Sneakin Deacon

i've dynoed the car numerous times, and in different gears, trying to narrow the problem down.

basically when i 1st get on it, it is rich 10.5-11.5:1 and around 4500-5000rpm it leans out and keeps leaning (16:1+!!!) it does this in every gear...the mechanical pump is keeping pressure though, entire pull it sits at 6-7psi.

i will note this is my 1st carb setup, it is a holley 780 with vacuum secondaries...they are opening up fully also. but i am very new to all of this and dont have any experience.

my best idea is that the cam is too agressive causing the richness when i get on it, and the secondary jets are too small. but i am open to all suggestions, this is just my theory as of now.

Cali99SS

iTrader: (1)

What size engine? How many RPM's to redline? What Cam do you have? How much vacuum are you pulling? Your power valve might be too big. And/or your primary jets are too big and secondaries are too small. I have a Holley 950 HP with mechanical secondaries. My cam is 254*/257* duration at .050 with .712 lift in a 383 spinning 8000 Rpm's. It has the factory jets and #52 high speed air bleeds and #74 idle air bleeds. I maintain about a .45 BSFC from 4000 to 8000 (didn't record below 4000 on the dyno).

MadBill

I think Cali is right; the clue is that it goes lean about the time the vacuum secondaries open. Lots of Holleys, especially if they've passed through many hands or had major parts swapped, end up with the wrong relative size jets on the primary vs. secondary side. There are rules of thumb, like: "Go up 8 sizes on the secondary if it doesn't have a power valve", but this depends on the size of the power valve channel restrictors in the metering body, behind the power valve. The PVCRs can range from maybe 0.035" (equivalent to ~5 jet sizes in the #72 jet range) up to 0.070" ( ~20 sizes! ). The first step would be to measure and correct this if necessary. Judging by your rich low speed AFRs, you likely need to go down ~4 sizes on the primaries and up ~ 10 on the secondaries!
Additional causes could be a plugged filter in the secondary float bowl, debris in the secondary needle, jammed float not dropping properly to open the needle, debris in the secondary fuel circuits, etc.
Another calibration approach would be to disable the secondaries and see how the mixture looks. If it seems OK all the way up (although obviously it will be down on power, running as a 2 bbl.), you could test it up to maybe 4000 RPM, changing jets for best power (usually ~ 12.7 -13.2:1), then re-connect the secondaries and test over the full RPM range, changing only the secondary jets to achieve best results.
BTW, the power valve determines only the inches of vacuum below which the additional fuel is delvered, not the amount/richness.

Sneakin Deacon

appreciate the help, i think i am going to pick up a holley adjust a jet so i can do the adjustments quicker and not deal with spilling fuel when i take the bowls off.

also im having a real hard time figuring out what carb this is aswell, the horn was cut off, and was told it was a 780, but now i am wondering. only numbers on the center section are 6931, R81 and P80...

nick_s

iTrader: (11)

okay - one thing some people misse dhere is that it is a Vac secondary carb - which usually doesnt have a metering block with replaceable jets (it has a metering plate) Also, you could have a heavy spring causing the secondaries to not open much or fully.

When asking questions like this, please give more info such as engine size, intake design, etc.

Sneakin Deacon

its a 355, super victor intake, 1 5/8 long tubes. cam is 242/248...dont remember the lift right off hand, on a 110LSA +2*

i have the yellow spring in right now. if i recall correctly its one of the lighter pressures.