Had fun with my good buddy Dr240z refining the tuning of his triple webers.

 

The last time we set them we used a fuel level of 25mm below the cover-body margin (as per Keith Frank's recommendation).  It turns out this is way too high and caused the cruise to be too rich... something that could not be tuned out. Idle and WOT were easy to set however.

 

This time we set the fuel level to 29mm and used a new rig to do this as precise but much faster than the pipette method or the caliper method.   

 

It turns out that a column of water 8' to 9' high "head" gives a pressure of 3.5 to 4 psi at the base  (simulating that of the fuel pump on a Z.) so we strung a funnel and hose near the garage ceiling and ran this into a carb lid on the floor. 

 

The carb lid sat on a clear plastic tub with water levels from the top scratched into it for reference. the top of the tub represented the top of the carb body.

 

By filling the system with water, it flowed through the needle valve at ~ 4psi. When the flow stopped, we measured the fuel level and bent the float tabs as required to get 29mm.

 

Once all 3 carb lids were the same, we continued tuning and finally dialed in the carbs to a powerful yet non-stinking/non-fume delight at idle, cruise, and WOT.

 

FYI: Gasoline would have been more accurate but water worked and was safer.  FYI the S.G. of gas is ~0.70 so a float would sink lower in it.

 

 

9' Head

 

29mm fuel level

 

The Doctor

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Edited August 9, 20159 yr by Blue

Quote

 A comment on using only water.

The pressure exerted by the water in the hose pushing down on the fuel needle to open it is 33% more than gasoline (assuming density of gas is 750kg/m^3)

The pressure exerted by the fuel float pushing up to close the fuel needle valve is ~ 33% more in water than in gasoline.

So using water is not very inaccurate as the two forces nearly cancel each other. The key point in using water is to use the table above to get ~3.4psi of head.

I am not sure that this is correct.  How much the floats are submerged in either water or fuel is determined by the requirement that the buoyancy force is required to a) counteract the force on the needle valve from the head of liquid and b) support the weight of the floats.

Let's assume the ball bearing in the needle valve is 1/8" diameter and the head is 4psi, then force on the needle valve from the head will be 4* pi / 8^2 / 2 or 0.05 lb.  This needs to be counteracted by the buoyancy force of the floats - but remember the floats are further away from the pivot that the needle valve and therefore the buoyancy force required is lower. My very rough calculation is the centre of the floats are 4 times further away from the pivot than the needle valve.  Therefore, the buoyancy force required is 0.0125 lb.

The floats are about 2" diameter and let's say 1/2" width and I think they weigh 0.05 lb (20 grams).  So, to displace their weight (i.e. to float), they would need to be submerged by (switching to millimetres) 19mm in water and 23.5mm in fuel.  To add the extra force to keep the needle valve shut then the numbers are 23mm and 29mm.  The difference of the last two figures is 6mm i.e. the fuel level needs to be 6mm higher than the water level to give the same force upwards.

Obviously, these numbers are only approximate - for example I have ignored the spring in the needle valve.

But, rather strangely, setting the float levels to 29mm in water would appear to be the same as setting it to 23mm in fuel i.e not very far away from Frank's 25mm. 

I agree Keith Franks 25mm recommended fuel height is due to the difference of specific gravity of fuel vs water.  Makes much more sense this way.

I am about to go through this process again. Reason.  Well I have not had to mess with my triple set up in over 10 years. Car starts and ran fine.  Well up until a couple of month ago. Seems starting the car up and letting fuel dry out in the fuel bowl about a dozen or so times gunked it up pretty good.  I lost all 6 of my accelerator pump jets and quick throttle inputs were, how you say, met with extreme hesitation. 

three carb kits later and a new 3 gallon ultra sonic heated cleaner and all three carbs were completely disassembled and cleaned and then sprayed with enough carb cleaner to destroy the ozone layer above houston.  

Now having said that. I set the plastic floats to 12mm when allowed to hang straight down resting on the ball in the needle and seat for a first try.  

 

I had fuel pouring out of my carbs barrels like an open hose. 

Now I am trying to get a handle on this.  I need to find out if maybe my floats are perhaps becoming a bit fuel saturated?  Not sure what exactly is going on.  But I had all of the fuel levels 22mm from the top using the clear little plastic tube method established by KF.  This is not because that is where I wanted them, but it was as close as I could get that damn tab to 25mm. 

I was thinking of building a setup like the one done above, then recalled this post!  I had forgotten about the fantastic level of nerdtasticness this forum generates. Gosh I love all of you! 

 

By the way, centerline says the float level should be 29.5mm from the top of the float bowl.  

 

 

Edited June 21, 20222 yr by Zedyone_kenobi