File this in the dont always understand all I know about it department.
While researching on the net looking for info to help diagnose/repair my
curious problems since my dual SU conversion, I came across a discussion on
ported vacuum sources vs. straight manifold vacuum source.
After reading more on when timing advance is required, and perhaps having
finally learned/absorbed enough on the topic, suddenly it clicked into place
for me.
I went out, hooked my timing light up to my 80 MGB LE, and noted that in
fact, when the vacuum line was connected, I was getting maximum advance at
idle. So I switched the line to the vacuum port on one of the SU carbs, and
noted that there was no advance at idle now, plenty of advance at partial
throttle, and the advance dropped off as the throttle was opened.
So, performance problem solved, fast idle problem solved, backfiring problem
seems solved!
Guess Ill go back and recheck all the carb setups now, and perhaps revisit
the timing issues. I had retarded the timing in an experiment to see how it
affected things.
I run 93 octane gas in the car to prevent dieseling. My engine has excellent
compression (150-148-150-150) and the higher octane has pretty much cured
the running-on problem. The timing spec is for 10 degrees BTDC. Ive been
told that I should advance the timing a bit to allow for modern gasoline
formulation.
Other than empirical testing, is there a good rule-of-thumb for timing
settings vs octane rating?
Thanks!
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