No idle - '67 F100 - 240 six

[brybar]

I have acquired a 1967 Ford F100 that probably hasn’t been running for 12 – 15 years. It is equipped with a 240 – 6, Ford (Autolite 1101) 1-bbl carb, with a manual choke.
This is what I have done to-date. Ignition system; new coil, cap, rotor, condensor, points, plugs and wires. Fuel system: filters, cleaned and rebuilt carburetor, cleaned and flushed fuel tank. Also replaced PCV and all vacuum lines. There is one vacuum line from carb to distributor. There are two ports on the intake manifold; one connects to the PCV valve and the other was blocked off.
So here is my problem. With everything connected as it should be (and initial carb settings per spec), I cannot get the engine to idle. It will always stall. I have taken the carb apart three times now and float level is good, and all fuel passages are clear. Everything that I have found on the internet would suggest a vacuum leak.
I got everything back together (again). With the choke on and the idle screw all the way in (not idle mixture), I was able to start the truck. After it had warmed up I pushed the choke in but it was still idling around 1200 rpm. I started turning out the idle screw and it started running very rough and finally died around 900 rpm.

Now here’s the kicker! I started the truck again (by turning idle screw back in). I then disconnected the blocked off vacuum line on the intake manifold and the idle speed picked up considerably. I was then able to get the idle speed down to 600 rpm, set the timing to spec and the old truck runs like a charm!! But the moment I put my finger on the open port of the intake manifold it dies immediately.
Now I have no idea what is original equipment on this truck, or if something should be connected to that spare port. It runs great right now, but I don’t think it’s such a good idea to have that port on the intake manifold sucking in dirt and such.
What am I missing? Any ideas?

[68 Ford Stepside]

Try unplugging the vacuum hose going to the distributor & see what happens. If you have full engine vacuum going to the distributor, it will advance the timing & make the engine idle up.

[brybar]

Thanks for your suggestion.
Unplugging the vacuum line from the distributor makes no difference at all. As I understand, when the fuel is going through the idle circuits of the carb, there would not be any (noticeable) vacuum on the port from carb to distributor.

Just to clarify, the truck is idling/running very well with the spare (normally plugged) port on the intake manifold wide open. As soon as I close off this port (finger over it) the truck stalls immediately.

[68 Ford Stepside]

The only thing leaving that port open does is to lean out the mixture. Your carb must be running rich at idle.Some of these problems just don't make sense.

[1972hiboy]

The only thing leaving that port open does is to lean out the mixture. Your carb must be running rich at idle.Some of these problems just don't make sense.

He' right, introducing fresh air into the intake manifold leans the mixture out. your carb must be dumping fuel or your choke is stuck closed maybe. Either way the carb is making a way over rich mixture thus your not able to get a smooth slow idle speed. If its leaking internally here are a couple things id check. Adjusting the idle mixture screw in leans the carb out. if you can turn that screw in until it gently bottoms out and the truck is still running its pulling fuel internally. CHeck for warpage between the air horn and the carb main body. double check float level and its correct to spec. check to make sure it isnt leaking thus becoming less boyant. Make sure you put your main jet back into the carb.

[brybar]

Sounds logical, the open port on the manifold is leaning out the mixture. But why?
I had previously done everything that has been suggested but went through everything again, just in case.

The problem still occurs and the choke is wide open (fyi: manual choke)

While slowly turning the idle mixture screw in the engine will stall, before it bottoms out. 1-1/2 turns out seems to idle best.

Float level has been verified correct (1 3/32) and the float is dry, no fuel inside of it.

When I look down inside carb with engine idling (manifold port open) I cannot see any fuel coming into the carb. So I assume that the only place it is getting in is through the idle port which is located below the throttle plate (which is closed).

And yes, the main jet is installed.

So, I'm stumped! Any other ideas?

[1972hiboy]

Ok, so you can shut the engine down with the idle mix screw. Thats good. that means your carb can achieve a over lean mixture that can not sustain the engine running. That means there is a sweet spot in the adjustment somehwere. with the vacuum port covered can you try adjusting the mixture screw out enough to get it to run right? Logically this is what you get from your symptoms. A open air source that is after the carburetor leans out whatever the carb has mixed in its venturi and can lower intake manifold vacuum. You want to make sure that all open sources for fresh air into the intake manifold after the carb are sealed for proper carb operation. Open vac lines, intake gaskets, vacuum advance diaphram ( in distributor) power brake booster( if applicable) all need to be checked for vac leaks. vac leaks typically increase idle rpm. Which would go with your first problem of not being able to find a low idle rpm. Opening the vac port could be compensating for a overly rich carb adjustment. I would recomend check ALL sources of fresh air into the intake manifold be checked for leacks. Use a vacuum gun to check vac advanced diaphram. you can crimp the line going to a vac booster ( if applicable) to take it out of the system. and try doing some more carb adjusting. Remember, when you screwed in the idle mix screw it shut the engine down. That means the carb has the ability to run lean enough to not sustain the engine running. you should be able to adjust it where you need to be.

[guhfluh]

What is the base timing? How did you set it? Pulling the plug from the intake manifold and inducing a vacuum leak not only leans the AFR, but also lowers manifold vacuum, which lowers the spark timing some. Your main issue is most probably still the too rich AFR.

[brybar]

It took quite a bit of tinkering with both the low idle and the mixture screws. The truck is now running, as it should, with the spare manifold vacuum port blocked off.

I have ended up with the mixture screw only out about 1/2 turn. Quite a difference from the starting point of 1 1/2 turns. I'm not sure I would have gone that far without your suggestions.

Thanks for everyone's help!!

[1972hiboy]

Awesome! Thanks for posting up you got it working again

[fonfixer]

I have the same set up, with same problem. Scratched my brain for the longest time and finally asked my brother for help. He tinkers with the carb and gets the same results. Finally he suggests to reverse the vac lines on the intake and it smoothed out just fine. The moral of this story....just because "it was that way when I got it" doesn't make it right, also it helps to have a fresh pair of eyes every once in a while.

[paulzcrazy67]

i got a 240 6 that the vacuum line is hooked to that spare port And the spark valve ??(ventri valve?) port on the carb for the vacuum line is plugged. mine keeps blowing out the diaphram for the vacuum advance on the distributor. or i got sold two bad ones in a row. is it jetted for the right elevation?
is there a little spring clip on the post for your rotor it should be clamped to the post under the rotor. that little thing took me around the block on my 240 a couple of times before i figured it out.(spring retainer clip i could only find one at a tractor parts house).. mine ran fine for almost a year after going through it. now it starts erratic and runs erratic. not sure it'll idle fine but gotta flutter the pedal for a while before it will not die when i puit on the gas... good luck