10 ft of 1/4" tube on a stick
a little bit of engine oil
The most accurate gauge you can buy make
Seriously a stick about 4ft long
Tape the tube to it in a "U" shape with one end about 2 ft longer than the other
Suck some oil into the tube until it fills the bottom 2 feet of the "U" section.
To attach it to the throttle bodies you will need a couple of nipples, you can make these from grease
nipples with the tip filed off the thread to let the spring and ball out.
You will need to wire the tube to the nipples or it will let go when it's hot or use rubber for the last few inches. EDIT - sorry, KW Roy, didn't see that you'd already covered this.
Here's the write up I found years ago on a BMW site for making the fluid manometer. There's a few caveats when using one of these:
- If your TBs are waaay out of balance, the one producing the most vacuum will suck all the ATF fluid into that cylinder. Smoke aplenty!
- If the TB vacuum ports get really warm, the tubing may soften and fall off one side with the same result. Smoke
To cure that problem, I found some plastic connectors that permitted me to attach a few inches of tubing that was less affected by temps, and didn't want to fall off a vacuum port.
- Others have built these home made manometers and implemented small reservoirs near the top of the instrument to capture fluid if it gets sucked up one hose toward that cylinder.
Having said that, I cannot imagine any costly manometer or other instrument being more accurate, unless you want to measure the specific vacuum at each port. Which in our case isn't of much value, since what you're attempting to do is set the vacuum on right and left to be the same.
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Written by Hugh Kenny
Friday, 01 March 2002
Since joining the Airheads shortly after purchasing my first-ever Beemer in October of 2001, I've found the AirList to be an invaluable source of information and direct feedback as I learn my way around my new 1984 R100RS. One of the first things I do when acquainting myself with a new mount is a basic tune-up. Lurking on the Airlist provided a bunch of helpful tips to supplement (and correct) the tune-up info in the Haynes and Clymer manuals.
One of the slickest tune-up tips I came across was Tom Rowe's mention of a ridiculously cheap and easy-to-build differential manometer (vacuum gauge) for balancing carburetors on vacuum port equipped Boxers. I'd read about the Twin-Max (aprx. $80) and have used the $40 CarbStix on my 4-cylinder Hondas, but for less than $4, I was able to build a twin carb synchronizer that is 16 times more sensitive than my mercury vacuum gauges and can be assembled from common materials available at almost any hardware store. I built it and my R100RS loved it - it really smoothed out some bands of footpeg/bar/mirror vibration that the bike had, even after using the CarbStix.
I posted a 'Thank You' note for the idea on the AirList after I tried it and got even more valuable feedback from Jay Carpenter and a request from John "Beetle" Mailleue, ABC# 5657 to write a tech article for the AirMail. After spending some more time in the garage incorporating Jay Carpenter's ideas, I figured I'd go ahead and write-up a description of how to build and use the $4 Carb Synchronizer, because it REALLY works. Super cheap, super accurate, super easy to build and super easy to use - CLASSIC Airhead technology!
Credit for the original idea has to go to Marty Ignazito of the powered-parachute crowd, he came up with the idea to balance the twin Bings on a two-stroke Rotax and his original write-up is at
www.powerchutes.com/manometer.asp. If you try this idea and like it, send Marty a thank-you e-mail at mdipe@mcleodusa.net .
Here's the Materials List for the $4 Carb Synchronizer Tool:
• 20 feet of clear plastic (vinyl) tubing - inside diameter big enough to slip on the vacuum nipple of your carb (3/16" i.d. worked for my bike, but it's tight, maybe 1/4" i.d. might be better). 15 cents per foot in the plumbing section at my local 'big box' hardware store, Sutherlands.
• A yard stick - Home Depot sells an aluminum yardstick for under $2, but you can make a perfectly satisfactory gauge with a 3-foot piece of 1" wooden lathe for next-to-nothing. (For a 'professional'-looking gauge, I actually used a yellow aluminum 4-foot rule, but that was wretched excess at $5.)
• 3M/Scotch/Whatever - clear mailing/packaging tape. You should have some of this left over from the Christmas mailing season; otherwise around a $1 a small roll (and you won't need much).
• 2 short nylon zip-ties - You should have these in your garage. If not, buy them in bulk for cheap in the wiring section of Home Depot, Sutherlands, Ace Hardware, etc. - you'll use them and wonder why you didn't have them before.
• A tiny amount of automatic transmission fluid - Actually, just about any fluid works, including used motor oil, colored water, 2-stroke oil, etc. I chose ATF because I already had a gallon of it and (most important) it is really thin and is RED (which looks WAY cool as the indicator fluid against my fancy yellow ruler) and ATF won't hurt the engine if it accidentally gets sucked in the carb's vacuum port.
Building the Balancer
Fold your 20' of vinyl tubing in half and mark the center point. Lay your yardstick down flat on a convenient work surface (kitchen table or floor). Place the center point of the tubing at the bottom end of your yardstick (there is generally a hole at the top end of the yardstick - put the center-bend of your vinyl tubing at the opposite end of the yardstick from that hole). Carefully run the tubing up each side of the yardstick, making sure that the tubing makes a smooth, non-kinked bend at the bottom.
Use the clear packing/mailing tape to fasten the tubing in place on either side ( left and right ) of the yardstick. Thread the zip-ties through the hole at the top of the yardstick and snug the left and right side tubing to the respective sides of the 'stick with the zip-ties. You should now be able to hang your yardstick from the hole in the top ( I use a bungee suspended from a hook in the garage ceiling). The tubing runs around the perimeter of the yardstick and about seven feet of tubing hangs down from the left and right sides of the 'stick. I fold a piece of tape around each end of the tubing like a little flag and mark the left side with an "L" and the right side with an "R" using a magic marker.
Now, put one side of the tubing in the container of automatic transmission fluid and, using the other side of the tubing like a drinking straw, suck ATF fluid about three feet up into the tubing. Maintaining suction for a second, pull the tubing out of the ATF container and then raise BOTH ends of the tubing above the top of the yard stick. Temporarily fasten both ends of the tubing high enough that the ATF drains down to the loop at the bottom of the yardstick. I recommend leaving it overnight so that all the bubbles, etc. work their way out.
Once the ATF has settled into the bottom of the tubing, the balancer is almost ready to use. If everything has gone according to plan, you should now have a nicely graduated rule hanging from the ceiling with a thermometer-like tube running up both sides, with the red "thermometer" fluid about half-way up each side at identical heights. Cut a piece of black electrical tape just long enough to cover the width of your ruler and use it to mark the height of the fluid. Your super-accurate Carb Synchronizing Tool is now ready to use.
Synchronizing the Carburetors
BEFORE attempting to adjust the carburetors, you MUST have the other basic systems of the engine working properly: ignition properly adjusted (timing spot-on, advance mechanism operating properly), valves adjusted, compression within specifications, air filter clean, pollution lines leak-free, etc.
Warm up the bike with a ten-minute ride. Park the bike next to the Carb Synchronizer. Loosen the throttle cable adjuster lock nuts with a 10mm wrench and slack off the throttle cable adjusters until there is absolutely no tension on those cables - they should be loose. Disconnect the vacuum tubes that run from the carb nipples to the airbox (or remove the blanking screws or plugs from the vacuum ports, this varies between bikes - check your manual) and connect the right and left Carb Synchro Tool tubes to the respective nipples. Pick up a straight-blade screwdriver that fits the carburetors' throttle-stop screws and start the bike.
Synchronizing the Throttle Stop Screws at Idle
Unless you are incredibly lucky, the red fluid will begin to move, rising on one side of the ruler and dropping on the other. The fluid rises on that side of the bike where the throttle butterfly is more closed, which raises the intake vacuum. Gently turn that side's carb's throttle stop screw in (clock-wise) until the fluid movement stops, then just a touch more, until the fluid moves back toward equilibrium. As the fluid approaches the balance point (which you have marked with the black electrical tape), turn the throttle-stop screw counter-clockwise and STOP the fluid movement right at the balance point. Now check the tachometer for idle speed. If the idle speed is between 900 - 1100 RPM, you have the throttle stop screws perfectly balanced for idle speed. I like a 1050 RPM idle speed. If your idle speed is too high or too low, carefully adjust BOTH throttle-stop screws until the idle speed is within the 900 - 1100RPM range and the Carb Synchro fluid remains STOPPED at the balance point. Check to be sure you still have slack in both throttle cables. If so, the idle throttle stop screw synchronization is complete.
Important: the bike should NOT idle in the garage for more than 5 to10 minutes after being warmed up without a 'cool down' ride around the neighborhood or use of a BIG, high-speed fan, blowing cooling air over the engine.
Note: At this point, you may wish to adjust your Pilot Mixture Screws. Since this is not an absolutely necessary part of synchronizing the carburetors and is covered adequately in the various shop manuals, I will not address it here. If you do the Pilot Mixture adjustment, re-check the throttle stop screw balance and idle RPM and re-adjust as explained above.
Synchronizing the Throttle Cables
Now you must synchronize the throttle cables. Amazingly enough, neither the Haynes nor the Clymer manuals discuss this procedure, yet it is essential to smooth running, vibration-free highway-speed operation of your bike. Both throttle cables should still be slacked-off and their lock-nuts loose and the twist-grip will have a bit too much free-play as a result of loosening everything up for the idle adjustments. Carefully turn the throttle cable adjusters counterclockwise until MOST (but not ALL!) of the slack is removed from the cables, then spin the throttle-cable lock nuts down (clockwise) until they just BARELY seat. Re-check - there must STILL be some slack in BOTH cables. You should still be able to move the cable adjusters and lock nuts easily with your fingers. Start the bike. Idle speed should be the same as you have originally set it. Turn the handlebars to either side. Idle speed should not change. If idle speed or balance changes, you have TOO much tension on the cable(s) and must dial in some slack. If everything checks out, you are ready to synchronize the throttle cables.
With the engine running, slowly roll the throttle on until the engine speed picks up to 2500 RPM. Hold it there and check the Carb Synchro fluid. If the fluid is moving (as is likely), turn DOWN (clockwise) the throttle cable adjuster on the LOW side of the fluid until it stops, then moves back toward equilibrium, while holding the RPMs at 2500. As the fluid reaches the balance point, turn the throttle cable adjuster counter-clockwise and STOP the fluid movement as it reaches the black-tape-marked balance point. Snug the throttle-cable adjuster lock-nuts and re-check the balance by rolling on the throttle. If the fluid does not move from the balance point as the RPMs come up, shut off the bike and tighten the lock nuts. Re-check, including turning the handlebars to either side. If the fluid remains balanced, you now have perfectly synchronized carburetors. Disconnect the Carb Synchronizer Tool, reconnect the bikes's vacuum hoses (or replace the vacuum port screws/plugs) and go for a test ride.
I think you'll find, as I did, that well-balanced carbs make for a smoother Beemer.
Click here for a drawing of the balancer. It's a bit big, 89k, so download only if needed.
Hugh Kenny #6051
Cheyenne, Wyoming
Last Updated ( Sunday, 19 November 2006 )
