View Full Version : New 408 tuning

06-13-2012, 09:09 PM
My question is I'm tuning the carb and it is running rich at slow speeds but good mid to high. Checking the plugs and they got that real nice tan look, but still can see some carbon build up around the inside of the plug. I dropped down four jet sizes in the front and left the rear alone. If I let it sit and idle it doesnt load up. When I drive around slow it loads up and spits and pops out the pipes. It will clean out when you drive at normal speeds. I'm thinking power valves are opening real early. My mech vacume gauge says 20" the lap top says 5" on the MAP sensor. I have a MSD timming box. I'm thinking of trying a 2.5" PV and a 10.5" as well and compare plugs, it has 4.5" in it now. Also the rear has a power valve I think I might plug it and just tune the rear with jets. The carb is a holley 950 ultra HP mech secondary, double pumper. Any advice? Also this is a new motor, I'm just starting to tune on it. I played with the timing a bit and the curve 6 works well. The motor runs great, other than the loading up.

Pop N Wood
06-13-2012, 10:01 PM
The MSD box reads absolute pressure, not vacuum. So 5 PSI of pressure probably equals 20 inches of vac.

One suggestion is to ditch the pills and put in your own timing curve. Take a look through the stickies at the top of the carb section to get some ideas.

If your timing is right and I assume you have the idle screws properly adjusted, then I would be tempted to try a larger idle air bleed and/or smaller idle feed restrictor. I believe they are both replaceable on the HP carbs. I would get everything else right before messing with the air bleeds though.

I would also suggest a wideband O2 sensor. That will tell you in short order what needs to be tweaked.

06-29-2012, 03:37 AM
depending how big your cam is I see alot of people having this problem don't create enuff vacuum to keep the power valve closed. It is the opposite of what most think the power valve is normally open and it uses vacuum to keep it closed. big cams that only create 2-3 inches at idle or low rpm it will open loading up as you cruise. You need about 1.5 inches over the power valve number to work correctly I normally run a 2.5 just to be safe for street driving.

06-29-2012, 11:04 PM
Put a vacuum gauge on it and see how much it pulls at idle and putting around town.

What are the specs on your cam?