We have the EGT probes mounted about 1-2 inches from the exhaust manifold
flange, per the gauge instructions, with the probes on the #1 and #4 header
pipes. We use them to adjust the mixture in the front and rear SU's (H6's)
and to get an idea if we are running lean or rich. "Normal" temps are in
the range of 1100 to 1300 degrees. We try to get about 1200 degrees at WOT
on a long straight. 1400 degrees would be way lean and anything less than
about a 1000 degrees would be rich.
We also use them for trouble shooting. At Elkhart Lake last September we
had a problem where the front carb would go way rich on the front straight
and in the carousel. The engine would really "load up" and then clear out.
We saw the #1 EGT drop to 8000-9000 when it loaded up. First found a
sinking float in the front SU but that didn't solve the problem. Solved the
problem over the winter by making some isolator blocks for the carbs and
also some carb jet "locks". We had not had the problem before last
September but had put new Cambridge motor mounts in over the summer when I
did the sleeve and piston replacement. The Cambridge mounts are quite a bit
harder than the Moss or Vic Brit ones which we had previously broken.
I think our main problem was the carb jet "sinking" due to a resonant
frequency vibration from the harder Cambridge mounts as Kas reported some
time ago in a post when he saw carb jets sink at a certain RPM on a dyno.
The combination of the isolator blocks and, mainly, the carb jet locks has
solved the problem. We did an airport test and tune to try and duplicate
the long straight at Road America and did not have any problems. Also, no
problems at Gingerman and Blackhawk (shorter tracks).
The EGT's were invaluable to pointing to the front carb going way rich.
Just took a little while to figure out why. Have been running about 1300 on
the long straights now with the throttle shafts replaced (were very loose)
whereas before the new throttle shafts we were at 1150-1200. I had the
wrong needles in the carbs (stock SA's that were set up into the pistons,
what came on the car). Now have RB's per Kas's recommendation with his G3
cam and our compression of about 11:1. Hope to do another airport test and
tune next month to see what the temps look like with the right needles.
Suspect we will be back to the 1150-1200.
Tim Murphy
1961 TR4 #317
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