Howdy folks,
I'm new here but have been reading for a while and learning alot. There's good people here sharing good information. I have a '84 F250 with a 351w that I've been working on to use as a hunting truck. I have replaced the timing chain and cover, water and fuel pump, thermostat, harmonic balancer, plugs wires, dist. cap and rotor. I have also removed all the pollution junk and ac components. I have also capped off the (stovepipe) vacuum heater that went to the carb. choke as the tubing was open to the exhaust port of the intake causing exhaust to be injected in the carb.
Now to my current problem.... When the engine warms up (temp. gauge midway in the normal range) the engine will increase rpms and idle very fast. A friend told me it may be the ported vacuum switch that is mounted into the thermostat housing. He said if this switch is faulty it will send full vacuum to the vacuum advance on the dist. causing the engine to run faster in an attempt to cool itself. This switch is yellow with 3 vacuum ports (apparently that was important to id the correct part).

My question is can I just remove this switch and plug the hole in the thermostat housing and if so, what do I need to do with the 3 vacuum hoses (connect 2 and plug the one to the vacuum advance ??) or is this switch absolutely necessary to make the engine run correctly?
I would like to remove any unnecessary hoses, wires, etc. as the south. tx. rats will chew off everything.

Sorry for the long first post but I wanted to give as much info. as possible in hopes for some helpful feedback.
Thanks.

 

the switch is not that bad to have, I know you dont care about emissions on this truck, but by advancing the timing short term it does cool off the cyl head temps.

 

The only way to know if that is really the problem, is to get under the hood and unplug the vacuum lines to that part when it's idling high correct?

I am not totally convinced it's not your choke. What pulls it off now that it doesn't have heated air going to it? If you are going to keep it that way, your best bet is to convert it to a manual choke.

 

If the carb is a 2150 2bbl it may have an electric choke on it already. Many did but they weren't always hooked up. And they were usually 7 volt units so would need to be powered by the S terminal on the alternator.

As for the vacuum advance, your friend is right about how they work. And if the temp unit goes bad it will give the symptoms you are getting.

 

Being that your truck is a 1984 351, it would have come from Ford with a "feedback" carburetor and engine computer. The engine computer found on these models controls all emissions, ignition timing, and the air/fuel mixture at the carburetor. If your distributor doesn't have a vacuum advance, then this is what you have. The yellow ported vacuum switch you speak of would then be an EGR (emissions) vacuum switch.

If you happen to have a Duraspark II ignition (pre-1983 and 351 H.O. models) or any other distributor with a vacuum advance, the ported vacuum switch you have may be used to switch between ported and manifold vacuum, although it will usually be red or black. This switch is not absolutely necessary, and although it does serve a useful purpose, your engine can run fine without it. Simply remove it from the top of the thermostat housing and plug the hole with a pipe plug. Then, simply run a single vacuum house from the distributor vacuum advance to a ported vacuum source on the carburetor. You may completely delete the hose that ran from the ported switch to the manifold vacuum source.

The stock Motorcraft 2150 carburetors never used a fully electric choke. They used a thermostatic "hot air" choke with "electric assist."

The way the thermostatic "hot air" choke works is this:

Clean, filtered air from the air cleaner is pulled through the "fresh air" tube (a rubber hose that is connected to the carburetor air horn and attaches to an aluminum tube) and into the bottom of the choke stove chamber on the exhaust manifold, where the air is heated up when the engine is running. From there, the heated air goes back up through the insulated "hot air" tube (attached on the top of the choke stove chamber on the exhaust manifold) that connects to the choke cap. As the air gets hot, it closes the spring in the choke cap, which allows the choke plate to open up as the engine warms up to run on a leaner mixture.

The "electric assist" portion of the stock choke system found on the Motorcraft 2150 does not work on its own like an aftermarket electric choke. In fact, it doesn't have to work at all in order for the choke to be effective. It is only there to "assist" the hot air choke in temperatures above 60 degrees, where it helps the choke come off sooner for cleaner emissions. It doesn't work at all when the weather is colder than 60 degrees. It connects from the choke cap by a wire that connects to the back of the alternator. This part of the choke is secondary, meaning your truck will run fine without it, but the choke will NOT work correctly without the choke stove and choke tubes in place and functional.

 

Thanks for all the suggestions and input. I will pass this info along to my friend as he has a better understanding of how all this works.
I'm not sure which carb. I have but it is a 2bbl Motorcraft. The dist. does have vacuum advance on it which would mean that I do have the ported vacuum switch. To see if this vacuum switch is causing it to idle fast, should I disconnect the vacuum lines as Franklin suggested?

If the choke you are referring to is the box where the tube from the hot air pipe connected, there is no electric wire connected to this device.

Thanks again for the help.

 

A 2bbl Motorcraft is probably the 2150. And, you should follow Franklin's advice and pull the line to the distributor when the engine has sped up, and cap the line with your thumb to minimize the vacuum leak. I'll bet the engine speed drops when you do it.

If there is an electrical tab on the choke, the black thing on the side of the carb where the hot air tube went, then you have an electric choke. Lariat is right in that it was used as an assist for the hot air tube, but it is electric. And, I've been able to get by with it on a truck where the hot air tube rusted out. It may not have been as good as it would have been with the full system working, but it worked for me. However, you shouldn't put 12v to it as it is probably rated for 7v. Ford wired them to the S, or stator, terminal of the alternator to get that voltage.

 

Thanks for the response Gary,
I will give this a try and let you know what happens.

Thanks again.