570 secondary vac tuning

[LJDB 103]

I have a slight delay when flooring it from cruising in any gear or any speed. I think its the secondaries opening but changing from one spring to another makes the car feel flat. I have the short yellow spring in ATM , flooring it from standstill the car pulls strong and no delay or flat spots ect. The next spring up doesnt feel as strong. Can I tune the primary accelerator pump to compensate for the secondaries opening quickly with a longer duration pump cam. I currently have the orange in hole 2 , it stops pumping around 1/2 throttle?

[gerg 10,871]

Your acc pump only should relate to what's happening in the primaries. Your secondaries need to open late enough that they flip open quickly, any earlier and they'll take too long to work and with the sec throttle cracked open a tad, that's where the flat spot occurs. It's not flowing enough air through to pull sufficient fuel out. I'm assuming it's a 351, so then I have to ask at what rpm are the secondaries opening? At a glance, it sounds like you need to go stiffer on the vac spring.

 

EDIT: just realised it's a 6-cyl, being in a 6-cyl thread and all.

[PRO250 1,506]

Have you moved the jetting around to see if you can get rid of the issue? 

Some 250s ive had cant cope with a 4 barrel, most will dip a fair bit in there power curve onto the seconderys but if you lift the spring or jet it down it will bog somewhat, if a widebad was fitted it would show a issue in the tune 

Some ive had have had this issue with a vac. Its very hard to get the best of both worlds a good clean tune from a stand still vs a mid throttle mash, you just flatten it and this was one reason I don't like having no secondery pump with a double pumper you can work around it with not much fuss the vacs you cant tune as much. 

Even one of my clevos with a vac had this issue. It works well from a stand still but a mid throttle mash it will pull it down just a touch. Cliping springs can be made to work a little different to a stock spring assortment, ive clip 3 of my vac cars to get the best out of them. Holley will say not to do this and some here will say the same but its worked for me.    

[PRO250 1,506]

Check ball is another thing to look at if it has one. If it runs one they will open slowly if you don't run one it will more or less flop open. on some engines it works others it will not, but is worth a try. Holley also say no to removing the check ball, but car to car it can be made to work.

[gerg 10,871]

^^^ Yeah that's pretty much spot-on. The stock 570 might be a bit too much carby for a 250 so go stiffer in the secondary spring (or clip the current one at a pinch). This makes sure that air velocity through the primaries is at its max before the secondaries cut in.

 

Jetting secondaries on those carbies can be tricky as they're a meter plate (I think) on that side. They can be drilled but once you do that and go too far you need to replace the plate.

 

Another method to overcome secondary bog (after you've got your spring set up as good as you'll get it) is to raise the secondary float level a smidge. This gets the mains flowing a little earlier. Go in 1/4 turns of the float screw with each adjustment.

 

First port of call is to shove a stiffer spring in. I'd go at least a purple or stiffer. Scroll down the Holley instructions to see their spring rates on a 350 CID engine:

 

https://ac17cb7e3cc3506f0d23-7386afb7d61f5e5af0e5a817d2877bfe.ssl.cf1.rackcdn.com/199r8219-2rev.pdf

 

Or alternately:

 

That chart shows yellow as the weakest spring but Holley shows a white one that's weaker still. Anyway, your yellow one is too weak and is allowing the secondaries to open too early, causing a drop in flow though the primaries and leaning out.

[LJDB 103]

its not so much a bog its more a delay I don't feel a loss of power just nothing happens then it kicks in and goes. if I put a stiffer spring it doesn't delay but it takes longer to rev out. the car feels stronger with a lighter spring. what I should have said is can I tune the pump to allow me to run a lighter spring. its in a 250 4 speed with 3.7's

[gerg 10,871]

What size squirters are you running? You can draw out the delivery for longer using smaller squirters. Acc pump tuning is ok but using that as a means of covering up an unrelated issue is really a band-aid fix.

 

Also note that 570 CFM is a lot for an engine that would struggle to swallow more than 400. Your carby is perfectly suited to a warm 302.

 

I think you should keep the spring stiffer and tune the primaries ie: jet up a tad or open up the PVCR if your jets feel ok in the midrange.

 

What jets are you running?

 

What is the stiffest spring you've put in?

[LJDB 103]

31 squirter with orange cam in pos 2, 64 primaries  70 secondaries 6.5 pv. silver/standard spring is the heaviest iv tried. feels better with the short yellow.

[gerg 10,871]

64/70 jets sound about right, depending on how big their air bleeds are these days. They sound bigger than stock jets. Maybe you could drop your squirters down to 28 or even 25. 31s are a bit big for an engine that size.

 

It's much cheaper to fork out for a spring kit than to buy them individually. Maybe go one stiffer than your silver one? Hume Performance does pretty much nothing but Holleys and will post them to your door. They've got a pretty good range of bits online.

 

One way to eliminate the problem of your secondaries opening is to wire them shut and/or disconnect the actuator. That way you can get your primary tuning spot-on and then work out what point you want your secondaries to open up.

[LJDB 103]

I wired the secondaries and found the delay is just the secondaries coming in hard. It felt alittle doughy on the primary's and the butterflies where soaked from a massive pump shot so I changed to a white cam and a 28 squirter and it felt more responsive. When I get a chance I'm gonna do a plug chop on the primaries to see what jetting is doing. Should I do it at full noise so the PV is in also or cruise to check just the jets

[gerg 10,871]

Just cruise for now, PV tuning can come later if needed. I reckon you may need to try less jet on the primaries, say 61-62 or so.

 

To reduce the secondary delay, you could raise the float level a smidge.