can only get 14* of advance at 4k rpm

[fletcher0780]

I was helping a friend time his 86 930 today and we ran into a hurdle. With both vacuum lines disconnected and plugged, idle set a 950 we could only pull 14* BTDC at 4k RPMs. Idle timing showed 7* BTDC. We connected bot vacuum lines and got almost 26* at 4k RPMs. it seems like mechanical advance isn't giving us enough. I pulled the cap an rotor and everything seems to move freely. The cars runs well and has no bogging. We're a bit stumped and don't want to dive in too deep since it's running well, but I think we're missing out on that extra 10*. anyone have any suggestions?

[911st]

First it sounds like your system is functioning properly if you follow proper timing procedure.

At idle when everything is connected there should be about 9-12 deg's of vacuum-retard at work and timing should be near Zero. As soon as you accelerate this vac-retard will go away taking timing to about -12 deg. Then as the rpm's increase mechanical advance would take it to about 26 deg.

At cruse there may then be a bit of additional advance but nothing like an EFI can can achieve. On boost timing should then be retarded to a safe level of about 18-22 deg’s.

The dual pot 930 distributor is an interesting puzzle and study.

It seems to have vac-retard, boost-retard, mechanical-advance, and I think even a little bit of vac- advance that all work together depending on rpm and load.

The most interesting part is how the double connection pot on the distributor may function playing each side off each other.

First we need to understand that the pot on the dist is connected to the manifold. One side is a 'ported' connection just past the throttle plate. At idle this side sees vacuum. However, as soon as throttle is applied the throttle plate moves past it and vacuum quickly goes away.

The other side of the pot is basically connected to the boost side of the intake manifold.

I believe the way the US 930 dist works is:

At idle, one side of the pot sees a strong vacuum signal and the other sees little to no signal and full vac-retard is achieved which pulls about 10 deg's of timing.

Then, as soon as throttle is applied the connection to the manifold moves to the other side of the butterfly and this vacuum quickly goes away. Thus, neither side of the pot is seeing much of any pressure or vacuum and timing is quickly advanced about 10 deg as vac-retard goes away.

Then as the rpm’s increase, mechanical advance brings timing up to about 26 deg's.

At cruse, one side of the pot sees boost and the other side now sees vacuum. This wants to pull the timing toward advance. I do not know for sure but this may advance timing a little past the 26 deg. However, we cannot achieve this in our driveway.

Then when at speed as we accelerate with boost, both sides of the pot start to see presser. With this I suspect the pot pulls back to an intermediate-retard-position that should put timing at about -18 to -20 or so.


I have spent a lot of time trying to understand these distributors but could still be mistaken.

[fletcher0780]

I guess I misunderstood the timing directions. I thought I need to warm the engine up, disconnect both distributor port lines, adjust idle to 950, rev engine to 4k RPM and set timing to 26 BTDC.

I can only get to 14 BTDC befor running out of adjustment. If I set it to 14 BTDC at 4k RPM, leave the lines disconnected and check timing at idle I'm at 7 BTDC, so I only get 7* of mechanical advance. Is this correct? I haven't even gotten to the point of checking vacuum advance/retard yet.

[911st]

Maybe I did not understand your problem.

I believe that with the lines disconnected you should be able to get 26 deg at 4000rpm. If you can not I guess I would ask if there is something keeping it from going further. Or I would wonder if the dist had been out for some reason and might be misindexed by one tooth.

Idle should be about 950 with the lines connected. Timing should be around zero deg's +/- a deg or two.

With the lines disconnected, idle should be faster than spec as there would be no vac retard and timing should be around 10 deg or so.

[fletcher0780]

Thanks.

If I reindex the distributer to get 26* at 4k with the lines disconnected, my idle timing will end up at 16* with lines disconnected since mechanical advance is only giving me an extra 10*. mechanal advance should be closer to 20* correct? everything seems to move OK in the distributer, but maybe it's time to pull is and give it a closer look.

[911st]

First, the more I think I know about the 930 ignition the less I seem to.

Having just 7 deg of mechanical advance measured at the crank seems short. If it is 10 deg might work.

If you are getting 26 deg at 4k, 16 at idle with vac disconnected, and then about 6 deg w vac connected triggering the fac retard it might be ok.

However, I am rusty on this.

Not sure if the following helps but on of the third page below might be saying this is ok.

Sorry if I have not been more help.

Originally Posted by cole930

AGAIN GUYS, THE 930 ROW DOES HAVE RETARD !!!!!!!!!





Cole

Originally Posted by cole930

Keith,

I'm the one that posted the 930 Graphs. Here is the one for the Advance Curve





Cole

Originally Posted by cole930

AGAIN GUYS 930 ROW TIMING 29 DEGREES @4000 RPM






"These are from the Porsche Factory Turbo Maunal"


Cole