Thermoquad Tuning & Troubleshooting

OK, as you may know...the step up metering rod piston is operated by a cam and lever under the plastic bowl in the base plate. This cam/lever cannot be seen unless you take the carb to pieces.

The "cam" is part of the primary butterfly shaft

The step up "piston" adjuster screw rides on this cam/lever, and as the shaft rotates, the cam lever raises the step up rod "piston". Therefore, the piston is operated mechanically as well as Vacuum released.

Now..

.as you also know, the "piston" is spring loaded and held in the top plate by a small retaing plate.

OK - Heres the important bit......This means that the adjuster rod and piston can only travel so far "up" , b4 contacting the retaining plate.

Unfortunately....IF the adjuster screw is adjusted clockwise too far (raising the rods)...it lengthens the adjuster screw to a point where the piston cannot travel up, and the adjuster screw interferes with the cam lever a sit rides up.

Basically you can make the step up adjuster 1" long, but theres only
3/4" travel available!!

To check this, you need to remove the phenolic bowl and check the "cam" and make sure the shaft is not riding the "cam" up against the adjuster and forcing the piston hard up against the retaining plate.

Heres a test:

If you plant your foot, making the carb "stick"
Then, undo the piston retainer top plate

If the piston "jumps" out of the sleeve, AND THE BUTTERFLIES RELEASE, then the cam at the bottom has been "cramming" the adjuster screw against the top retaining plate.

The fix?

I would then:

Remove the bowl and check to make sure the cam is not broken, the piston adjuster is not snapped, or the butterfly shaft is not bent.

Good luck!!