Carb Balancing - Carbtune

[Ebme Geek]

Recently I replaced my cam chain, after putting it all back together I tried to balance the carbs, I had borrowed a carbtune, all seemed to be going ok until I tweeked a screw the wrong way, the balance got worse, but the tick over went UP !!  .  This made me suspicious of the gauges    swapping pipes 1 & 2 suddenly showed quite a diffence, so I ended up trying to balance this pair by looking for the same difference when swapping and unswapping the pipes. Spent ages trying to set up a half way house. Then repeated this for 3 & 4, then between 2 & 3,  I am sort of happy where I ended up 
 
Since then I have got some T pieces and commoned them together, with the below result, here you can see 3cm of mercury difference between channels, the spec in Haynes is all to read within 1cm Hg.
 
So be warned, check your tool before using it   
 

[red98]

always calibrate my gauges before use by connecting to the same cylinder in turn and adjusting if needed  ....i have the dial type which are easily adjusted not sure if you can adjust the mercury type ...

[Ebme Geek]

These are not mercury, they have stainless rods in them and actually work by drawing a small bit of air over them so by rights, they are actually a flow meter not a vacuum gauge.
 
A mercury manometer is the 'laboratory standard' and as long as the column does not get split does not need calibration.
 
I intend to re-do mine with dial type vac gauges, cause as yet I have not seen all 4 up at the same time (that means anything)  .  And will keep my T pieces in my Fazer box of bits, don't intend to get caught out like that again 

[Fazerider]

These are not mercury, they have stainless rods in them and actually work by drawing a small bit of air over them so by rights, they are actually a flow meter not a vacuum gauge.

A small amount of air does find its way past the rods, but it's not the flow that is being measured... the pressure difference at the two ends of the rod acts against the spring connected at the bottom end. So it is a vacuum gauge, just a slightly leaky one.
If you can get in there it may be possible to match the sensitivity of the four columns by tweaking the springs, but it's not something I've investigated with mine as they were OK.
You can get false readings if a rod jams, that's why they emphasise the need to make sure they're all jiggling slightly as you make the measurement.

[Ebme Geek]

Ok, that makes sense, not being mine I was not going to open it up, and have returned it letting him know what I found.
 
But you have raised a question in my mind, I thought (dangerous) I had seen when held upsidedown rods slide out, lay on back they stay there, only go back when stood upright, so gravity only no springs, I could be wrong. I will ask to have another look at it this evening.

[simonm]

daft question time.  Do you connect this to the same place that a scottoiler vacuum doodah fits ?  if so do you need to compensate for the drop in pressure across that cylinder ?


I'm probably talking about the wrong nipple/nozzle or summat, if so just ignore me, I know nuffin.

[Dead Eye]

Yes, you connect to the same place as the Scott Oiler connects - why would there be a drop in pressure?

[simonm]

Yes, you connect to the same place as the Scott Oiler connects - why would there be a drop in pressure?

Because the scottoiler uses some of the pressure to push oil to the chain ?

[richfzs]

Ah no....

The scotoiler uses vacuum to open a on/off valve, is all. The oil flow is then gravity operated, with the flow rate controlled by a manual valve. That's my understanding anyway!

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[simonm]

Ah no....

The scotoiler uses vacuum to open a on/off valve, is all. The oil flow is then gravity operated, with the flow rate controlled by a manual valve. That's my understanding anyway!

Sent from my Nexus 7 using Tapatalk 2

Thx for the explanation.  I guess there is no difference in measurement then even with 2 ft of pipe attached to it.  You learn something new every day 

[Dead Eye]

Even then, you disconnect the Scott Oiler to be able to use the Carbtune so it wouldn't affect balancing of the carbs

[Fazerider]

I took the back off mine this morning. This is what the insides are like:





That's with it held upside down so gravity is simulating about 14cm of mercury.
You can see the bottoms of the rods have screw adjusters so the columns can be matched.

[Jimmygixxer]

 

What actually makes the carbs go out of balance?

[red98]

What actually makes the carbs go out of balance?

ohhhh....thats a very good question  .....i dont know but i would have a guess at....wear and tear 




     

[Dead Eye]

Could be different amounts of crap in the float bowls or levels of crap / build-up in the jets and airways - loads of different options. Best described by red98 as general wear and tear

[simonm]

Even then, you disconnect the Scott Oiler to be able to use the Carbtune so it wouldn't affect balancing of the carbs

 so you balance 4 carbs so that the vacuum levels are the same and then change the vacuum level on one by attaching a scottoiler.  Wouldn't attaching a scottoiler affect the pressure level on the carb it's attached to ?

[richfzs]

It's a vacuum level, not a volume - you could attach anything you wanted to that pipe, and as long as there were no leaks, it wouldn't affect the operation of the carb. (although, if you'd attached a huge volume, the carb would take a while to create the right vacuum level, so there would be rough running until the level was achieved)

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[simonm]

It's a vacuum level, not a volume - you could attach anything you wanted to that pipe, and as long as there were no leaks, it wouldn't affect the operation of the carb. (although, if you'd attached a huge volume, the carb would take a while to create the right vacuum level, so there would be rough running until the level was achieved)

Sent from my Nexus 7 using Tapatalk 2

Thanks for clarifying my fundamental misconception 

[Ebme Geek]

Thanks Fazerider for putting up that picture,
I still have not managed to get my hands back on the one I borrowed, it was a different vintage ?  did not have the swaned in sides, same width top to bottom.
Hope it has the same type adjustment. hopefully find out soon.