TOPIC:

Quick question guys that use these. Back in the day they were filled with mercury and very accurate and also were adjustable.
H&S decided that one of us might try eating them so no more mercury. I then changed to the hoop ball bearing method when my original mercury carbtune died and have been very confident with it. A mate bought one of the new bar carbtunes and out of curiosity I borrowed it to see if these were as accurate as the originals. Although they have dampers incorporated they are not adjustable.
So to test if the bars read equally I set bar 1 to 4 up consecutivly to carb one. The difference between bar readings was as much as 25% all for the same carb.
Am I missing something or are they really that inaccurate now? or are they adjustable but in an unobvious way?

Hi Warren if it's the Morgan carb-tune you are talking about i can only say cut the damping inserts in the pipes to different lengths to attain the same vacuum on each bar it will take a few minutes but will work, i will now have to look at mine and see what variations there are in the columns.

Hi Kev, am I remembering right that the mercury ones had a set up adjustment inside so that once set the four columns read the same for 1 carb. Then you would be confident a given vacuum would read them same on all 4 columns?
Couldnt find anything on this latest version. Im not bothered as it wasnt mine and will continuewith the hoop ball method.

If you had doubts about the Carb Tune you were using couldn't you just compare the reading it gave you to a vacuum gauge? 20 inches of vacuum on the Carb Tune should be 20 inches of vacuum of a vacuum gauge.

Rick H.

Warren3200gt wrote: Hi Kev, am I remembering right that the mercury ones had a set up adjustment inside so that once set the four columns read the same for 1 carb. Then you would be confident a given vacuum would read them same on all 4 columns?
Couldnt find anything on this latest version. Im not bothered as it wasnt mine and will continuewith the hoop ball method.

We're still running some ancient V&H mercury sticks with modern thick-walled silicone tubing for the vacuum lines. It came to slmjim decades ago sans tubing and mercury, but back then mercury was easily available. We use a 17.5 pilot jet inserted into each tube to ensure vacuum is equal on all four columns. The jets effectively dampens flutter. Each column pulls from a common pool of mercury, and all four silicone tubes are equal length. We're really less interested in absolute vacuum than in equal calibration across all four columns. Could a similar setup be used on the Carb Tune device?

Rick H. wrote: If you had doubts about the Carb Tune you were using couldn't you just compare the reading it gave you to a vacuum gauge? 20 inches of vacuum on the Carb Tune should be 20 inches of vacuum of a vacuum gauge.

Rick H.

Similar thought. A handheld vacuum pump has s vacuum gauge. Connect a pump to a Carb Tune port & pump vacuum to 20 inches (or whatever). Mark where the column rests at that vacuum. Repeat for each column.

Or,

Try using a few cheap windshield washer/emissions system plumbing T's to make a manifold to create a 5-port connection - one for the pump, the other four for simultaneous connection to each Carb Tune port.

Good Ridin'
slmjim & Z1BEBE

Warren3200gt wrote

The difference between bar readings was as much as 25% all for the same carb.

That spread is quite a surprise. I've had a Morgan Carbtune for years and am happy with it so am thinking there must be something wrong with the one you used. I would suggest contacting the manufacturer . They must had precision data on hand.

Helps to clean them occasionally.

Guys, as I said I only borrowed them out of curiosity to see if they were as accurate as I remember the mercury ones being. They dont appear to be so without some trial and error set up. I moved onto the ball bearing hoop method after my original set died.
If anyone is unaware of this method I will explain with the aid of some pics.
The inner ring is sealed and the ball bearing hangs centrally under gravity and is your datum point. Pic 1
The outer ring is open at both ends. Attach one end to the base setting carb, normally number three. Attach the other end to carb one. Start the engine and adjust the idle to about 1500 /2000 rpm. If carb one is out of balance to carb 3 the outer ring ball bearing will swing to one side or the other depending on wether there is more or less vacuum. Pic 2.
Adjust your slide so that the outer ring ball bearing is back centrally directly under the ball bearing in the inner ring. As in Pic 1.
Do the same for carb 2 & 3 and 4 & 3. Your carbs are now perfectly in sync. Beautifully simple and 100%
Accurate.

Clever setup. Just guessing here but does the ball need to be the same diameter as the hose ID? Otherwise it would not seal and the ball would be bypassed.

A disadvantage is that you don't know the actual pressure.

FYI - I use gauges purchased from Z1E.

Ehhh Hardrockminer, you took the words out of my fingers as I was typing. I use a Carb Tune, but will have to run a comparison against a vacuum gauge to make sure it's accurate.

Rick H.

I like that Warren can you give us the spec, bearing and pipe size and how many loops in the set up,

hardrockminer wrote: Clever setup. Just guessing here but does the ball need to be the same diameter as the hose ID? Otherwise it would not seal and the ball would be bypassed.

Hardrockminer, the ball bearing needs to be smaller than the tube inner diameter so that it moves freely with bypass otherwise it would just get sucked into the carb. Vacuum bypass doesn't matter as its a sphere the presented face of the ball bearing is always the same area and the vacuum applied to it is always equal regardless of any bypass.

Rick H. wrote: I use a Carb Tune, but will have to run a comparison against a vacuum gauge to make sure it's accurate. Rick H.

You just need to test as I did Rick using the same carb for each column and comparing the readings. The actual value doesn't really matter they just need to be equal.

kevski wrote: I like that Warren can you give us the spec, bearing and pipe size and how many loops in the set up,

Hi Kev, I can take it apart and measure everything if you like or seach on ebay. I bought mine from the guy that makes them probably in the late 80's but I believe they are still available. Cost about 5 quid back then.