Measuring advance curve

[mhackney]

Hello all, I posted earlier this week for the advance curve for my 1970 911S (0 231 159 007 distributor). Now that I have the curve, I attempted to determine the curve for my dizzy. First, I transferred the factory curve to Excel. Then I measured my advance at 5 degree intervals. Since I was working alone, I hooked up my tach (Gastester Pro's tach). Then I dialed the advance on my Sears timing light and reved the engine to get Z1 aligned with the tick mark. I then read the rpm and entered that in the spreadsheet. I measured from 5 degrees to 30 degrees.

So, here is the results plotted in Excel:



So, is this totally bogus? Can I draw any conclusions from this test? How is this normally done - increment rpms and dial in the timing? I guess that would require 2 people to manage.

I am trying to diagnose a fast idle issue and working through the Check Measure Adjust. The car idles at 900 when warm but not up to temp. At temp it idles at 1600rpm. I reset the points and the dwell is 35.5 degrees.

Cheers,
Michael

[Mo_Gearhead]

No expert here but if I read the chart correctly you are getting 4 degrees of "mechanical" advance at about 1300 RPM? Is that in addition to the initial timing at idle? Not sure what it is on your car (2-4 degrees?). So that would put you at about 6-8 degrees at around 1600 RPM? Doesn't seem to out of line to me, but backing off initial timing slightly may be helpful.

The curve you plotted closely follows the factory specs. The change in idle speed, once warmed, is a backward correlation. The RPM must advance FIRST to bring in more advance so, I dont believe the timing is causing the fast idle if that's the demon you're chasing.

Got carbs or other induction?

[mhackney]

Mo_Gearhead, it is 4 degrees at 1500 rpm - those are 200 rpm tick marks. . My idle is 1500rpm so that is where I had to start. The fast idle is the issue I am trying to fix. At idle, I am supposed to be at 0 degrees for the 2.2L 1970 S engine. Also, the advance at 6000 rpm is supposed to be 30 degrees. I didn't have the guts to go up to 6k rpm (!) but at 5400 I was at 32 degrees.

The 70S is an MFI system.

thanks!
Michael

[Mo_Gearhead]

Ok then, Zero at idle. (I am a carb. guy, others may assist in lowering idle with MFI?)

Well, assuming this is a used distributor (not rebuilt?) it looks like the advance springs are weak. Makes the advance come in too quickly. This happens over time; wear/temperature all take their toll. The pivots where the advance weights attach/swivel can wear. I would think that the only fix would be to send the distributor to a shop that repairs/reconditions them. (Assuming parts for your type distributor are available), they can configure the curve to like new.

Perhaps others will offer particular shop recomendations? Good luck.

[304065]

2,2 liter E and S have an identical advance curve. Yours is same as mine.



Expressed in terms of CRANK rpm and CRANK advance:



Expressed as a table:



Of course it's the high-limit values in the table. If you are getting MORE advance than that it's dangerous. Using the values for advance, write down the engine RPM and compare it to the table. If the RPM is higher than in the table, retard the timing.

[911pcars]

mhackney,
I haven't seen the term "Dwell" used to describe ignition advance (on the vertical axis of your chart). It's used mostly to described degrees of dist. rotation when the ignition points are closed. Your reference to setting the point dwell is also a little confusing in light of the dwell reference on the advance chart.

Other than that, I'd say your advance curve is within factory specs.

What is your objective, to adhere to published specs, maximize power or reduce the hot idle rpm?

If the former, I think you're okay.

If the latter, a workaround is to reduce the static timing. You didn't say what you set it at, but if you kick it back a couple of degrees, the hot idle speed will drop. However, so will cold and warm idle speed.

You might have a hot idle air leak around the intake manifolds. Check for cracks on any vacuum hoses. Use any of the air leak detection methods in the archives (propane, carb cleaner, etc.).

Sherwood

[mhackney]

Thanks gents. Sherwood, my primary objective is to solve my fast idle problem without screwing anything else up! The engine pulls strong and is responsive. It starts easily and runs well to and through warmup. I set the dwell at 4 degrees at 1500rpm (the lowest idle I could muster). I picked 4 at 1500 off the published advance curve (you can see that on my blue line which came directly from the graph John posted above).