Dry carburetor auxiliary jet...primary fine.

casey_waAugust 22, 2005

1983 Honda Civic 1500 w/ 122,000 miles.

Carb is a 3-bbl Keihin that was flooding gas to the point of gas getting into the oil. Removed carb and rebuilt. Installed, adjusted float levels, changed spark plugs, drained oil...idles great. Test drove finding the engine is starving for gas over 25 mph. Rechecked float levels and re-adjusted auxiliary (the primary was fine). Noted that at idle the primary jet is spraying gas into the carb but not the auxiliary jet. When throttle if further advanced to higher rpms the primary jet sprays gas stronger but the auxiliary jet remains dry.

Am wondering if the lack of auxiliary jet supplying gas is the cause of my Civic not getting enough gas when driven over 25 mph? At what point should the auxiliary jet kick-in t do it's job?

I am a do-it-yourself guy an am wondering what I might be overlooking.



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Adjust the linkage - these carbs are ultra complex, IMO, the shop manual is a necessity.

    Bookmark   August 22, 2005 at 12:44PM
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A side note since I just looked closer at photographs that I took while rebuilding the carb. The primary float/chamber supplies the fuel to the two large barrels of the carb (the auxiliary to the very small third barrel). Of the two larger barrels only one is supplying gas thru the jet and the other is not. I suspect there is a blockage and will need to remove and clear out. Interested in your thoughts before I do this again. Thanks

    Bookmark   August 23, 2005 at 12:15PM
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>I am a do-it-yourself guy an am wondering what I might be overlooking.

To have gotten formal instruction and training on doing this repair PRIOR to attempting it? VBG...

Carburetor repair is going to be just about a lost art, practiced by a select few. Even though I've rebuilt dozens of this carb, attempting to remember every detail about one without having it in front of me is really going to be tough.

First is the "auxillary" your referring to the thrid barrel? Is the throttle plate opening on it when you are looking into it for fuel flow? Are you sure your secondary port throttle plate is opening and it flowing fuel? Just because the primary has varying amounts of fuel flowing sitting in the stall at different engine speeds, does not mean it's flowing enough fuel. Did you remove the jets when you did the "rebuild"? How did you clean the main metering passages while the carb was dissasembeled? Did you use compressed air to attempt to blow the passages clear? If so I have some bad news, some of these carbs had encapsuled check balls in the main metering passages, and if hit with unregulated shop air pressure, you could actually seat some of them and permanently block the passages. While I'm pretty sure I'm thinking of the one Nissan carb at the moment, they had a screw that was hollow, and created one of the fuel feed passages. The reason I mention that is some of the carb screws still create fuel flow passages, and the screws are different lengths to accomplish this. Did you match up all of the screws during dissasembly, and then ensure they went in to the correct holes? Too long of a screw in the wrong hole could block off an important passage.

Other carb tricks... Were the venturi's removable during dissasembly and cleaning? If so were they removed and the o-rings replaced? The the venturi's staked back in, oh and made sure they were turned the right direction so that the fuel passages lined up?

Does your Honda have an O2 sensor? If so then fuel metering is controlled by pulsing large and small vacuum leaks, and the carb is basically set to be over rich, and the balancing act of controlling the vacuum leaks creates the correct fuel mixture. Did you check this system to see if the vacuum controls are stuck in the lean position?

Good luck, somethings really should not be a DIY project no matter how confident you are. This old Honda fuel control system is one of them, and your going to be hard pressed to find a tech today thats comfortable even working on this, especially going in after the fact..

    Bookmark   August 23, 2005 at 12:32PM
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I read you to say you are looking into the carburetor while advancing the throttle. That being the case I can't argue that the carburetor may be giving too much gas. But, I do suggest you take a look at the exhaust to be sure there is no black smoke. I say this because I had an 88 Dodge Ram 50 with a 2.8L engine that would fade whenever it was loaded, trying to increase speed or going into a hill. It ran fine at constant speeds. I figured it was running too lean and tried to get someone to work on it, no takers and a new carburetor is several hundred $s and I would not take on the replacement myself anyway. To the point of my saying double check, I found surfing the web someone with the same problem who told me the the second barrel (aux?) was injecting too much fuel. He suggested I plug the venturi on that barrel, the right hand side from the driver's seat. I did so, and bingo, the engine ran great. Another piece of information here is that with the axillary fully disabled the engine ran fine, good as "new" as far as I could tell from the driver's seat. Alas, I've traded it in on a new Chevy Colorado, now all my vehicles have fuel infection, and I think that's a good thing. The GM employee price sale and cash backs produced some great buys, even on a fuel efficient Colorado 2.8L with Manual. While on the subject of GM/Chevy, I got the traction control 2WD with automatic locking differential (a lot less money than a 4WD, but with some of the benefits). Anyone have any experience with an automatic locking differential? I'd like to have one on my garden tractor too... still another subject.
Good luck with the Honda carburator.

    Bookmark   August 23, 2005 at 3:53PM
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Check the repair manual. The 3rd jet on your Keihin carb may be an air bleed, not a fuel feed. The Keihin carbs on motorcycles have air bleeds for the low speed fuel adjustment. Opening up this bleed leans the mixture, just the opposite of a fuel feed.

    Bookmark   August 24, 2005 at 12:02AM
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First Google :


The Keihin 3 barrel carburetter appears to be a conventional compound carburetter with an additional single pot carburetter used to meet EC regs by providing a rich mixture to a chamber in the cylinder head (? like a Ricardo swirl chamber in diesels ? don't know), to ignite a leaner mixture in the main combustion chamber.

The site doesn't say, but I think the auxillary barrel is primarily used for starting, particularly in cold areas, where a conventional choke would exceed the allowed emission levels.
So I don't think it would affect performance per se, it would just be harder to start if it was too lean.

If acceleration is poor you would look at the primary chamber in the compund carburetter (the second chamber opens when cruising to let more air in and lean out the mix).

    Bookmark   April 8, 2011 at 1:19AM
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had an 83 colt with a mikuni carb? hot water choke pellet stuck and would not come off choke. burned up cat. took off aircleaner and watched choke open and close right before my eyes. never acted up again.

    Bookmark   April 11, 2011 at 12:19PM
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