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1983 GPz 750, green
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Discussion Starter #1
So I have several 1983 Kawasaki GPz 750's.
They use the Mikuni BS-34 carburettors.
At idle, say 800 to 1000 rpm, what level of vacuum should I expect at the carb suction port?
Does anyone have a number? Does about 8 inches (203mm) of Mercury (Hg) vacuum sound about right or is this way out?
You mechanics with the vacuum gauges should be able to tell me, right?
Thanks for any replies.
 

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Yes that sounds about right. A KZ750 should have 22cm of vaccum at idle and less than 2 cm of difference between any two cylinders. Sorry I did not have a GPz750 manual on hand.
 

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1983 GPz 750, green
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Discussion Starter #3
Thanks WFO, I suppose it varies a bit depending on idle speed and atmospheric pressure as well as carb model. Maybe someone knows for the Mikuni BS-34 if it is significantly different than 220 mmHg you said for the Keihin carb on the KZ750. The reason I asked was because I'm making a digital version of the tried and true highly accurate mercury manometer and need to know what sensor range to select. No mercury involved.
 

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Very interesting project. Begs the question, if it is digital can you not make it to have whatever range you want?

Yes it would vary with idle speed and atmospheric pressure, but I think the reason many manuals won't spec a specific number is because of those factors plus what is really important is the balance as opposed to the specific vacuum.

My ZN1100 uses BS34 carbs and so does the KZ1100. My manual does not spec an idle vacuum but it does show a picture of the OEM vacuum tester 57001-1152. The picture shows that the range starts at 20 and goes up to 60 graduated in 2cm increments. My manual also says balance must be less than 2 cm Hg difference between any two cylinders.

Hope this helps.
 

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1983 GPz 750, green
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Discussion Starter #5
The range of operation depends on the type of sensor not the computer. The computer just reads the values the sensor puts out.
Yea they don't specify a specific number, probably because it 'kinda depends' on various things. As long as all the carbs are 'close' ie within 20 mm of each other should be good. I plan on having them all within 5 mm (0.5 cm). Its one of those things that I like to be as accurate as possible. I had a mishap with mercury once and seriously don't want that to ever happen again, its bad stuff outside of its container. Plus I noticed that sometimes the mercury separates it self from the rest of the column and its hard to read. Serious warning, dont ever rev the engine when its connected to a mercury manometer! Besides all that I just love making new stuff and playing around with computer stuff and motorcycles.
 

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Understood. Well by the sounds of things you need a range of 20-60cm. I would think that if you are never going to use this device at different elevations (ie on a mountain top), you could probably narrow the range.
 

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1983 GPz 750, green
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Discussion Starter #8
Just asking, so under what conditions would the guages shown in the picture above, by Davidr63, ever read a vacuum of 25 inHg (84.7 kPa)? Would you get a reading like that under wide open throttle 10,000 rpm? Again just asking. Is the vacuum guage useless after 10 inHg on a motorcycle? And why even read positive pressure for a carb sync tool? Curious minds would like to know.
 

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1983 GPz 750, green
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Discussion Starter #9
Figured it out... you can use the device to measure tire air pressure too. Great idea except I only have two tyres.
 

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I've used one of these with good results, but you have to buy the different "cone" attachments, on some bikes a lot easier to push straight into the air stack
 

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1983 GPz 750, green
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Discussion Starter #11
My understanding is this: Vacuum would be highest at idle or at high rpm with rapid closing of the throttle. I found this at another web site:
"Vacuum and Throttle Position
A cranking engine will normally produce about 3 to 5 inches of mercury, or Hg. Vacuum increases and is highest at idle when the throttle position is closed or very slightly open. This results from the restriction of air flow that cannot move in great volume from the air intake to the manifold. As the throttle opens, more air enters the intake, causing a decrease in vacuum."

A carb sync tool is used to set idle conditions not for high rpm conditions. So the range of operation would be zero to maybe 15 inches Hg (381 mmHg) vacuum. A sync guage that reads positive pressure would be useless, similarly the engine could never produce more than about 10 inHg under idle conditions.
To my way of thinking, these dial guage sets will only ever see zero vacuum (no vacuum) to 15 inHg. The rest of the gauge's range is totally useless.
This is the reason pod filters are problematic, not enough restriction to air flow causing the vacuum point to shift.
Anyone want to chime in on this, any thoughts?
 

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1983 GPz 750, green
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Discussion Starter #12
Friday night humour for the lads. Imagine having a speedometer on your motorbike that had markings on it ranging from -200mph ( negative 320 km/hr) going all the way up to 600mph (960 km/hr). But the bike could only ever achieve 7 steps back per minute (that's like .0625 mph reverse) and 65 mph at full throttle going forward with a properly tunned 100cc engine.
 

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This might help to explain the need for greater range. Also, some Kawasaki service manuals ask you to blip the throttle when you are done synching, to see if the needles all settle back down to the correct readings.

42721
 

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By Alex_GPz: ... At idle, say 800 to 1000 rpm, what level of vacuum should I expect at the carb suction port? ...

There's a KZ750 chain drive pdf manual floating around the internet.
It may be linked at KZRider or 750Turbo dot com.
Get the one w Yellow/Gold cover & color wiring diagrams.
There's also a poorly copied black-&-white version, so be sure to get the good one.
 

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1980 Kawasaki 750 LTD
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By Alex_GPz: ... At idle, say 800 to 1000 rpm, what level of vacuum should I expect at the carb suction port? ...

There's a KZ750 chain drive pdf manual floating around the internet.
It may be linked at KZRider or 750Turbo dot com.
Get the one w Yellow/Gold cover & color wiring diagrams.
There's also a poorly copied black-&-white version, so be sure to get the good one.
Not on either of these sites? Will search net
 
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