Holley 770 Street Avenger adjustment

Hello all,

I recently replaced my Holley 750 with mech secondary with a new 770 Street Avenger.
Mainly because the vacuum secondary and electric choke which increases starting and drivability.
Installed it straight from the box and she started up and run all ok.
Previously i was running very rich on my double pumper, and although i re-jetted several times, coming down from 78 to 70 on the primary it just refused to run any leaner.
At idle i could easily set it from anything 10 to 18 AFR no problem, but during cruising i am lucky to see 11. Mostly 10 - 10.5 AFR.
When hitting it or at WOT it just leaned out far, probably due the re-jetting i did but came back to more decent figures after the initial hit.

Anyway, new carb new mixture.
When the engine reaches normal operating temperature it idles around 17 - 17.5 AFR, is this a problem?
I tried to adjust the idle screws (2-corner) and can bring it down but bringing it down very far comes with vacuum loss, which is already not great at idle. (now 9.5-10 Hg at idle in neutral)
When speeding up normal, fast or WOT the mixture is good all over, 12.5 - 13.5 so no issues there at all.
But when it comes to maintaining speed it runs too rich, around 11 average i would say at speeds of 30 - 70 mph.
I want to try and bring this down, the carb came from factory with 72 jets on the primary and a 6.5 power valve.
I have changed this to 70 jets and a 5.5 power valve but nothing really changed.
My timing is set at 20 deg initial, 18 deg mech advance, all in @ 2500 rpm. (black bushing)
No vacuum attached at the moment, but the can is still installed on the distributor.
Now, due to the 4.10 ratio rear axle the engine rpm is above 2500 rpm at 50mph.
Wondering if i should recurve the distributor to add more timing earlier? (See attached advance curve i have now)

With the previous carb i tried the vacuum can on the manifold vacuum but this adds 10 deg at idle already so i had to retard the timing and try but it run like crap.
After i tried it at ported but started to ping at light loads. (with initial timing back at 20 deg)
I have not tried the vacuum can yet with the new carb.
I was thinking it will be better to ask you guys for some advice on what can be done.
My feeling says try the vac can on ported vacuum to add some timing when at cruise, but as this was unsuccessful with the previous carb i am a bit lost.
If you have some tips to share with me it would be highly appreciated guys.

See some specs:
1969 Coronet R/T
440 cid, street/strip cam (unkown specs), headers, auto tranny, 4.10 rear axle with LSD, Holley Street Avenger 770, MSD RTR distributor.
AEM wide band AFR sensor in the driver side exhaust.

Current distributor curve and vacuum canister curve in the picture.

Thx for reading!!

I have a 770 ultra avenger, and in my case the idle feed restrictions were really small, I think maybe 0.025"
I needed to make them slightly larger (richer idle / off idle transition) Either of the carbs can be made to run correctly, but you mat have to change more than jetting.
You may want to get David Vizards Holley tuning book too?

https://www.amazon.com/Vizards-Modify-Holley-Carburetors-Performance/dp/1934709654

Thx 451Mopar,
How did you make the idle feed restrictors larger?
Is there anything required to do this? new screws or so? I knew i could get in trouble with the 2 corner idle adjustment compared to the 4 corner system i had.
I went to do some touring today and i must say i love the way it drives now, i am catching myself all the time i am driving it like i stole it.
It picks up smooth and goes smooth trought the revs, but if you hit it, it comes in hard and loud.
Drives like a beast, and good to know the mixture is spot on during speeding up.

Wietse said:

Thx 451Mopar,
How did you make the idle feed restrictors larger?
Is there anything required to do this? new screws or so? I knew i could get in trouble with the 2 corner idle adjustment compared to the 4 corner system i had.
I went to do some touring today and i must say i love the way it drives now, i am catching myself all the time i am driving it like i stole it.
It picks up smooth and goes smooth trought the revs, but if you hit it, it comes in hard and loud.
Drives like a beast, and good to know the mixture is spot on during speeding up.

Click to expand...

Sounds like you'll need to increase your pvcr and decrease your primary jetting. If your rich condition is in transition, reducing the IFR would help.

By increasing the pvcr you get more fuel coming through when the power valve opens right?
And to offset that get smaller jets.
It only runs rich when maintaining a constant speed, when accelerating the mixture leans out to healthy numbers.
I do have the book from David Vizards, but there is so much info coming at ya if you are looking for something specific.
I did not check but i believe the pvcr are not the replaceable type which means i need to drill them out, not sure if i want to do that
If i f*ck something up or it does not work out...i think i got a problem.

Originally I was afraid to drill (use pin drills, you just turn them by hand, no actual drill needed) and experiment too.
I bought some metering plates with the screw in restrictions. I think the plates even came with a few different sizes of screw-in restrictors.
I used the adjustable metering plates to try different adjustments, and then duplicated the restrictor size diameter in the stock metering blocks with the pin drill.

I just checked Holley their website but they only offer a replacement metering block, which will be same.
I can get my hands on a drill set ranging from 0,0135" to 0,157", not sure if these sizes will do because i have no idea what size holes we are talking about here for the pvcr?
Also i will need to drop the primary jets another size or 2? Go to 68 or even 66.

For the lean idle, i also could get another primary metering block for my carb, swap over the primary jets and install a power valve blank right, or does that not work on a secondary vacuum carb?
Would give me back the 4 corner idling.
If not, leaves me no other option then running a drill throught the ifr's

I have the quick fuel ones. Holley owns Quick Fuel

https://documents.holley.com/99-34-8.pdf

Roger that, so i could buy this metering block and install it to see how it changes.
After i can play with the ifr and pvcr to alter any readings that are not good.
Also a set of 66 and 68 jets i will include so i can lower the cruising afr.

first thing I would do is connect the vacuum advance and take another reading.

lewtot184 said:

first thing I would do is connect the vacuum advance and take another reading.

Click to expand...

I will try that first, but go for ported vacuum right?
The 20 deg initial is exactly were it wants to be to idle nice, if i add it on the manifold port it pulls another 10 deg advance which makes me adjusting the intial to 10-12 but then it drives like ****.

Regarding the Quickfuel metering blocks, will the 34-8QFT fit my carb? It states 4160 carbs on Summit racing.
https://www.summitracing.com/int/parts/qft-34-8qft/overview/
I did find the 34-10QFT also on Summitracing, which states 4150 carbs?
https://www.summitracing.com/int/parts/qft-34-10qft/applications
The other thing is, do i only replace the primary metering block with a quick fuel, or do i need to get a secondary model as well? (Like the 34-9QFT)

Wietse said:

I will try that first, but go for ported vacuum right?
The 20 deg initial is exactly were it wants to be to idle nice, if i add it on the manifold port it pulls another 10 deg advance which makes me adjusting the intial to 10-12 but then it drives like ****.

Regarding the Quickfuel metering blocks, will the 34-8QFT fit my carb? It states 4160 carbs on Summit racing.
https://www.summitracing.com/int/parts/qft-34-8qft/overview/
I did find the 34-10QFT also on Summitracing, which states 4150 carbs?
https://www.summitracing.com/int/parts/qft-34-10qft/applications
The other thing is, do i only replace the primary metering block with a quick fuel, or do i need to get a secondary model as well? (Like the 34-9QFT)

Click to expand...

do ported first. i'd also bring the centrifugal in quicker. try a light and medium spring together. you can't calibrate a carb until the ignition is correct. with the low vacuum you have I think the engine wants the timing in quicker. also check the vacuum advance to see if it's adjustable.

Thx lewtot184, I'll give it a try first tomorrow if i find some time.
The vacuum canister is the non-adjustable type (round housing) that came with the MSD RTR distributor.
It only goes by the chart given above.

Wietse said:

Thx lewtot184, I'll give it a try first tomorrow if i find some time.
The vacuum canister is the non-adjustable type (round housing) that came with the MSD RTR distributor.
It only goes by the chart given above.

Click to expand...

just for giggles try a small allen wrench in the vacuum canisters nipple. I know some of those gm type canisters are adjustable.

Ok, so i had hooked up the vacuum canister to ported vacuum and made a test drive but nothing changed.
I did notice a little bit ping at very light acceleration, so back home i checked the timing and found the vacuum can advances the timing at idle already with 5 deg, so 25 deg advance at idle.
I retarded the timing back to 20 deg with the vacuum can attached, which sorted the pinging issue.
But again the AFR does not improve and maintained around 11 during cruising speeds of 30-60mph.
Below 30mph it stays a bit more lean, maybe 12-12.5.

I did play around with idling and was possible to get a better AFR at idle of around 15 which i think should be not an issue.
Also had to adjust the high idle for the choke control as it idling too slow with the choke active.
Now it idles better when cold.
Later on when the engine is cold i will try again and see how the choke and high idle works, for now it is back in the garage as there is a storm coming in with a lot of rain.

seems odd that throwing some timing at it didn't make the slightest difference. I don't think it's float level, fuel pressure, power valve because it doesn't run rich at idle. 72 jets in the primaries shouldn't be a problem. how much cam is in it? do you have access to another a/f meter?

The old carb did exactly the same, stuck at 11 at cruising speed.
Now, with a new carb i was expecting this "magic" limit could be broken but apparently not.
The 72 jets i pulled out and installed size 70 but did not change much, or nothing at all.
The cam...that is the big mystery of this engine as i have no idea what cam is in it.
Today i managed to get 10Hg vacuum @950 RPM, so to me that indicates it is quite a radical cam.
Even if it just idles it stumbles a bit every now and then, it always has done it with all different settings, rpm and even with the new carb it still does it.
For the rest it has an Edelbrock Performer RPM manifold(dual plane), some unknown brand headers which seems to be "tuned" ones comparing them with pictures on the web. (equal length runners)

I only have this a/f meter, it's quite new and is self calibrating every time it powers up.
It shows realtime changes and does show readings from 10 to 18.

Last thing left i could try before ordering stuff is to change the mechanical advance curve on the distributor.
See below charts, currently i have the 2 blue springs installed, Chart D.
I was thinking to move to Chart E, where i will be all-in on mech advance at 2100-ish rpm, where now i it is all in at 2500 rpm. (Have the black bushing installed, 18 Deg total)

i'm thinking the last time I did one of those distributors I used the black bushing with a light and medium spring; can't remember the colors. engine was a 440 with the 284/.484 cam and it worked well. your vacuum advance must have a pretty light spring in it. too bad the spring tension isn't adjustable. could there be an issue with the O2 sensor?

The kit that comes with the distributor has a set of blue (light) springs, set of grey (light) springs and a set of grey (heavy) springs.
I think tomorrow i will find some time and change the spring setup for a light and a medium spring and take it from there.
Another thing is, first time i checked timing, which was set for me when they installed the new distributor it was at 10 or 12 deg. (with the same 18 deg mech advance)
I played around with it and found it more responsive at 20 deg and left it there.
What sounds more common to you in this case? 12 or 20 deg?
I've read quite a few topics were others mentioning 15-20 deg on a 440 is quite normal as they like the advance when running on premium fuel.

To me the O2 sensor works just fine, it's always changing readings when touching the throttle.
When adjusting the idle screws you can see the reading changing the same direction i am playing with the screws.

in your case I would just set the total timing and not worry about the initial. if I remember correctly the black bushing allows 18 crank degrees in the distributor. if total timing is set at 36 degrees then you will have a least 18 degrees at idle; maybe more depending on the springs.