The OEM Aisan carb and its related emissions control systems are well engineered and durable; one known exception is a leaking AAP diaphragm, which might be considered a limited life component like tires and timing belts, but far cheaper!
Before going to the extreme of putting on a different carb, I would not give up on finding what has gone wrong with the OEM carb or its ancillaries. Anything else is likely to present a new onslaught of drivability problems; and if emissions checks are required to license this vehicle, a properly functioning OEM system is the path of least resistance.
A choke only works when an engine is cold, right? Once it warms up and the chokeās valve gets āverticalā, it stays that way until the engine cools off. Do I have that right? In which case the choke doesnāt really matter to my high-idle-when-warm problem.
I had been ignoring the choke because I had no trouble starting, but yesterday I took a closer look at how it was working and found its mechanism on the outside of the carb seriously stuck. A few ounces of carb cleaner and spray lube, and many workings of the mechanism with my fingers, I unstuck it completely (and got a free solvent high! When I self-check out at Walmart when I buy this stuff the computer stops and waits for the cashier to make sure Iām old enough.). By then it was dark and I was hungry.
Now that the choke is unstuck it opens and closes all the way (hooray!) and the idle speed climbs without limit. I canāt let it idle it goes so fast. The throttle doesnāt move, the acceleration pump doesnāt engage (though it still leaks if I move it by hand), the throttle valve doesnāt move. If I close the choke valve by hand I can slow it down, even stall it. Iāve disconnected the AAP. Iāve tested for vacuum leaks. I suspect Iām missing something obvious. I hope one of you-all sees it.
Well, after all this, Iād concentrate on that dark fuel window on the bowl. Can you pinch the fuel line to the fuel pump and see if the bowl level comes into view?
I can see through the sight glass now. The fuel level is at the top of the range the manuals say is acceptable with the engine off; I can see air in the āearsā: the higher parts on the right and left side, and just a bit below the lower part. Pinching the fuel line made no difference; a stroke of the acceleration pump caused it to drop lower; it stayed there for the few seconds I watched.
I bought a re-build kit, a GP Sorensen 96-615B (aka Standard Motor Parts 1626BS). It has no instructions. Iām supposed to take the carb apart, identify every part when I take it off, find its match in the kit. I suspect I can do it. Am I too optimistic?
I used a multi-compartment muffin tin and made notes and labels as I rebuilt carbs.
Before delving into the carb innards:
What do you find when to take out the accelerator pump plunger: is its leather dried and shrunken? is any debris in its bore?
Re the AAP: did you take unscrew the three screws that hold its housing onto the carb and remove the diaphragm and its spring? any debris in there or the openings into the carb itself? If the diaphragm is broken did you replace or remove it? If not replace, did you block off the vac hose that connects to the little pipe on the housing? Did you also block off the rest of that vac hose before it goes into a little pipe somewhere near the intake manifold?
After I disconnected all the hoses and wires and was inspecting the throttle cable (is that a clip ring? Love to pop those off then search for them.) it started raining, then the sun set. I ran out of metaphorical gas. Iāll start again tomorrow. The worst thing I can do is lose my patience.
The kit has replacement pumps. I paid for all of it so Iāll use all of it. I disconnected and plugged the AAP hose a few days ago, which made no difference.
The power valve at the bottom of the throat bowl, the old one has a brass washer at the top that holds on the spring. The new one has a nylon washer with a slot in the edge that it could be inserted (and possibly removed). I thought it may be a protective cover for the brass washer at first, but there isnāt one. Will nylon really hold up?
Iām watching some Youtube videos. They donāt have the same parts I have. My kit has about a dozen loose o-rings. Are they associated with a specific part that I have to replace (the kit has rebuild parts for 3 pumps, for example)? Or should I be on the look-out for places to use them?
I took out the Idle Mixture Adjustment Screw, a screw at the bottom of the carb that sets the depth of a needle valve that feeds a barrel. Chilton tells me to turn it 2.5 turns back from fully closed; the kit instruction manual says 4.5. Which is it? Or do they have different definitions of āclosedā? At 4.5 turns I can see the needle sticking into the barrel.
I canāt budge the needle assembly closest to the fuel inlet. Itās a relatively large part for this kit, 22mm long, 10 mm diameter, brass. The Youtube videos show it screwing right out. The ears that hold the floats make it hard to grip. I donāt want to crush it, even if I do have a new one. Iāve drenched it in various penetrating solvents and lubes.
The kit has an
acceleration pump
auxiliary acceleration pump
power valve
needle for the fuel inlet
rod & diaphragm assembly for the choke
gaskets
washers, o-rings, retaining clips, ball bearings
I can figure out how to use all of 1-5, most of 6, but many of part 7 I havenāt removed nor can see. Should I be searching them out? For example, the slow jet (part 53) has an o-ring. It looks okay. None of the o-rings in the kit look like they would fit. This jet isnāt in the rebuild kit. And the primary venturi (part 47) also does: should I take it out and see if one of the o-rings I have will fit?
I also have a spare ābootā (so the instructions name it), a piece of
rubber that works like a bellows: itās ribbed and compresses and
expands. The accelerator pump had its own. This one is longer. I
canāt find it on this worksheet. I donāt see anything like it in my carburetor.
Before I rebuilt the carb I unplugged the AAP and blocked its hose, which made no difference in my idle. When I replaced the diaphragm and rod I found no debris or varnish or anything apparently wrong.
The carb was clean except for a thin layer of varnish at the bottom of the bowl that washed off with carb cleaner easily. The fuel inlet filter was clean, too, as well as the chamber in which it resides, not even varnished.
Rebuilding the carb solved the idle problem. Because I had the idle adjust screw turned all the way out, I stalled at red lights: the idle was 300. After I turned it up to 700 (spec) I realized that years of a high idle had trained me not to have to hit the gas when starting after a stop.
Did a leaky AP cause the engine to compensate for lost fuel by increasing the idle? What would be the mechanism?