Confuzed1

iTrader: (4)

Hi all,

I'm going to need to set up my Holley 750 for a small roots blower. I've done research, so I guess I'm clarifying with all of you about what I've read on different web sites about modifying this carb, also trying to find out if I missed anything.

I've heard that I could simply "jet" the carb up several jet sizes and run it - at least initially to get it running.

The downfall to that is it will run rich all the time, and the power valve will not sense true manifold vacuum, so that extra bit of fuel during heavier loading won't be there. - Also the accelerator pump shot is currently 30cc, but I may need to increase it to a 50cc pump on at least the secondary side (maybe primary too) in order to not run lean when mashing the pedal. Not good because after doing more reading recently, it seems I may need to raise the carb 1/4" with a spacer in order for the pump arms to clear. (Hood clearnce is tight)

So - to sum up what I think I know:
1. I need to reference the power valve to "intake" vacuum.
2. I'll need to jet up at least 3 jet sizes.
3. Most likely need to install 50cc accelerator pumps

- That's what I know, but is there anything else I should be looking at?
- Any suggestions on how to initially set it up to at least be safe (not run lean and detonate)??
- Will I need to do anything with idle air bleeds, etc. ??

I'll be installing a wide band O2 sensor to assist in the tuning, never used one before but I hear I almost need it to tune it right.

Any input welcome!!

lukn4trbl

Yes

Probably. Depends on boost, and engine size @ max RPM.
Your wide band will indicate this best, but you'll need to read the
plugs for best results. Also note the HP on dyno, or MPH trap
when racing.

Not necessarily. That can be figured afterward. You might be able
to tune it out using pump cams and nozzles.

So far, so good. Make sure your air filter can provide enough flow
at peak HP RPM. Get the air as cold as possible coming in.

Fuel pressure gauge is good insurance and smart for tuning.

- Any suggestions on how to initially set it up to at least be safe (not run lean and detonate)??

Back down the advance timing about two degrees for every 6 PSI of boost.

Run a good fuel, and correct octane.

Start with a bit of boost, and check the plugs after wide open throttle
blasts. Increase the boost, and then check the plugs again. Keep the
AFR on the rich side until you can tune it under load on a dyno, or track
ultimately.

Double check vacuum lines and gaskets, especially between the charger
and carburetor. You don't want any air leaks causing lean conditions.

Confuzed1

iTrader: (4)

Thanks lukn4trbl for the reply.

Only reason I said "Most likely" needing to go with a 50cc accel pump is because the Holley carbs I've seen at Jeg's - etc, all have 50cc accell pumps on them. But I can see where trying different cams could work. I'm kinda hoping to get this carb at least in the ballpark so far as calibration goes before trying to dyno it.

I only plan to run around 8 pounds of boost. It's the engine in my sig. The blower kit I bought has a pulley to supposedly produce 5-6 pounds of boost on a mild SBC 350. But I'll change pulleys and run up to 8 pounds if I don't have to back timing down too much and can still run pump gas. I've been told the limit is around 12 pounds with this small blower.

I also have a MSD boost retard box I plan to install. I'm HOPING to fit this under a 4" cowl hood, so the biggest air cleaner I can run will be a drop base 12-14 inch diam. air cleaner with a 3" thick element....again - I HOPE.

Besides knowing if my fuel pressure is dropping under load, what else would a fuel gage tell me? I'll be running a 130 glhr pump with an Aeromotive bypass regulator. Hoping I can get by with the stock lines but I'll see.

lukn4trbl

It's a good tuning and trouble-shooting device. Ultimately, it's a visual safe
guard in the event the pump/regulator fails. It could save your motor!