Idle Speed

[schultzy29]

Hello all....I have bought three books on Bronco repair but I am still fairly automotive illiterate. Anyway, my '72 (three in the tree) is idling at like 4000 RMP's when in Neutral and when I put the clutch in to shift gears, then when it's in any gear it feels as though it is picking up speed or on cruise at like 30-40 mph.

I messed with the fast/slow idle screws a little and something seems to work at times and then it won't work at times and goes back to it's "old self". The carburetor is relatively new but I'm stuck as to what it could be.....any thoughts?

Not sure if this is descriptive enough of the issue.....Thanks!

-Jason

 

[BroncoJoe19]

after it warms up does it kick down to a normal idle?

The automatic choke should have a stepped throttle that holds the idle higher until it warms up.

IF your choke is sticking, that could be it.

Otherwise a vacuum leak could cause a high idle.

Checking for vacuum leaks.

Some of the sensors and some of the actuators either read the amount of vacuum, or use vacuum to move controls, therefore it is important to the proper functioning of the engine that your vacuum system is fully intact. Otherwise one may get codes falsely indicating that there is a problem with a controller/actuator, or a sensor when in reality it is caused by a lack of vacuum.

Look at your rubber vacuum hoses (they are about 1/4 - 3/8 inches in diameter, if they are dry rotted or cracked, replace them. IF they slip on/off of their connectors too easily, they may have stretched out a little bit, cut the ends off, and put them back on. Check the PCV valve, hose and grommet.

You may listen for a leak. Sometimes using a cut off piece of garden hose is a good aid to listen with. Sometimes a cardboard tube (like from gift wrapping paper) is helpful. Just remember that whenever you stick your head inside a running engine compartment, that you are not wearing any loose clothing or jewelry that can get caught.

Another suggestion is to use a spray can of carb cleaner and spray around the bottom of the intake manifold, and the base of the throttle body assembly. IF there is a leak, you should hear a change in your engine.

Keep a fire extinguisher handy, or use water instead of carb cleaner (it works, but not as well.) Be aware that if you use water and the wires get wet and it runs worse... your wires may be bad, and the water is allowing them to arc instead of jump the gap of the spark plug.

Another suggestion is: to take a propane torch, turn it on (but don't light it) and move it over the vacuum hoses and connections. The engine RPM's will increase if there is a leak and the propane gets sucked in.

 

[Yardape]

I'm curious to know what you call the fast/slow idle screws. Can you post a picture of the screws?

 

[Seabronc]

ditto on Yardapes question. That must be some exciting ride /emoticons/[email protected] 2x" width="20" height="20" /> . You either have a linkage problem or a carb problem. You shouldn't be messing with screws on a carb unless you know what you are doing. What carb is on it? 2BBL or 4BBL?, Manufacturer?

Good luck,

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[Bully Bob]

Yep.., as above and...,

At rest., (key off) you should be able to look down the air horn (throat) of the carb & see....the choke plate should be fully open (or nearly so).., And the throttle plate (deeper down) should be fully closed.

There's mixture control screws at the (outside front) very bottom of the carb. There's linkage adj. screws on the linkage for idle & choke settings.

That much RPM is a major problem......such as the wrong base gasket or spacer....jamming the throttle plate...

Best guess scenario..

Also.., a combo of things.., one being the timing too far advanced, in addition to carb. problem.

 

[schultzy29]

Okay, thanks to all that responded, I will take a look at all of those things. The "screws" I am speaking of is the one that "bully bob" spoke to at the bottom of the carb, the other is just the ***** that allows the throttle to return fully back to it's "idle" position or you could ***** it down more so that it doesn't come back all the way, I'll try to take a picture of it, I saw it in one of the repair manuals and it said you could turn it a little so I did but nothing too crazy.

Let me see what I can work on tonight and let you know. Thanks again for all the help.

 

[Seabronc]

Okay, thanks to all that responded, I will take a look at all of those things. The "screws" I am speaking of is the one that "bully bob" spoke to at the bottom of the carb, the other is just the ***** that allows the throttle to return fully back to it's "idle" position or you could ***** it down more so that it doesn't come back all the way, I'll try to take a picture of it, I saw it in one of the repair manuals and it said you could turn it a little so I did but nothing too crazy.
Let me see what I can work on tonight and let you know. Thanks again for all the help.

The ***** on the bottom is the idle mixture adjustment. It only has an effect on the fuel air ratio at idle. It should only be adjusted when initially setting up a carb. It is not possible to make the engine run at 4000 RPM with just a poorly adjusted idle mixture. You have a more serious problem with the carb than that.

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