Using AFR gauge and how to know which carb adjustments to make?

[fairlaniac]

I bought an AEM AFR wideband sensor - gauge and installed it on my Fairlane. I'm not a good carb tuner, I do ok but there is room for much improvement. So my maiden run using the gauge. I am around 12.4 idle, 11.3 cruising and 14.4 hard on the throttle. I saw a 15.xx while on the throttle once but couldn't duplicate it. I notice a 10.xx reading when cruising and that instant I hit he throttle hard. So it goes 12.4 - 10.xx - 14.4. Where do I begin? I'm going to pull my bowls as I lost the paper I had the jet sizes and PV size written on.

Thanks,

[Drew Pojedinec]

Reverse the cruise and WOT and you’ll be nearly there

[chilly460]

I’d go bigger secondary main jet as you’re lean at WOT, maybe see bit smaller on the primary jet as you’re rich at cruise.  10:1 is the squirter hit when you go WOT

[Joe66GT]

Some good reading here...

https://www.yellowbullet.com/threads/holley-4150-hp-metering.490296/#post-9219091

https://www.yellowbullet.com/threads/carburetor-guys-lets-have-a-discussion-on-emulsions.435314/page-2

[fairlaniac]

Some good reading here...

https://www.yellowbullet.com/threads/holley-4150-hp-metering.490296/#post-9219091

https://www.yellowbullet.com/threads/carburetor-guys-lets-have-a-discussion-on-emulsions.435314/page-2

Thanks for the links but even these are a bit over my head at this time. I’d like to understand when I see the numbers on my gauge what do I need to change, how much of a change and what to expect from the change. I don’t know when to make a jet change to a primary, secondary or both?

Thanks!

[Drew Pojedinec]

To start, what carb are you using?
It is a 750 double pumper correct?  list number?

[fairlaniac]

To start, what carb are you using?
It is a 750 double pumper correct?  list number?

Drew, it is a 750 DP 4779. I currently have:
Primary jets - 72
Secondary jets - 80
Primary power valve - 55
Secondary power valve - 65
primary squirter - 28
Secondary squirter - 31

FYI - I converted this to 4 corner idle circuits about 10 years ago. It's a milled horn.

I am playing with this one but am sending you my 850 DP to look over. I'm using this 750 as a learning tool and then when I get my 850 back, I'll tune that.

Thanks!

[chilly460]

During cruise you should be more towards 13.5-14:1 as it's light load.  At WOT, you should be around 12.5:1 depending on what gas you run, so as Drew said, you're currently "backwards" from where you'd want to be.  Your cruise is rich and lean at WOT.

Since you cruise on the primary jets only, I'd reduce them two sizes to start, it should lean it out.  Since the secondary jets come into play when you go WOT, I'd go up four sizes to start, two to make up for the jets that you reduced on the primary side, plus two more to start to richen it up a bit.  I like taking little jumps, you could go up six sizes as well and see what that does directionally. 

I don't worry too much about idle as a big cam, in my opinion, can skew AFR at idle.  I'd tune for mixture for max vacuum at idle and leave that alone for now. 

Drew knows a LOT more than I do, he may be able to chime in. 

[Drew Pojedinec]

If it was me....

I’d toss a vac gauge on and see where my cruise vac is.
It is possible the power valve is opening.

I also don’t bother using secondary power valves unless for some reason you are cruising with the secondaries open.

Also make sure secondary transfer slot is not exposed. Basically it can idle fine at low vac, cruise with high vac pulls on the slot making it really rich. Same reason I’m not a huge fan of 4 corner idle.

Your 72 jets are in the neighborhood so I wouldn’t make any big changes there until I ruled out the other things.
I’ve also been know to plug all power valves and go for a cruise with no high power runs. Get the jetting nailed down with no enrichment. Add enrichment later for WOT usage.

[fairlaniac]

If it was me....

I’d toss a vac gauge on and see where my cruise vac is.
It is possible the power valve is opening.

I also don’t bother using secondary power valves unless for some reason you are cruising with the secondaries open.

Also make sure secondary transfer slot is not exposed. Basically it can idle fine at low vac, cruise with high vac pulls on the slot making it really rich. Same reason I’m not a huge fan of 4 corner idle.

Your 72 jets are in the neighborhood so I wouldn’t make any big changes there until I ruled out the other things.
I’ve also been know to plug all power valves and go for a cruise with no high power runs. Get the jetting nailed down with no enrichment. Add enrichment later for WOT usage.

I've never heard a recommendation to cover the secondary transfer slot? What is the theory there? Also I had a local company TRC Custom Carbs provide my 4 corner based plate a few years ago. They also drilled a small hole in each butterfly.

I'm waiting now for some gaskets. Tore the one fuel bowl gasket. Ordered some earlier, here on Tuesday. So no testing til then.

[Drew Pojedinec]

Theory is as noted above.
More vacuum=pulls harder on any fuel restriction=rich cruise. Your primary idle circuit should provide sufficient fuel until the mains come online.
Typically with a 12.5:1 idle, 13.5:1 in gear, leans out until mains start where it temporarily richens up a touch.... cruise should lean out until power valve opens.
How lean depends largely on your manifold distribution and cam action.

Just try it man

[6667fan]

Doug, is the engine a 482? I can’t recall which engine you have going between the Fairlane and the other car. If so that 750 might be small for that motor. I know you mentioned sending Drew the 850.
Once you get that carb back a trip to a shop with a chassis dyno would be money well spent.

JB

[fairlaniac]

Doug, is the engine a 482? I can’t recall which engine you have going between the Fairlane and the other car. If so that 750 might be small for that motor. I know you mentioned sending Drew the 850.
Once you get that carb back a trip to a shop with a chassis dyno would be money well spent.

JB

Jim, I have the 750 DP I had on my 390. I just want to get some cruising runtime on the engine and it is a 484 in the Fairlane. I was looking at going to either of the two dyno shops within an hour or two but both are closed. Non-essential so that is why I'm poking around myself. I plan on hauling the Fairlane to Springfield for the Fairlane Nats but I want to get it running well before then. 7 weeks seems like a long time but life, work and running the club all take time away. A dyno shop may not take place until August or so.

[6667fan]

Hi Doug, I know you are taking some action with the 850 carb , however I have a Quick Fuel Q series 950 that would be very good on your 484.
Let me know if interested.
JB