Throttle Body Sync or Balancing

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If you ever sync'ed the carbs on an old Honda we know what a difference it can make in smoothing out a engine. Why doesn't the Honda Service Manual have a procedure how to sync the throttle bodies? I doubt a Honda shop would try it, they would just buy new factory set throttle bodies ($$$).

Anyway, I've been researching it and have an idea on how to do it but would like any advice on how to do it without permanent regrets.

First off, pictures!



This is bottom side of the throttle bodies, cable connection of far left, TPS on the far right. The adjustment on the left will change both throttle plates and the TPS's position. The middle adjustment would only change the throttle plate on the right and the TPS's positon, so it must be what is used to sync the throttle bodies. Honda has painted the 2 adjustments and the TPS and warns not to adjust.

Also shown is the tubing for the MAP Sensor. It senses from both throttle body intake sides and routes to a 3 way tee and then to the MAP sensor. I would think this would be a good place to measure intake vacuum for each cylinder. I got gauges, tubing, splitters & tees. Another possible vacuum sensing points would be the Evap Vacuum hose line that is next to MAP Sensor line. To be honest, I may have labeling the wrong set as MAP sensor, it may be the Evap Vacuum line. It has a similar routing where it goes to each intake and it from a 3 way tee. Here is a pic of the top of throttle bodies that shows the 3 way tees.



So, my general procedure would be:
1. Measure TPS Volts on a idling warm engine at ECU....just in case.
2. Strip NT done to throttle bodies and route capped tubing to exterior for later vacuum gauge use. One cylinder would go a gauge only. The other cylinder would go to a gauge and the MAP Sensor. So now the MAP sensor would only have one cylinder operating it (Good? Bad?).
3. Determine method to reach a wrench/driver to operate middle adjustment screw. Taped or magnetic 1/4" drive with swivels and extension is my first idea. If this isn't possible, this adjustment will consume many days of trial and error.
4. Reassembly NT. Ride and warm up engine.
5. Connect vacuum gauges to each cylinder intake. Pop the throttle a few times to see how synced vacuum is and adjust at idle to match as close as possible.
6. If needed adjust TPS to volts measured in step 1.

Maybe some of the auto & motorcycle mechanics have some experience with this idea. Would I want to cap the Evap Vacuum line while sync'ing? It seems like it would act like an equalizing line between the two intakes if it remains connected.

Anyway, thanks for any help.
 

mikesim

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Are you having a problem that you are trying to correct? If not, this is a can of worms that is best left unopened. Your procedure sounds logical enough but, if it ain't broke, don't fix it.

Mike
 

DirtFlier

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That screw head is impossible to see when the throttle bodies are mounted. And getting some sort of socket & ratchet into that tiny space will be a real battle. I've looked at this before and decided it wasn't worth the trouble. The adjuster screw is almost facing straight down so turning it in either direction is problematic. My guess is that at the Keihin factory where they are assembled, they are adjusted on an air flow machine and not on a running engine.

My bike with 62k miles is still returning 50-55 mpg on long trips, starts easily, and doesn't hesitate on acceleration so why bother? :rolleyes:

ps. good call MikeSim!
 
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If it ain't broke, don't fix it. You also don't have any idle adjust ether. If you have had the carbs completely apart then yes, but unless you have any real symptoms leave well enough alone. If I remember correctly (Been over 2 years since I have owned a NT) the service schedule does not call for a TB sync.
 
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I also probably would not adjust unless I was having a problem.
I sort of remember someone having issues trying to use the vacuum lines. It may have been they were trying to get a vacuum source for cruise control hook-ups but my point is are the MAP lines in the correct location to measure the vacuum needed for syncing?

Brad
 
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dnktng
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I also probably would not adjust unless I was having a problem.
I sort of remember someone having issues trying to use the vacuum lines. It may have been they were trying to get a vacuum source for cruise control hook-ups but my point is are the MAP lines in the correct location to measure the vacuum needed for syncing?

Brad
The MAP sensor tubing is connected to a throttle body port downstream of the throttle plate. That is the correct side, and it will be vacuum, but will it be correct enough for syncing? I'm not 100%, but it should be. I wouldn't need quantitative values, just comparing one side to the other should be adequate. Do other throttle bodies that have ports designed for syncing have to be located in a specific location behind the throttle plate, or at a specific entry angle? I don't think it needs to be perfect location to measure vacuum, but both throttle bodies vacuum sensing points definitely need to be in the same location to be comparable.
 
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Please have a look at my posting on the UK site which I have just finished before I saw this one.
You should not do what you are wanting to do!
It is just too difficult for a an unknown result.

Macka
 
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dnktng
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Please have a look at my posting on the UK site which I have just finished before I saw this one.
You should not do what you are wanting to do!
It is just too difficult for a an unknown result.

Macka
Thanks for response. Huge help and confirmed some of my thoughts on this topic.

I agree 100% with your technical review concerning the problem with only one TPS, the need to split the MAP sensor to measure vacuum, that the only adjustment possible between throttle bodies is mechanical and no computer/ECU adjustment is possible, and that it would be extremely sensitive and difficult to make a mechanical adjustment to a running engine to improve throttle body syncing.

I emailed the UK shop that claimed to have performed a throttle body syncing to try to get any info on how it was accomplished. This was several months ago. I did not receive a reply.....not surprised.

Good info and thanks again.
 
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My guess is that at the Keihin factory where they are assembled, they are adjusted on an air flow machine and not on a running engine.
Seems to me that I've read that this is true for a bunch of manufacturers. I've heard that BMW throttle bodies are synced on a flow bench.
 
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