to check my logic or plans.

 

Oh, I wrote more but then found I was getting some confusion in my post, so rewrote it, 2 or 3 times. I understand entirely. I found that if I tried to explain what I know, I would proof read it and find that I had screwed it up with a wrong word. Finally I just went for "keep it simple". I found the charts that Edelbrock shows in literature helped keep it straight back when I tuned my 1405. I used the kits too.

Just trying again.
If the metering rod is down in the jet, fuel flow through the jet is reduced. If flow is reduced at a given throttle position, a leaner, more economical air / fuel ratio results. Thinner rods restrict less than fatter rods, so thinner rods = richer.
Metering rod springs try to lift metering rods up out of jets for higher flow. Stiffer springs like 8 keep the rods up above the designated (say 8 hg) vacuum level. A softer spring like a 7 will let the rod stay in the jet above that designated (say 7 hg) vacuum level.
Heavier throttle means more air so then more fuel is needed for proper A/F ratios to make power. Heavier throttle results in lower manifold vacuum.
That plain looking vacuum gauge on the column beside the tachometer is of more value to me than the tachometer itself ... but the tachometer looks sexier.

 

Yes, I'm thinking that a vacuum gauge would be a very useful tool to have in the cab.
So ... an 8 hg spring is kept pulled in the jet as long as the vacuum is higher/greater than 8 hg. Throttle increases/opens, rpm increases and vacuum decreases. When vacuum falls to 8 hg or below, the spring counteracts the pull of the vacuum and the metering rod is lifted out of the jet. What perplexes me is that with a 5 hg spring the same m.rods are showing an A/F ratio of about 14:1 at cruise speed. With the 8 hg spring, the A/F ratio is 12:1. That tells me that the m.rods are lifting slightly in the jets. I just didn't think that at cruise speed my vacuum could be so low. That's why I need a gauge! I guess that for the sake of tuning this carb I could run a long v.tube into the cab so I can read and know what the v. is at cruise. Huh! I need a tach as well!
I really like the performance of the 8 hg spring, but know that that rich A/F ratio would definately affect mpg on a trip.

 

I think you'll be surprised what you see driving with a vacuum gauge in cab, it'll tell on you.