Tuning the Stag Carburetors Using Colortune®

by Tim Buja, TKBuja@insightbb.com

Setting the air-fuel mixture using a Colortune involves replacing the spark plug in one of the cylinders with the special Colortune plug. The Colortune plug has a transparent window that allows you to see the flame color in the cylinder as it fires. An overly rich mixture will have a yellow flame and will be accompanied by black smoke (and noxious fumes) from the tailpipes. As you lean it out to the optimum stoichiometric mixture, the flame color will change to Bunsen blue. A mixture that is too lean will still have a blue flame, but the brightness will decrease as the mixture is weakened. If the piston dampers in each carb are working correctly, you should see the flame color change from blue to yellow when you open the throttle, and then change back to blue as the engine comes up to speed. Read the Colortune instruction sheet for full details on how the system works.

In order to set the air-fuel ratio correctly, you need to know which carburetor feeds which cylinder before you can start making adjustments. In this section, all references to “right” or “left” are made when sitting in the driver’s seat looking toward the front of the car.

The Stag V8 engine is fed by two Zenith


Stromberg 175CD SEV carburetors mounted on a two-plane manifold. The right carburetor feeds the lower plane, which connects cylinders 1, 4, 6, and 7. The left carburetor feeds the upper plane, which connects cylinders 2, 3, 5, and 8. There is a small balancing passageway that connects the two manifold planes, so you may see some small effects of adjusting one of the carbs in a cylinder not directly connected to it. Be aware that the cylinders in the Stag V8 are not numbered per the standard method (#1 at the left front), as illustrated in the figure below:

               front of car

cylinder cylinder Key: L: Left Carb
l #2 #1 r R: Right Carb
r #4 R #3 l l: Cylinder fed by left carb
r #6 L #5 l r: Cylinder fed by right carb
l #8 dist #7 r dist: Distributor

Firing Order:
1 – 2 – 7 – 8 – 4 – 5 – 6 – 3

Note that the #2 (left front) cylinder is used to set the ignition timing, not the #1 cylinder as in most other engines.

Colortune® is a registered trademark of Gunson’s Ltd., Pudding Mill Lane, London E15 2PJ


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