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Custom10 05-24-2010 08:35 AM

Jet Change Effected Idle
I changed the primary rod and jet combo on my 1405 eddey carb to lean it out in cruise mode by about 8% from were it was. Does this effect the idle quality of the motor? I did not think it did but now the idle quality is ruffer and it seems to surge a bit, the driving charcteristics are good with plenty of off idle throttle responce and no missing or boggs etc. I used a new top plate gasket kit and changed the primary venturi gaskets and the excel pump squirter gasket as they were in the kit. This is a fresh 383. I checked to make sure that did not disconnect anything or put things back together right including plug wires, linkages, hoses etc. I have been trying to set adjust the idle mixture screws, used a vac gage, checked the timing, moved it to 18 deg BTDC. It was idling allot smoother before, the only other thing that changed is that I went from premium gas to regular just before the swap? thanks

kpcrossfire6 05-24-2010 09:16 AM

Pull out your idle mixture screws, take an air hose and hold it into the hole and blow air in their for a couple seconds on both sides, clean the ends of the mixture screws etc. then reset the screws again. also it doesnt hurt to take some carb. cleaner and put a little bit in the fuel bowls as well.
good luck,

XNTRCI-T 05-24-2010 10:33 AM

There are three screws on the 1405 that affect idle... the two idle mixture screws and the idle air flow screw that opens the throttle plates slightly to gain additional airflow when needed on some engines. If the air flow screw is used in setting the idle, you may expose enough of the transfer slots to activate flow through the primary circuits. In that case, changing the jets and/or rods may affect idle quality. Otherwise, it shouldn't. At least, that's the way I understand it. I'm running two 1405s on my 6-71-huffed SBC.

cobalt327 05-24-2010 10:42 AM

Usually when I hear "surge", I think of a lean a/f mix or too much advance.

You're correct in thinking that changing the primary jetting shouldn't affect the idle quality. Is there a chance the float level got changed during the work that was done?

If you find that the curb idle screw is far enough out to raise the idle speed that the carb is transitioning into the cruise circuit, you might want to hook the vacuum advance up to a manifold vacuum source (if not already).

This will raise the idle speed and then you can use the curb idle screw to lower the idle speed and by default lessen the transfer slot exposure.

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