Throttle Cable Synchronization

[montmil]

There have been a few recent threads and commentary regarding the R65 Airhead's propensity to vibrate within the 4-5K rpm range. Nature of the beast? Not as much as some people may think.

The inherent design of the engine does tend to cause slight vibes when cruising within the above noted rev range. But there is more to it than just checking motor mount bolts or adding bar end weights in an attempt to lessen those annoying vibrations.

Having two R65s in the shop provides me with a benchmark vehicle plus a mule. So it was when I rebuilt the carbs on the 1981 R65. I promote the Bing tuning techniques of Hugh Kenny as they are well written, easy to understand and provide excellent results. However, Hugh's helpful techniques do not end at the carburetors themselves; the throttle cables must also be synchronized so that each cylinder is pulling the same manifold pressure and not "fighting" each other, causing... vibrations. This is the key to a smooth cruising BMW.

My 1981 R65 has a barely felt vibration between 4-5K rpm. The 1983 has yet to have its carb cables synched. It buzzes uncomfortably at the same revs and speeds where the '81 is much more mellow.

If your Bings are balanced at idle and just off idle, there's no need to touch the idle mixture screws nor the throttle arm stop screw. All adjustments will be done with the threaded cable adjuster screws.

The throttle cable synchronization portion of Hugh's essay follows. There's a link to the full article at the end.


[size=14]Synchronizing the Throttle Cables
[/size]Hugh Kenny Airhead Beemer Club #6051 Cheyenne WY

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.   Hugh

Here's the link for the entire Bing balancing article:
http://www.airheads.org/content/view/183/98/

[Adrian]

Thanks for this info montmil. I've downloaded the article to my computer for future use. I've always used a twin vac gauge on my bike - following my Haynes manuals procedures. This has always given reasonable results but the vibes still dominate even when they're reduced substantially. I can see how this "new" procedure could be much better. I look forward to trying it out when the old girl is back in one piece again.

[Barry]

Assuming we are talking about finely balancing an engine that is already running reasonably well I differ slightly from the very first part of Hugh's and most every other written up method in one respect.

  If there is already the correct amount of slack in the cables I leave well alone and don't go in for slackening off cables any further. It's disturbing the the reasonable setting that you already have and I just can't see the point. Most written up methods would have you do the idle balance with a large amount of slack but if the carbs won't balance properly with the correct amount of slack say 1-2mm then it's utterly pointless to do the idle balance with a larger amount of slack and then adjust them back to 1 - 2mm again. The idle balance ought to be done with the correct amount of slack because that's how you are going to ride the bike. If more slack makes a difference then it's telling you that new cables are needed.

Of course if the the cables or carb settings have been disturbed then you can start with a large amount of slack to rough in the settings but the final idle balance should be done with the amount of slack that you intend to ride the bike with.  It would be just kidding yourself to do it any other way.

[donbmw]

When I sync my crabs I adjust the cable to have both carb open as the come off idle and the check through the full throttle range and find that the carb stick shows the same reading from idle to 5000 RPMs.                                              

[montmil]

The existing slack in the throttle cables, with the engine at idle, disappears as soon as the throttle is rolled on.

With this technique -and cables properly adjusted at approx 2500 rpm- you are actually removing a bit of slack but leaving enough that there is no tension on the cable at idle.

The manual-specified cable slack is only a starting point- just like the number of "turns out" on the idle jet screw. Final adjustments may be different than the manual's specs.

[montmil]

The existing slack in the throttle cables, with the engine at idle, disappears as soon as the throttle is rolled on.

With this technique -and cables properly adjusted at approx 2500 rpm- you are actually removing a bit of slack but leaving enough that there is no tension on the cable at idle.

The manual-specified cable slack is only a starting point- just like the number of "turns out" on the idle jet screw. Final adjustments may be different than the manual's specs.

[Barry]

With this technique -and cables properly adjusted at approx 2500 rpm- you are actually removing a bit of slack but leaving enough that there is no tension on the cable at idle.
 

Monte,

It's only this bit I have a problem with "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"

I know that it's all about leaving enough slack so there is no tension at idle. What I'm saying is that disturbing the cable balance by putting more than enough slack in just to set the idle balance is artificial and utterly pointless unless you intend to ride the bike like that. And yet most carb balance procedures start by telling you to do just that. It makes no sense unless the cable balance is miles out to start with and that should not be the case for a routine service balance.

The way I do it is check the idle balance with the manometer if it needs adjustment at all it will be a very slight turn on the throttle stop screw. This is insignificant in terms of it's effect on the cable slack.

I then raise the revs to 2500 rpm and adjust one cable or the other to achieve balance. Again it's going to be a fraction of a turn on one of the cable adjusters. I already have about the right amount of slack because I didn't mess it up when I did the idle balance. The fraction of a turn on one adjuster is not going use up a significant amount of slack so that's the job done. I can if I wish choose to increase or decrease the overall amount of slack slightly by deciding which cable to adjust.

Al bets are off if you have been riding around with no slack in the cables but who does that ? it would rev up every time you turned the bars.

[montmil]

What I'm saying is that disturbing the cable balance by putting more than enough slack in just to set the idle balance is artificial and utterly pointless unless you intend to ride the bike like that.

I'm not sure I understand what you're attempting to say, Barry. The discussion is in regard to balancing throttle cables with the engine at much higher revs than at idle; which will achieve smoother highway cruising speeds. Idle speed should have already been set and will not be compromised. You seem to have segued off track and into a different topic.

As stated by Hugh Kenny in his article, "turn the throttle cable adjusters counterclockwise until MOST (but not ALL!) of the slack is removed from the cables." Just how is that adding slack to the throttle cables? Perhaps you misread.

As you are aware, the idle quality is set via the idle mixture screws and the throttle stop screws with the engine running at idle revs; usually around 900-1100 rpm. As such, making changes to the throttle stop screw, which does move off its stop at just above idle, seems counter-productive to a well adjusted idle speed; even though this discussion is about cruising, not idle, speeds.

I would ask that you reread the article and perhaps get a clearer mental image of what is occurring.

[Barry]

Monte

I've explained it as clearly as I can but I can say it again. The article starts with - "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"

What I'm saying is for a routine carb balance I don't  "loosen everything up for the idle adjustments" because it's pointless and I find it unnecessary to start a routine cable balance at 2500 RPM from that position. It just creates out of balance cables before you start and therefore more work. It's much easier to do a 2500 RPM cable balance with the adjusters  left untouched. That way you will likely need only a very small adjustment.

I fail to see how that is a different topic.

Also doing a routine 2500 RPM balance my way you get an idea how far out of balance the cables were. If you have "loosened everything up" then you have no idea. They could have been spot on.

The whole point of my post is that  I do want it to be a discussion with new ideas instead of just following written procedures which repeat the same mantra ad nauseum.  I always try to think about what I'm doing and I have found from experience that "loosening everything up" is unnecessary when the cable slack and balance was pretty close to start with.

We are all free to balance carbs any way we like so lets at least have an open mind about it.

[donbmw]

I have gone back and read this article, the Clymer's manual and the BMW manuals I have. In all of them this is clear as mud. BMW manuals only uses a tool for syncing when adjusted  idles. On points models they use the shorting clyinder  procedure for adjusting throttle cable. On electric igntion models they set the throttle cable free play .5 to 1.5 mm for syncing.

Here is how I have sync my carbs. First off get the engine to operating temp.  Check and adjust if need the throttle cable free play to just a slight amount. Same with the choke but make sure they are going all the way off. Adjust mixture. Adjust idle so the fluid in carg stick is at the same level for the RPM at idle.  Roll on the throttle and watch the fluid level in carb stick. If both are not the same loosen or tight free play as need to get the same amount of level on the fluid as you open the throttle. Also I will check from idle to about 4000 RPM to see if the level is the same for both carbs.  Which if the are adjust to both carbs to be opening at 1500 to 2500 RPM's the  carbs will be the same all the way to full throttle.

In balance carbs it is all about having the right amount of slack in both cables for the carbs to balance out.  Remember that no two cable will be the same exact length that is there is adjustment for the slack.

Don

[Adrian]

I also have a problem when sync the carbs. Having checked valve clearances and set the slack in the cables to be the same, I then watch one operating arm on the carb with a finger on the other. As I slowly take up the slack on the twist grip I can see one arm moving as I feel the other moving. That seems to work real well as I can't see both operating arms too well at the same time. Then I attach my vac gauge and at tick over and slightly above (about 200 reves) I use the throttle stop screws to get a balance. This gives me a reasonable smooth - but not totally smooth engine up to about 3500 revs and a very smooth enging above this. However, I can not get the vac gauges to level out above 200 revs as the gauge tubes start to seperate a little as the revs climb. My question is having read all the above is this. Because I can't get the carbs to balance through the whole rev range is it possible that I need a carbie service kit? I always check my plug colours when I do a balance and they are always okay.

[Adrian]

However, I can not get the vac gauges to level out above 200 revs as the gauge tubes start to seperate a little as the revs climb.

Sorry this should read 2000 revs.

[Barry]

Check and adjust if need the throttle cable free play to just a slight amount. Same with the choke but make sure they are going all the way off.

Don,

You have captured exactly what I have been trying to say. If you check and find you have the right amount of slack at the start go ahead and sync the carbs. As long as you make sure the throttle plates go all the way off to the stops there is no need to mess things up by introducing a whole load more extra slack.

[donbmw]

Barry

There is no way you can check to see if you have the right amount of slack in the cable with out putting some type of carb sync tool on the bike. To have the carb in balance you will not have the same amount of slack in both cables.

Don

[Barry]

Barry

There is no way you can check to see if you have the right amount of slack in the cable with out putting some type of carb sync tool on the bike.

Don

What I meant was you can check there is some slack in the cables i.e. enough to carry out a throttle syc. You just lift each cable at the carb adjuster and if there is about the right amount of free play you can go ahead and do the sync with a manometer.

I thought that was what you meant when you said "Check and adjust if needed the throttle cable free play to just a slight amount"