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Throttle Sync/Big Brass Screw Setting


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I am working on a throttle sync for my 1999 1100 RT and using a Carbtune.

My question is regarding the setting of the brass bypass screw and the readings on the carbtune.

I did not record the amount of turns needed to set the screws when I started (oops). I set them at 3/4 turn out and got the idle to 1,000 with the gage showing roughly 22 on one side and 24 on the other. It ran very smooth at speed.

I then noted that an RT should have the screws turned out 1.5 turns so I reset them and got the readings back to the same place and it idles and runs just as smooth?

Which is right and should I be more picky with the 22/24 readings?


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Morning hellmutt


You have something amiss there if you are getting the same idle RPM with the BBS at 1.5 turns as at .75 turns.


It should balance cross sides with the BBS set to about anything between almost closed and about 2-1/2 turns out but the more open the BBS the more air (should) be let in therefore increasing engine idle RPM.


If the engine idles the same RPM at .75 vs 1.5 you probably have plugged BBS air passages under the BBS screws. (maybe remove the screws and use a cat. & 02 safe carb cleaner to clean the passages out.


The 1.5 out from seated is a good place to start the balance process but nothing cast in stone saying they must remain there. All those BBS screws do is let in more or less idle by-pass air (around the throttle plates) so things like worn throttle plates or worn throttle plate shaft or worn bushings can let in more air so the BBS will need to be turned in a little to compensate.


Are you absolutely sure the throttle cams are fully seated against the throttle stop screws during the IDLE balance phase? You need to verify that before continuing the TB balance.






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Thanks for the quick reply.

After I checked the valves I used one of the feelers to check under the throttle stop screws (I forget which feeler fit), however it was similar on both sides...........looks like I should set them both to NO gap and start again.

I am a little confused about the cable setup; am I right to understand that when the cable from the throttle gets into the bike it then reaches a joint where it splits into a "Y" that connects to both throttle bodies? Some of the info for earlier bikes seems to indicate that the cable goes to one side then another cable reaches over to the other throttle body?

This bike had 4K miles on it when I got it and only has 26K on it now; I had a major service done at about 14K.

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Morning hellmutt




You POSITIVLY need some slack in the throttle cables on each side to allow the throttle cams to FULLY seat against the throttles stop screws. If you can get a feeler gauge of any thickness in between the throttle cam and stop screw- STOP right here and find out what is holding them off the stop screw (probably tight cable(s).


First you need to make sure that the choke isn’t partially on! That will hold the throttle plates open a little.


On the cable splitting or being continuous from one side to another? The very early 1100’s had one continuous cable that ran to the left TB then wrapped around that cam and continued over to the right side. Your later 1100 should have a boden box (splitter box) with the main cable running into that then splitting there with a (separate) short cable running to each side TB.


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Thanks D.R.

Its back to the garage for me.

Luckily Connecticut is being treatened with rain and chill for the next week so I will be able to do this at my own (slow) pace. I will report back.

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Just go to the right handlebar and undo the throttle cable lock nut, turn the barrel adjuster in to get some slack at the throttle bodies (i.e. add slack to let the throttle stop screws rest against the stop plate). Resync.

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One point to emphasise is do not adjust the throttle stop screws. They are factory set using a flow-bench - messing with these can give real problems re-setting the throttles.



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Thanks Boys, I got back at it today.

Made sure the cables had some slack (the choke was tight).

On idle of just slightly less than 1,000 rpm I got the Carbtune to within less than the 2cmHg that the CarbTune manual calls for, above that the reading is dead level.

On the road it runs as smooth as a sewing machine motor.

At idle it is not as smooth however I would have to pull a plug wire to make it run like a Harley.

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