I have a new Holley 4160 on a 305 and I'm having problems with a rich idle. When I hook up a timing light it indicates a miss on every spark plug wire at low rpm. When rpms go over 1000 the exhaust clears up and the miss goes away. Everything in the ignition system is new except for the coil. The coil checks out within specs when I do the service manual tests.

I've replaced the power valve just in case it was leaking and I've double checked the float height. The idle mixture screws seem to have little effect on anything.

Now I guess my question is, could the idle mixture cause a miss from being too rich? Or is it the coil just won't put out enough spark at low rpm? Or am I overlooking something else?

 

Tell us more about your ignition system. What is it, exactly?

A weak spark can cause a miss at idle that will go away at higher RPMs. Idle mixture screws not having an effect is typically due to the throttle blades being open too far (either primaries or secondaries - check them both). At idle, the primaries should uncover the transfer slots by about the width of the slot. To check this, you have to take the carb off, turn it upside down, and look from underneath. Set the idle speed screw to obtain that uncovery (make sure the choke is completely off when you do that), put the carb back on, and adjust the idle with the mixture screws only.

 

Sorry for not posting more details. I've got an Accel 59107 HEI distributor (vac advance, coil in cap), bosch 8mm wires, and Delco R45TS plugs. The coil is an OE part, I'm not sure how old it is. The car is an 84 but I've only had it a few years. I've tried setting base timing from 3 to 12 degrees. It seems that as I advance the base timing the miss gets worse.

Thanks for the advice, I'll have to check the throttle blades and see if that's a problem.

 

Forgot to mention, the cap and rotor are new Accel parts.

Anyway, I readjusted the throttle blades and now I seem to be getting somewhere with the mixture screws. However, its still too rich and its missing when the screws are adjusted to max vaccum. I think they're about 1 1/4 turns out.

 

How could it be getting max vacuum if it's missing?

 

I meant to say I adjusted the idle mix screws to about the highest vacuum reading I can currently get on my gauge.

If I go leaner I can clear up the exhaust quite a bit, however that worsens the idle quality and drops vacuum.