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Old 10-15-2012, 02:14 PM   #1
limeymike OP
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Carb Sync - Shorting Method(?)

I know starting a carb sync thread is like starting an oil thread but....

Is there a video around anywhere that shows the steps to sync carbs using the shorting method?. I've read Snowbums description but I ain't picturing it right.
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Old 10-15-2012, 02:48 PM   #2
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Do you have the tool?



Which motorcycle do you have? Any modifications?
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Old 10-15-2012, 03:15 PM   #3
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Standard R80RT ('86)

No tools yet, I'll have to make some. Do you have a larger picture?

So as I understand it you plug one end of this tool onto the spark plug and connect the other end to your spark plug lead. Do the same on both cylinders and then when you want to stop one side firing you short it out by placing a screwdriver between the cylinder head fins and this tool. The idle will drop to 500-600-ish and should be adjusted so both cylinders idle at the same speed when it is the only one running. (yes?)
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Old 10-15-2012, 05:29 PM   #4
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Yes.

According to;

http://realoem.com/bmw/showparts.do?...57&hg=13&fg=11

the carbs on your bike are 64/32/357 & 358. You should check this, these numbers. They are the same carbs BTW on the r80 GS and R100 GS. (not sure what vintage, maybe the same vintage as your bike). The #'s on these newer carbs are not stamped on the side plate like on my older R90/6 carbs but are stamped somewhere. I think on the top somewhere. If the #'s you find are different let me know because not all this info I have given you would then be correct. The methods talked about here are the same though.

The initial idle mix setting according to the Bing Manual, which I am looking at, is one half a turn out. You may find the bike runs better with some other idle mix setting but the starting place is 0.5 turns. I find I don't do as well with the mix setting as some others. I usually leave it where the book says to start and only increase or decrease in small increments later to try and get the plugs reading right.

The main jet size is 135
The needle jet size is 2.66
The idle jet size is 45
Clip notch is #3 from the top

These carbs are the ones that run rather lean because of emission concerns and they may respond well to a different jetting or even more idle mix screw adjustment. I can't see in the book exactly what an earlier R80 uses but it looks like bigger main, 145, and bigger idle, 50. Those changes would be a lot I think. You might just try a bigger main some day or the bigger idle. But again I don't have a newer bike, I have an older one. I think also your vintage BMW has the air injection ports and plumbing? A lot of riders take this stuff off.

The tools shown in the picture are from the Northwoods Web site. He sells these for a few bucks but you can make them cheaper. He must solder the nipple from the plug onto the 4mm stud but you can make do with another nut and lock the stud and nut together on the end so they don't vibrate off. If you did the extra nut and solder they would be extra secure.

The tools are installed on both plugs. Tighten them down a bit because they will come off. The Bing book doesn't say what idle speed you want to end up with, I guess that's in some other manual. Most today are happy with anything around 1000 rpm. I don't ever want anything over 1000 rpm on my bike. I think as low as 850 would still be allowed but they never give you anything less than about 950 in the book I believe.

With the bike warmed up after a 20 min ride and the plug tools installed each side is shorted out momentarily and the idle speed screws adjusted so that the end result is the balanced idle needed. After an adjustment on one side the motor is raced and the other side tried. The racing the engine needs to be done because a lot of gas gets pooled in the carbs and you will have uneven running.

This is all the idle speed. That is the opening of the butterfly when the throttle is off at idle. When the bike is not warmed up yet the throttle and sometimes the enrichners will need to be used a little to keep to bike idling well enough till it warms up.

There is also an off idle setting. This is done with the cable adjusters on the top of the carbs. Basically both cables need to actuate at the same time. They need the same amount of slack on both sides. This amount of free play in the cable can be seen by lifting each cable outer sheath from the cable adjuster and noting the amount of free play till the cable tightens. The cable free play can also be felt in the twist grip. And you should be able to see that both carbs start opening at the same place when the throttle is held at that point.

Cable free play and the throttle looseness before the cables start opening the butterflys should both be as small as possible. Works better and gives more maximum opening when the throttle is held to it's stop.

While doing all this there is a danger of over heating the engine because all this idling on the center stand does not flow enough air over the engine to keep it cool. A large box fan will help if you have one and also once you get this routine down just not being at it for more than 5 mins will be of some help.

You may find that after all this work and then you go for a long ride the bike is now idling too fast. Happens because the bike wasn't warmed enough when you did the setting or I believe because the jets those carbs have are too small. You can try resetting the idle speed screws while on this longer ride. I sometimes just take an afternoon to do this, a little adjusting and then some more riding. Or you may want to up the size of the jets a little.

Try to avoid ridding the bike with the plug shorting tools installed. They will come loose and there is danger of electronic ignition problems if they do. The type of problems are not just a temporary poor performance but the need to replace expensive parts of the system.

Let us know how it goes.

I think I covered it all but if I remember any other stuff I'll post some more. I usually leave somehting out so stay tuned and ask questions.
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Old 10-15-2012, 05:36 PM   #5
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Sorry about the small picture. It's really the lack of contrast because of the weird blue back ground. If I ever get my own camera I'll do better. This picture is from Northwoods Airhead Web site.

The tool is nothing but a long 4mm screw of the same thread as the nipple from the spark plug. There should be a new nipple in each package for a new plug. I save these for making the tools. The long threaded rod is from the neighborhood hardware store. It was originally a screw that you cut the head off of. Also don't forget to get the extra nuts.
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Old 10-15-2012, 05:42 PM   #6
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The bike will not idle at 1000 rpm on one plug, if that's what you are aiming at. I personally don't pay any attention to what the bike idles at on one plug. I set the one plug running so each side sounds the same and I end up with the idle speed I want. The bike may idle on one plug at 600 or 700 rpm but the end result is 1000 rpm. The end result is what counts.

Many years ago when we had good gas this was easier to do and today I have a Harmonizer that helps a lot but I still use the shorting method sometimes.
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Old 10-15-2012, 05:46 PM   #7
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Wasn't that the procedure specific for points ignitions? Personally, I woudn't use this method for 1981+ electronic ignitions.
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Old 10-15-2012, 06:01 PM   #8
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Plug shorting works with all carbureted system as far as I know. The idea is to balance the carbs. No reason I can see this doesn't work with electronic systems if you are using the tool which was invented so we don't blow the electronics up.

What you are remembering is probably that we used to pull the plug wires on points ignition bikes and balance the carbs by letting one side not fire at all. This will harm electronic ignitions and many of us had to learn this the hard way. I blew a Dyna III system doing this.
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Old 10-15-2012, 06:05 PM   #9
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Here is Jeff Trapp's website for the tool that disston is talking about - http://www.northwoodsairheads.com/Tools.html . Comes with written instructions that are pretty clear, just takes a little practice and fine tuning of your technique.
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Old 10-15-2012, 08:06 PM   #10
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disston; Great info, thanks for taking the time. My R80 is dead stock and still has the air injection system in place but that will probably be taken off as part of a winter project. I have used the $4 home made balancing tool and have heard both positives and negatives about the electronic tools so I wanted to try this method to try and improve running and quell some vibration/buzziness. I like the fact that it takes into account more factors than just vacuum.

craydds; Thanks for the link.
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Old 10-15-2012, 08:46 PM   #11
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I have a Motion Pro Mercury Balancing stick. It works I guess. Bounces all over, I don't have any luck with it. My problem with my carbs though, I finally put new throttle shafts, O-rings, screws and butterfly plates on. I very carefully set it up correctly. The plates have a certain way they have to be out of the 4 possible ways to put them in, only one way is correct. And so I have had much better luck tuning the carbs. But now I also have the Harmonizer which I like but I don't think mine is working right or something. I get a much different idle speed with the Harmonizer hooked up and slower idles speed when I have the vacuum ports plugged. It acts like the Harmonizer leaks all the vacuum it is using, causes the high idle. I can still use it. It balances the carbs off idle much better.
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Old 10-15-2012, 10:33 PM   #12
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This discussion has not covered the most important part of the whole process which is making sure that the throttle cables are synchronized. Just looking at the free play in the throttle cables is not good enough.
I would use a manometer at around 2000 rpm for this, Bing carbs are very sensitive to this. You can do it by ear and by feel, but a manometer is best.
If your mercury manometer is 'bouncing all over the place' then you need to put some restrictiors into the rubber tubes.

Ideal idle speed depends on whether the bike has a heavy or light flywheel, heavy flywheel bikes can idle slower than the lights.
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Old 10-16-2012, 12:58 AM   #13
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^^^^
Oh man you had to go there

http://www.advrider.com/forums/showthread.php?t=623553
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Old 10-16-2012, 01:34 AM   #14
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I was not really trying to get into the my tool is better than yours type of argument.....it's just with bings it's the actual synchronizing the throttles that will make the difference when you are riding the bike, getting the idles synchronized does not really make so much difference when you are on the move.


I think it goes back to the 70s when with the heavy flywheel bikes, mucho bragging rights could be gained by tuning your bike so it idled reliably at the lowest possible revs!

A manometer is the easiest may to tune in your ears and senses so you can hear and feel that the engine is setup properly and working well. Some people seem to find it difficult to use any tools.
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Old 10-16-2012, 06:56 AM   #15
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Quote:
Originally Posted by batoutoflahonda View Post
Maybe I should have done a search on the topic first. A little passionate debate is always good.
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