n8dogg

My 72 LT1 has a Holley 4160 that I converted to a 4150 with changeable jets in the secondary metering block. The engine is original with factory manifolds and internals.

Cruise and light load I'm seeing between 13.5 to 14.0 AFR, which I am fine with.

Medium load I am seeing 12.0 to 12.5 AFR, this must be when the power valve kicks on.

WOT I am seeing 10.0 AFR. This is too rich for my blood and likely robbing power.

The primaries have #69 jets and the secondaries have #71 jets.

Timing is set at 12 degrees base, vacuum disconnected. I have not determined what the total timing is.

I have the Holley Jet set, #64 is the smallest.

What do the rear jets need to be to keep the WOT AFR the same as medium load/when the power valve is opened? The same as the primaries? Smaller than the primaries? My goal is 12.5 AFR WOT, maybe the carb is too big for the application. Factory LT1 carb was a 780cfm!

Thanks for the help!

gkull

Why would you not use vac advance?

The question of is it to big is....... 4150 could be a smaller 600 up to a 850 VS carb.

As to what jet to use............... If we don't know what jet size it has now how could you recommend a replacement size.

So figure out your carb size and rear jet size and then re-ask the question

Big2Bird

You need to set the timing curve first. 36* total mech advance, and an 8* vacuum for a total of 52* at 3000 RPM, no load. (cruise)

A baseline for jetting to start would be 72 primary, and 76 secondary. You want it richer at WOT.
If 70 primary is fine, then secondary should be about 74

n8dogg

Looks like you may have missed some of the information I provided in my OP. Current jets are 69 in front and 71 in back. Carb size is a 750. I do use vacuum advance. I said the base timing is set at 12 degrees with vacuum disconnected. Vacuum was reconnected after the base timing was set.

Yes, the carb had 72 in front and a metering plate equivalent to a 76 in back. I converted the carb from a 4160 to a 4150 with changeable jets in the back VS a metering plate and installed 71 jets due to the AFR being pig rich (9.8 AFR) at WOT. Dropping jets 5 sizes in the rear made very little difference in WOT AFR.

Big2Bird

You WANT it rich at WOT. No one cruises at WOT.

TheycallmeDave

OP, bear in mind if you have a cam with a lot of duration overlap , that it will run richer due to the unburned gases in the cylinder . Just thought id throw that out there. You can play with the jet size but I see a bit of over-richness on the secondary at WOT kind of an insurance against possible detonation especially with the cheap quality gasoline on the market . Im running a tad rich on my secondaries and notice no performance hamper .

MelWff

I wonder if you have an internal leak in the carburetor. I have a 70 LT-1 short block with original solid lifter cam, early angle plug heads, RPM Air Gap and a Holley 3310-1. Primary jets are 72, secondary are 78, front and rear power valve 8.5. No indication of running rich.

n8dogg

This is well known. I appreciate the feedback. 10:1 AFR at WOT is OVER RICH and power robbing. Ideally on a N/A car you want 12:1 to 13:1 AFR at WOT for maximum power.

The cam is a solid lifter crane cam with specs very close to the factory A wideband 02 sensor actually measures stoich for the fuel that is being used and will be fairly accurate under load. I ignore the lambda/AFR reading at idle because of the overlap.

A fuel leak is possible, how ever I recently went through the carb. Cleaned it and replaced the gaskets, diaphragms, power valve etc... I've had the bowls and plates off a few times since to change jets etc... Gaskets all look good. It would have to be leaking in the main body somehow, where I can't see it.

Have you used a wideband 02 sensor? That is a 780cfm carb. Back in 1970 fuel efficiency wasn't a big concern. Big carbs helped sell cars, especially performance oriented cars. Lots of talk online about the 780 being overkill on a small block. Many changed jets for more fuel efficiency and have not noticed any power loss.



FWIW I just got done changing the jets again. Holley's website suggested a classic holley 600cfm for a 350ci with redline of 6500 RPM. The jets used in their 600cfm is a 65 primary and 69 secondary. So I installed 65 in front and 69 in back. My cruise and light acceleration AFR is spot on hovering around 14.0 AFR, lambda fluctuates between .9 and 1.1. Moderate acceleration and WOT only leaned out very slightly. So the primary jet change made a noticeable difference. The secondary jet change made very little difference. I went from 72's to 65's in the front and 76's to 69's in the secondary. Either there is a leak in the secondary circuit or the carb is too big or the air bleeds are too small.

Big2Bird

Sounds like your getting there quick.

MelWff

what is the list# of this carburetor? The leanest 780 from the factory in 1972 used 68 primary, 73 secondary, 6.5 power valve, and yellow spring

mikem350

Just another thing, float levels OK?

Interesting that the primary change made a significant improvement at wot, I will remember that tip!

Barry's70LT1

For many years I've been using a 750 DP (R4779). It came with 75 P and 76 S. It was way rich at cruise, so I "restricted" the idle/transfer slot circuit until I got good plug color. (No O2)

Then I worked on WOT and ended up with 71 P and 79 S.
Power valve is 6.5.

The carb is now very responsive, great mileage and performance.

Note: Many years ago I tried a Holley 600 CFM to see how that worked.
It was on there for about one day. The performance was dreadful, took forever to go thru the gears.

Jartanyon

Tagging along - I had the same problem dropping almost 10 jet sizes on my Summit 600 with no change in my rich 11.5 WOT. My primary had an effect and settled on the 67 jet for the best mixture.

n8dogg

The list number is 3310-4, it is a 750, vac sec, and elec choke. Comes with 72p and 76s.
Floats are good, barely trickles out when the plugs are removed with engine hot and running.

Changing the primary jets made an improvement at cruise and light load only, not WOT. WOT is still pig rich at 10:1 AFR even after dropping 7 jet sizes.

The car is running great on 65p and 69s. Still pig rich at WOT but cruise, light/medium load areas feel really good. Don't get me wrong, the car pulls OK at WOT, but I know that power is being robbed due to it being over rich.

n8dogg

I was thinking about putting some copper wire in the secondary metering block. I have my doubts that it will do anything since the jet change didn't affect WOT AFR.

Good to know about the 600. If it hinders performance than nothing is gained. Leaves very few other options for downsizing the carb.

If I really want to nail down the WOT AFR, I can always play with the air bleeds and PVCR's. Not sure if the effort is worth it to me for the small % of time I am going WOT. It's not like I'm racing against anyone either.

MelWff

the fact that you have dropped 7 jet sizes on secondary with no effect doesn't lead you to believe there is something not right somewhere in the secondary components. A gasket not sealing or wrong gasket, something warped, power valve not tight, the secondary metering block wrong or bad casting, etc.

n8dogg

It is odd, there may very well be something wrong. However it would have to be in the main body. The secondary metering block is very simple, 2 gaskets, no power valve, and 2 jets. I've checked several times, the gaskets are correct and not damaged. I could try the smallest jets I have, 64s and see what happens (just to see if AFR changes). I'll do some research first.

claysmoker

What size are the PVCR's? They could be way too big and have the effect you are experiencing at WOT.

n8dogg

I'm not sure what size the PVCR's are. The carb was untouched, so what ever the factory diameter is for a 3310-4 4150 750cfm carb.

To change the PVCR's you'd actually have to drill/tap them to take air bleeds.

Either way, the AFR when the PV opens is acceptable at 12.5:1. My concern is when the secondaries open, it goes super rich to 10:1 AFR.

Haggisbash

I am curious, MelWff mentioned the secondary spring size,you haven't said what spring is fitted or whether the secondaries are opening all the way? What happens to the A/F ratio if you tie the secondary shut?