timmyf

I have a 1991 Mustang that I switched from EFI to carb. It has stock bottom end, x303 cam, Edelbrock Performer RPM intake, Holley 600 double pumper carb, Edelbrock RPM heads, Holley red fuel pump, MSD distributor, MSD 6-AL2 ignition box, MSD blaster 2 coil and MSD 8.5 mm wires. All these parts are brand new. It misses really bad at low RPMs and the plugs are black. It idles fine and seems fine at high RPMs. The ignition timing is set at 12* and I'm getting 6psi fuel pressure at the carb. Any ideas what could be causing it to miss?

67mustang302

Regulated to 6psi at idle? What's your idle vacuum and what power valve is in it, and what jets does it have?

timmyf

There's no regulator on my pump, but the pressure is measuring at 6psi. Idle vacuum is 13, 6.5 power valve and 69 main jets and 71 secondary.

joeymustang

i had a pcv from my carb vacuum inlet to the valve cover...had some hissing and a bit of fouling, plugged the carb and put in a breather instead...problem solved

does it accelerate from stand still fine (i.e. when you push it with your hand how does it sound)

timmyf

I've unhooked all my vacuum components and plugged all the ports and took it for a drive and it still acted the same. If I rev it by hand it sounds fine, but when I'm actually driving it, it misses.

67mustang302

Are you sure there's no fuel leaking past the power valve, and are you sure you're getting good ignition?

timmyf

I'm pretty sure the ignition is working good. I tested it the way MSD says to and it seemed fine. As far as the power valve goes, if I turn the idle mixture screws in all the way and the car stalls, is that a sure test that it's good? Or should I actually disassemble the carb to see?

67mustang302

Have you tried jetting it down? You jet spread should be around 6-7 sizes between front and rear. Since the rears are 71 the front should be around 65. Try going down 2 sizes to a 67 on the primaries and see if it helps.

What model carb is it, does it have an HP body with adjustable bleeds?

Just as reference, my 302 has AFR 165's and a Crane cam, and with my Quick Fuel 650 DP the out of the box jetting of 68/74 and the factory air bleed sizes is was stupid rich and was getting a rich misfire at low rpm. Right now it's at 63/69 with larger air bleeds and runs fine.

John Doev1

Is it a off idle mis or does it mis at idle also. If it's falling on it's face at throttle tip in then the accelerator pump may be the problem also.

timmyf

Thanks for the input. My carb is a Holley 600 DP (part 4776). I'll try changing the jets this weekend and see what happens.