> I have a neighbour who is trying to help a Roadster owner tune his car for our
> local air care program (Vancouver BC). He is getting frustrated dealing with
> how Datsun sets up the timing of the Roadster. Locally we have difficulty
> passing air care tests without resorting to cats or other methods to cheat the
> test. The neighbour wonders why no one has addressed (unless they have and we
> don't know about it) the problem with the original design of this car's
> ignition
> system (as he sees it). He cannot understand why Datsun designed the ignition
> system to have a initial timing of only 20 degrees and a distributor that can
> only advance a total of 15 degrees. He has never seen a car that has such a
> high inital degree setting. Usually cars are developed with an inital setting
> closer to 10-15 degrees then a full advance of dizzy advance of another 25
> degrees. He has tried to recurve the dizzy to allow for near 25 degrees of
> timing and then set the intial at closer to 10 degrees. He wanted to know:
> 1) at what RPM is the distributor at full advance?
> 2) has a distributor be developed to do this yet?
> 3)if not why not? why rebuild dizzies to original Datsun specs (that he deems
> wrong)?
Not knowing the specifics of your local emission standards it's had to
address the question other than to say that in most cases the cars are
required only to meet the emission levels in effect the year it was
manufactured, or the year the engine in the car was manufactured if a later
motor has been swapped in. That being said I know of a number of US domestic
engines that use about the same 20 (crank) degree initial and 32 to 35
degree total advance amounts. In my 40 plus years of playing with both
domestic and foreign cars I've also seen a wide variety of combinations of
mechanical and vacuum advance curves.
You didn't say what year and model the roadster in question is. During the
late '60s and into the early '70s Nissan And most other manufacturers)
changed the methods and often the tuning specs from year to year in order to
meet the changing emissions levels. Fortunately, for most of us, cars of
this vintage are exempted from even the OEM specs because of their age and
relatively tiny numbers still on the road. Another problem is that the
advance plate assembly wasn't designed for longevity and without frequent
disassembly and lubrication tends to get real sloppy which doesn't help
either. Fortunately the plate assembly may still be available fairly cheap
via your local parts department.
The factory service manual might have the total mechanical advance specs,
but frankly the books of the time on the roadsters weren't the best. I do
remember that there were at least two different mechanical advance cam
setups with differing amount of max mechanical advance. I've always set the
total advance on both U-20 and L-series (510 and Z-Car) motor at around 32
degrees total (vacuum advance disconnected and either using extra marks on
the crank pulley or more recently dial back timing lights) and letting the
initial fall where it may.
If the distributor is in good shape, minimum slop in the shaft and advance
mechanism clean, lubed and moving smoothly, and the carbs are in good
condition and adjusted properly, the car should pass a test based on period
levels unless there are other issues with the engine. Making it pass
later/lower levels is another story.
Regarding newer distributors, Gary Boone offers a later, breakerless unit,
adapted to the U-20, but I'm not sure if he alters the factory advance
curve.
FWIW,
Ron
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