Triumph Repair: 76 Spitfire Metering Needles, stromberg carbs, spitfire 1500

Question
Hi Howard,

I have a 1976 Spitfire 1500 with a single Stromberg 150 CD carb. We have deleted the catalytic converter, hot air hose, and a host of other emissions equipment that have introduced a significant amount of airflow and leaned out my a/f ratio to where i cannot get it to run rich with everything on the carb set to rich.

My question is this; Are there "richer" needles available for the stromberg 150 cd? I know this is common in SU's but can't find info on ZS's. I'm not afraid to file if I must, but I would much rather purchase something made to run a stronger mixture. Thanks so much for your help and take care!

Answer
Hi Hugo,

Removing the emission control items does not lean out the carburetor. The factory does have it set quite lean in an attempt to clean up emissions. First check to see if you have the 3mm Allen screw down in the top of the carb. Some were plugged to prevent any tampering with the adjustment. There are different needles available but I don't know the letter/number of which has what profile.

I have made my own needles for the Stromberg carbs but it is a lot of work. If you are in an area that you don't have emission standards to keep, You can change the profile of the existing needles by making a hub to hold the ball end of the needle and put the hub in a drill press or even a hand drill clamped in a vice. Then sent up a magnifying glass over it and mark the needle with a magic marker every 16th of an inch and mike each line and write it down. This gives you the existing profile in diameter. Then you need to do a little math to get the square inch area that each 16th of an inch has. Then measure the diameter of the fixed jet hole in the carb and figure the square inch of that. Now subtract the square inch of each 16th from the square inch of the jet and you will have the square inch volume of the open area which is the area of fuel. And thus a map of the profile of the needle in square inch area of fuel.

The carburetor is an air flow controlled carb so the more air that passes through it, the more fuel is delivered. It also is close to the range of RPM. So you need to decide at what RPM you want to make richer and by how much. Depending on who you listen to the max horse power with mixture alone is from 15% to 40% richer then stoichiometric (the ideal mixture for that fuel in that engine)
Assuming the factory was close to correct on mixture (I don't think so) You can with your figures now add a % of fuel volume by reversing your math to come up with a wanted diameter of the needle in a certain area (RPM range)

Then you need a die grinder to grind down each line in the area you want to alter the profile. Miking each line as you grind on it. If you grind too far the needle is trash. You end up with a stepped needle but when finished it is easy to smooth it out with a light use of the die grinder  or Dremel tool and a little Emory cloth.

It takes me 8 hours of work to alter one needle. I had received a DOE grant to build a alcohol still and convert a Triumph TR-7 to run on straight grain alcohol and I needed to alter the profile of the needles to get the mixture correct.

You can't just drill out the jet as that upsets the mixture profile. The needle must be changed.
If your piston has one of the plugs You might search for another piston that is not plugged. I have been told by friends that they have drilled out the plug and can adjust the needle then.

Howard