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So let me ask,, how do you increase or decrease the engines vacuum? What I’m saying is no matter what the actual engine vacuum is that if you set the gauges up to read the same on the work bench and then attach them to the vacuum ports and then adjust the throttle bodies to read the same across the board you will have sync’d your throttle bodies. On my fjr depending on the day I see between10-12 inHg at the gauges, is that the actual vacuum ? Don’t know w/o actually calibrating the gauges on a master gage. I guess I’m not sure of what you are trying to tell me.

So do you just take your gauges off the bench put them on your bike and just adjust the throttle body vacuum to 9.8inHg across the board ? what about the “master” #3 t-body ? What gauges do you use ? Just tryin to understand here.

the vaccum of ANY engin is a Direct result of rings/ valves/ compression etc

a new NOT broken in enginw will NOT read as same engine Broken in ...............

nor will it read after Uv achieved best reading on 1 engine...........SAME reading on another

the gauges first came out when the CB750 air cooled engine was introduced............

the headache religion busting circumstance was at idle a minimun reading was suggested

yet by time the gauges were near the same the engine had gotten HOT & HOT idle resulted in

very different idles that flucuated greatly

U are by bench or mityvac setting to a false ZERO ................

set the same at a min usually resulted in an advantageous of idle throttle response

instead of a stumble

out of adjusted in many instances does NOT result in a a Stumble

a proper adjustmen does NOT help anywhere else other than idle

No Less it DOES NOT result in a power gain thru out RPM range
 

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So let me ask,, how do you increase or decrease the engines vacuum? What I’m saying is no matter what the actual engine vacuum is that if you set the gauges up to read the same on the work bench and then attach them to the vacuum ports and then adjust the throttle bodies to read the same across the board you will have sync’d your throttle bodies. On my fjr depending on the day I see between10-12 inHg at the gauges, is that the actual vacuum ? Don’t know w/o actually calibrating the gauges on a master gage. I guess I’m not sure of what you are trying to tell me.

So do you just take your gauges off the bench put them on your bike and just adjust the throttle body vacuum to 9.8inHg across the board ? what about the “master” #3 t-body ? What gauges do you use ? Just tryin to understand here.
There's really nothing to understand here. I try not to overthink it. You do it your way, I'll do it mine.
 

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Ok Russ, not trying to be argumentative or bust your butt here, just trying to understand what you’re doing to see what I’m missing here, learn something maybe, ya know.
 

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I have and it is basically the same !
 

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To perhaps shed a little more light on it, every engine's condition and valve clearances will not be the same. Sync will change with changes in valve clearances, slowly over time. The service manual also says to check valve clearances and engine idling speed before starting the sync. Valve clearances at minimum could have different vacuum than at maximum. Idle speed change will change the base vacuum reading. It goes further to say check ignition timing, which..... meh... not adjustable. The Gen3 manual has no reference vacuum reading specified.

So, Gen1/2..... If your idle is at 1000-1100 rpm, check the reference (#3), and if it is close, you can tweak #3 perhaps and/or tweak idle, the vacuum may be different at 1000 rpm than it is at 1100.... OK to try, but I wouldn't guarantee it will be 250 mmHg.... so, close is OK it's not that critical. Sync all to be equal to the reference.

IF way off, say 150 mmHg, then something needs attention, depends on mileage, how the bike was ridden, etc.... better do a compression test/leakdown test.
 

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The Gen3 manual has no reference vacuum reading specified.
I'm thinking herein lies the confusion. My manual, a Gen2, specifies a vacuum value. So I set them to that. Others are looking at a Gen3 manual. No value, just willy-nilly set them wherever you like.
 

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If you want to know the true vacuum of the engine, one would connect a vacuum gauge at one of the other vacuum hose ports, not at the sync port....... Gen3 sync is done with the reference screw that is marked and turned all the way in.
 

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The main difference here is that I calibrate my vacuum gauges before Using them so they read all the same BEFORE putting them on an engine to sync.

So let me ask,, here we go,, ,, If on a gen 1/2 the idle vacuum on the “master” isn’t at the specified 9.8 in/hg what do you do ? Do you adjust that to the 9.8 in/hg and then set the remaining 3 to match ? Gen 3 has no spec number asRay said, jyst match the master setting. Just curious.

So for me after 40 some years of doing this (mostly all Honda’s) it’s always been check the valves, check the timing, set the idle speed, calibrate the gauges and then set the carbs/throttle bodies to all match. Never ever set things to a prescribed vacuum number because just a small twist of the throttle stop or valve adjust throws that all off.

I’ve never had an issue ! Be it at 2 in/hg like on my 82 cb750f which is it’s spec number and is to low to use my CarbTune or at 12-15 in/hg like on my old Goldwings so long as they’re all the same. It’s what I do !
 
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