View Full Version : Mixture Control

05-18-2011, 01:47 PM
TR-6 runs fine but seems a bit rich according to plug color. When I try to adjust mixture, nothing happens. Manual says when you lift the piston 1/4", idle should speed up and then die off. When I fooled with SUs back in the '50s, that's the way we tuned them, but these Stromberg's don't make any difference what I do, even when I screw the needles all the way in or back them way out. The idle speed just falls off right away, indicating too lean. Any ideas?

05-18-2011, 02:07 PM
You could stick with reading the plugs. Adjust them leaner, take it out for a 10 mile run and check the plugs. Rinse and Repeat...... This would be a fun testing method.

05-18-2011, 02:17 PM
Make sure that the needle is still threaded onto the adjuster. From fully Clockwise with the Allen wrench which is full rich to the disengaged point is slightly less than 2-1/2 Counterclockwise turns.
Start out about 1-1/2. Lift the air valve about 1/8 inch and observe the IMMEDIATE reaction of the engine. Taking a couple of seconds to decide will cause the engine to flutter and give the false impression that the mixture is lean.
BTW, I rely on the reading the plugs after getting the mixture close with the air valve. The reason is by driving I go thru a variety of conditions. Very little idling, a lot of acceleration, a little downshifting and some cruising.
About 30 miles on old plugs should be enough to develop a color change.

05-18-2011, 05:33 PM
When that happens on my TR3, it usually means there is a problem somewhere else. Valve lash, ignition timing, clogged air filter, etc.

If you can't find anything else, double-check the valve timing. I've never seen it on an LBC, but I have seen other engines slip a tooth on the timing chain, which will cause the plugs to show rich (or even carbon foul).

05-18-2011, 10:01 PM
Make sure that the needle is still threaded onto the adjuster.

That's my first reaction as well.