Big Time Wierd Ignition Timing Anomaly: HELP!

[bfranklin]

Hi all, I've been trying to figure this one out for a while and it baffles me. Maybe someone out there has the answer:

When I first got my 73 911S, it idled pretty high (i.e. 1500-1800 rpm). Looked into the problem and found that my ignition timing was showing as WAAAAY advanced. I don't have a timing like with an advance feature (yet), but I'm talking like at least 20 degrees advanced, probably more (Z1 was way way over). The car idled high, but ran well, ran relatively cool, had lots of power, and didn't ping at high rpm. Alarmed, I set the timing back to spec, 5 degrees BTDC with vaccuum attached. Idle dropped alot (almost too low), car had no power, and ran significantly hotter - all these are symptoms of over-retarded ignition. At first I thought the distributor might be off a tooth, but then I realized that distributor orientation has nothing to do with it - the fact of the matter is that when #1 fires, it has to fire way before the Z1 mark is topside for the car to run halfway decent. Again the car doesn't ping when it is set up this way.

My question is...in a nutshell - "what the hell is going on with my car?"

I have three conclusions:

Scenario 1) My fuel mixture is somehow way off (rich- my plugs are sooty) and somehow with the big overlappy cams in the 2.4S advancing the timing is changing the way the mixture burns blah blah - this theory makes no sense to me, but I don't know enough about what a large amount of ignition advance does to a car - but when it comes down to it, the way the car runs is simply ignition timing, valve timing, and fuel mixture. I also don't understand why it doesn't ping at such high advance (Z1 moves further into advance as well as revs rise, so I assume centrifugal advance is fine).

Scenario 2) Somehow, some way, the Z1 mark is lying to me - i.e. #1 plug fires, piston is at TDC or near there since I really couldn't tell in this scenario, and the Z1 mark is just 20+ degrees to the left on the pulley. Is this possible? I really don't know much of the car's history, but I do know the engine was rebuilt about 20K miles ago. Can the pulley be misaglined? If so this seems like a likely culprit.

Scenario 3) I've unknowingly entered the Twilight Zone and I'm nervously expecting at any moment while driving to see a little demon/gremin creature pop up onto my hood with a bundle of wet brake lines in its hands.

I know its not my timing light, its the propery of my good friend Marc Weintraub (our early cars live in the same NYC garage, and we tinker with them endlessly) and it works just fine on his car. I truly hope I'm just dumb and totally missing something very simple, please tell me what it is? It's driving me nuts!

Many thanks in advance to any advice/wisdom,

Brad

[This message has been edited by bfranklin (edited 10-10-2001).]

[Bill Douglas]

Hi Brad.

I would guess the advance on the distributer is stuck. I think you can test these by twisting it to see if it is "spring loaded".

Bill '79SC

[Early_S_Man]

Brad,

First of all ... your timing spec was WRONG! The correct setting is 5° Before TDC with the vacuum hose OFF!

I concur with Bill's conclusion! Disassembly and cleaning your distributor would probably be a good idea!

I suggest you read and print the following thread for testing your distributor's advance curve:

http://www.pelicanparts.com/ultimate/Forum3/HTML/009677.html


------------------
Warren Hall
1973 911S Targa
1992 Dodge Dakota 5.2 4X4 parts hauler

[bfranklin]

Distributor advance stuck...hmm that could be a culprit. As I said before, the marks do move as I rev the engine, so the advance is doing SOMETHING, but perhaps its not advancing enough, i.e. it may only move 10 degrees. Thanks Bill for shedding some light on this idea, and thanks Warren for your forceful yet excellent post!!

Plan of action: Purchase a timing light with timing advance function, set my timing back to 5 degrees BTDC with the hose *off*, and use the advance function on the light to compare with the factory advance curve.

Stay tuned

Brad

[marc weintraub]

I chose number three. After all, I know both cars.

[tmctguer]

i'll defer to the wiser ones on this BBS, but after i sorted out the distributor, timing, ignition, vacuum, etc. on my new "old" car, and got those items correctly, i found that my MFI was totally out of adjustment.

get the basics established, then you can work backwards to solve other problems that may have been masked by PO's by over compensating related systems.

hey, it worked for me.

[Wayne 962]

Do a quick check - test the total advance (you do need an adjustable light for this), at 6000 RPM. That's really the only way to see where you're at...

-Wayne

[bfranklin]

Thanks guys, timing light with advance is on order, along with compression tester, MFI tools, and have received a gunson gas tester already, so once I make sure everything is right with the timing I'm gonna do the whole "check, measure, adjust" routine on my MFI. Hey I guess I should look at the bright side of things, once I get all this stuff right I should have a much better understanding of the cars ignition and MFI systems

Oh by the way, I was reading through the Bosch manual and it mentions some protractors as special tools, does anyone know where I can find these?

Brad

[Stinky 911T]

I still have not gotten over my fear of hanging my head over an engine running near the red-line to check the timing. The noise is horrendous! Porsche must not think it will hurt the engine or they would not specify it at 6000 RPM. I wonder if fan blades ever fly off when someone is checking the timing? How long can I run my engine at 6000 RPM before the neighbors call the police?

I realize that I am not being logical here, but is everyone else comfortable with this?

------------------
Bob Thayer
1971 911T
Robert.J.Thayer@ue-corp.com

[911pcars]

Brad,
You don't need a fancy "advance" timing light to verify total ignition timing, merely more convenient. Just check with your basic timing light at 6000 rpm (actually, full advance should be in by around 5000, but 6000 insures you aren't getting more).

If you're not detonating with the current setup, I would have to concur with Bill Douglas' diagnosis in that the mechanical advance weights inside the distributor are hanging up and not returning to the rest position.

Sherwood Lee
http://members.rennlist.org/911pcars

[bfranklin]

Hey Stinky911T, what's really fun is if you pull the air box off and have stacks that occasionally backfire like mine (yet another problem I have to figure out) I can just see the day when the garage workers see me run past screaming with my hair on fire...

Thanks Sherwood, all you guys are great for helping me out. I'm learning more and more about these cars every day, and the more I hear the more this makes sense (a problem thats been bugging me for quite a while). I'm not sure how I'd see how many degrees of advance I was getting without an "advance" timing light or a degree-marked pulley, but no matter, I just ordered one of the fancy timing lights yesterday anyway.

Brad

[This message has been edited by bfranklin (edited 10-11-2001).]