For those few hardy souls (SOLs) who have followed my pinging exploits, I
believe I have solved the problem:
I purchased a rebuilt 25D distributor, and swapped it in this morning.
The pinging is virtually eliminated (I am running a lean mixture at the
moment, which I think accounts for the very few rattles I heard).
The interesting difference is in the mechanical advance. The stop on my
old one was marked "19", which I take to mean 19 degrees total mechanical
advance. The rebuilt one is marked "10", or only 10 degrees! The
springs are also stiffer, and the weights look the same (I didn't take
apart the rebuilt one).
The timing mark is much steadier and moves more in accordance with my
expectations. I'm running at about 11 degrees initial advance at 1000
RPM... it idles much better... Oh, the vacuum unit on this one doesn't
seem to give it as much idle vacuum advance, either.
The upshot is that I am running strong at lower RPMs without that
disturbing racket above 3K.
What happened? That's the disturbing question. I mean, I've had this 19
degree one on since I got the car, and the pinging just seemed to start
recently.
Well
1) The compression was low before the rebuild, so large amounts of
advance were not causing pinging.
2) After the rebuild I tuned by ear. I was probably in the 5-8 degree
initial advance range.
3) When the rings deteriorated a bit I tried to get power with advance.
This started the pinging and accellerated the ring deterioration in #1.
4) Since the blow-by was intermittant at this point, and pinging was
mostly evident while blow-by was severe, I thought it was due to
excessive leaning of the mixture because of my crankcase ventilation
routing. It could have been an advance problem as well, though, masked
by the other symptoms.
5) It was only after replacing the piston that I became terribly
concerned with timing and mixture. I then started using book values
instead of tuning by ear.
6) During all this confusion, the springs on the advance could have weakened.
zzzzz (hey wake up, it's almost the end!)
For those of you who warned me about my crankcase ventilation system
routing, I have a work-around.
K&N sells cute little filters which sit on the end of the crankcase
ventilation tube. It's just like one of their air filter except 2" in
diameter or so. It comes with a supply of oil which could do 2 of
the full sized filters.
It flows well enough to dissipate the current crankcase pressure (almost
none) and traps the hot oil vapors. I may have a clean engine
compartment soon! (well, after I replace the clutch master cylinder).
For those of you going to Berkeley this weekend, I'm looking for:
1) SU fuel pump bracket and rubber collar for MGB
2) Reasonably priced rear Armstrong lever shocks (MGB)
3) Triumph engine rebuild parts, rear leaf spring.
4) $10 running MGA Twin Cam (driveable)
5) A good time.
John M. Trindle | jtrindle@tsquare.com | Tidewater Sports Car Club
'73 MGB DSP | '69 Spitfire E Stock | '88 RX-7 C Stock
Home Page: http://www.widomaker.com/~trindle
"Lowery's Law:
If it jams - force it.
If it breaks, it needed replacing anyway."
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