Make it run lean

onefastmustang · #1 (**permalink**) 07-25-2007, 06:50 PM

Good evening fellow cobra enthusiasts.... I need some guidance..

I have procured 2 different motorcraft 4300 carbs, both remanufactured from Holley.. Neither of them has been able to be dialed in lean enough to pass smog. Is this a crap shoot to get one that works or is there something I can do to assist them in achieving the right AF ratio. My mechanic friend told me today to just keep returning them as only 1 in 4 usually good but what a PIA...

Jac Mac · #2 (**permalink**) 07-25-2007, 08:13 PM

From what I remember of the 4300 which was OE on Falcon GT etc here in NZ & Australia is that it was sensitive to changes in the manifold vacuum etc. If your engine has any mods such as header's/cam/compression, the baseline settings ( Assuming Holley are using them) are going to be a mile away. If this is the case your mechanic is going to have to get up close and personal with the carb to recalibrate the PV plunger spring setting- check that the correct jets etc are fitted and ensure that the float system is working freely and set correctly along with attempt to get the secondarys calibrated to open at the correct time/load. The jet for the secondarys is a tube with one end swaged down to a certain size-thats all- that press's into the carb body. Also remember that these carbs saw duty on 302w-351w-351c-429/460 with different size venturis for each application, so your chances that you have the 'correct carb ' are thin.

The float design made the setting critical along with the float material being subject to deterioration from different fuel blends etc.

Along with all that many of the engine variants were fitted with 'smog' pump and exh port air injection to add to the mix.

Good Luck.
Jac mac

427sharpe · #3 (**permalink**) 07-26-2007, 12:03 AM

Make a small hole in the intake side of the PCV line which can be corrected after smog. A friendly shop will usually run a 'check' for free if you ask them so that you can adjust the size of the leak.

onefastmustang · #4 (**permalink**) 07-26-2007, 12:33 AM

So what your saying is drill a hole in the bottom of hose and the extra air would lean it out. sounds like something to try.. I have an LM-1 so I could measure the affect on the AF.. Thanks for the idea.