Tuning carbs with gauges

Hi, Have just refitted my twin amals and balanced them using Davida gauges, it ticks over nicely and at tickover they sit bang on with both needles the same on each gauge, I open the throttle a bit and the needles both move in sync and at the same speed of movement indicating that the slides are moving at the same time and the cables set just right.
When I go up the rev range and hold it about 3000 the needles start to fluctuate and give different readings one going higher and the other slightly going back, If I have to be honest, I can't remember if this happened before when I've used my gauges, maybe it has, but somewhere in my mind says the should be the same at any given rev number.
I read a few articles on the 'net and there is a school of thought that because the pulses are big due to the cylinder sizes, (and there being only two), this can affect the readings at higher revs.
I would be grateful if anyone could shed any light on the way the readings should be higher up the scale, as I said it's ticking over cleanly and running well.
Cheers

I gave up trying to set my Commando's Amals using vacuum gauges.

I found it was easier and just as effective using the "rods (drill bits) under the slides" method.

i dunno, but i'm thinking a slight mismatch may be a problem at the drag stip but probably won't be noticeable during daily driving.

L.A.B. said:

I gave up trying to set my Commando's Amals using vacuum gauges.

I found it was easier and just as effective using the "rods (drill bits) under the slides" method.

Click to expand...

Hi LAB, I did the initial set up with 1/4" s/steel rods and went from there. I had had the gauges for ages and like using them , and have done so on other machinery. Just can't for the life of me remember how they are meant to read higher up the scale.

rbt11548 said:

I did the initial set up with 1/4" s/steel rods and went from there. I had had the gauges for ages and like using them , and have done so on other machinery. Just can't for the life of me remember how they are meant to read higher up the scale.

Click to expand...

I believe the important thing is to get the readings approximately equal. The actual vacuum readings you get will depend on how much the slides are opened so it varies with the throttle position.

L.A.B. said:

rbt11548 said:

I did the initial set up with 1/4" s/steel rods and went from there. I had had the gauges for ages and like using them , and have done so on other machinery. Just can't for the life of me remember how they are meant to read higher up the scale.

Click to expand...

I believe the important thing is to get the readings approximately equal. The actual vacuum readings you get will depend on how much the slides are opened so it varies with the throttle position.

Click to expand...

Yes that is correct, at tickover they are equal, on opening from closed they are equal, but holding steady at about 3-3.500 the needles start to wander apart until there is maybe 7-10+ increments of a difference.
It probably looks more dramatic on the gauge face than there is a problem, as the each increment is only .01 of a bar which is about .147psi, which I suppose is virtually nothing, although I stand to be corrected.
I just was wanting to know if it was normal for slight differences at steady mid to high rev speeds, or am I concerning myself over nothing.
Cheers

rbt11548 said:

Yes that is correct, at tickover they are equal, on opening from closed they are equal, but holding steady at about 3-3.500 the needles start to wander apart until there is maybe 7-10+ increments of a difference.
It probably looks more dramatic on the gauge face than there is a problem, as the each increment is only .01 of a bar which is about .147psi, which I suppose is virtually nothing, although I stand to be corrected.

Click to expand...

Vacuum gauges are usually graduated in inches or cm Hg?

http://www.hellotrade.com/davida/vacuum-gauges.html


I think accepted limit is the readings should be within 2 cmHg of each other.


1 bar = 75.0061561303 cmHg = 29.5299830714 inHg

Ideally the vacuum readings would be the same from idle to full throttle. Mine vary a bit from initial pickup (moving from the idle screws to the cables lifting the slides) as the tension sorts out on the cables. Last time I decided that I'd like to have the readings even with the cables fully supporting the slides (as at cruise or accelerating) and not during that brief moment as the slides lift off the idle screws. I've been using a mercury manometer but just picked up vac gauges I could actually use on the road, because I need another wormhole for my time...

Speaking of which, I have new throttle cables and thought I would experiment with routing them to see if I could get the tension to take up a bit more evenly in that transition stage.

But I have to say it runs grand how it is.

I would attempt to set the slides so that the readings were even at 3000 revs and at idle.

Overthinking. If it idles smoothly and gives you good throttle response, call it a day and ride it.

L.A.B. said:

rbt11548 said:

Yes that is correct, at tickover they are equal, on opening from closed they are equal, but holding steady at about 3-3.500 the needles start to wander apart until there is maybe 7-10+ increments of a difference.
It probably looks more dramatic on the gauge face than there is a problem, as the each increment is only .01 of a bar which is about .147psi, which I suppose is virtually nothing, although I stand to be corrected.

Click to expand...

Vacuum gauges are usually graduated in inches or cm Hg?

http://www.hellotrade.com/davida/vacuum-gauges.html

1 bar = 75.0061561303 cmHg = 29.5299830714 inHg

Click to expand...

I think accepted limit is the readings should be within 2 cmHg of each other.

Hi Lab definitely BAR same as these



As quoted , I may indeed be overthinking, as although it looks quite a difference on the gauges, when you do the sums, using your 2cmHg figure it may only work out about . 75 cm

Overthinking is why I'm here!

Whitworth Ranch said:

Overthinking is why I'm here!

Click to expand...

:lol:

What amount of vacuum are you getting at idle?
I have a set of EMGO's that give all sorts of indications of valve timing .ignition etc depending on where the settings are.(will upload pics once I can get into Photobucket)
Put a balance tube between th JS carbs and set the highest (and equal ) amount of vacuum at idle off the mixture screws. 10 inches seems about right on mine
Seems to work well
Regards Mike

Finally got into photobucket here are pics of my EMGO's they have a plastic needle valve inline in the tube to regulate the vacuum suck so the needles are readable and not all over the dial as can happen.

Brooking 850 said:

What amount of vacuum are you getting at idle?
Regards Mike

Click to expand...

Idle's great on mine with both gauge readings the same, but was just a bit concerned when I went higher up the rev scale that the readings were starting to become different to each other.
I was just wondering what would cause the change in readings from the same at tickeover and rising off idle to starting to become different higher up the scale.
The general census is, from what I can gather, that due to the amount of variables involved, big pistons, pauses, etc., that I maybe thinking too deeply , and it is fine as it is.

I would say the vairation between cylinders can be differant, tappet clearance,compression, timing ,inlet shape [these varie] cam lobe shape and wear in the parts will cause a differant vacuum..get it the best average..thats all you can do.
Remember that 3000 rpm on the stand will be only a smallish throttle opening, on the road to maintain 3000 depending upon the gradient will much larger..so a 3000 unloaded vacuum balance will be completly differant with load...it's all compremise!!