Functions of Carburetor

Carburetor

Single Carburetor

Carbs work amazingly well, considering that they are just basically a "scent spray" device!  They have had many years of continual refinement. and they evolved into really quite efficient accurate devices.  They have some big advantages over fuel injection, that is of benefit to people that modify cars, or bikes or for people building drag cars / bikes. 

Advantages:

• Cheap, simple to understand and light! (race use)
• Easy to swap some jets to change fuelling characteristics
• No electrics or battery required.  (Some race vehicles have none or only small battery because of weight considerations)
• Work well in most situations
• Can be swapped from one engine/car to another pretty easily
• More instant throttle response is possible than with Fuel Injection as we don't have to wait for sensors to tell a computer what's happening and wait for it to calculate a result!

Disadvantages:

• They can ice up in cool damp weather!
• They do not have as accurate control of fuel / air mixture as modern fuel injection so  can easily kill catalytic Converters...
• Fuel can vaporise or boil in summer making hot restarts difficult.
• The essential for fuel draw "venturi" always restricts airflow slightly reducing max possible power
• Fuel drop out occurs in the manifolds causing higher emissions and fuel mixture dependent on manifold temperature. Fuel sat in the manifold as condensed droplets goes through the engine largely untouched!

All stock units are Aisan-manufactured dual-throat single-choke downdraft-type carburetors.
All throat sizes are 27/27 mm. 3K and 4K venturi diameters are:

• 21/24 mm for single carb
• 20/22 mm for twincarb motors (3K-B, 3K-D).

Jet sizes are:

• 0.99/1.75 mm for single carbs
• 0.84/1.40 mm for twincarbs.

Throttle is operated via accelerator cable with the progressive secondary throat operating on a mechanical linkage, although there is a weighted vaccuum-style "flapper" butterfly in the secondary throat as well. These carbs flow around 150CFM.
Some carbs have an ignition-operated solenoid switch to prevent run-on (Dieselling), I suspect that this came in around 1975 (with the KE30), however some later model cars have been released from the factory with no solenoid? I have found a paragraph in a Toyota workshop manual that suggests this is part of the -C motor emissions system.
These original carbs are quite poor, having very little mixture adjustment, and it would seem they have poor fuel atomization qualities too (whoever heard of a standard carb running rich on a worked 4K?). There's isn't really much you can do as far as modification is concerned. An open-side design air filter, or Ram-Flo, will help throttle response tremendously compared to the stock snorkel-type filter housing. It has been suggested that jamming the secondary throat "flapper" butterfly open (put a spring on the weight arm or something) could help, but just seems to reduce throttle response at low RPM and make the car more noisy up high, maybe with some slight midrange improvement in power.

Aftermarket

One of the easiest mods for these cars is to change the carburetor
A Weber 32/36 DGV is the most common aftermarket downdraft around. Usually you'll need to jet down a carburetor like this, unless someone has already done the work for you, or you have a fairly modified engine. For a 3K/4K, start at around:

• 130 mains
• 170 air jets
• 50 idle

and take it from there.
Basic tuning tips: If the car "stumbles" forward but revs eventually climb, you're running too rich. If the car just cuts and dies, you're running too lean.
Other carbs suitable for these motors include:

• Weber 32/26 DFV (aka Holley 5200)
• Holley 180
• Weber 32/34 DMTT
• Dellorto DHLA40 or Weber DCOE40 in single or twin configuration
• Dellorto DHLA45 or Weber DCOE45 in single configuration.

One of the best carbs I think you can find for one of these is a Weber 28/36 DCD from a Mk1 GT Ford Cortina. You probably wouldn't want to go much bigger than that. I also have a Nikki 28/32 off a Mazda of some kind that I'd like to try out.

Fitting aftermarket carburetors

If your non-standard carb has a fuel return line, block it, or T it back into the fuel line before the fuel pump. If you are looking for somewhere to flow the crankcase ventilation tubes, it is "suggested" you obtain a charcol canister and flow the hoses into these to be emissions legal. You could also put the crankcase vents back into the air cleaner as per original, but I don't think the carby is a very good place for engine oil. I suggest you make yourself a catch can, or get a rocker cover breather.
If you end up with a carb that has a different stud pattern, making a mounting plate and cutting gaskets isn't hard, so you could theoretically fit any carb onto the manifold, as long as you taper the throats (or expand the original manifold) to create smooth flow. The adapter plate on my Weber is about 15mm thick.
Other sources of bolt-on carburettors (ie: same stud pattern) for these cars include:

• Celica motors (18R, 2T, 3T) which are still Aisan carbs.
• Datsun L motors, apparently these Hitachi carbs are just as crap as Aisans.
• Mazda Capella and other cars with Mazda piston motors (1300, 626, etc).
• Holden Gemini Nikki carbs

I have heard mixed reports of these Nikki carbs, some people say they are the bee's knees, some people would rather bolt on a DGV. I have managed to score one, one day I'll kit it and let you know.

Sidedraft

New manifolds can be purchased to adapt sidedraft carbs to a K motor, Available from such places as Repco (Made by Redline and Lynx). Also you can use a Redline linkage kit which can be purchased new as well. The bolt patterns are the same for Weber/Dellorto/Solex side-draft carbs. A new manifold like my Redline one will have to be machined to fit with the extractors. Make sure you have extractors before fitting twincarbs or you will get inlet reversion.