Running a Holley UltraHP 750 on a very mild 454

[G_skillz81]

Guys, hi

hewb type questions here/my first BBC

i recently purchased a 1970 Chevelle (SS clone) form the US

Has 12bolt with 4.11's and a Muncie m21. Runs. Haven't tried to run it outside due to weather, does move one it's own around the shop. Runs rich as hell on the q-jet

engine is a 454 with stock milled heads, edelbrock Performer intake, Rochester q-jet, 2 1/8 hedman headers, cam dynamics stage I cam (222/235 @.050)

we're trying to replace the q-jet with a Holley ultraHP 750 double pumper. Jetted to the smallest jets we have it floods under 2500rpm. We milled an adapter to get it on the spread bore intake, but it seems that tunning it to idle and run smoothly on low rpm isn't going to work. The engine doesn't seem to be pulling enough air at low rpm.

Keeping the q-jet isn't really an option, it's worn out and in generally nasty condition...and no one here works on them. Holley's are more "common" by comparison

i'm thinking we need a dedicated squarebore intake for the Holley, unless someone can tell me how to tune it on a spread borbore.

if the UHP750 is too overkill for this application we can use a street avenger 670 (there's one for sale locally - cheaper option than getting a new intake ) http://www.summitracing.com/int/parts/hly-0-83670/overview/ although it is still a squarebore carb.

So should I swap the intake and keep the 750 double pumper, or go with a milder carb?

[Chevelle598bb]

Keep the 750hp and change the intake to a square bore. The spead bore to square bore adapter could be jacking up the vacuum pulling on the idle mixture slots since the primary and secondary bore sizes of the adapter are different. When that is taken care of you can move onto the tip in and part throttle. Just a tune of the jets and power valve should get you very close to where it needs to be. I would put the carb back to start first and then start tuning from there.

[Derek@ModernSpeed]

How much overlap does the camshaft have? Does it run rich at part throttle and under 2500 rpm or idle and part throttle?

[sam 74]

You need tune your idle circuit so you have a decent running starting point to tune off idle. Adjust the mixture screws as necessary to get the motor idleing decently, you may need to get additional air supply and need to drill butterflies if they aren't already, before you do to the carb make sure you dont have to much transfer slot exposed, that's the easiest fix and causes headaches for a lot of people newer to carb tuning. Once transfer slots are set then you can adjust the mixture screws and modify the butterflies if need be. That's where I'd start.

[Derek@ModernSpeed]

On that particular carb, the transfer slots shouldn't be exposed at all.

[Erik Beckett]

Verify your idle screws on the sides are only turned out 1 1/4 to 1 1/2 turns out. This makes a huge difference in idle and air mixture. Alot of guys turn then 2 turns out because it idles higher/better but thats just dumping more fuel in and causing a rich condition. Start from here then adjust each side 1/8 turn at a time until yiu get the highest vacumm reading. Then I turn them in a 1/16 of turn.

Thanks, E

[hipockets]

You should run the proper intake, without an adaptor plate. a performer rpm air gap is much better suited to your engine.

Then, to start, you must have you timing correct. Base, curve, and full advance. What distributor do you have? Start with 14 degrees initial timing at 600 rpm or so. You want full timing of 36 (to start with) with the vacuum advance unhooked. Springs for advance weights as needed for the full timing to be in by 2500 rpm. Then hook your vacuum advance to full manifold vacuum.

For the carb, start with setting the idle screws as follows;
Start out by putting a small piece of plain writing paper (.003) between the throttle
butterfly adjusting screws (Idle speed screws) and the stops, let the throttle close,
now back off (counter clockwise) the idle screw until you can just slide the paper
out, now go back in (clockwise) 1 turn on the primary and 1/2 turn on the
secondary. To start with anyhow, we want the primary and secondary idle speed
screws to be set 1/3 - 2/3rds. The primary end of the carb is designed for idle and
light cruise so it only makes sense that we should set the carb up using mostly
the primary end circuits to accomplish a good idle. Always adjust the Primary and Secondary idle speed screws proportionately as
described in the initial set-up section to achieve final idle speed. So if you need
to add 300 RPM you would add 200 RPM with the primary idle speed screw and
100 with the secondary idle speed screw to maintain balance.

Then adjust the mixture screws as follows;
Now screw in the four corner air mixture screws until they gently bottom, your
seating against a small rubber o-ring so be gentle or you'll crush and possibly
tear the o-ring apart, which will cause a vacuum leak into the idle circuit and
make adjusting impossible. Back out 1.5 turns
Primary and 1 turn on Secondary end. Notice here how we’ve again loaded the
Primary end of the carb with more fuel.

Then adjust the accel pumps;
With the throttle Wide Open, use your 3/8
open end wrench and push down on the pump arm. You should have just a
slight amount (.015-.020) travel left in the pump arm. Using 2-3/8 wrenches
adjust the pump arm until you get some clearance, very little is needed we just
don’t want the pump arm bottoming the diaphragm in the pump.
With the throttle in the closed position the pump arm should be just touching the
adjusting bolt stop.

Once you start the engine, set the fuel level in the window to 1/3 from the bottom, both bowls the same. If you have fuel dripping out of the boosters at idle, you may have to lower the fuel level to the bottom of the window.

Then adjust the mixture screws;
If it tries to stall as your lowering the idle speed DO NOT hit the throttle, tap
gently on the accelerator pump arm on the primary bowl…. did it get better or
worse?
The engine is talking to you, it’s telling want it wants, if it got worse it’s still rich
and you need to go 1/8th of a turn in (clockwise) on the Primary idle screws. If it
got better or RPM increased the opposite is true, so you'd obviously open the idle
circuit a 1/8th of a turn (counter clockwise) as your engine is looking for more
fuel.
Use the same procedure for the secondary end of your carburetor, tap adjust
until there is virtually no change in the idle quality.
No tap both pumps at the same time, same procedure except now reduce your
adjustments to about a 16th of a turn and make your adjustments to all four
screws equally.
So we balanced the front A/F ratio, then the back and then the total.
This may take several times around the carb to get it right. Making all
adjustments equally to each of the 4 screws.

Proper ignition timing must be done first, and your ign system must all be working properly. Wires, plugs, cap, rotor, coil, and steady 12.5-14.5 volts. Otherwise you will never have the carb work properly.