B20a3 Carb Adjustments Write-Up and ECU Error Codes
By: TripleLude
(From: http://www.preludepower.com/forums/showthread.php?t=264926)

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This write-up is meant to solidify some facts about the dreaded B20a3 engine. This mostly is meant to cover some basic adjustments of the carburetors. I have successfully rebuilt my carburetors and tuned then to the optimum capacity with the tools I have. I took all of these pictures of my own engine bay. Please keep in mind that your engine bay will look slightly different than mine. I have removed some parts that I feel were unnecessary for my use of the car. For example: the big, black, plastic box-looking thing that was on top of the intake manifold, and the thing that looks like an EGR thing that attached to the box (jeeze what was that thing called anyway), and some coolant hoses that went to the manifold. I would recommend you leave this stuff on your car or at least PM me before you take it off.

(1)
First off, i will introduce the 1988 Prelude S engine bay featuring the SOHC cylinder head with dual Keihin side-draft carburetors; aka the B20a3 engine. yeah, its been a little while since i cleaned mine.



In Yellow: This is the whole carb assembly containing both carbs, linkages, choke, and other necessary components.

In Red: This is the general area where the throttle cable attaches to the carbs, and where the idle adjustment screw is located.

In Green: This is the general area where the choke is located.

(2)
Here is a closer picture of the carb assembly. You cant actually see the screws in this picture. It is meant to clarify where you can find this stuff in relation to the engine bay. note: adjustment screws not visible in this pic.



In Red:
The arrow expresses where you will find the Idle Adjust Screw on your stock carbs. This is meant for adjusting the idle when the engine is fully warmed up. When adjusting idle: Clockwise=higher; CounterClockwise=lower. Be sure you take the car for a drive before adjusting this because the choke may stay on, which will cause miscalculations. Sorry about the low quality picture.



In Orange:
Under the vacuum hoses is the Choke Adjust Screw. In most cases, this screw has a tamper-proof plug on it (dark orange). This adjusts how far the choke will engage when the engine is cold. From my experience, it mostly affects how easy it is to start the car when completely cold; in the morning.

If you push the plug back with your finger, the choke butterfly's close. If you push the plug forward, the choke butterfly's open. This plug is difficult to remove and requires removal of the carb assembly. If you look closely, the screw is attached to a tab. If you can grab this tab with some needle-nose pliers, you can bend it just a hair in the direction you need to adjust it. Be careful not to break anything, don't pry either.



In Green:
Below the goldish-color thing(valve), in the second pic, is the Fast Idle Screw. This controls the idle speed before the engine has reached its normal operating temperature. It can only be adjusted correctly when the engine is cold. You will probably want the idle to be a little higher when the engine is cold, as compared to a fully warm engine. To adjust fast idle speed: Clockwise=lower CounterClockwise=higher.





(3)
B20a3 ECU error codes:
I figured that this is worth putting here too. The B20a3 PGM-Carb system has a totally different set of ECU error codes than normal Honda's. This is why I took the time to make a small list of error codes and what they mean.

Error Code ::: System Indicated

1 ::: Oxygen Content
2 ::: Vehicle Speed Pulser
3 ::: Manifold Absolute Pressure
4 ::: Vacuum Switch Signal
5 ::: Manifold Absolute Pressure
6 ::: Coolant Temperature
8 ::: Ignition Coil Signal
10 ::: Intake Air Temperature
14 ::: Electronic Air Control

If you get code 7, 9, 11, 12, 13, 15, or greater than 15; This means you have read the codes wrong or you may have a faulty ECU or electrical connection going to the ECU.

(4)
Thermostat
The carbed lude is very sensitive to coolant temperature. If the engine runs too cool, it will lose power and efficiency. The engine temperature is dependent upon the thermostat.

There are different rated thermostats available for the B20a3, but not all of them reflect the proper operating temperature. Proper temperature is 192*F (or 88.88888*C). Even the Honda dealer carries thermostats of differing temperatures. Regardless, use only the 192*F rated thermostat on the B20a3, no matter what the circumstances. Bryan_Patrick pointed out that the dealership offers three different rated thermostats that fit the B20a3. Only one is actually the correct one.