750 WEBER TUNING

[jayh]

My 750 is finally finished now aside from the weber tuning.I'm currently studying any information I can get my hands on as to how to tune emulsion tubes and their effects on various rev ranges.After reading Higgy and Romus's Weber mods pdf. I decided to test the float level of my carby vs the holes in the emulsion tube (f21) to see what holes were above float level,at float level and below float level,and the total volumes of air that these holes allowed to pass.I compared these to the 906 f27 and 750 f36 tubes and found that the vertical heights of holes are basically the same in the lower rev ranges,when float levels were taken into account.They all have a row of holes sitting about 1 mm above float level,these would be the first to start emulsifying as the main circuit cuts in.and they all have a row of holes well above the float level to delay the onset of emulsification,I'm unsure of the other effects of these upper holes on the transition,maybe someone could enlighten me ?.The f21 tubes that I'm currently running have no more air bleed holes below float level,and seem to bog down a bit toward the top end,I would say they are leaning out too much ?.The f36 and f27 tubes are essentially the same ,with a slightly different outside diameter(what effect would this have ?).The f36 lacking the upper row of holes ,but having its float level set 4-5 mm lower which corresponds to the next row of holes down the tube.Almost Identical spacing to the 906 but different no. and diameter of holes for the smaller capacity.The F36 has one more set of holes further down the tube and the F27 two more sets plus a third set on the customised tubes of Higgy and Romus.
I was wondering what other effect float level would have on the carby other than raising and lowering fuel to match holes on the et's,would it effect the progression or idle circuits ?
I have noticed that Geomac has posted again recently and seem to recall him saying in an earlier post that he would post a picture or diagram of his emulsion tube spacings one day,they would be really handy to compare with the information I currently have ,before I start turning my f21's into a block of swiss cheese.
I know that the 750 is over carbed with the 36 chokes ,but smaller chokes are getting hard to come by and I would like to have a good crack at sorting the tuning without swapping them .I'm running a K&N and Contis so this should help.
So heres the plan so far:
fit 60 idle jets,160-165 main jets
raise the float level to bring on earlier emulsification
Block at least one of the four 120 bleeds above float level (the 906 only has 4 x 115 and the f36 2 x 110) to bring on earlier emulsification
hopefully these mods will cover the lean hole at transition.
Try and get diagrams or pictures of others modified et's that are working
drill several more holes at a yet to be determined diameter and height below float level.
It all sounds easy but I've no doubt that I'll be tearing my hair out before I finish.
Any help would be appreciated.
PS. I don't consider 'throw the weber in the bin' to be helpful

[higgy]

The f36 and f27 tubes are essentially the same ,with a slightly different outside diameter(what effect would this have ?).

The diameter of the tube has a direct effect on the amount of fuel available. Larger tube= less fuel and faster emulsification

The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.

In the first three cases above, once fuel flow is established it is greater than it would be with fewer or smaller holes. Visualize wind blowing spray off of the top of water waves. It doesn’t take much pressure difference to cause the velocity of the airflow through the bleed orifices to have significant velocity in the orifice, even approaching sonic (1100 F.P.S.) if the orifices are small. The phenomena of critical flow is what limits the total air flow through an orifice and allows tuning by changing bleed size.

Essentially, the emulsion effect will richen the low flow and the air bleed size, main well and nozzle restrictions will control the increase or reduction of high flow. Again, the desired air/fuel ratio is the primary purpose of the bleed system. "Improved emulsion" is an oxymoron if the modification of air bleeds to "improve emulsion" results in an incorrect air/fuel ratio in some range of engine operation. Correct proportioning of all the different bleeds (and, of course, the idle, transition and power circuits) will give the correct air/fuel ratios over the total range of speeds and loads and a flat air/fuel ratio characteristic at wide open throttle.

seem to bog down a bit toward the top end,I would say they are leaning out too much

smaller Ac jets will help here by restricting air flow to the ET

drill several more holes at a yet to be determined diameter and height below float level.

more later gotta walk the dog

[jayh]

Thanks Higgy, There's a lot of information out there with much of it conflicting .It makes it extremely challenging to get to the bottom of the problem.

[higgy]

Most of it is fairly straight forward, The emulsion tube unfortunately is largely a matter of trial,and error, due to the nature of its function. Add to that the fact that parts are not easy to find and it can be a challenge to get it correct. The very first thing you need to do is get your fuel pressure under control. Weber DCNF's require a pressure of around 3psi. Having the ability to vary it between 2.5 and 3.5 can largely make up for the DCNF's lack of a power valve. Romus and I also found a good filter and a Grose jet worked best.

[jayh]

Got the holley reg fitted.Grose Jet ?

[higgy]

Want to get a Grose Jet?
Send an email to johndtitus@q.com
$18 each plus $5.99 for shipping priority mail if you are in the US (a bit more for overseas)
A Grose jet uses a ball bearing to control fuel flow rather than a rubber nipple

[higgy]

More on the Grose Jet

At 08:27 PM 9/2/2008 -0600, Henry Speer wrote:
>>"I have purchased a pair of Grose Jets as they were highly recommended by the local British Car Expert. However, in researching this on the Internet I ran into several negative comments about Grose Jets. It seems they are prone to sticking and flooding the carb. Now I am wondering if I should stick to needle valves".

Grose Jet on left - Standard float valve on right


I happen to like Grose Jets. I've been running Grose Jets for about 20 years, close to 200,000 miles.

The original float valve has a brass pin with a conical point sealing against a conical seat in the brass valve body. The pin is somewhat triangular in shape to allow fuel to flow down past the sides of the pin when the valve is open. There are variations of the pin, some having a spring in the middle, some with rubber (Viton) or nylon tip. The spring loaded ones are supposed to be less sensitive to vibration and resonance frequencies. Elastomer tips are intended to make a more perfect seal, which might be important only when the engine is not running but fuel pressure is applied (such as immediately after switch off). The fixed conical tip of the pin is subject to long term wear that could eventually make it stick closed. Elastomer tips could wear faster or possibly deteriorate when exposed to the odd compounds in modern motor fuels.

The Grose Jet has two spherical balls inside, a large one on the bottom that will be contacted by the actuator arm, and a smaller one up inside (out of sight) that is pushed up against the seat to make the shut-off seal. The all metal construction makes these parts wear and corrosion resistant. The balls are free to rotate to any orientation, which gives them a very long wear life. The all metal construction may in some cases be susceptible to certain resonant vibration frequencies causing overflow, especially if the system has abnormally high fuel pressure.

I had two problems with my Grose Jets, probably neither one being fault of the parts. Early on one of the jets came unscrewed and fell into the float chamber causing severe flooding and overflow of one carb while driving on the expressway. That was a 5-minute fix on the shoulder of the road, no big deal. The SPL does not show any packing washer for this piece. I can't remember if I had used a fiber (or lead) washer the first time, but more recently I certainly do not use any washer in that joint. The float actuator arm in the SU H-type carburetor can be bent to adjust the float level, so there is no need for to adjust the height of the float valve body. There should also be no need to worry about sealing the valve body to the chamber cover, as you are screwing a brass bit against an aluminum part, both of which are slightly compliant for making a good metal to metal seal. With no packing washer the parts can be screwed in tight to never work loose.

The next problem came after about five years in service. At some point I had installed an aftermarket fuel pump that may have been delivering too much pressure, possibly in the 5 to 7 psi range. It ran that way for at least a few years without incident. One day while autocrossing it would overflow the front float chamber at mid range engine speed and a certain hard turn. I finally got it to repeat the overflow condition at about 2500-3000 rpm while stationary in the paddock. I installed a cheap pressure regulator right after the pump and cut it back to 3 psi, and the problem went away. Some years later the fuel pump was replaced, the new one ran at lower pressure (4.5 psi max), so the pressure regulator was removed.

About 100,000 miles after first restoration the carbs were rebuilt with new throttle shafts but keeping the same Grose Jets. Then it has been run another 114,000 miles without incident. I just installed another new fuel pump as preventative maintenance, and have run it another 3000 miles since, still using the same 20 year old Grose Jets. I tend to like things that are long lived and maintenance free.

One of the key factors for successful use of Grose Jets is to have clean fuel. Apparently a tiny bit of rust, small enough to go through the screen in the float cover, might cause one of the balls in the Grose Jet to stick either open or closed. If you physically knock it loose or blow it our with air pressure it works again. I have never had this problem. My "secret" is that I drive a lot and run several hundred gallons of fuel through the system every year, so it always stays flushed out clean. I have a small fuel filter 1" diameter x 1" long in line before the fuel pump. I been able to run 100,000 miles without clogging this little filter, so the fuel tank must be damn clean at all times.

If you did have a problem with dirt screwing up a Grose Jet, the solution would be to install an in-line filter between the fuel pump and carbs. A plastic see-through filter is cheap and easy to install with hose connections. If it gets clogged you can change it easily. If it gets clogged too often you would need to remove the fuel tank for a good rinse out. Meanwhile, to minimize rust in the fuel tank, keep the tank mostly full when parked, especially if it is in storage for months at a time.