I installed a Holley 4160 Vac secondary on my 351w. The carb has been rebuilt with 68 jets and a 31 squirter. At idle my widebands show it to bounce from 15 to 16 which is a little lean. When i accelerate slowly the wideband climbs slowy up the lean side untill it bars out at about 18. When I punch it it leans out at 18 and then drops down to about 11 and bogs and starts to miss. I'm not sure what to change to make these symptoms go away. Do I need bigger or smaller jets? Bigger or smaller squirter? Lighter or heavier secondary springs? Im at a loss, its drivable and has a little power when you get on it but Im sure it has more to offer once the carb is tuned right.

Thanks, John

 

You need to play with one "circuit" of the carb at a time. Start with idle and open the idle mix screws up until you get to best idle, which should be around 14.0, but certainly richer than 15 if your wideband is reading correctly.

Once the idle is dialed in then cruise in town above 30 MPH and see where you are, and then ease on up to 55 to 60 MPH, but don't "punch it" as that opens up the power valve and you want to be testing only the main jets. If it is still around 18 then you need to go larger on the main jets. I would go up one size at a time until I got to about 16:1 at 65 MPH, but anything from 14.5 to 16.5 should be acceptable, with 14.5 giving a bit better throttle response and 16.5 giving better MPG.

Once the main jets are sized right then you need to dip into the throttle enough while cruising to get the power valve to open. But, you must know what PV you have, meaning where it opens, and be running a vacuum gauge so you know when it opens. As the PV opens your AFR should go from the 14.5 - 16.5:1 from above, to 12 to 13:1. If the truck falls on its face before the PV opens then you need a PV that opens at a higher vacuum.

Then, when all of that is right you need to get on it enough that the secondaries open, and your AFR should be somewhere between 12 to 13:1. If not, change your secondary jets/metering plate.

 

I forgot to say, it has a 6.5 power valve. It came new in the rebuild kit. So your saying to tune in three steps. 1- idle mixture screws.
2-jets.
3-power valve
4- secondary jets.

How do you change the size of the secondary jets on these and does going up in size increase fuel or decrease it?

 

I think that's 4 steps, but yes. The idle mix changes the cruise AFR a little bit, but mostly at low speeds. So it is adjusted first. Then dial in the primary jets to fit your needs, with 14.7 being the best mix in theory, but 16 or so gives better MPG and the PV will fill in when you ease into the throttle. However, you will note that overall AFR goes up, meaning leans out, as the throttle is gradually opened. IOW, the AFR at 50 will be richer than at 70 if the PV hasn't opened. And you don't want the cruise AFR to get higher than about 16.5 or 17.0 as the throttle response will decrease when it gets that lean.

 

The 1460 uses a metering plate rather than replaceable jets, and you have to buy a new plate with larger orifices in order to enrichen the secondaries. But most secondaries are already pretty rich, so that might not be an issue.

 

Haha whoops, 4 steps. Alright tomorrow after work I'll do those test and let ya'll know the findings.

Thanks, John

 

A long steep hill will keep your road speed down when testing. A level roadway would be good for the main jetting/part throttle testing. Stepping on it gradually on a long hill will be good for testing the power valve. Flooring it on a steep hill will be a good place to test the secondaries.

 

Good points, Dave. Thanks.