5.0pntydrpr

Hello all,

I have recently installed a vortech v1 s-strim supercharger into my 66 fastback.

The engine is a 5.0 from an 89 GT with the following mods:
Comp Cams XE270HR cam
GT40 iron heads
Weiand Stealth intake
holley street hp 650 carb (list 82651) ((Wonderful blow through carb btw))
Hooker long tubes with 2.5 inch dual exhaust with magnaflow mufflers
stock 289 distributor with pertronix ignitor I and flamethrower coil.
1 step colder plugs at .034", 10* initial timing with 30* total timing.

I have no problems tuning at any rpm and throttle position above idle.

My problem is my wideband shows 11~11.5 afr at idle. I've tried everything: I've adjusted the transfer slots back to stock holley settings, almost completely closed the throttle, opened it farther than stock, nothing I do will produce a 13.5~14.5 idle mixture when the engine is warm. When the throttle blades are at holley's settings, i get the leanest reading: 12.8. But i'm idleing at 1600-1800 rpms which is too high - I would like to idle around 900-1100.

The throttle screws are working in that if i screw them almost all the way in, the engine almost dies, but the AFR is still rich. This tells me that the power valve is fine. I have also removed it and checked it - it is also fine. I also checked for vacuum leaks and have found none.

The vacuum advance is hooked up and working properly, but i have adjusted this also to play with my idle speeds so that I could reposition my throttle plates - to no avail.

Any input or ideas are welcome. Maybe I missed something easy? Do blow through carbs usually idle rich?

Gun Jam

Mine is the same way. Ive come to the conclusion you'll never get a good run at 14.7 arf idle. I think this is due to valve over lap common with performance cams. You have an incoming intake charge that partly escapes from the closing exhaust valve and causes a rich idle.

Now that said you should be able to obtain a 14.7 reading by adjusting the 4 idle mix screws one on each side of the metering blocks (i think thats a 4 corner idle carb) turning these screws in should reduce the volume of emulsified air/fuel mix entering the idle ports and cause the afrs to lean.

How far out are the idle mix screws?

5.0pntydrpr

Yep mine has a 4 corner idle adjustment. I can totally follow what you are saying about the valve overlap with a performance cam.

I can also follow what you are saying about the 4 corner idle screws emulsifying the air and fuel entering the idle ports, but I currently have them backed out about 3/4 turn from seated, and if i go in any more, the idle suffers severely and the engine starts to bog. If I back them out anymore, the the idle goes pig rich.

I drilled out the idle air bleeds to .091" and it helped a little bit, not enough. Any other ideas?

Gun Jam

my wide band shows 11.5 at idle as well. I have QFT with adjustable air bleeds and have been trying different ones. It came with 70 pri and 73 sec idle ABs. It doesn't take much to make a dig bifference in the way it performs at cruise speed or off the line. The idle circuit is plays a huge roll even at speeds above 70mph under normal load. Changing your IABs is going to effect your cruise AFRs negatively (if it was perfect before) so in an attempt to correct for a lean cruise one might reduce the high speed AB size in an attempt to get a richer mix from the boosters....Im not a 100% sure if that would help at all.

That said what is your vac readings at idle? What power valve do you have installed now?

68dustin

I haven't been able to get mine in the 13's either. I'm running a blow thru with 10psi. Cruising usually is around 11-12 too. But every time a pull a plug they never look like they are running rich.

67mustang302

A lot of it is cam. I'm running XFI and idle at 13:1, that's as lean as it tolerates without causing issues in some places.

Domestic engines typically don't like lean idles.

5.0pntydrpr

Gun Jam: I believe you are exactly right. When the throttle plates are more closed while cruising, the idle circuit carries more weight so to speak. Today, my cruise was at about 15 to 15.5, so i will be ordering some smaller air bleeds and installing them. Vacuum is very respectable at 20 in/Hg, and i have a 6.5 power valve that works right when i need it.

68dustin: Yeah, I think i'm just going to have to accept a richer idle, as the cruise and all other rpm air fuel ratios are good. I will probably take the secondaries down 2 sizes as i am right at 10.5 to 10.8 and i would like to be at about 11.3-11.6. Advancing my initial (and consequently my total) timing 3 degrees today helped out the idle more than i thought: about 12.5; not to mention quite a bit more power...

67mustang302: I think you hit the nail on the head with the cam. Gun jam mentioned earlier that because of the valve overlap, just before the exhaust valve closes, the intake valve begins to open and some of the fresh air and fuel mixture might be sucked out of the exhaust, triggering the o2 sensor to return a richer reading. This is one very possible cause...

67mustang302

I'm glad you understand that, a lot of folks get confused by this and it's frequently talked about on the CPG forums for XFI. The fresh charge pulled through the overlap at idle has tons of unburned fuel that causes the O2 to read rich. The O2 is just an ATTEMPT to measure air/fuel ratio (in the chamber where it matters), but actually only reads after effects and can be tricked, by things such as overlap. It reads the exhaust stream, not the chamber.

The chamber may be at 14.5:1 when combustion takes place, but at 14.5:1 in the chamber it may read 12.5:1 in the exhaust because of overlap. You have to give the engine what it wants to be happy, not necessarily what you think it wants on a gauge.

Most domestic (American pushrod V8 engines) typically want to run richer than what most people think.

5.0pntydrpr

67mustang302: Thanks for the confirmation on my thoughts there.

All: I just had a thought: now that we've already established that the "rich" idle may not actually be rich, but just 'reading' rich, then if my WOT reading is 10.8:1, then what is the possibility that I'm actually running leaner? The reason i ask is that if I actually am running 10.8:1 at WOT then i would like to jet my secondaries a little smaller pushing for 11.5:1. BUT, if I'm pressuring the intake charge, and the intake and exhaust valves overlap, then could my o2 sensor be returning a richer reading? Should I leave well enough alone?

I really, and I mean REALLY, don't want to blow my engine due to detonation. I'm already at 33* total timing at WOT with about 7 lbs of boost at 5500 rpm. I can't hear any pinging, but i'm not sure if I should push it. Recommendations anyone?

barnett468

my long post just evaporated so this might be sloppy, sorry

test again for san air leak using flammable brake cleaner. plug all vacuum fittings at carb. check at intake at head. you might have a leak in the vally area which won't show up.

that cam should idle at 900.

you must have no vsacuum to distributor sat idle. if the ported vac has vacuum ast idle at 900 rpm you might need to drill your front throttle plates. we'll see .

your eng should due with the mix screws in all the way. since it does not, either your idle set screw is holding the throttle too far open or the secondary butterflies are too far open.

if your secondaries are set correctly and your idle set screw is all the way put asnd your engine still runs you have an air leak.

i would disconnect the dist vac

turn the idle down as far as possible.

advance the distributor until the car just started to stumble or the rpm no longer increases then retard it 4 degrees.

if it still starts fine when hot it is good.

if it turns over slowly when hot then retard it 3 more degrees. this is where it wants to be.

that cam likes around 34 max timing depending upon compression.

what is your static or dynamic compression?