Engine flooding / running rich issue.

Engine flooding / running rich issue.

I have been chasing an intermitant issue with my Motorcraft 4300 (square bore) for a few months now.

It would start up fine and run with no issues. Then I head out on the highway and after a number of K's down the road (I'm in Australia so no miles here lol) I would back off and the thing would run very rich (I have a O2 sensor on one bank of the exhaust). Normally it runs lean heading down the highway but when it starts to play up, it would run very rich and start to miss fire and blow smoke and at idle all be it a very rough idle.

I know the carby is fine as I purchased it fully restored and rebuilt (by a carby expert). 

I also rechecked the float level.

My car is a 351C H code that I converted from 2V to 4V using a local "small port" 4v inlet manifold to match the 2v heads.

I also rebuilt the distributor as its vacuum advance was sticking.

In doing some research I found a few video's talking about the secondary inlet valve on these carbs is known to be a problem as it is only held by gravity. Also they are prone to issues when fuel pumps are feeding at a higher pressure than the base 5 or 6psi. Last year I replaced my original 1972 factory one with a stock replacement so I suspect it is pushing a little more than 6psi.

I found this video that talks about the secondary inlet. See from about the 5:30 min mark.

- https://l.facebook.com/l.php?u=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DcxN1OxF9858%26t%3D479s%26fbclid%3DIwAR2Xl1T23Yt8s-rg64Fb5DqbfB8utdzyy5C_MQxTpnHbFjc33Lom1QxgHPg&h=AT0uys9ymYRmHxCM5NlAPybp6JYfnaih9OlkEoqavKL4cxSgg-Mz_OYc0M-ZUtz24bVze1GQMlMrCcpHMMkM2QqC_jjaa33PMYZxwjgBzcJJsBuW7-2N_DHuqLsW5Cta7nSfHQ

As the car is a driver and not a drag racer (stock FMX trans, 2.75 diff etc), I felt that if I follow the advance then I wound have any issues as I am unlikely to be driving the card hard enough for the fuel in the bowl to run out!

Basically the advice was to either replace the secondary inlet valve with a bung or block up the factory one.

From https://www.carburetor-parts.com/75-45 (Mikes Carburetor Parts)

I opted to modify mine my removing the needle and soldered up the hole.

To do this I found a small screw that fit in side the valve snugly and then proceeded to add solder.

As the valve is brass the solder adhears to the brass nicly and a standard soldering iron with a fine tip did the job.

I then reinstalled into the carb and reinstalled back into the car.

I have now been on 2 drives trying to make the engine play up as it was before the change and thankfully, it no longer seems to have the issue.

Of note, when I parked the car after a drive before the change, the garage would always smell of fuel. Now it does not.

I suspect a combination of a slightly higher pressure new fuel pump (stock off the shelf type) plus the gravity design of the needle in the secondary value was the cause of the issue.

I can now comfortably drive it on long runs with out the concern of it failing and or playing up.