pagefaulted

iTrader: (1)

So I got a vacuum gauge and timing light and ready to tune! But I have some problems/questions.

Setup:
12a stock port
MSD wasted spark
disabled vacuum advance
No trailing
Holley something-something

1) If i move the dizzy to full retard it is still slightly left (advanced) of they yellow timing mark when using a timing light on leading 1. Is this normal?
2) Will enabling trailing help with MPG or power? I've been seeing conflicting reports.
3) Is it a good idea keep vacuum advance disabled? Again, conflicting answers.

j9fd3s

iTrader: (3)

1. no? full retard on the distributor should be like 20 degrees... if you have the pickup wires backwards on the MSD the timing will move as the engine revs. also weird things can happen if you have timing light with a dial on it. if you have one of those send it back to 1962. or set it to zero.

2. the trailing doesn't do much on a stock port, but it does help with both power and mileage, and it reduces the load on the stationary gear teeth. on a bigger port, the trailing does a lot more. there is no gain for removing the trailing, except simplicity.

3. depends. the vacuum advance works at part throttle, so it helps part throttle mileage. @wot, there is no intake vacuum, so they aren't doing anything. the "it depends" part is that some intake setups, like the wrap around weber/dellorto ones, work very well in the midrange and may not need more timing.

Rogue_Wulff

Minor correction to Answer 3 above. The vacuum advance works by the vacuum port being above the throttle plates. As the throttle plates open, it starts pulling vacuum. The more air flow thru the carb, the more vacuum it creates. Even at WOT, there is vacuum going to the vac advance, when it is PROPERLY hooked up.

j9fd3s

iTrader: (3)

i put a gauge in the line from the distributor, and um that's not what it did. car was a totally stock 83.

basically it was manifold vacuum except the ecu switched the vacuum off @zero throttle.

Rogue_Wulff

I'm guessing this wasn't an RX7, since 83 didn't have an ECU.....
Computer controlled cars are different than non-computerized cars.

Try that same test on a correctly hooked up, non ECU controlled vacuum advance, and it will do as I claimed.

However, your test does have some merit. Even computer controlled setups don't use manifold vacuum ay the vac advance during idle........

trochoid

iTrader: (6)

You both bring up interesting questions. The ecu is for emissions control, not engine control in the 1st gens. The RX7 FSM is the first and only manual I've seen that does not tell the user to disconnect and block the vacuum advance line to set timing. That is one part of rotary tune ups I've never had an answer for.

j9fd3s

iTrader: (3)

sorry substitute Emissions control unit for ecu....

i actually did test a real 12A powered, stock, california emissions test passing, 1983 limited edition rx7.

the advances saw manifold vacuum, except at zero throttle when they are tuned off

if its hooked up like you say it is, why do they even need it? couldn't it just be done with the mechanical advance?

Rogue_Wulff

There is also a mechanical advance, but it doesn't start advancing until higher in the RPM range (like above 2500), where the vacuum advance comes online just above idle. It's for lower RPM drivability and MPG.

California emmission systems are not native to me, so I have no clue how they are setup. I'm talking about federal standards, like the rest of the country got.

Old school stuff uses "Ported" or "Venturi" vacuum for the advance, and it does function as I described.
Electronic engine/emmision controls use manifold vacuum and electronically meter the vacuum to the vac advance, in most cases. Some computer controlled carbs do not even have the "Ported" vacuum ports on them.