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Home Tech Papers: Tips & Tricks Adjusting The Carb Pilot Screws

Adjusting The Carb Pilot Screws

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This is gonna' be lengthy...just preparing ya'...but, if you're gonna' do it...do it right...smile.

First things first. We're gonna' have to remove those soft plugs that conceal the pilot screws. These can be drilled-out if you are "REAL CAREFUL", but it is a touchy subject. I know that there are a few folks on this forum that have drilled these out...mine do not have them, hence I don't know the exact size or depth they drilled to. Drilling is scary unless you know what you're doing. If you drill too deep it is possible to meet the surface of the pilot screw head and damage it to the point where you will not be able to adjust it or worse...grab the screw head and forcibly seat it, i.e., damaging the pilot screw seat. So, if you elect to go this way...wait until some of the other forum members post their experiences with "drilling" these plugs. Otherwise, you can do it with an "extractor", available at most machine shops and auto parts stores. Drill a shallow hole into the plug and use the extractor to back/pull the plug out. The Honda manual lists the following procedure:

1. Center punch the plug and drill into the plug with a 5/32" bit; stop 1/8" into the plug to prevent drilling into the pilot screw.

2. Force a self-tapping screw (4mm) into the drilled plug and continue turning with a screwdriver until the plug rotates with the screw.

3. Pull out the screw head with pliers to remove the plug.

4. Use compressed air (get a can of compressed air used to clean computers if you don't have an air compressor. Make sure all metal shavings are cleared from the passage and that you can see the pilot screw head. Do the same for the other carb.

Now, I don't know the cycle of your last service, but you should ensure that both carbs are synchronized. I would recommend that you do this on your next service as it requires a vacuum gauge; if you have one...give me an email an I'll try to explain the process and "vacuum readings" you should be looking for. If you choose to do the vacuum test yourself, you'll need a vacuum gauge that can read a + or - factor within a 40mm or 1.6 inch range.

If you pull your gas tank and remove the air cleaner tube from the throat of both carbs, you can check it the "poor-man's" way by looking down the bore of both carbs and eyeball the "slides" in each carb. Very slowly twist the throttle (engine OFF) and observe the slides...both should "rise" together in unison. If one begins rising before the other...get 'em sync'd. If they're off by just a tiny bit...let's go on.

Put the gas tank back on and connect all fuel lines and vent line. Before you put the tank back on...do a quick check of connectors and a little cleaning and maybe a light coat of wax on the frame rails.

NOTE! If you do some cleaning/waxing with the air box connector tubes off the carb throats...use some soft (lint free) cloth to cover the carb throat openings to prevent anything from falling into the carb throats. Your best bet would be to replace the air box connection to the carb throats before cleaning.

Now to adjusting the pilot screws. You indicated that you were having a little popping when you "got OFF" the throttle, this would lead me to believe that the idle circuit is running a tad on the lean side. Let's understand that the pilot screw is controlling air on the pilot circuit. Once you shut down the throttle, you "fall off" the primary circuit and your pilot circuit is doing the work of managing the amount of fuel/air mix that you normally have at idle.

Hopefully you have a tachometer...not the handlebar kind, but access to one that can read a + or - factor of 50rpm. Keep in mind that your scooter has a dual fire coil (meaning the coil is firing both plugs, even though only one of the cylinders is actually firing. Some auto tachs will read double the rpm value (I have one...smile); i.e., 2000rpm on the tach is an actual reading of 1000rpm. So, if you're reading a difference of 100 rpm...you're actually reading a 50rpm difference. If you don't have a tach...you're gonna' have to use the ole poor man's method of using your ear to detect minor deviations in rpms. We want to start with the scooter idling at 1000rpm + or - 100 rpm. Your engine should be warmed to operating speed for this process.

1. Turn each pilot screw "in" until it "GENTLY" seats. Do not tighten, please do this gently so you don't damage the "seat".

2. For a '95 or earlier ACE, turn the pilot screw out 1-3/4 turns on each carb. For a '96 or later, turn the pilot screw out 1-1/2 turns. If the scooter isn't at operating temp...go for a short ride and warm her up.

3. Recheck the idle speed with the tach to approx. 1000rpm.

4. Turn each pilot screw 1/2 turn "out" from the initial setting shown above. If the idle speed "increases" by 50rpm or more, turn each pilot screw "out" in 1/2 turn increments until the engine speed "CEASES" to increase.

5. Readjust the idle speed back to approx. 1000rpm. Hopefully you already know where the idle speed adjustment is. If not, it is the black, knurled knob on the leading edge of the front of the left carb. It's the only black, plastic knob there, so it should be easy to locate.

6. Turn the #1 (rear cylinder) carb out until the engine speed drops by 50rpm. At this point, turn the screw an additional 3/4 turn "out" and re-adjust the idle back to 1000rpm. Perform the same steps for the carb for cylinder #2 (front). Take your scooter out for a ride and see if this doesn't correct your popping noise.

If the noise is still prevalent, turn the screw "in" by half-turn increments and readjust the idle (if necessary) and re-run. If this stops the popping...you've found your "happy mark", if it reduces it, but does not stop it...turn the screw "in" another 1/2 turn and re-run. If it seems to get worse by turning "in". Return to the orginal setting as detailed above and back the screw "out" by 1/2 turn on each carb, readjust the idle (if necessary) and perform the same procedure. I can't see or hear your scooter, but I suspect it is lean on the idle circuit but performing both the above procedures (turning screws "in" first, then trying turning them out from the base starting point (detailed above in items 4-6)), you should be able to get the popping to cease or at least reduce it.

Once you're satisfied..recheck the idle speed and adjust as necessary. I wouldn't worry about a "plug test" unless you spend the bulk of your commute at idle. I think Don has a FAQ re plug tests.

If all of this is a bit much for you...you're going to have to convince the dealer that the pilot circuit is off-base and have him remove the plugs and adjust. Bear in mind that the pilot screws are plugged because the EPA doesn't want you spewing emissions on their precious highways while idling in traffic and some dealers will be "a##holes" and refuse to remove the plugs due to "legal stuff".

Good luck on your journey. Don't be afraid to tackle this...I know this is long, but this is an easy procedure and one that "even a layman" should feel comfortable in "learning".
so we can talk.

Have fun!

NOTE: Just for the record. Turning the screws IN......leans the mixture. Out richens the mixture.

 


Submitted by Ronn


Last Updated on Monday, 06 April 2009 22:45