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.

Addendum, August 27, 2015:
Be aware that in recent years, since the patent has run out, there are some copy "Grose Type Jets" on the market that may be poor quality and problematic.