How many of us are honest enough to admit that they use the distributor to
help adjust the idle speed when all else fails?
I've spent much time adjusting the timing just so with lamp and markings and
then just flung all the tools and started moving the distributer about when
I couldn't get the idle below 1000 with the perfect timing. It's just so
much easier than fixing the real problem of shaft leaks etc. Seems to run
fine too.
Unfortunately now I have a really rough idle so it's carb rebuild time.
Along with O/D rebuild, and Diff rebuild and rear hub fix, and rim
straightening... sob... :^(
Mark Hooper
-----Original Message-----
From: Randall Young [mailto:Ryoung@navcomtech.com]
Sent: Friday, June 27, 2003 10:14 AM
To: triumphs@autox.team.net
Subject: RE: Ign advance and cam phasing for TR4
> I've always heard that conventional wisdom says to advance the
> ignition at idle until the revs stop increasing, and then maybe
> back it off slightly.
Rupert, with respect ... that's crazy. Even with a stock cam, the idle
timing is nowhere near "max revs", because cylinder pressures are way down
which slows down the flame front. To put it another way, if maximum idle
speed was what was wanted, the vacuum advance would be mostly active, since
the manifold pressure is low.
The situation is even worse with a radical cam, since cylinder filling at
idle is apt to be erratic, and mixture tends to be way off (which is why
radical cams idle badly), both of which affect timing.
I don't have the numbers handy, but I know that engine builders look at
total advance (static advance plus max centrifugal advance, ignoring vacuum
advance if present) rather than idle advance.
Failing that, the most important thing is that the engine never knock under
normal operation. One acceptable way to chose a timing setting is to start
conservative and do a "road test" with heavy throttle at low rpm. According
to the TR2/3 shop manual, "light pinking" is acceptable, but personally I
like to play it safe and back off two degrees from where I can hear it.
> Set up the dial indicator and yep, sure enough, it looks my
> intent to advance the cam by four degrees actually turned into
> retarding the cam by four degrees.
Just a thought ... have you double-checked the marks on the pulley ?
> My question is, would a retarded cam cause the engine to want
> more ignition advance,
I'm not certain just what effect a retarded cam would have on optimum static
advance. It is likely to change the optimum advance curve (odd that you
didn't mention that ...).
Optimum advance is all about getting maximum cylinder pressures at the
proper point in the compression stroke, thereby producing the most torque
and power, without causing knock. Thus, anything that changes the speed of
the flame front, like cylinder pressure during combustion or changing the
shape of the combustion chamber, is apt to affect the optimum ignition
timing.
Randall
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