Walbro 39 carburetor on WJ Spider demystified:

There was some confusion about the Walbro 39 carburetor on the Spider units so here is more info to get things clarified.

 

  • # 1 is a jet screw - should be adjusted 1 1/4 from fully closed. See this for most performance tuning. 

  • # 2 is a idle screw - it should be a fully closed - turn clock wise all the way. This increases your idle and eliminates most vibration.

  • # 3 is a screw that will fine adjust the length of your throttle cable - if you push the trigger and there is delay in power increase - turn this screw clockwise to shorten the cable action. I actually use this screw to increase the idle so the engine is running at 5 % of power when the throttle trigger is released as I try to eliminate all vibrations.

  •  # 4 is a cable stop that will rough adjust the length of your throttle cable 

Troubleshooting Walbro 39 carburetor settings on Spider models:

Symptoms Reason Fix
Engine vibrates and idles slow Idle screw (2) not adjusted properly  Turn idle screw (2) clockwise all the way in. 
The idle RPM is too high and engine pushes while at idle Throttle cable too short Turn screw (3) counterclockwise till engine runs smooth but does not push while at idle. If this does not help the cable stop (4) may need to be adjusted - move it further away from the leaver
I start pushing the throttle trigger but the engine RPM will not start increasing until the throttle trigger is pushed in 1/3 Throttle cable is not adjusted properly - too loose Turn screw (3) clockwise or move the cable stop (4) closer to the leaver
Engine does not run smooth  The mixture too rich Turn jet screw (1) 1/8 clockwise
Engine does not react quickly enough to follow fast throttle changes. The engine hesitates to accelerate fast enough. The mixture too lean Turn jet screw (1) 1/8 counterclockwise
Engine stalls when full power is applied The mixture is way too lean Turn jet screw (1) 1/4 counterclockwise

See more improvements and technical info here