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Discussion Starter · #1 · (Edited)
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 carberetors. I have succesfully rebuilt my carberetors 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 unnecesary 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 carberetors; 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 throtle 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 butterflys close. If you push the plug forward, the choke butterflys open. This plug is difficult to remove and requiers 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, dont 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.


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.

Bryan_Patrick said:
I have the correct Honda part number for the 88C(192F). The part number is #19300-PB2-817. Please add this so others can order theirs direct and without the hassle I went through with my Honda Dealer.


I hope this is simple and strait forward. Let me know if I missed something. Extra thanks to Bob_Nova, Dano, my dad, and all you preludepower members.

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Some extra info and pictures below. Use as reference if you need it.
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Idle Fuel Mixture: But I'll advise not to touch it unless you really know what you are doing.




Fast Idle Screw: Raise or lower the idle for when the engine is cold. This prevents it from stalling before the engine is warmed up.


Choke Adjust Screw: If the car has trouble starting when engine is cold, or maybe it runs a little rich when engine is warm. Adjusting the choke may be necessary.


A/C Idle Speed Adjustment Screw: Basically, the A/C compressor draws power from the engine and lowers the idle. Boost the idle speed with the following screw.


Jets (meter fuel):








 

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great effort man, i dont own a carb, but i did once and i could not ind the stupid chocke screw!
it was awful till it warmed up!
this will come in hand for me in the future im sure!
 

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you dont know how good this write-up really is to making others understand our particular "issues" that they don't have with their FI ludes.

One question though, when you removed the air chamber and the air valve how did you replace these and what did you have to use to by-pass/remove them? From what I read these parts where on the car to help silence or "calm-down" the natural sound of the carbs, how does the sound of the car sound now compared to stock? How does it run compared to stock, if different at all?

About rebuilding your carbs, what carb cleaner did you use to soak the carbs in(your's look really clean) and also, did you have to remove the choke and thermovalve from the left carb before cleaning?

And one last Q, what is the diameter of the vacuum lines you used on the carbs and vacuum manifold? What other sizes of vacuum hose did you end up using for the other hoses?

thanks!
 

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Discussion Starter · #4 ·
good questions bryan_patrick.

ok, i think it is called the air suction valve but i may be wrong. idk what did it, but the car is a lot louder than it was stock. it sounds quiet at idle but heavy throtle makes it roar. i have a port and polished IM too, but i did all of these mods at the same time so i really dont know which makes it so loud.

here is how i said it to one carbed luder who PMed me:
TripleLude said:
before i begin you should know that removing some things can cause engine problems or CEL codes to pop up. i am still testing my car and yes i still have some problems to fix; like my valve adjustment, O2 sensor, CEL code 4.

1) air sucktion valve
well, this depends on what you need for your car, and your intended purpose. if you compare my IM to how it came stock, i have some stuff removed like the black plastic piece and stuff. i believe it is called the air valve or air suction valve. i forgot what its purpose was but i know that it puts pure exhaust gases strait into your intake.

when removing it you will find that the black piece has a hose attached to it. if you follow the hose, it leads to the bottom of the airbox on the carb side. this needs to be plugged for removal. try to plug the hole on the airbox and remove the hose if you can. on mine, i cut the hose to about 3-4 inches and used high temp RTV to glue a synthetic cork into it. yes, it is an unused, synthetic, wine bottle cork and it fits perfect.

you will also need to plug the EGR-looking metal pipe that comes from the exhaust manifold (btw it doesnt come out easy). you should unhook it from the exhaust mani and put a real plug that fits in the hole, and torque it properly. mine isnt like this but it will be shortly.

next you should notice the vacuum line that connects to the valve on top. just unhook it and plug it nicely. for the time being, i left the vacuum hose on the car and plugged the open end with a nail.

removal of the part itself is easy. i think it is held on by two or three bolts. be conscious of where each bolt comes from though. i think at least one bolt needs to go back on because there is a bracket there that uses the bolt (but maybe i am wrong).

2) IM coolant hose

this is meant to heat the IM so that the carbs can warm up. most stock carbs have been tuned to work best at a certain temperature. i would say just leave it on. there isnt a whole lot of power gain from it that can be worth the trouble of how the car warms up. just dont get this confused with the thermowax valve (the main thing controling the choke). it also has some coolant hoses going to it.

3) intake plumbing

i took off the intake pipe and replaced it with a 3 inch diameter intake flew-piece. i also used a 3 inch coupler adapter from autozone. it's nothing major. just please be sure to leave the stock air filter in. if you want something better, go buy the K&N filter for our car.

eventually i will be making a high-flow cold air intake like the Si ludes have. after i test it i will do a write-up on it and maybe sell some too. i have plans in the making i just need to decide which route to take.

well, thats enough info for now. keep in mind that this stuff may not be the best thing for you and it may even cause a problem. i dont know of any problems now but something unexpected may happen.
rebuilding the carbs are a different matter and it may be impossible to completely rebuild them. the screws are made of really soft metal and combined with age they strip real easy. therefore, not everything on my carbs was rebuilt. i never got to soak the carbs. the most important parts are the needle and seat, float, and that general area.

on certain areas, i was only able to scrub the exterior of the carbs and spary carb cleaner on them. for other parts that were removed, i used acetone and sometimes sprayed carb cleaner into a container and soaked small things.

about the vacuum hoses, i have no idea what size they were. i used some that i already had in my garage. i did keep a lot of the stock vacuum lines though because they were in decent shape. i just cleaned them with engine bright and inspected them for leaks.

enough typing already. :p

-joe
 

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Thanks for your explanations, very thorough! As to what made your car a lot louder it was the removal of the air suction valve and air chamber, not the porting of the IM. Both are, according to the manual and what Ive been told, there to quiet things down on the motor. However Honda figured out a setup like this would quiet things down is beyond me.

This new increase in sound, was it the engine running louder or was it more of an intake roar; kinda like listening to a set of Webers roar as you go WOT? Does this loudness, in your opinion, make the car sound better or does it detract from the normal running sound? I must say I wouldnt mind it sounding a little meaner any way I can get it. After all though, our carb'd cars sound better, induction-wise, than the FI cars do.
 

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Discussion Starter · #6 ·
i forgot to mention i have the stock exhaust with the exception of a hollow catalytic converter. and my EGR is plugged.

there is maybe a tiny bit more of intake sound now but it isnt a 'wushhh' sound like the Si's get with an intake. idk how to explain the intake sound. up until i heard the engine roar like it does now, i hated it. there is something so 'pleasurizing' about it that my Si doesnt even have. you can here my Si in the video in my sig. the most surprising fact is that my Si has a high-flow exhaust system and my S has a basically stock system.

it is deffinately worth the sound. let me also note that if you remove the plastic box and dont plug the port, it sounds like crap. so if your gonna do it, do it right. besides, nobody wants all those exhaust fumes going into the passenger compartment.

edit: i'll make a video. the car still needs work though so dont make fun of it. :D
 

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I like to see a video of it....Im curious to see what sound it makes...yes, the stock S has a better sound thana stock FI any day of the week.!

And like you, my EGR has been plugged up for over 2 years no..no easy way of clearing it out short of taking the manifold and carbs off and manually cleaning the ports out, unless you know of a easier way to open them up?...LOL
 

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man I'm so glad I got rid of that shit :D
 

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holy crap thank u guys.....this is the one thing ive been waiting for....my lude is a 1990 s and it has a high idle of a steady 3000 rpms.....hopefully with all this ill be able to fix that finally....if you know exactly wat i have to do please let me know.....thanks again great for carb'd luders
 

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also check the throttle controller. Its located on the dashpot where triplelude has labeled(in green) where the fast idle screw is. Take he vacuum hose off of this, if the idle doesnt dramatically rise, then thats your problem. If the high idle stays constant when warm even after doing this, you might want to check your O2 sensors and/pr a/c control dashpot
 

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My '90 B20A3 prelude 2.0 S idle speed gradually increased until it was revving near 3K at stop signs... All was fixed by replacing the idle controller dashpot. A simple vacuum check on the old dashpot showed it would not hold a vacuum. No vacuum, no work. Now I am on to installing a fresh B20A3 longblock and want to either replace the dual Keihins with Webers or completely overhaul the Keihin carb system. Any members with supporting information are welcome to write me directly at [email protected]
 

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also check the throttle controller. Its located on the dashpot where triplelude has labeled(in green) where the fast idle screw is. Take he vacuum hose off of this, if the idle doesnt dramatically rise, then thats your problem. If the high idle stays constant when warm even after doing this, you might want to check your O2 sensors and/pr a/c control dashpot
The Thottle Controller was the culprit in my Lude's high rpm problem! Easy to test, just connect a tube to the diaphragm and suck on it to see if it will move or lose vacuum and return. Mine was so bad, I could barely move it. PITA to replace though... consider removing the valve cover to get to it.
 

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Discussion Starter · #15 ·
My '90 B20A3 prelude 2.0 S idle speed gradually increased until it was revving near 3K at stop signs... All was fixed by replacing the idle controller dashpot. A simple vacuum check on the old dashpot showed it would not hold a vacuum. No vacuum, no work. Now I am on to installing a fresh B20A3 longblock and want to either replace the dual Keihins with Webers or completely overhaul the Keihin carb system. Any members with supporting information are welcome to write me directly at [email protected]
I overhauled my kehnins and its probably just as much trouble as Webbers would be, minus the price.
 

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I overhauled my kehnins and its probably just as much trouble as Webbers would be, minus the price.
I have a pair of Keihins sitting on a junkyard intake manifold that I could rebuild and then swap with my current system... what about that left carb and the thermowax choke controls in it? My carbs are working great except the choke acts like it is stuck open, i.e. hard to start when very cold. Once warmed up, it runs great.
 

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Discussion Starter · #17 ·
I have a pair of Keihins sitting on a junkyard intake manifold that I could rebuild and then swap with my current system... what about that left carb and the thermowax choke controls in it? My carbs are working great except the choke acts like it is stuck open, i.e. hard to start when very cold. Once warmed up, it runs great.
You can bend the choke tab a tiny bit forward. That will make the choke come on more when cold.
 

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synchronize carbs?

i bought an 89 prelude S and the idle adjust is fully wound there is another screw to the left of the carbs that is fully unwound and on the autozone website it says to synchronize the carbs before you adjust the idle. so do i need to synchronize?.... basically what should i do
 

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you will know if you need to syncronize....if your cr runs like a total piece of shit, usually so...but the carbs almost never go out of sync unless they were rebuilt or screwed with improperly. if you need them sync'd..best bet is to take them to a pro...unless you have all of the necessary equipment?
 

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Discussion Starter · #20 ·
Don't try to sync the carbs yourself. It is a helluva procedure on a 192*F engine. Don't ever touch the sync screw.

If the carbs are out of sync, it will definitely idle like crap. The idle might fluctuate or stick in very strange ways. Basically, one butterfly will open more than the other, causing more air to two on the cylinders and less air to the other two.

Some people don't realize what they are getting into with these carbs. You can't just pop the hood and start turning screws. You have to know which screws to turn and the proper procedure for adjusting them. There is a good reason why I haven't documented many of the other carb adjustments. People simply can not handle them.
 
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