Before the emission controls were put on the roadsters, the distributor had
15 degrees of internal mechanical advance. If you set the ignition timing
at 16 degrees BTDC and then rev the engine you will see - with the timing
light - that the ignition advances or in simpler terms, that the spark
happens sooner. At full mechanical advance, you would see the timing light
fire at 31 degrees BTDC in the above example. In order to meet emissions
regs - readings were taken at idle at the time - Nissan retarded the timing
at idle. Emission controlled cars are supposed to be timed at TDC - 0
degrees. The emission cars still need 30-35 degrees of ignition advance in
order to make power so Nissan changed the distributor so provide 35 degrees
of mechanical advance. This way they could time the car at 0 degrees at
idle but still have the required amount of advance at speed. The problem is
that the cars don't run well with the smog distributor. They don't make
good power right off idle don't idle well and are very prone to overheating
when timed at 0 degrees. Nissan tried to fix that problem by adding the
thermal modulator on top of the water neck which is supposed to open up a
vacuum passage when the engine gets too hot. That vacuum goes to the vacuum
advance unit which advances the timing until the engine cools back down.
The thermal modulators are often not working or have been disconnected by
people who thought that it's better to just disconnect that emissions crap
not knowing what they are really doing. Many of those folks then time the
car at 16 degrees not realizing that they now have 51 degrees of total
advance which is way too much.
The solution is to put a new distributor cam and weights into the
distributor so that it will provide the correct amount of total advance as
well as the correct advance curve - the amount of advance at any given
engine speed. This is called recurving the distributor. It is very easy to
do, you just need the correct parts which, I think are still available from
the vendors. One other option which I am considering is to change
distributors entirely. The later electronic distributors like those used on
the B-210 can be adapted to the Roadsters and can also be recurved with many
different curves being available.
You can tell how much advance your dist. has built into it by removing the
breaker plate so that you can look inside the dist. You will see a number
stamped on the plate at the bottom of the cam. Most common are 7.5 and
17.5. This number is the degrees of advance the distributor has in camshaft
degrees. Multiply that number by two to see how many crankshaft degrees
that amounts to. 7.5 x 2 = 15 degrees which is a non-smog distributor. You
can set the timing at idle at 16-20 degrees. 17.5x2=35. You must time a
car with that distributor at 0 degrees or risk hurting your engine.
There is more info on dist. swaps at
http://home.att.net/~jason510/Dizzy_FAQ.htm
This is L-series info but it applies to the roadsters as well. It just
might not be cook book simple in an R or U series engine.
Brian
-----Original Message-----
What does it mean/do, when you say you re-curved the distributer?
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