[Lip Ripper]

I have a 750 vacuum secondary on my 66. I just don't think it's giving me what it should. I was running too lean. Plugs looked brand new after several hunderd break in miles. I removed the 72 jets and upped it to 74's and now the plugs look nice and light tan. I don't think i'm getting all of my secondary throttle. I tried to gauge it with a paper clip on the diaphram shaft but it fell off twice in a row. I'm ordering the secondary spring kit "holley 20-13". I'm curious what others are using for carburation on their 390.

 

[Jonzo77]

what rpm are you expecting to run?

 

[Jonzo77]

holley says 750-770

 

[Jonzo77]

Holley Performance Products 770 CFM Four Barrel Carburetor 0-81770

 

[Lip Ripper]

I've put in a tech report to holley and I'm waiting to hear back. These 390's don't wind real tight without a lot of modifiying. I'm running 10 to 1's, Comp cams 270H, Performer intake. Mallory unilite, stock manifolds. Stock GT heads, crank and rods.

 

[Jonzo77]

i guess i can mod the specs then... i think you might be better off putting a mechanical secondary into it... Holley Performance Products 800 CFM Four Barrel Street/Strip Carburetor 0-4780C

but...Holley Performance Products 750 CFM Four Barrel Street Carburetor 0-80508S now that i know what you have in there...

 

[Lip Ripper]

That's exactly what my thoughts were but Holley seems to push towards the vacuum models. This is what's promted me to make this post. To see who is happy with what.

 

[Jonzo77]

usually for a full size car, you want vacuum. i don't know why, but thats what most companies recommend...

 

[Zephyrman]

Mine wasn't a 390, I had a 360 in my 68 F100. I swapped on a used Offy 4 barrel intake and a rebuilt Edelbrock 750 carb. IIRC it was mechanical secondaries on it. Had great throttle response, lots of torque with a four speed in it. Would still drink fuel like it was going out of style, but I had a lot of fun in that truck.

 

[Lip Ripper]

Holley replied and said to increase the shooter size and go with the yellow spring on the diaphram so I will. I've been reading about this and it's interesting how tuneable the carb is. All I've really done before is re-jet mine.

 

[Jonzo77]

that's why i always liked them. that and ford used them for factory carbs on various vehicles. like the 200 inline 6 in fairmonts, and the 4bbl's on shelby's

 

[paso ranchero]

With the stock exhaust manifolds you may want to consider a 600 carb. If not, at least upsize the exhaust with free flow muffler if you haven't already. I have a 390 with a 750 Edelbrock and headers in a 67 Ranchero and I'm thinking about downsizing the carb. Cheaper than upsizing the motor.

 

[Lip Ripper]

I think the GT manifolds have pretty decent exhaust flow. So far I have been fighting a lean condition. I went from stock 72 jets to 74 and this really helped the plug readings. Today the spring kit will be here so I will install the short yellow spring per Warren at Holley. The squirter will be here tomorrow. Right now it has a .025 nozzle and Holley has recommended going up to a .035. Doing the math this should give me 29% more fuel from the accelerator pump delivery and make it more responsive. I'll let you know what happens after the tuning. Oh, another thing I found is that the accelerator pump arm has some wear at the shaft. I lengthened the screw about a turn to take up the slack. According to Holley you should have .010 to .015 of pump arm travel left at WOT. I will order a new pump arm and then re-adjust.

Okay, here's my follow up. I changed the pump nozzle from.025 to .035 and installed the short yellow spring from the vacuum secondary kit. What a difference! The car responds way better than before. These modifications came from Warren at Holley tech and were easy to do. Total cost of around $27.

LR