When I start my 1975 chevy 454 it appears that the choke sticks, motor races at rather high rpms, it doesn't slow down after warm up. If you put it in gear it will go about 25-30 miles per hour with no foot pressure on the pedal. We replaced the vacuum hose and the little "pot" looking thing that the choke linkage hooks to. Is there a simple way to unhook the choke and see if that is the problem?

It sounds like your fast idle cam is sticking. When you press on the throttle before you start a cold engine, as the choke closes it pulls a cam into position to set the fast idle. Normally, after the engine warms up, if you tap the throttle hard enough to momentarily speed the engine up, it releases the fast idle cam so the engine resumes a normal idle when you release the throttle. Then you can drive off at a normal speed.

If this doesn't work at the outset, & your engine resumes a normal idle after you drive it, it's coming off the fast idle cam sometime, but is sticking at first. This can be caused by wear, improper assembly, or most likely crud in the mechanism.

The only way to see if things are working properly is, with the KEY OFF & a cold engine, remove the air cleaner, & examine the choke & throttle mechanism closely. Watch what moves, & where it goes when you press the throttle to set the choke & fast idle cam.

With most carbs you'll see a little curved gadget with some notches in the edge that rotates as the choke closes. When you release the throttle, an arm with an adjusting screw will will come to rest on one of the notches. The notched gadget is the fast idle cam, & it's supposed to rotate back to where it started when the engine warms, & the choke opens.

That's the fast idle mechanism. Sometimes a shot of carb cleaner, followed by a shot of WD-40 is all it takes, but if it's improperly assembled, badly worn, or something is bent, you may need more. Also, sometimes adjustments are made by bending one or more of the little rods that complicate the mechanism. In any event nothing should bind as the mechanism operates.

olroy, I will go out and check what you are talking about.

quote:

Sometimes a shot of carb cleaner, followed by a shot of WD-40 is all it takes

I have had better luck with Dri Slide as a lubricant after the cleaner. When the volatiles evaporate, a film of moly mumble-ide is left behind to keep things working smoothly. Tri Flow is really good, too.

On these old engines 30 seconds after a cold start you should slightly depress the gas pedal to kick of the ckoke. It may help to clean it as above and my feelings for WD-40 are negative as it eats rust and whats left over is pitted and dry bare metal.

The little "pot" thingy is a bimetallic heater coi - it works like a thermostat in reverse. Check to make sure it's getting 12VDC withthe ignition on. The heater disengages the choke as it heats up.

As olroy and bill h mentioned, the first order of bidness is to clean the choke by spraying the choke plates, shaft, and carb throat with choke cleaner, then spray the choke shaft with teflon or silicon.

You can test operation with the engine off by setting the choke and seeing if kicking the throttle disengages the fast-idle cam.

After cleaning and lubing, check to see if the throttle return spring is strong enough. Sometimes a new spring takes care of what you have.

The choke valve should be open on a warm engine, and that should move the fast idle cam so the fast idle screw on the follower is free of it. Things should be free enough for that to happen, and you should feel spring tension when you try to close the choke valve.

Is your heat riser valve working? If you have a hot air choke, proper heat riser operation is important.

Warning: Technobabble follows:

That year could have either a hot air choke, or an electric one. The hot air is more common. Operation is similar, in that a bimetallic spring inside a black phenolic cap moves the choke mechanism from semi closed to full open. When the gas pedal is depressed on a cold engine, the spring pulls the choke closed and sets the throttle on a fast idle cam, allowing the engine to run without dying until it gets warmed up and has the proper (unchoked) mixture. After the choke spring is warm, a little throttle will release the fast idle cam follower from its cam, and the normal idle screw will control idle speed. Since your fast idle is operating, I would bet that the choke spring is doing its thing. As it gets hot, it should release, if allowed to.


I am not sure what the pot thingy is, but one that has a vacuum hose on it is called a vacuum break, which keeps the engine from dying if the throttle is advanced too rapidly before the engine is sufficiently warm. It basically senses really low vacuum and pulls in more choke. Mine has two of them. There is a another pot thingy there that is an idle load compensator, which performs overlapping functions, and yet another pot thingy which is an actuator that raises the idle if the engine starts to die when warm, from a heavy load.

Of course, yours may be a little different. I am speaking from recent memory, thinking of what is on my 84. I think I have a 76 manual somewhere that might have info a little more pertinent to yours. But Qjets are pretty much the same.

carburetor Mine is like this, tried all suggestions and it still runs WIDE OPEN when you start it, guess it is rebuild time.

Call me, and I will try to walk you through a little more trouble shooting.

Thanks for the offer bill h, but I have another 454 that I can swap the carburetor from one to the other. The other 454 is sold and the guy doesn't care if I swap.

If this doesn't work out I will holla at you.

There are sometimes minor, sometimes large, differences among carbs that look the same; swapping can trade one issue for others. Make sure both are EXACTLY the same, before you commit to the trade. I think that the issue with the current carb is likely minor, but that's easy to say, as bill h and I come from the Carburetor Epoch.

Yours, BTW, doesn't have the electric choke heater; likely there's a lever on the side not shown that connects by rod to the heat riser valve. The heat riser valve diverts warm air from the exhaust manifold into the carb to cause better atomization of fuel when the engine's cold.

But I doubt that's at all related to the problem - something is causing the throttle to hang up on the step cam. It could be the choke isn't the culprit, the throttle plate and primary carb throat is.

With the engine off, hold the choke open and spray carb cleaner down the primaries (the throats on the right in your photo) and operate the throttle linkage (you can see the throttle plates down inside the throat); a brownish residue (varnish) is common, especially for engines not operated regularly.

Varnish causes the throttle plate to stick. As the engine warms, the throat expands, and allows more clearance, permitting the throttle to operate more-or-less normally.

If it were I, I'd also call bill h for that walk-through.

Rusty, mine is not the electric heat riser, and I will compare the two carbs closely before swapping them. If that doesn't work I will holla at bill h.

As always thanks to all that replied.

The carb model/version number is usually in the back, stamped or embossed in one of the creases...

The heat riser valve works like the electric heater, but instead, exhaust manifold heat expands the bimetallic coil and operates the choke.