Open secondaries vs drilling hole in primaries

[bandit70]

Have a rich issue at idle.  Suspect larger cam in it from stock but not positive.  Timing is set to right before ping.  Can get rid of richness by opening secondaries a little but then I lose my idle mix screws and they are non responsive.  Anyone know if I will lose the idle mix screws if I drill holes in the primary plates for extra air vs opening the secondaries?

Holley 3310-4    4 barrel, 750 cfm, vacuum secondaries

[firefighter3931]

Drill The primaries....then you will be able to slow down the idle speed and the transfer slot will no longer be exposed....the idle circuit will now function. 

I've done many like this to compensate for a low idle vacuum condition. I use a 1/8in drill  bit 



Ron

[Chippa]

I had my 750 3310-4 off last weekend and set the primaries to .20 transfer exposed and then just cracked open the secondaries until i acheived the idle setting i wanted....

Car seems to run ok, is there any probs setting it up this way??? that is adjusting/opening up secondaries to get some more air in..set mixtures to highest vac reading  so i know it was metering  the idle mixture ok or am i off the mark  

Chippa  

[bandit70]

I lose my idle mix screws when I open the secondaries by even a little.   They only respond when the secondaries are all the way down and only a square is showing in the transition slot.   This makes the car way too rich though.     Need a way to lean it out and keep my mix screws active.

[Chippa]

If you set your primaries to square T slot and then crack open your secondaries more to get the air i dont understand why you cant control your richness as you should be only on your idle circuit and not drawing fuel from your main as well....

It sounds like you have gone thru all the first steps,

timing set
fuel pressure/float level(this can cause you probs if not right)
adjusting mixtures to highest level with Vac gauge

Main jets for cruising
PV for enrichment (this shouldnt come into play at idle as they feed the boosters once there is enough velocity to pull fuel thru them say >2000rpm)

I guess like Ron said you need the extra air to compliment the already mixed fuel coming thru the idle circuit but this is where i get a bit stuck as thought you can acheive this by crackin the sec open??

This is where i wish i had a newer carb with adj AIB's and IFR's etc to really play with things more

RON or CHRYCO please chime in if my thinking is wrong, i;m still learning the dark art of carb tuning haha

Bandit70 heres some good reading for you
its quite a long read but i liked it

http://members.tccoa.com/392bird/tuning.htm

Chippa

[tan top]

Drill The primaries....then you will be able to slow down the idle speed and the transfer slot will no longer be exposed....the idle circuit will now function.  

I've done many like this to compensate for a low idle vacuum condition. I use a 1/8in drill  bit  



Ron

yep    best plan  

one particular cam i was trying ,   needed drilling the primarys ,  was about to , but checked edelbrock web site , as i had swapped to a thunder  , they dont sell spare throttle blades , so decided against it , & swapped cam again as it was a bit to radical for a tight converter i had anyway ,   .  not got that problem with holleys , they sell all spare parts  

[bandit70]

I called Holley yesterday and they said if you open them too much it goes onto it's circuit and why I lose control of the screws.   But I am losing them with it just a tiny tiny bit lifted.   Only completely seated will get the screws working.   They suspected an air leak which I am going to try and hunt down today.   I am not sure what to do here besides drill if I want to solve this and not get into a whole big can of worms on this.    The car runs fine with the secondaries out(maybe a little rough idle but not too bad) but the reason I started digging into this was because those mix screws did not work.    I could just put it all back and live with no mix screws.

What is the deal if you are not on the idle circuit?   I could not notice any big detriment except a little rough idle but nothing I could not live with.  Is there a big deal with needing it on the idle circuit?

[bandit70]

No vacuum leaks that I can find.   

[firefighter3931]

What is the deal if you are not on the idle circuit?   I could not notice any big detriment except a little rough idle but nothing I could not live with.  Is there a big deal with needing it on the idle circuit?

The idle circuit is designed to be leaner. Basicly, when your car is at idle and there is no load on the engine so you don't want the mixture to be too rich. Ideal idle mixture is 13.8-14.5 AFR from my experience. If your idle is too rich you will foul the plugs and can eventually wash down the cylinder walls resulting in ruined piston rings. Excess fuel in the oil is not good either as it will ruin the bearings.  

Vacuum secondary carburators are designed to idle on the primary venturies....there is no 4 corner idle provision on a 4160 style carb. So, you have to make it work with the primary side of the carb and that means drilling some holes in the butterflies to allow more air in. This will close off the exposed tranfer slot and allow the idle circuit to function.  

Personally, i would not idle any engine i wanted to stay healthy on the main circuit of the carb for any extended period of time. The idle mixture has to be lean enough or you will hurt the engine  



Ron

[Chippa]

Yes i agree with you Ron as i get how the idle circuit works but the thing i'm wanting to understand is with a vac sec if you SET the primaries to where it should be so that you are only in the idle circuit then opening the secondaries a little should negate the need to drill holes in the butterflies yes???

Chippa 

[bandit70]

Makes sense.

Update:  took of the carb again and found some binding in the secondaries that would not let it seat all the way.  It was only closing enough before to where a square was showing on the t-slot(most say this is where it should be anyways) but could not get it lower.   I now have the secondaries fully closed off and tested again.   Car will not stay running now unless you open up primaries agressively.   It did not like having those secondaries completely closed.   

Without considering opening the secondaries for the rich issue I am having, where should these be set?  I have seen two differing points on this.  Some say to leave a square in the transfer slot and others say to just turn the screw until it unseats the secondaries but does not open them.   I am probably going to drill the holes tomorrow unless I think of something else, but what is the offical spot for these secondaries to be set at?  Closed or t-slot showing a square?

One other question, I have my timing set to right before pinging on full stomp but could the secondaries not fully seating contribute to this too? 

[Chippa]

When you took the carb off are you looking at the position of the T slot on the primaries or secondaries?

You only want to concentrate on the primaries,dont try and set the T slot on the sec as they are higher up in the venturi.....get the Prim' set then if you can adjust the Sec' screw to get desired idle (i made up a little tool to adjust the sec' while motor was running)

Read thru the link i sent you i think it explains it well,

[bandit70]

Thanks, I'm reading through it right now.    The tslot square I am talking about is on the secondaries.  

You have to pull the carb to adjust the secondaries(real pain in the ass when trying to fine tune).  The screw is underneath.  So really no practical way to set the primaries then fine tune the secondaries without pulling that thing a 1000 times.  I think I saw an adapter you can buy but there has to be an easier way to just set this.    

[Shields14]

i have a 670 avenger all floats and pressure checked and good  on 440  just start to get into the sweet spot with the air fuel screws and it bottoms out. any help with that would be great i have had the carb on 2 different 440s and the same with both  have new performer intake and new gaskets all thru..

[Chippa]

I just modified an old flat blade screw driver and used a 3/8 ring spanner and was able to turn the adj screw without pullin the carb off...

Pull the carb off once,set primaries to show T slot square and then do not change setting ,put back on and if it doesnt want to idle adj secondary shaft screw as i have done above

[bandit70]

Being able to turn that screw while it is mounted would be nice.   I'm going to try and rig something up.