Hi everybody,
My big debut at autocrossing is coming up on Feb. 28 and I'm trying to get
my Spit tuned up so that I at least won't be embarassed by poor mechanical
performance. The drivers performance is another matter! Any way I'm a
little confused about the vacuum advance on my distributor. I have a '76
distributor with what appears to be a vacuum advance unit ( the vacuum
canister has the input on the front like every advance unit that I have
ever seen), it originally had the Lucas constant energy ignition (a
magnetic pickup and a seperate module mounted on the firewall shelf) which
has since been replaced with the Crane optical system. So here's the
questions:
1) At idle with the vacuum connected I get about 12-14 degrees of advance,
so if I have the timing set at 10 deg BTDC and I pull the vacuum line off
the timing instantly retards to about 2-4 deg ATDC. If I give the car some
gas the timing retards even more to about 10 deg ATDC and then as I hold it
steady at about 2000 RPM in moves back towartd 10 deg BTDC. This seems to
indicate to me that my springs are too strong or the weights are stuck and
are not allowing the timing to advance. Is this correct?
2) If I disconnect the vacuum line and plug it and then set the timing so
that it reads 10 deg BTDC with no vacuum advance installed the distributor
advances to about 30 deg. BTDC and stops there regardless of any further
advance in rpm. This seems like the mechanical advance is working to
advance the timing as engine speed increases? Driving the car like this
gives much quicker acceleration, but... the engine seems very noisy, like a
lot of valve noise, no miss but almost like a volkswagen diesel or
something, the car has had a recent valve job and has the 7.5:1 pistons
and I'm running 93 octane is this detonation that I am hearing? Is the
timing being overadvanced and causing this?
3) Finally as an experiment... my car has the flame traps that have been
discussed recently. One vacuum line runs from the bottom of the carb
through the trap and to the distributor, another runs from the top of the
carb through the trap to the egr valve. I thought that I had perhaps gotten
these lines switched so I interchanged them. The results were that the
vacuum advance and the mechanical advance now seem to give a cumulative
effect and provide about 45 deg BTDC of advance, but there seems to be so
much vacuum to the EGR that it is on all the time even at idle which causes
the car to miss very badly. So this is probably incorrect any thoughts on
this?
4) Do I perhaps have the incorrect distributor? Was an advance unit used
on US spec cars and should it be plumbed differently to make it function
correctly?
I appreciate any help that anyone can give me on this, I'm sorry it's so
long but maybe this will answer a lot of questions other people have as
well. I'll be racing as number 44 in H stock on the 28th, kind of cheesy I
know but hey someones got to carry the banner!
Glenn Trunnell
'76 Spitfire
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