Syncing carbs R100s

[steve750Four]

Hi all new to this site & new to these bmw twins, I have finished restoring a 1977 R100s back to stock standard now its time to Sync the carbs looking at finding the best way to sync the carbs seems to be quite a few different methods which would be the best & correct method any help in this area would be greatly appreciated.
As soon as I work out how navigate the site I’ll up load some photos. Cheers Steve.

[Kurt in S.A.]

The two primary methods are: 1) use a differential pressure gage. Your carbs should have vacuum standoffs stiff out the side, capped by a small screw. 2) plug shorting. This must be done correctly so as not to damage the coils. It's not harmful done correctly.

Process involves:
- warm engine with 15-20 minute ride
- place fans in front of engine while running
- ensure some slack in throttle cables
- turn idle mixture screw to get maximum RPM on the cylinder; screw controls a idle gas circuit
- set RPM the same on each cylinder; use differential pressure gage or shorting method
- set tension in throttle cables to achieve same RPM; typically done around 1500-2000 RPM; use differential gage or shorting method

Kurt

[SteveD]

Welcome. Let me know if this helps?

http://rossmz.blogspot.com/2008/12/tuni ... r-bmw.html

[steve750Four]

Thanks for that, yes very helpful now to put it into practice. Cheers Steve.

[Wobbly]

A novice doesn't want to leave things to "chance" and try to tune by ear. Still lots of good information here so far. Good carb tuning will help your bike run it's very best and eliminate a lot of the hiccups and stumbling encountered while starting from a stop. Here's a blow-by-blow....

• Adjust your valves. Probably 80% of the beginner questions and confusion is becasue the valves have self-tightened and lost their proper clearance. That's the first thing you check on an Airhead when there's ANY trouble.

• You've got to have a tool. Build one of these for under $10...
http://www.powerchutes.com/manometer.asp
I filled mine with red ATF to make the levels more easily visible.

• Locate the carb sync vacuum hole, which is on the front of the carb, hanging straight down, and is sealed with a very small screw. Don't confuse it with the idle mixture screw which is next to it, but up inside a recessed hole. Leave the vacuum hole screw in place for now.



• Set both mixture screws to 1-1/2 turns out.

• Go get fresh, high-test gasoline.

• This parts trips people up. You need your engine thoroughly warmed up in order to get the best results. Go for a 10 mile ride. Yes, it takes that long to warm this engine !!

• When you come back, park your bike in the centerstand, with a box fan in front blowing on the engine. Remove the 2 vacuum hole screws. Plug your vacuum gauge hose ends over the vacuum ports. Start the bike.

• PART 1: Adjust the idle speed screws until the tach reads 1000 RPM AND both fluid levels are at the same height. You should have noticed a definite improvement in the quality of idle.

• Now check that BOTH throttle cables have free play in the cable housing at the twist grip. This should be roughly 1 to 2mm when the twist grip is fully rotated forward (toward idle).

• PART 2: Turn the throttle until the engine speed reaches between 1500 and 2000 RPM. (A speed just above idle.) Now adjust the throttle cable housing adjusters (on top of the carb) until BOTH fluid levels are at the same height again.

• Now re-check that BOTH throttle cables have free play in the cable housing at the twist grip. This should be roughly 1 to 2mm when the twist grip is fully rotated forward (toward idle).

(To summarize, you not only want balanced carbs at idle, but also just above idle. That second part is what prevents the hiccups at take off.)

• Remove the vacuum tool and hang it up for next time. Replace the 2 vacuum hole screws back into their tiny holes. Don't forget to plug these holes or the bike won't run worth a toot. Now, where are those 2 pesky screws ? They were just here ! (This is the part that takes the longest.) Darn, they were just here ! Two hours later after sweeping the floor and crawling around on hands and knees, you'll find them on the bench.


If your bike fails to idle correctly....
• Common issues with older bikes include partial failure of the spark plug wires, plug caps, and spark plugs. Replace the wires with METAL CORE wires, NGK 5K Ohm plug caps, and NGK BP7ES plugs, which will total to less than $20. You might do it anyway as cheap insurance.

• Gummed up carbs. You might try cleaning your entire fuel system with a product called StarTron, and running only name brand, high test fuels with the additive cleaners.

• Set the ignition timing using a strobe lamp at HIGH RPM using the "F" mark.


Hope this helps.

[Beemerboff]

I have one of the electronic vacuum gauges, but can get them as close as the gauge will measure on full sensitivity by simply putting a fingertip on each butterfly lever and having someone twist the throttle.
Actually you can get just as close single handed, with a bit care.
Dont find it makes much difference to the way it runs unless you start a mile out, setting the flow at higher revs with a flow gauge like a Synchrometer seems to give better results.

[Airbear]

When syncing the carbs by ear I like to get down behind the bike so I can get an idea of balance at the exhaust when the throttle is just cracked open. To do this by myself I tape a long stick to the throttle grip, but sometimes a friend will provide assistance.

[Gurstang]

I need to sync my carbs as well. I was going to try it building a manometer as linked here: http://www.powerchutes.com/manometer.asp

I believe my vacuum ports are on the side. I haven't tried to remove the screws yet since it looks like it will be a pain. Is the shaft I have marked here in red part of the screw? or part of the carb body?


If that is part of the screw, how do you attach the tubes of the manometer to the carb?

[Kurt in S.A.]

Yes, that's the vacuum takeoff for synching. You will need to find a piece of tubing that fits over that port tightly...otherwise, find some hose clamps to squeeze them...maybe zip ties would work??

Looks like they may be kinda frozen. I'd attack with some Kroil and/or heat...let that wo it's magic and then use a screw driver with a tight fight into the slot.

Kurt in S.A.

[gspd]

I think this is a subject that has been way over-discussed in the past.
Crikey! it's a simple 2 cylinder engine...after all.
Warning: This post will take way longer to read than it actually takes to do the job (sigh!)

First off, there is a distinct difference between 'syncing your carbs' and 'adjusting your carbs'.
Before attempting either, it is important that your valves are OK, air filter clean, ignition system ok and timing dead on (S at idle AND F at high revs). Many people seem to think that F at high revs is all that counts, but unless your timing retards to S at idle, you will not get the idle smoothness and instant starting that a well tuned airhead should have.

If, and only if, all the above are OK, you can now proceed to carb adjusting and syncing.

IMHO, you should throw out your manometers, vac gauges, fluid hoses, and all that stuff. They are totally unnecessary to achieve PERFECT carb operation on airheads. On second thought, don't throw them out, they may come in handy on many 3, 4 (or more) cylinder bikes or your buddy's 440-6pack 'Cuda. You can set up cooling fans if you want but the whole procedure should only take a minute or two.

I can (and you can too!) set up your carbs in less than one minute with only a flat blade screwdriver and a 10mm key!!! and they will pass the "manometer test' with flying colors if you decide to check them that way afterwards. Guaranteed.

Be sure your bike is warmed up to normal operating temperature...

Before attempting adjustments, make sure there is TONS of free play in both throttle cables (the butterflies must be held open only by the throttle stop screws, not the cables).

QUICK PRE-ADJUSTMENT TEST: Screw in the mixture screw on LEFT carb until that cylinder stalls then back it out about 2 turns. Now do the same on the RIGHT cylinder. This will establish that both carb idle circuits are working and that your carbs are in fact adjustable.

CARB ADJUSTMENT FIRST: ...with engine running....
1) TURN OFF LEFT CYLINDER Turn the mixture screw all the way in, just lightly seated, with very little pressure, or you will break off the tip and it could be a bitch to fix.
2) ADJUST RIGHT CYLINDER MIXTURE (screw in until it stumbles and then back it out, bit by bit in very small increments, until it won't go any faster. Now slightly back off the screw that holds the butterfly open and repeat the procedure again. Imagine you are tuning a single cylinder engine for now. Once you achieve a slow but steady thump-thump-thump on only the RIGHT cylinder, you're half way there.
3) Get the LEFT cylinder running again and TURN OFF RIGHT CYLINDER by screwing its mixture screw all the way in, but this time note how many turns out it was and where its final position was as if on the hands of a clock; i.e. about 2 turns out, 7 o'clock. This can be easily done by counting in half turn increments as you screw it in.
4) Repeat step 2 on the LEFT cylinder, remember, slow and steady thump-thump-thump.
5) Turn the RIGHT carb back on by returning the mixture screw to the previously noted position.

You should now have your engine thump-thump-thumping (smooth idle) on both cylinders. Experiment with the butterfly stop screws until you have the idle speed you want (950-1100 rpm) keeping in mind that a too slow idle speed will contribute to backfiring and popping from the exhaust during deceleration on most airheads. To get your ear accustomed to 'tuning', experiment by cranking one carb up a 1/4 turn and back down, then the other. You will quickly HEAR the sweet spot where the engine is running perfectly even on both sides.

That sweet spot will be very obvious, even to an untrained ear. HINT: On some bikes, taping the clutch lever to the handlebar eliminates transmission noise and makes the actual engine easier to hear.


NOW : THE SYNCHRONIZATION
Throttle cables must be in 'as new' condition. You can't sync cables that have bends in them (where the black outer sheathing meets the metal adjuster). Also, both throttle cables must be floating. I.e. If you put one hand near the throttle end and one near the carb end, you should be able to slide the cable back and forth through the bike. It should not hook up anywhere around the steering head or under the gas tank or fairing.

To sync carbs, simply take out all but a smidgeon of slack in both cable adjusters. Then, as you take up the slack by VERY slightly twisting the throttle, both carbs should open evenly. Now zero in on that sweet spot, where the butterflies transition from throttle stop screw to cable tension. Experiment by pulling slightly on one cable by hand, then the other, and then both together. You'll see what I mean. When you get it right, that transition will be imperceptibly smooth. THERE! You're done.

NOTE: Trying to bench sync your carbs at higher RPMs is TOTALLY pointless.
At cruising speeds, being a bit off on the cables makes absolutely no difference because, although 1mm of adjustment makes a huge difference right off idle when the butterfly might only be a few mm open, 1 or 2mm makes no perceptible difference once at partial throttle (20 or 30 mm open).

No matter how precisely you sync your carbs, once locked down, they may be off a bit at certain ambient or engine temperatures. Hence, my invention below. This my homemade adjuster, for toolless tweaking. It allows me to fine tune my synchronization without tools while stopped at a light. As far as free play in the cables, I like almost zero play. It makes the transition from closed throttle to open throttle a lot smoother in real world riding conditions.

Nuf said?