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Discussion Starter #1
This thread will be specifically on carbs. Old school technology. As some of you have read from a few threads here and there about my attempt to get jiggy with a JDM Dual carb setup. The carbs are Keihins CV (Constant Velocity). There is much I still have to learn about them, but I thought it would be a good idea to post the information that I have accumulated over the last couple of weeks or so. Eventually I will get up off my ass and get some pics of it so there will be a better idea of what I am talking about. I know from those of you in Europe, Asia, Australia, and Africa that have given me most of the help on the Emmissions routings and CV carb mods that there are probably more of us carb guy's out there that might need the info and maybe we can have some decent discussions on a topic that is dear to my heart........carbs. Someone hand me a hanky. Sorry sometimes one of me gets a little emotional. . And if you find something that I have written that you feel is wrong. Let me know. God knows Im not perfect.

Links to discussions:
carbed d-series motors?
Performance Cams for D15B4?
The ZC Engine
DA-series Honda Integra Specifications (1989-1995)
DOHC ZC website
CRX Resource Featured CRX

Manuals dealing with Dual CV Carbed Honda's:

* there may be others so if you know where to find them let me know and I will post them. You can also find these and other manuals in the .pdf manual thread in the Party and Bullshit forum.

Info from posts:
From "Current Dseries projects" thread:

Sat Oct 25, 2003 8:31 pm Post subject:
Got my JDM ZC SOHC Dual Carb/Intake manifold set-up yesterday. Dual Keihin's and it almost looks brand new. The intake's runners runners are BIG!
It fits the y7 head perfectly. There are two bolt holes that need to be drilled. JDM only uses 5 of the 7 holes that the U.S. use. Other than that it's a straight swap as far as the bolt pattern. I am looking for any info on these as far as vac. lines, cfm flow, and links to info. If you know anything I would appreciate it.

Sat Nov 01, 2003 12:31 am Post subject:
Well just got through putting the dual carb/intake manifold (PM-4) back together again. Didnt really have to do too much to it. Cleaned the intake manifold. Took off both carbs and put new o-ring gaskets on the bases where the carbs meet the manifold. Also took off all the vac lines, changed the fuel lines, and removed the choke. Once it was all together again I realized that I needed to drill 2 extra bolt holes on the flange where it meets the head because JDM only uses 5 instead of the 7 USDM uses. I probably could have gotton away with just removing the 2 extra studs in the head that hold the IM on but figured I would be safe about it. Once I got the 2 holes drilled it fit perfectly. A new IM gasket and grade 8 nuts , put it on the D15y7 hybrid conglomeration. I felt like Dr. Frankenstien. It looks mean as shi* once installed on the y7 head. Soon I will be done and my next step is to put it in an 87 civic.

Wed Nov 05, 2003 9:42 pm Post subject:
Took the carbs apart last night.pulled the slides out and polished them. Polished the throats of the carbs and rounded the bottom edge of the slide and smoothed it out. Shaved 1mm off the top of the plastic spring retainer to alow slide to move higher. Also cut the top 3 loops at the top of the spring then extended the spring back to original size allowing for less resistance and faster throttle response. I plan on getting the next size main jet and needle. May check out Dyno Jet or another CV carb jet kit. Connected the 2 Black/yellow stripe wires coming from the 2 Slow jet fuel cutoff solenoids and soldered them together w/ another wire that will lead to the Ignition wire. The Primary slow jet solenoid that also carries the ground for both solenoids now has a screw down connector so I can ground it. We will see if this works to allow fuel to the carbs at start-up. Still have some work to do, but getting closer. Also just bought a pair of DellOrto Sidedraft carbs.

From "Rejetting dual carb Keihin" thread:
Thanks to Dual Carb

Posted: Mon Nov 03, 2003 9:30 pm Post subject:
This is probably an old thread, but I came across it and have some useful info...

I have the ZC SOHC Dual Carb in my 4G4D, with the same carb setup as you. I will give you some hints on modding those carbs, purely coz you drive a honda

First, get two motorcycle cone filters, the ones for performance bikes. Then, remove your stock airbox completely, and block off all attached piping. DO NOT block off the breather hose that goes from your valve cover to your stock airbox, get a breather filter for this one. Remove mesh guaze covers and spacers over carb throats. Take the two cone filters and push their rubber ends THROUGH the spacers. Bolt the spacers with filters attached, back onto carb throats, without the mesh covers. File them in the dustbin... you might have to bend two little copper pipes next to throats slightly inward to accommodate the cone filters. Don't worry, they are purely for athmospherics. If you are not sure about the size filter, take a spacer, drive to your bike hop-up shop and compare sizes before buying. If you can, get filters with the rubber ends slightly larger than the hole in your spacer, to ensure a snug fit. You might have to use super-glue to make them stick... do not worry about a CAI. Carbs respond much better when they are sucking in air from 360 degrees, as opposed to from one direction only, such as a CAI. The gains from two cone-filters on the throats is much bigger than what you would gain from a CAI. Cold air is great for FI, but free breathing is GREAT for carbs...

Next, you do the Keihin CV Carb mod: You will see two silver lids on your carbs, where it says KEIHIN. Carefully remove these lids by unscrewing the screws that hold them in place. Careful not to drop any screws or dirt into the carbs once you've taken the lids off. Underneath you will see two long springs. Take out the first spring, measure it, and then cut off 3-4 coils of the spring. Now, stretch it to the original length... repeat the process with the second spring. Make sure they are the same length as before they were cut. Next, look at the open carbs again. You will see two little plastic stoppers on top of the two pipes that the springs fit over. Mine were black, so yours might be too. Remove and file them.

Now, take the springs and put them back where they came out of, with the uncut ends facing DOWN. Carefully replace the carb lids.

What you've done: The springs control the rate at which the diaphragm piston valve rises when you push the accelerator. After the mod the valve rises much quicker, due to lessened spring density. This vastly improves throttle response. The plastic stoppers prevent the diaphragm valves from raising above a certain point, to keep fuel consumption low. The higher the diaphragm raises, the bigger the needles open, the more fuel you get into your engine. The increased air from the cone filters and the fuel from the quicker lifting and higher lifting diaphragm means a cool 10-15HP increase (if you have a decently built engine already, otherwise, expect less)...

If you want to go even more, you can machine one (1) millimetre out of the carb throats (you will need bigger butterfly's), and source larger jets from Honda bikes or Harley Davidson bikes. They use the same carbs... But this is going extreme. Harleyheads get an extra 20HP from machining and rejetting their carbs. But then they are not very streetable, and your fuel consumption goes for a dive...

But then again, performance means more air-fuel is needed.

Posted: Mon Nov 03, 2003 11:36 pm Post subject:
Thank you for responding with this info. Great information too.
1. What about all the vac lines? Which can I plug up, and which do I have to use?
2. The electronic slow jet(2 electronic sensor type things on the sides of both carbs. One with 2wires the other with 1wire, both same except for the wires? and the fast idle valve with the green and yellowstripe wire and two nipples for hoses. How to hook them up, or do I need to? I assume one of them shuts off the fuel?
3. Any pics?

Again thanks

Info from emails:
Thanks to Mark McCandless


I found any motorcylcle shop could supply keihin jets to play with the
carbs. I got a set from a scrap yard and did some cleaning to smooth the air
flow out a little. some european hondas were a 1.6l SOHC Dual Carb also,
never got my hands on one but i'm convinced there a slightly bigger bore. To
be honest there isnt very much you can do, the setup works very well as is.
Fittng a direct replaement K&N helps high end, fitting custom airbox's and
cone filters makes it even worse. Theres more gain to be had from exhaust


Since these posts I have modified the carbs a little and I am almost done taking off the emissions equipment. And soon will post information on my modification of the inner air vent solenoid which shuts off the carb vents when the ignition is off so fuel vapors wont escape from the carb.
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