What is the recommended diameter of the Idle Fuel Discharge Port ?

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Dumindu
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What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Dumindu » Mon Oct 16, 2017 9:29 pm

My car is a 1987 Toyota Tercel DX Wagon 1452cc 2bl manual driven car. Carburetor of it is a Aisan 14120 Two Barrel carburetor.

I heard that the Idle Fuel Discharge Port size affects for the fuel efficiency of a car. There are upper port and a lower port in the primary barrel and a one in the secondary barrel.

What is the recommended diameter of the Idle Fuel Discharge Port (orifice) for upper and the lower discharge ports in the primary barrel and that in the secondary barrel?

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xirdneh
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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by xirdneh » Tue Oct 17, 2017 12:36 am

I have worked on quite a few carbs on the 83-88 4wd tercel wagons. That carb number does not ring a bell in my mind. I cannot remember the most common number but I think it ends with 40. I am curious about what your looking for. The only holes I have ever been concerned about are what I call the primary and secondary jets. Two brass screws with small holes in them. They are located near bottom of chamber in front of carb the float sits in. Most primary's are marked 101 but I have seen 102 and 103. Secondary are usually 159 but have seen 160 and 162. I believe they are millimeters 101 would be 1.01mm
I did have one car that had a 106 primary. Engine had more power but mpg ran around 26mpg. I prefer around 30mpg so I use the 101's.
Love those Tercell 4x4 wagons but they sure suffer from road noise.

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Dumindu
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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Dumindu » Tue Oct 17, 2017 2:09 am

Thank you very much. This data is very important. I couldn't find this anywhere in internet.

I am planning to put a stainless steel rod of this diameter in to the hole and weld the orifice with gas and then remove the rod. Would this be successful ?

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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by xirdneh » Tue Oct 17, 2017 8:53 am

So are we both talking about the same holes?
If so I am assuming you want to change the diameter of the hole making it smaller. I ordered a set of small drill bits from amazon that cover the main jet range. I solder the hole to plug it then redrill. Bits are delicate so I use a small hand drill (I think its called a pin drill) its slow and takes a while.
Love those Tercell 4x4 wagons but they sure suffer from road noise.

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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Petros » Tue Oct 17, 2017 4:39 pm

I am trying to understand you thought process, motiveation and terminology. the engine will not run properly with out the correct fuel air mixture, and the range that it will run correctly is very narrow. who ever told you that if you lean it out is an ignorant Neanderthal and knows nothing about combustion thermodynamics (do not take any advice from that person). If you make it too lean your fuel economy will get worse, not better. Emissions will also go up, and it will be undriveable. the idle jet only affects the mixture at idle, and will have very little effect on fuel economy. if that is too lean it will want to die when you accelerate from a stop off idle, and it will largely undrivable.

the only "idle jet" is low on the barrel with a fixed size, but it has a screw adjustable needle valve. this idle mixture adjustment screw is set at the factory and sealed with a soft metal plug. If necessary you can drill out the plug (on the outside of the lower carb body), and adjust the screw, but that should not be necessary. Unless the idle fuel port is plugged with crud, which means the whole carb has to be dismantled, clean out and rebuilt and adjusted properly.

the fixed jets in the barrel that are screwed into place (they are brass, and you can not weld it with stainless steel) that Xirdneh is talking about are best replaced with different size jets, or you can DIY change the size as he suggests. there are low speed and high speed fuel jets, as well as air mixture jets (sometimes called compensation jets) that also affect the fuel air ratio and various engine speeds and throttle settings (since you do not generally drive around at full throttle, the intermediate speed jets and air compensation jets are critical). This btw is why all car manufactures have abandoned carburetors for fuel injection, which has much more accurate control over the fuel air mixture, increasing economy, and power, and reducing emissions.

The factory spends a lot of time and money working all this out to optimize fuel economy, drivablity and emissions. But there is a small range that can be adjusted, but I would stay within what the factory found out. Those would be the sizes that Xirdneh has discovered.

But the first thing you need to do to improve economy is make sure it is running properly, all components are in good condition, no vacuum leaks, your tires are all the same size, good quality (cheap tire have higher rolling resistance), the alignment is good, and none of the brakes are dragging. if any of this is out, you will not improve your economy no matter what you do to the carb. Also, make sure your spark plugs, wires and cap are all in good condition, and your ignition components are up to snuff. I have a weak coil, it runs fine when warm. I have driven on it for over a year, and the occasional misfire I know affect my economy. I really need to replace it before the really cold weather comes or it will not want to start.

short of replacing the engine or induction system with a modern EFI fuel system (very difficult and costly), the best you can do is just make sure everything is in top condition. You can also set the spark timing at about 10 deg BTDC, that will help a bit. Also, make sure your vac advance is good, it does not affect how it drives, but could effect your part throttle efficiency if it is bad.

Good luck.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

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Dumindu
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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Dumindu » Tue Oct 17, 2017 5:14 pm

I disassembled the carb totally for my interest to see the vacuum lines. I removed all the aluminium plugs to see the vacuum lines.

When I do this foolish thing, that Secondary Barrel Idle fuel discharge port was damaged by the drill bit when I was drilling the port plug. Now the diameter is around 2.5mm which you mentioned should be nearly 1.6mm. The holes in the primary side were not damaged.

This is the place I was talking about. (This image taken from the internet: not of mine)
_low_discharge_ports.JPG
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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Petros » Wed Oct 18, 2017 3:32 pm

I do not recall that there is any idle port on the secondary port, the throttle should should not even open on the secondary until you are above a certain speed with the throttle wide open.

The idle mixture is adjusted in the primary barrel only.

There may be some secondary fuel ports in the secondary barrel (I just do not recall seeing one), if they are damaged your best option is to replace that part of the carb assembly (or replace the whole carb). these ports size and location are very sensitive. You possibly can use epoxy or low temperature solder or metal, fill the whole damaged area, and than redrill it to the correct size. you have to make sure any interal passages are also redilled.

BTW, it is not normal or routine to dill out all the plugs when rebuilding a carb.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

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Dumindu
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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Dumindu » Wed Oct 18, 2017 7:07 pm

I have done that just for my interest. To see the line connections. I have different types of aluminium plugs (Hammer rivets) to replace the plugs.

I am planning to weld the hole and re-drill it with 1.5mm drill bit and make the hole slightly bigger by the drill (by press rotating the bit over the hole outer circumference).

My other idea is to make a stainless steel rod of 1.6mm. Then fill the plug area with a mixture of 2:1 of Construction cement : Plaster Parris (It tolerate heat and easy to remove) . Then put the rod to the hole orifice (until it immersed in cement). Then let to harden. Then weld the orifice with a acetylene welder. (since the Stainless Steel is not welded by a gas welder I will able to remove the rod leaving a 1.6mm hole)

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Petros
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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Petros » Wed Oct 18, 2017 7:48 pm

this is in the throttle body of the carb, which is cast iron as I remember. welding on it may result in warping or distortion of the body, I do not think that is a good plan. something with a lower melting point, like a brazing rod, might be better.

Or, no welding, drill out the hole to a larger size to fit a large soft plug, and than drill the correct size port in the plug. I would install the plug with loc-tight or epoxy or something similar, just to make sure it seals and does not work loose.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

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Dumindu
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Joined: Wed Aug 02, 2017 12:34 pm
My tercel:: 1987 Toyota tercle 4wd DX Wagon 1452CC
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Re: What is the recommended diameter of the Idle Fuel Discharge Port ?

Post by Dumindu » Thu Oct 19, 2017 8:37 am

Today I met a engineer at a local engineering workshop.

Similar advice that of Petros was given by him. Thank you Mr. Petros.

His idea was to make a short brass plug that having a 1.6mm hole and insert to the plug cavity and place it attached to the orifice. He also adviced not to weld with acetylene.

This is the hole I am having the trouble. I have removed the throttle and its shaft.
Photo0196.jpg
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