Rebuilding Carb: What Jets to Use? Other Carb Mods?

General discussion about our beloved Tercel 4WD cars
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The Professor
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My tercel:: 1987 Tercel SR5 4WD wagon
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Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by The Professor » Wed May 15, 2019 12:53 pm

I picked up a couple spare carbs and have been tearing them apart to build into a nice replacement.

I've done some reading and it sounds like most carbs came with a 103 primary, but a 101 could be swapped in for better fuel economy. One carb had a 159 secondary jet and the other had a 162. Which should I go with?

Are there any other modifications to the carb that people normally do?
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

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Petros
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Posts: 10887
Joined: Sun Jan 28, 2007 6:31 pm
My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by Petros » Wed May 15, 2019 5:45 pm

I have owned a number of Tercel4wd cars, and I have noticed that one of them, a 1987 that I bought to fix up (front end clip was crunched) had a completely stock engine but appeared to have noticeably more power than other T4wd I have owned. I suspect the larger jets came in the later production model years to try and keep up with Honda, Mazda, Nissan, etc in the power department.

So, use the larger jets if you want more power, and the smaller ones if you want more fuel economy. likely the economy increase is not much, 1 or at best 2 mpg.
'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)

The Professor
Top Notch Member
Posts: 408
Joined: Wed May 12, 2010 6:38 pm
My tercel:: 1987 Tercel SR5 4WD wagon
Location: Western Washington
Contact:

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by The Professor » Thu May 16, 2019 8:03 am

The larger jet came off a carb from a 4A in a Corolla liftback. So I could use a 101 on the primary to pick up maybe 1 or 2 mpg and the 162 on the secondary for a little more WOT power?

No issues with under or over fueling with this? (I'm a FI guy, this is my first ever carb rebuild, heck, never even tuned one before!)

Any other tweaks or modifications I can make to the carb while it's out of the car and disassembled?
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

xirdneh
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Posts: 1932
Joined: Thu Aug 04, 2005 11:38 am
My tercel:: 87 tercel 4x4 wagon w/reringed engine, 83 tercel 4x4 wagon w/salvaged engine and 4.1 Diff's
Location: seabeck, washington, USA

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by xirdneh » Thu May 16, 2019 10:58 am

I had a tercel that had much more power than all the others i have messed with. The primary jet was 106. Pretty sure but its been a while. Mpg was around 26. I prefer the 30 mpg jets 101.
Love those Tercell 4x4 wagons but they sure suffer from road noise.

The Professor
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Posts: 408
Joined: Wed May 12, 2010 6:38 pm
My tercel:: 1987 Tercel SR5 4WD wagon
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Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by The Professor » Thu May 16, 2019 11:44 am

Anyone have a spare 101 jet? I've got two 103s (plus whatever is in the running carb on the engine).

Could I pair the 101 and the 162, or is that too much of a spread? (I know nothing about setting up carbs).
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

xirdneh
Highest Ranking Member
Posts: 1932
Joined: Thu Aug 04, 2005 11:38 am
My tercel:: 87 tercel 4x4 wagon w/reringed engine, 83 tercel 4x4 wagon w/salvaged engine and 4.1 Diff's
Location: seabeck, washington, USA

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by xirdneh » Thu May 16, 2019 2:01 pm

I should have a one o one but will not be able to check till saturday
Love those Tercell 4x4 wagons but they sure suffer from road noise.

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Petros
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Posts: 10887
Joined: Sun Jan 28, 2007 6:31 pm
My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by Petros » Thu May 16, 2019 7:32 pm

consider that sometimes going too lean with jets results in lower fuel economy, you do not have a lot of adjustment range before the efficiency goes down. Also, check the function of the jets in the FSM, it is sensitive to that spread for different speeds/load conditions. And there is also what is known as the "emultion jet", an air jet that allows some premixing of air and fuel to regulate air/fuel ratio. how these work in a mystry to me (it has been many decades since I studied it), but that getting these air jets to the correct size will also affect power/economy and drivablity.

if you have a 4a engine in your car you may want to use the jets from the 4a carb. OTOH, if you want to try the smaller jets from the 3a as an experiment to see if you can improve economy by all means try it. the problem with the factory carb, unlike the weber, you have to pull it all apart to change out the jets. The weber has a handy cover you remove and you can replace all of the jets with little effort.
'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)

The Professor
Top Notch Member
Posts: 408
Joined: Wed May 12, 2010 6:38 pm
My tercel:: 1987 Tercel SR5 4WD wagon
Location: Western Washington
Contact:

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by The Professor » Fri May 17, 2019 7:52 am

The car still has its original 3A, I just grabbed the carbs off a 3A and 4A because they're similar and not as common as they once were in the local yards.
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

User avatar
Petros
Highest Ranking Member
Posts: 10887
Joined: Sun Jan 28, 2007 6:31 pm
My tercel:: '84 Tercel4wd w/extensive mods
Location: Arlington WA USA

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by Petros » Fri May 17, 2019 7:21 pm

it might even be possible to use the larger carb (I think the 4a has larger throat) on the 3a to gain a bit more top end, but that would take some experimenting with the jets. OTOH, those tiny 8 valves in the head will inherently limit the max power output. but sometimes with only 62 hp, every little bit helps.
'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)

The Professor
Top Notch Member
Posts: 408
Joined: Wed May 12, 2010 6:38 pm
My tercel:: 1987 Tercel SR5 4WD wagon
Location: Western Washington
Contact:

Re: Rebuilding Carb: What Jets to Use? Other Carb Mods?

Post by The Professor » Mon May 20, 2019 8:26 am

I'll have to bust out the calipers and measure the throat diameters, though I'll probably just stick with the 3A parts to keep it simple. There appears to be an extra vacuum port on the 4A carb near the idle mixture adjustment.
1975 Subaru SuperStar wagon
1984 Subaru Turbo-Traction wagon
1984 Subaru Turbo-Traction hardtop
1987 Subaru RX 3-door
1987.5 Subaru RX 3-door
1987 Subaru RX Type-RA 3-door
1987 Toyota Tercel SR5 wagon
1999 Subaru Forester S
2002 Subaru WRX sedan

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