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Haven't tried any of these but.......

Digital displays are great for displaying static or slowly changing values. Trying to understand something that is rapidly varying is another thing entirely.

Analogue displays or columns of liquid on the other hand.............

 
Or one of the kits that has four mechanical dial gauges? I wonder if they read precisely enough?

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71i926VVueL._SL1200_.jpg


These go for pretty low cost - well under $100 on Amazon...

 
Ross, analog gauges of that caliber, +/-2% FS at best. Especially at that price point. I'd apply vacuum to all 4 at normal FJR vacuum level (whatever that is!) , mark the faces with Sharpie pen.. Note the delta between. Then use that delta when performing TBS..

 
They don't have to be accurate since they can be "calibrated" to each other by hooking them to a common source as suggested by DC. Absolute readings are less important than relative. The gauges have to be reproducible and have to be sensitive enough - no good if an important difference between two cylinders is only a small needle deflection. A small enough orifice in the line or large enough vacuum reservoirs for each gauge would stabilize rapid needle fluctuations. Might try a set.

 
I personally prefer the manometer style gauges (fluid filled tubes, like the Motion Pro unit). I have a set of gauges and they don't match, nor can they be adjusted on the fly to "calibrate" them when you hook them up to a common vacuum source. The manometer gauges can be connected to the common vacuum source, leveled out across all four, and then when making adjustments to each TB/carb you don't have to remember how much offset each gauge had before you started. You just get all four tubes to the same height and off you go!

I must add that the gauge set I have looks like the second picture Ross posted, and they are a cheap set. A high end gauge set may be a different story. The advantage of the gauges is that you can measure absolute vacuum if you're that anal about the adjustments. Personally, I just get them to all match and call it good.

 
When I owned an oilhead BMW R1100RS I picked up a TwinMax electronic differential vacuum gauge. It wasnt digital, but it was highly sensitive, and therefore an accurate way to balance the two cylinders of those engines. I still use it for my VStroms L-twin engine, though that is not nearly as balance sensitive as the old BMW boxers were.

It is possible to iteratively balance two cylinders at a time (of an I-four engin), but the need to keep swapping hoses around, and never seeing what all 4 cylinders are doing, made me buy a Morgan CarbTune for the FJRs. It is accurate enough for these engines, has permanent metal rods to display the pressure in each column, so no need to mess with liquids, and they only go for about $120.

 
Northwoods Snowman said:
I personally prefer the manometer style gauges (fluid filled tubes, like the Motion Pro unit). I have a set of gauges and they don't match, nor can they be adjusted on the fly to "calibrate" them when you hook them up to a common vacuum source. The manometer gauges can be connected to the common vacuum source, leveled out across all four, and then when making adjustments to each TB/carb you don't have to remember how much offset each gauge had before you started. You just get all four tubes to the same height and off you go!
I must add that the gauge set I have looks like the second picture Ross posted, and they are a cheap set. A high end gauge set may be a different story. The advantage of the gauges is that you can measure absolute vacuum if you're that anal about the adjustments. Personally, I just get them to all match and call it good.
Click to expand...
What percentage of full deflection do you typically see on your gauges or have you tried them on the FJR's TB?

With cheap dial gauges, they all get hooked to a common source and they are "calibrated" using a Sharpie marker on the glass dial face of each gauge (or on tape if the dial face is plastic). As I mentioned, they have to be reproducible (i.e. come back to the same position if vacuum changes and then changes back). Cheap gauges often have enough friction in the mechanical linkages so the needle doesn't come back to where it should. Sometimes you need to tap the dial face to overcome the resistance and that is no good! I would want to know more about the meter range. Ideally, you would like the "correct" vacuum to be at least two thirds deflection of the needle. This gives you decent sensitivity when you are trying to equalize the readings for each cylinder. Doesn't matter with the manometer types because you are just measuring differential vacuum (between channels) rather than absolute for each individual channel. If deflection is only 10-20% of the full scale, it will be very difficult to match the four channels precisely.

 
Fred W said:
When I owned an oilhead BMW R1100RS I picked up a TwinMax electronic differential vacuum gauge. It wasnt digital, but it was highly sensitive, and therefore an accurate way to balance the two cylinders of those engines. I still use it for my VStroms L-twin engine, though that is not nearly as balance sensitive as the old BMW boxers were.
It is possible to iteratively balance two cylinders at a time (of an I-four engin), but the need to keep swapping hoses around, and never seeing what all 4 cylinders are doing, made me buy a Morgan CarbTune for the FJRs. It is accurate enough for these engines, has permanent metal rods to display the pressure in each column, so no need to mess with liquids, and they only go for about $120.
Click to expand...
A differential gauge is probably the way to go unless you want to spend big bucks on an accurate (electronic or mechanical) absolute vacuum gauge. A home-made liquid manometer or a device like the Morgan CarbTune is probably the simplest; especially if absolute vacuum is not (as) important. I saw the electronic gauges when looking for the CarbTune and thought I would ask if anyone had tried them. While mercury sticks are favored by some, I wouldn't use them. Materials are hazardous and could cause real damage if the liquid gets sucked into the engine! Water (or transmission fluid) in a manometer wouldn't do any harm and either is actually far more sensitive because of the much lower density.

 
Last edited by a moderator:
RossKean said:
Northwoods Snowman said:
I personally prefer the manometer style gauges (fluid filled tubes, like the Motion Pro unit). I have a set of gauges and they don't match, nor can they be adjusted on the fly to "calibrate" them when you hook them up to a common vacuum source. The manometer gauges can be connected to the common vacuum source, leveled out across all four, and then when making adjustments to each TB/carb you don't have to remember how much offset each gauge had before you started. You just get all four tubes to the same height and off you go!
I must add that the gauge set I have looks like the second picture Ross posted, and they are a cheap set. A high end gauge set may be a different story. The advantage of the gauges is that you can measure absolute vacuum if you're that anal about the adjustments. Personally, I just get them to all match and call it good.
Click to expand...
What percentage of full deflection do you typically see on your gauges or have you tried them on the FJR's TB?

With cheap dial gauges, they all get hooked to a common source and they are "calibrated" using a Sharpie marker on the glass dial face of each gauge (or on tape if the dial face is plastic). As I mentioned, they have to be reproducible (i.e. come back to the same position if vacuum changes and then changes back). Cheap gauges often have enough friction in the mechanical linkages so the needle doesn't come back to where it should. Sometimes you need to tap the dial face to overcome the resistance and that is no good! I would want to know more about the meter range. Ideally, you would like the "correct" vacuum to be at least two thirds deflection of the needle. This gives you decent sensitivity when you are trying to equalize the readings for each cylinder. Doesn't matter with the manometer types because you are just measuring differential vacuum (between channels) rather than absolute for each individual channel. If deflection is only 10-20% of the full scale, it will be very difficult to match the four channels precisely.
Click to expand...
I don't remember for sure. Last time I did the FJR was a year or two ago. I have the Motion Pro unit and what ever the height was at it was plenty sufficient. I want to say it was 30-50% but don't quote me on that. The tubes are pretty tall for their diameter. I blip the throttle to make sure idle comes back to a consistent reading and closing the throttle at elevated RPM causes extra vacuum that can suck the fluid out of the gauge if you get too overzealous. The gauge is sensitive enough that the level will oscillate a very little bit even with the orifices are installed in the lines. For less than $100 it has paid for itself between the FJR and my old ZR-7 I originally bought it for.

 
One other point about digital displays. They are without doubt much easier to 'read' than analogue gauges and give the impression of being more accurate.

However their accuracy is entirely down to whatever type of strain gauge is used to obtain the initial pressure to volts (or whatever), plus the internal circuitry to convert from analogue to digital.

 
Donal said:
One other point about digital displays. They are without doubt much easier to 'read' than analogue gauges and give the impression of being more accurate.
However their accuracy is entirely down to whatever type of strain gauge is used to obtain the initial pressure to volts (or whatever), plus the internal circuitry to convert from analogue to digital.
Click to expand...
Again, absolute accuracy is immaterial for this application. Adequate sensitivity in the particular measurement range is critical - as is reproducibility. Likewise, all four channels have to read the same as each other even if there is a calibration error. Some sort of signal damping is also needed for a digital gauge - either mechanical (small orifice or vacuum reservoir) or electronic via an averaging algorithm. You are absolutely correct in your comment about perceived accuracy of a digital readout. I have a nice digital tire gauge that consistently gives me the wrong reading! (Although it looks good!)

 
Differential analogue stops any uncertainty. No accuracy concerns at all.

(Click on image for larger view)



And homemade costs peanuts.

 
Last edited by a moderator:

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