OE Carburetor Modifications
   by Sean Dunlop
   seand@multiline.com.au
   and Nick Reilly
   njr@ts.roke.co.uk

Sean's suggestions:

   Try modifying the spring that regulates your vaccuum secondaries on the carby. This model responds well to cutting the spring down by 2/3 or even try driving with it totally removed. This allows your vacuum secondaries to kick in much lower in the RPM's, and is a very noticable modification. Recommended for all 4-barrel Nikki carbies.

   OK - Remove the air filter for easy access. Looking at the carby from the right side of the engine, find the vacuum bellows which operate the secondary butterflies on the carby. Remove the four corner screws and gently but firmly separate the cap from the fixed section. Inside is a spring and a rubber bellows connected to the vacuum secondaries by a metal rod. This mechanism operates the secondaries via a vacuum signal received from the airflow inside the carby. The spring dampens the secondaries response to the vacuum signal. By reducing the spring size you can bring the secondaries on quicker and lower in the revs.

   For 7's with stock exhaust try 2/3 or 1/2 cut spring and with exhaust mods try 1/3 or no spring at all. Also recommend a replacement air filter element with a performance oiled foam or K&N. Also, try using a hole cutter and drill 1-3/4" holes around the outside face of airfilter housing.

   Also, if you have a solenoid fitted to the vacuum bellows then disconnect the single wire connected to it. This will ensure that the vacuum signal does not become interupted.

   These mods are the basics of extracting the most out of a factory S1 [1st gen]:

• Exhaust- Free flowing
• Carby- Cut or remove spring and solenoid wire.
• Airfilter- Performance foam/K&N
• Ignition- Upgrade to 82 or later electronic + coils ; Try 2.5 - 7.5 degrees more advance, leave the L-T gap the same.

Nick's results:

   I removed the spring from the carb (appears to have no solenoid on secondaries on mine, although there is a solenoid further back on the carb; I think it is the "richer solenoid") and cranked the timing up 5 degrees on friday. The result is truely incredible bang for the buck (i.e., a fair bit more power for less than 15 minutes with a screwdriver, spanner and timing light. The bad news is I enjoy the power and managed to get through half a tank in 50 miles (and 10 mpg Imperial = 8 mpg US !), but this is only 'coz I was giving it full throttle every chance I got! I think the modifications wouldn't make any difference to fuel economy if it was driven half sensibly! Now all I have to do is avoid overtaking policemen at 100 mph whilst trailing flame out the back!