Should I change secondaries spring?

[CobraSix]

I have a Holley 390CFM 4V with vacuum secondaries. I'm at a crossroads with tuning it. The primaries are pretty much jetted almost perfect. The power valve comes in nicely without bogging unless I'm at really low RPMs and loading the engine up (like going 20MPH in 4th gear trying to accelerate up a hill).

My only issue is with my secondaries. They transition in nicely without bogging. That makes me think they are set about right. My only problem is they are coming in too early and my mileage is really suffering as a result. Part of it is I like the feel of the secondaries. Makes it fun to drive, but my mileage has only been averaging a mere 20MPG. So I ran an experiment with a vacuum gauge attached and I tried to limit my hard accelerations and limit the use of my secondaries. My mileage when up to about 25MPG combined highway and city (70% highway). So, it leads to believe that if I go to a stiffer vacuum diaphragm spring, I'll be able to increase my mileage.

My question is will that make my secondaries transition in more poorly since they seem to be coming in almost perfect now?

Thanks.

Slade

 

[addo]

I can't see why it would. When they open, the A/F ratio in their bores will still be the same.

Then again... amateur thoughts approaching! If they open later, youre running a greater velocity through the primaries; more air, same maximium jetting = leaner so marginally higher RPM - possibly. Upon opening the secondaries, the A/F would drop, but bog? I couldn't tell you.

 

[Guest]

Before you change it, you need to take me for a ride 8) I'll buy the gas!!

 

[80broncoman]

I think you are right on the money. You found the biggest factor in fuel economy.... ones foot .
I was always told if you ever feel the secondaries "kick in" on a vacum sec carb what you really fell is the bog of transistion between the two.
You might be surprised to see what happens if you simply disconnect the secondaries for a while. it might not help at all if you constantly have the pedal to the floor.

 

[CobraSix]

How do you disconnect the secondaries.

I'll clarify "feel" the secondaries. I mean, you feel the acceleration...you really don't feel any bog at all prior. But that reminds me that maybe I'm just not feeling the bog, so maybe it's there and I need to stiffen the spring up.

Al...anytime, you available This saturday? My wife will be in Boston. I think you'll be grinning, especially with my loud ass exhaust...

Slade

 

[wsa111]

slade, holley makes a secondary spring assortment as a package deal. if you can't find them at your local parts store, summit or jegs carry them.
give me your list # of your carb-its stamped on the left side of the primary choke air horn. with that # i'll look at my holley book when i get home & see what spring your carb came with. later william

 

[80broncoman]

Under the diaphram is the rod that attches to the sec linkage. just take off the the e clip and pull off the rod.

 

[CobraSix]

My local mustang shop has the holley kit. That is what I was going to get, along with a quick change kit.

Slade

 

[wsa111]

Slade, i don't know what you want??performance or ecomony?? according to my holley book, the 390 carb has a plain spring.on a 350 cubic engine the opening is from 2240 rpm -8160 rpm. i may be wrong but you might want to go to a lighter spring according to the holley book.
the spring i would chose would be the lighter yellow spring in the kit. if you spent all the money on an oz head i would chose the best performance available. but thats up to you. the only way to see if you get full secondary throttle opening is to remove the hood & look at the secondary throttle lever?? if you want fuel miliage man just keep your foot out of the throttle.
i'm not trying to be sarcastic but its your choice of the both worlds available. just my opinion. william

 

[CobraSix]

Generally, if your car is tuned right, especially on 6's, you can get mileage and performance. My biggest concern is that many people with the Oz are getting better mileage. I should be gettting better then 20MPG is basically my point. I was getting 25-26MPG before.

I think I figured out part of the problem. The V-8 throttle rod I used is a little too long for the lever arm and I'm at WOT at 70% of the pedal position. That means I do not have a lot of fine tuning room with my foot. I'm going to try to move the position down the rod some to shorten the lever arm so I can get WOT at just before pedal to the floor.

Slade

 

[xctasy]

........My only issue is with my secondaries. They transition in nicely without bogging. That makes me think they are set about right. My only problem is they are coming in too early and my mileage is really suffering as a result. Part of it is I like the feel of the secondaries. Makes it fun to drive, but my mileage has only been averaging a mere 20MPG. So I ran an experiment with a vacuum gauge attached and I tried to limit my hard accelerations and limit the use of my secondaries. My mileage when up to about 25MPG combined highway and city (70% highway). So, it leads to believe that if I go to a stiffer vacuum diaphragm spring, I'll be able to increase my mileage.

My question is will that make my secondaries transition in more poorly since they seem to be coming in almost perfect now?

Thanks.

Slade

With a Holley 4-bbl with vac sec actuation, the acid test is this. When you open the throttle at a part throttle cruise on the highway, you should feel a slight dead spot. Ifyou don't, the secondary system is not sprung hard enough.

The consensus from all experts I've consulted is that the quick operation of secondaries hurts mpg unless you have a 4180 600cfm carb like in a 5.0. They are set up different to earlier 4150 and 4160 series carbs. Go for a stiffer spring, and see your milage improve. It's that simple.

You have a classic Hobsons choice, damned to burn fuel with no flatspot, or a damned flat-spot with good economy. The 390 is an eccono carb for smaller engines like 215 Buick/ Rover 3.5, 2.6/2.8/2.9 Colognes. On a six it has quite good vaccum, and is opening too early.

Try the stiffest spring, and check mileage. Do some 30 to 50 mph in 3rd acceleratin runs. The flat-spotting heavy spring will be little different to the current smooth transiting spring, but fuel economy will be radically different.

Deano

 

[wsa111]

Slade, the holley uses venturi vacuum only to open the secondary throttle blades. Engine vacuum has no play with this carburetor like the early loadamatic system, on the distributor advance which used a combination of venturi vacuum & manifold vacuum to control spark advance.
You need to pull the hood off the vehicle or put it on a chassis dyno to determine at what rpm the secondaries are starting to open & what rpm they are fully open. Of course within the rpm band of your engine. Sure you can put a heaver spring in the vacuum housing. But if you want performance this is not the way to go. for example with too heavy a spring the secondaries may not be fully open till over 7000 rpms. Y ou need to evaluate what you have, which sounds very close to proper performance. I agree with jim, you may have the perfect combo now. You can go lighter on spring tension till it has a bog. then go to one spring tension higher.
You can control fuel miliage with your right foot.
I wish i lived closer to you i would love a test ride with your oz head. Best of luck william PS fix the linkage problem first so the throttle is not so sensitive

 

[CobraSix]

Yeah, going to make a run to Truvalue today so I can find something to move it down.

Slade