I just installed an Edelbrock avs2 on my truck. It is the 500 cfm, elec. choke model. Eddy's have a reputation for being bolt on and play, so I was surprised to have to richen up the mix screws more than a full 360* to drop down to the 12.6 to 1 air fuel mix that is required. I set it at the 12.6 to 1, and the next morning, after warm up, it was back to 12.2 to 1. Now, I set the mix with the air cleaner on, engine at running temp, and 3-4 miles later the air/fuel mix/ratio is back to 12.2 to 1. I also notice that at cruise speed, about 2,000 rpm, the carb leans out to 15.5 to 1. I understand that the lean situation is most likely due to a need for fuel that can be fixed with a larger jet size in the primary, but the mixture change is something I have never come across. I have searched for vacuum leaks by spraying around the base with starting fluid. No leaks. Does anyone have any insights?? Thx. Kevin.

 

The two mixture screws aren't for adjusting air fuel ratio. That's done with jets, metering rods, and step up springs. The idle mixture screws should be adjusted to get best vacuum at idle (using a vacuum gauge). Then you tune the carburetor for cruise, acceleration, and wide open throttle with afore mentioned components and the secondary throttle valve adjustment.

I'm assuming you have an A/FR gauge? Don't worry about your air fuel ratio at idle. If it's 11-14:1 after you've tuned the driving circuits, you're fine. And no carburetor is tuned right out of the box. We used to think they were, but A/FR gauges taught us otherwise.

 

I'll tell you the order I tune carburetors in. Good refresher for me also, since I'm installing a new one tomorrow. It happens to also be an AVS2.

First I hook up a vacuum gauge to the manifold vacuum port on the carburetor. I lightly bottom out the idle mixture screws, then back them off 1 turn. Crank the engine, and get it up to normal operating temperature. That's important, especially in cold weather conditions. I then set the idle speed, then I start backing off the idle mixture screws, about 1/8 turn each, until I get the highest vacuum reading. The idle speed will have changed, so I reset it.

Then I go for a drive. I try to start with cruise A/FR. If you see a lean spike while accelerating up to cruise speed, don't worry about it yet, as long as it goes down. If it doesn't go down to below 15, it'd be best to go home and jet up the primaries. This is a matter of taste, but it also effects fuel mileage. If you tune your cruise A/FR to 14.7:1 or higher, you're gonna have to change to 10" or higher step up springs, so you don't get lean spikes every time you ease into the throttle. But that's gonna use a lot of fuel. I like my cruise A/FR between 13.5 - 14:1, because it's gonna get higher as I go up slight inclines or lightly accelerate. That's achieved with primary jet and metering rod changes.

Then I check light acceleration from cruise. I correct lean or rich spikes with step up spring changes. The vacuum gauge comes in handy here. It'll tell you which springs to use. I have vacuum gauges mounted next to my A/FR gauges. But for tuning, you can just put the vacuum gauge under your windshield wiper while you drive. Just don't use the windshield wipers.

Then I check wide open throttle. If it spikes when you floor it, then adjust the secondary air valve. Every AVS2 I've tuned needed the secondary air valve retarded a bit. Once that's done, go to wide open throttle again, and check A/FR after the valve has opened. Any A/FR correction need there, is done with secondary jet changes.

And it's a good idea to record your tune, because it's gonna change with the seasons. I've got three different tunes recorded for mine.

 

Great stuff. Thanks a heap. I'll be trying your system out tomorrow. Kevin.

 

Great. Let me know if your truck blows up, so I can do mine differently.

 

I'm about to head over and tune. One thing I found while driving it this morning, fully warm, was that when I try to accelerate with the other cars from a stop, I'm having to use 3/4 or more throttle just to get a good acceleration. It doesn't go lean or spike, but takes an abnormal amount of throttle just to stay with traffic.

 

Do you have an A/FR gauge?

 

Hey Mike. Yeps, I have an AFR gauge, and a vacuum gauge, and a tach. So, I just had a very irritating conversation with Edelbrock. According to them, the only way I can get a common acceleration is by installing stiffer springs in the power boosters. I asked: So, for your carb to accelerate with the normal city flow of traffic, it has to go into power mode? He said 'I don't know what else to tell you.'

That can't be correct. That would destroy any hope for decent mpg. He couldn't care less if I kept it or sold it. Wow. I'm considering returning it to Summit for a refund, or changing it out for the Summit 600.

 

No, that sounds right. When I'm accelerating from stop to flow of traffic speed, my vacuum gauge shows that the metering rods are lifted on my Edelbrock, and my power valve is open on my Holley. As I speed up and my foot raises, I see the vacuum rise, and know that the rods are dropping. My Dent has about 20" of vacuum at idle, and about 14 at cruise. It takes very little foot action to make that drop below 7", which is when my metering rods start to rise. I don't know what carburetor you had before, but what your foot feels can be very different between carburetors.

Just tune it for A/FR. It will all work out.