Setting Advance Timing

[leadrsa729]

Hello -

I've never set the timing on an engine before, and I'm pretty confused. I have a 72' 911T and the spec book says idle timing should be 5 Degrees After Top Dead Center, but that the advance timing should be 38 Degrees @ 6,000 RPM.

First question -
Do I use the same 5 degree atc timing mark when I set the advance?

Second question -
If I have to rotate the distributor to change the timing to get the 38 degree advance, won't that mess up my previously set idle timing?

Thanks for all your help.

[Gogar]

Well, you can set the idle timing to start. Unhook the vacuum line from the distributor, and plug it with a golf tee.

Loosen the nut on the distributor, set up your light, and check it out. With the line unhooked and plugged, you should try to get the timing around 5BTDC.

Then, when you hook the line back up, it should drop to about 5ATDC.

Good!

If you're confused about high-rpm timing, you need to understand that the distributor as a set of weights and springs inside it that will spin as the engine turns faster and MECHANICALLY increase the advance.

So, set your idle advance and leave that line unhooked and rev the engine and see how much high-rpm advance you get.

IMO you need to check that 38 number, that doesn't seem right. There is a mark on the pulley that indicates 35 degrees BTDC. aim for that to start.

If your distributor's internal weights and springs get dirty or gummy, then your distributor may not be able to advance the full amount to 35BTDC, and you might check into cleaning your distributor.

There's a really great thread by "Gunter" about cleaning a distibutor.


What you might want to do is set your HIGH rpm timing FIRST, rev to 6000 and try to get to that 35 mark on the pulley, or 35 degrees if you have an adjustable advance timing light. Then, after you do that, check the idle timing and kind of let it fall where it may.

Just be careful with the HIGH RPM timing. Improper idle timing = crappy performance, but improper HIGH idle timing (as in TOO MUCH) = blown up engine.

[Rot 911]

Easiest way to set advance timing is with a timing light with a knob on it that allows you to set advance timing. The correct way to use one of these is to set it at what degree you want your engine to be at, and then it should show up at 0% before tdc on your pulley. For example If I want my engine to be at 35% btdc at 6000 rpms, I would set the timing light at 35% and when I take the car up to 6000 rpms it should show 0% on the pulley.

[SteveinTO]

Generally, the 5 degree number is set at idle or 800 RPM+-, with or without the vacuum lines disconnected & plugged. Do whatever the good book says in that regard.

The distributor adjusts with RPM until the advance is 38 degrees at 6000 RPM. I'm a little queasy about the 6000 RPM part, but that's the only way to guarantee you're not getting too much advance (which might damage the motor). Actually I think you get max advance earlier than 6000 RPM, but the only way to tell if the distributor is set up properly is to pull to 6000.

[911pcars]

"....IMO you need to check that 38 number, that doesn't seem right. There is a mark on the pulley that indicates 35 degrees BTDC. aim for that to start....."

5ºATDC + 38º advance = net 33º BTDC

Sherwood

[mpetry]

Sherwood, you mean initial timing of 5 degrees before top dead center, plus 38 degrees of total advance, equals 33 degrees AFTER Top dead center when all the advance is in, right ?

You can set the 5 degrees with static timing too. Not as reliable as using a timing light, but nice to know about if you get stuck somewhere.

Mark Petry
Bainbridge Island, WA

[911pcars]

Originally Posted by mpetry

Sherwood, you mean initial timing of 5 degrees before top dead center, plus 38 degrees of total advance, equals 33 degrees AFTER Top dead center when all the advance is in, right ?

You can set the 5 degrees with static timing too. Not as reliable as using a timing light, but nice to know about if you get stuck somewhere.

Mark Petry
Bainbridge Island, WA

Mark,
If the initial timing is 5º After Top Dead Center, during spark advance, the spark must occur sooner (advance 5º) to arrive at TDC. Any additional spark advance ("advance" as in spark before TDC) is added to the amount needed (5º) to get to TDC.

Thus, total spark advance in the example above is 38º (33 + 5). Total advance relative to TDC is 33º, or 33ºBTDC, the spec used to quantify ignition timing.

Setting the ignition timing with the engine at rest (Ign. ON) is the same as using a timing light (assuming the distributor or other vacuum or elec. advance mechanisms are not yet active). It's the occurrence of the spark relative to TDC no matter the rotational speed when measured.

Sherwood

[Walter_Middie]

You guys are going to confuse Leadrsa729 with all the talk of initial and total advance. I am surprised that you're learning how to set the timing on a 1972 911T. It's like buying a Steinway, then deciding to learn how to tune pianos. Most start with a 1965 Dodge Dart or something similar. But kudos for learning anyway. Here's a video that might help clear up a few basics. It's not a 911, but the idea is the same:

YouTube - How To Guide: Adjusting Ignition Timing

[Eagledriver]

I believe the number 38 comes from a note in the manual that says the timing must not exceed 38 degrees (BTDC) at 6000 RPM.

For your 72 this is the information: Idle timing with vacuum line CONNECTED is 5 degrees ATDC. Advance with vacuum DISCONNECTED is 30-35 degrees at 6000 RPM BTDC. The idle number can be compromised to get the proper 6000 RPM advance.

-Andy

[tobluforu]

Time by ear and feel and you really can't go wrong, well I guess you can but with these old cars, everything changes.

[Flat6pac]

The pod on the early distributors through 77 is actually a retard for emissions. 5 before TDC with the vacuum hooked up will retard to 5 or 10 ATDC. 38 degrees I ve never seen before as a setting.
Bruce

[Steve@Rennsport]

Originally Posted by tobluforu

Time by ear and feel and you really can't go wrong, well I guess you can but with these old cars, everything changes.

Au Contraire, Sir,...one can go VERY wrong by timing an air-cooled engine by ear. Such an exercise gets quite expensive in a hurry.

It bears mentioning that timing specs on these older cars were based on gasoline formulations of the day. In some cases, you cannot safely run maximum specified advance depending on the fuel in your area.

Caveat Emptor on that.

[psalt]

I've never set the timing on an engine before, and I'm pretty confused. I have a 72' 911T and the spec book says idle timing should be 5 Degrees After Top Dead Center, but that the advance timing should be 38 Degrees @ 6,000 RPM.


Hello,

The 5 ATDC @ idle and 38 BTDC@6000 rpm specs in the manual are crankshaft degree measurements for checking the timing on a running engine. The high speed reading is max advance to avoid detonation with the specified fuel and the idea that these number should be netted in incorrect. The 5 ATDC at idle is with the vacuum retard connected, which is a curb idle HC emission test device. The 38 BTDC@6000 rpm spec is to check the timing under load at high speed where there is little vacuum and the retard is irrelevent.

The proper procedure is to set the idle timing with the vacuum retard connected, then read the timing at high speed. If the timing exceeds 38 BTDC or does not advance into the 30's, you know something is wrong with the distributor and you need to test the mechanical advance and the vaccum retard to determine what is wrong. A false idle reading is common with a distributor with worn springs, especially with an elevated idle, because the advance will be effecting the reading.

[tobluforu]

Originally Posted by Steve@Rennsport

Au Contraire, Sir,...one can go VERY wrong by timing an air-cooled engine by ear. Such an exercise gets quite expensive in a hurry.

It bears mentioning that timing specs on these older cars were based on gasoline formulations of the day. In some cases, you cannot safely run maximum specified advance depending on the fuel in your area.

Caveat Emptor on that.

Yea I should not have posted that due to most users not having a clue how their cars runs. I have been timing my car my ear/feel for 19 years now, but I know my car-knock on wood.
Buyer beware in terms of what, gas?