i've got a 70' f250 with a stock 360 and a 4 speed (first is a granny gear). I picked up a 390 4v intake (large tube on passenger side front of intake). i left the bottom end alone but put a set of rebuilt 390 heads on. got my 4v intake on and topped it off with a holley 600 vaccum secondary carb #0-1850s. Okay, under regular driving everything works fine. nice and smooth. But when i'm under a little load and put my foot in the gas, it seems like everything stays the same. Runs just like I've still got the old 2v on it. I have my timing set at about 10 btdc. The dwell is set at about 30 degrees. I also put a screw in the secondary cam to try and pull the secondaries in manually but had really bad results. This on has me very puzzled. Any help would be great.

 

The secondaries on a 4 barrel vacuum secondary carb only open when the engine needs the extra air flow. This opening is controlled by air flow thru the primaries and the spring on the secondary vacuum diaphram. You can change the spring (there is a spring kit available), but if you get too light a spring in there you will get the same results as you did with the screw in the linkage (which is alway a loser for performance). Best way to be able to tell if the secondaries are opening is remove the air cleaner and you will hear them open. The other problem you are fighting is that the stock cam runs out about 4000 to 4500 rpm and that about when the secondaries will start to open with a stock 1850 carb. If you have single exhausr you are strangling the motor too. Air can't get in if it can't get out.

 

Put a hairpin or paperclip on the secondary vacuum plunger rod against the bottom of the vacuum housing then do a full throttle road test.
You should see that the paperclip or hairpin has been pulled lower on the
plunger rod if the vacuum secondary unit is working properly.
If the small cork gasket is missing or the diaphragm leaking the signal will be lost with no secondary opening, a too heavy of spring will cause the same symptom.
To "feel the secondaries kicking in" is a false statement used for many years with too light of spring plus meaning the carb isn't set up properly to the vehicle, you should have a smooth transition from primaries only to secondaries starting to opening.

 

well i am running dual exhaust but thru the factory manifolds. probably not as much air flow as needed. i'm not looking for seat of the pants performance or anything but i'm not seeing any gain over the stock 2v, if anything a loss in performance. it almost feels like the engine wont turn the rpms quick enough to use the extra fuel i'm giving it. i guess thats where the 4000 to 4500 rpm cam really hurts me.

 

I'm running a stock 352 through single exhaust and manifolds. I too converted to a very similar setup and love the results. However I have been messing with Holley's since Columbus brought over the first one! It should work just fine with what you have. Either you are expecting a big difference(which their isn't) or the carb isn't setup right yet. I did gain marginal fuel mileage, but the biggest difference is when high power is needed at high RPM.
What everyone has said is good advice.
You may(will) need to play with the primary jet(secondaries are fixed on yours), accel pump nozzle size, power valve, and secondary springs, as mentioned before. "Out of the box" is hardly tuned right for every application. Probably not what you wanted to do....I'd at least change to a smaller jet, I think most come with a #66 factory and I think that's a little big for most. It's been a while but I think I have a #59 in there...I've also messed with the springs and have a heavier one in there now.
One other suggestion...lose the points and go electronic ignition.

 

A old plow horse can only run so fast no matter what ya feed her or the rider, same thing with that stock 360. Compression, cam and especially the lazy ignition advance curve that will keep your added 4 barrels increased performance to a minimum.
Overall if your right foots heavier than the left you'll see a reduction in mileage over the 2 barrel.
Headers be it El Cheapo's with dual 2 1/4" plus a crossover with free flowing mufflers then call it good until ya make a purpose built motor for you needs.
Heavy truck with 3.50" (360) stroke doesn't cut it, 3.78" (390) better, 3.98"
(410/428) crank with good heads, compression, slight cam and headers with a ignition and carb tune then ya got something to smile about. Maybe a little over the top for some, JMO's.

 

If your not on the edge, you're taken' up too much space...

 

I had a 360 a few years ago, before I put the 390 in. It used a Holley 600cfm vacuum secondary, 390 "S" code intake, D2TE-AA heads, headers and dual exhaust.

I was running #68 primary and #72 secondary jets. It had the kit on the carburetor to change out the secondary metering plate to a metering block, so I could change jets. It had one lighter spring installed in the distributor, and I typically ran 10 degrees BTDC initial advance, with the advance in the "15L" position. That motor made quite a bit of power, more than a 360 really should. Fry the tires on command.

I hope my old setup provides you with some ideas or at least points you in the right direction. I would suggest, as above, some lighter advance springs and headers.

 

Hey everybody, thanks for all the advice. I blew out the secondary diaphram when I tried to put a screw in the secondary rod. I'm building the carb with lighter secondary springs (quick change secondary kit for the holley) and I'm going down to a #57 jet. Hopefully that will give me a good starting point. One last question though, anybody have an idea what power valve I should run. Ive heard go with the stock 6.5" and I've heard I should go with a power valve block off and go up a couple of jet sizes. I'm not on the up and up with carb setups so any help is needed. Thanks again.

 

Do not block off the power valve!!! You would need to increase the main jet by about 6-8 sizes to compensate for the reduced fuel flow under load. Not good for fuel economy! It is difficult to suggest a power valve without using a vacuum gauge and having more info. I would stick with the 6.5 or maybe depending on vacuum levels under light load, use a 7.5.