Diagnose my fast idle problem!

Maintenance/Repairs
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21

To disable the AAP, disconnect its vac hose somewhere along its route and plug the hose. Leave or install a short piece of vac hose attached to the AAP housing. Plug that hose well enough to keep gasoline from dripping out of it.

The AAP system is only supposed to work when the engine is cold, but if the diaphragm is broken it will cause excess fuel to be pulled into the intake manifold, downstream from the carb. I found my 79 truck worked fine with AAP system disabled, even in winter here in Duluth, MN, once I got the carb and choke adjusted.

Once you get excess fuel eliminated - by disabling AAP and also making sure the idle cutoff solenoid (if present) is working and the little o-ring on its working end is intact - you may just need to get basic idle mixture set back to normal, and adjust the various idle speed and fast idle speed screws. Those all may have been misadjusted over the years in efforts to solve the too-rich condition.

22

I can get my fingers on the acceleration pump with the engine running. When I pull it out idle slows. I havenā€™t found an explanation of how the AP works. Does it increase air or fuel? Since the throttle isnā€™t moving that isnā€™t why my acceleration pump moves - or does it not move, but fail to respond when something else causes acceleration?

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It increases the amount of fuel entering the engine, not air. The idea is to make the mixture more rich when the engine is cold. On cold starts you want some combo of more gas and less air. The throttle linkage is connected to the accel pump linkage. So what I expect is happening is that your manipulation of the accel pump is being transferred to a motion of the throttle plates, which will change the idle speed.

Canā€™t speak to your particular configuration, but the way the accel pump works on my truck, thereā€™s a small chamber with gasoline in it adjacent to the fuel bowl, and a rubber diaphragm that prevents the gasoline from leaking out and dripping on the engine. Pressing on the gas pedal causes a little arm to press on that diaphragm from the outside, which compresses the gas on the inside & forces some of it to spray into the carbā€™s air intake path. The gasoline path is from the fuel bowl through a one-way valve into the chamber, then out the chamber and up through an internal tube in the carb, where it exits near where the venturi booster is located. If I work the accelerator linkage by hand (with the air cleaner off) I can clearly see gasoline squirting out at the top part of the carbā€™s air intake.

Iā€™ve had that rubber diaphragm break from use several times over the years. The symptom when that happens (on my truck) is that cold starts are a little more difficult, and when I work the accel pedal linkage by hand I can see gasoline leaking out of that chamber, and dripping onto the engine below the carb. Itā€™s never caused an idle quality or idle speed problem tho.

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Then the AP shouldnā€™t move unless the throttle moves?

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On my truck the throttle and AP are connected (through linkage) together, so if either moves, the other does.

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That doesnā€™t seem to be the case in my truck.

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Before spending $400+ for an Aisan carburetor consider this

http://www.webercarbsdirect.com/product-p/wk746-38.htm

I have seen those kits installed on Nissans, Toyotas and Mitsubishis and the owners were happy with the performance.

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RT, do you have an Aisan carb? That brand was stock for Toyota trucks when yours was made. Aisan is the spelling. The name is cast into the front of the main body. In my posts I am assuming thatā€™s what you have.

We have discussed the AAP, which enriches the fuel mixtures when the engine is cold, but drastically enriches the mixture nonstop if its diaphragm is broke. That AAP is on a side of the carb and has one vac hose going to it. Three screws secure its housing to the carb main body.

The accelerator pump - call it AP - is different. It pumps a squirt of fuel into the carb whenever the throttle linkage is rotated. You do this by pushing on the gas pedal, or if at the engine by grabbing and rotating the assembly the gas pedalā€™s cable attaches to. The AP has a shaft that travels up and down inside a bore. A leather plunger attached to that shaft forms a seal against the walls of the bore. Itā€™s like a bicycle air pump, pushing gasoline instead of air.

With the engine off, if you look down the carb, you should see/hear/smell a spray of gasoline deep down in the throttle when you rotate the throttle linkage (or a helper floors the gas pedal.) Thatā€™s a sign the AP is working and the carb does have fuel in its float chamber. The sight glass on the front of the carb shows the level of fuel in there.

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http://mastertechmag.com/pdf/1988/09sep/198809IS_Aisan2Barrel.pdf

My truck doesnā€™t have an aux accel pump (aap), only a primary ap, so not able to be of much help for your aux pump. Maybe the above link will help for the toyota aux pump.

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My idle speeds up from 600 to 1500 if I let it run for 15 minutes. I never touch the throttle; it doesnā€™t move. If I pull back on the AP the idle slows down, the throttle doesnā€™t move. This would seem to mean that the linkage doesnā€™t work both ways: the throttle can make the AP move, but the AP canā€™t make the throttle move. If the AAP were the problem, pulling back on the AP would make no difference, right?

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When I shop, vendors tell me I have an Aisan. I canā€™t see anything cast into the front of the main body.

When I push the shaft of the AP in, gas leaks out of it (engine off). Is that wrong?

Iā€™d need a periscope to see through the sight glass with the air cleaner on. Even with it off the glass is so dark (Iā€™ve scrubbed it clean) I still canā€™t see.

While idling, the AP slowly engages, the throttle not moving at all (the throttle rotates a cam that pushes on the APā€™s shaft; I can move the APā€™s shaft without making the throttle move.) Is this wrong? What most likely causes it?

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RT: If the idle speed slowly increases over 15 minutes (from a cold start?) that sounds like the action of the choke is a factor. The bimetallic coil inside the choke housing winds/unwinds with changes in temperature. When you press the gas pedal to the floor before starting a cold engine, that sprays fuel from the AP into the carb, deep down, and frees up the choke plate from the various other mechanisms that link to it; the only force acting on the choke plate then is that coil. Depending on ambient temperature, the plate should close a little or a lot. You had mentioned elsewhere working on that choke. It may need some more attention.

As soon as the engine starts, vacuum will operate a diaphragm and linkage to pull the choke plate somewhat open. There may be more than one device that allows or forces the choke to open.

The coil in the choke mechanism gets heated, by coolant in earlier trucks like my 1979, but probably electrically in yours. It coils or uncoils and puts a light force on the choke plate. In normal driving, pushing and releasing the gas pedal frees up the fast idle cam, which is one of the several devices that mechanically link to the choke mechanism. Or you can do a quick jab of the accelerator to free it up. That lets that light force from the coil open the choke plate fully. The cam, rotating to its warm-engine position, allows the main throttle plate to close to its normal warm-engine positionā€¦ so the RPMs, which are faster with cold engine, to revert to normal.

If after 15 minutes you kick the accelerator pedal, do the RPMs return to 600 or so?

Re: the link between the AP and the throttle - you can probably operate the AP by itself, via its lever. If you operate the throttle, the APā€™s lever will also operate.

Gas should not leak out of the AP, if by that you mean itā€™s leaking outside. The AP should spray gas deep down in the main (smaller) barrel of the carb, where the only place it can go is into the intake manifold.

Youā€™re probably not able to see the carb well enough until you remove the air cleaner. I canā€™t think why the engine wonā€™t idle with the air cleaner removed but it might be some vacuum hose you inadvertently pulled that needs to be kept in place or plugged.

Once you have a better view of the carb, can you verify it is the same one you posted an exploded diagram of in another post?

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I replaced the plugs, distributor cap, rotor, air filter, PCV valve, and its grommet. (The old grommet cracked into pieces, one of which I had to pick out of the block.) I unplugged the hose to the AAP and plugged it with a golf tee (Haynes recommends golf tees). I put a couple of thick pieces of insulated wire in the arm of the AP that keep it from moving. The bellows of the AP are torn and cracked. (Does an AP come with a re-build kit?)

It still speeds up gradually while idling. Moving the throttle or choke valves with my fingers makes no difference.

Iā€™ve looked at the carburetor with the air cleaner off before. I posted that picture because it looks like what I see. Iā€™ve taken a close look at the replacement carburetor Iā€™ve considered buying to make sure it looks the same. It claims to be Aisan equivalent.

Later the same day: I can run with the air cleaner off if I plug the 5/16 ID (Ā½ OD) hose that connects to a white plastic plug on the air cleanerā€™s bottom. This would seem to be part of the mixture control system. If I let it breathe freely, the engine dies; if I plug it completely idle rises too high. Could the MC valve be plugged, causing this problem? Iā€™ll do the MC valve test.

The manuals say that no vacuum should be felt at the MC valve with the hose disconnected then reconnect the vacuum hose and check that vacuum is felt momentarily. The first happens, but when I reconnect the hose it blows out, not sucks. It may suck momentarily after blowing out.

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Itā€™s likely going to be impossible to get it to work like it should until all the minor problems, AP bellows etc, youā€™ve noted are corrected. Suggest to replace what needs replacing, bring the carb parts up to spec, before you fine tune it. On my Ford truck on cold starts it is normal for the idle rpm to be on the low side at first, then gradually increase in idle rpm as the engine warms up. Thatā€™s just b/c the engine runs better , and therefore idles faster, when warm. If it is idling much too fast as it warms up, that means the fast idle speed screw is probably set too high. Or the fast idle cam isnā€™t properly phased with the choke plate angle. Those are separate adjustments on mine. I had to mess with both of them over the course of a week, back and forth as one affected the other, until I was able to get a good compromise, stable idle when totally cold, but the idle speed not getting too high during the warm up phase. Eventually, like over the course of 10 to 15 minutes, the engine will warm enough the fast idle cam will kick the idle rpm back to what is set for the warm idle adjustment.

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Good advice! Iā€™d still like to know whether the behavior of my mixture valve is correct. Itā€™s not a part at the auto stores, would take waiting to get from the dealer.

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That large hose sounds to me to be part of the PCV (Positive Crankcase Ventilation) system. Not sure what you mean by MC Mixture Control system and the mixture valve - not shown on the exploded diagramā€¦

If fuel is leaking out of the bellows of the accelerator pump, find out whatā€™s wrong. The bellows is just enough to keep grit out, not fuel in. Sounds like fuel is leaking past the APā€™s leather plunger. Take the AP apart and see whatā€™s what in there.

I also wonder if the fuel level in the bowl is too high. Persist in viewing the fuel level through the window on the front of the carb. You will probably need a good light on it, and may need to have someone jiggle the engine to make the fuel slosh around so you can see it. If you donā€™t see fuel through the window and the car runs, that could mean the level is way too high. If so, the float may be bad (too heavy due to leak or absorption) or misadjusted. Youā€™d have to take off the top casting of the carb to get at it. The needle valve, controlled by the float, could also be involved.

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A different large hose runs to the PCV valve.

Itā€™s part of the emissions control system; itā€™s physically in the air cleaner but connects to the carb, at least indirectly. Quoth Haynes ā€˜To reduce HC and CO emissions, this system allows fresh air to enter the intake manifold on sudden deceleration.ā€™

http://www.autozone.com/repairinfo/repairguide/repairGuideContent.jsp?pageId=0900c1528004d55e

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Thanks for clarifying that. I donā€™t think the MC system has anything to do with the increasing idle speed.

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Like Rod_Knox I suggest ditching the Rube Goldberg carb and bolt on a Weber.
I like the progressive DGEV which has smoother throttle response and can be set up for better fuel economy:

http://www.webercarbsdirect.com/product-p/wk746.htm

I put one on an '81 Accord, along with a cam, header and non-CVCC head.
Also got an adapter to use the stock air cleaner with the heat pipe, which allowed for a slightly leaner main jet in the primary barrel.
They are easy to fine tune.

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That is clearly promoting himself and/or spam

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