Weird Timing Setup...

[unclebilly]

Hi Guys,

I searched the archives, looked in both of Wayne's books and the Haynes Manual.

I set the valves, changed the plugs, cap, rotor, points, changed the oil to synthetic.

My 77 911S has a sticker that says the timing it to be set at 15° after TDC. Trouble is, it won't run there very well and it will backfire on decel if I rev it. I set the dwell to .013" (split the difference between the .014" that other posters have suggested and the .012" that Wayne suggests in the engine rebuild book). It seems to want to run best at 0°. It also runs nice at 5° but won't start once it cools down.

Is 15° the right setting? It is possible that the timing marks have moved relative to the crank (on Jaguars and GM stuff, it is common for the outer ring on the harmonic dampener to spin). I'm not sure the the crank shaft is one piece on the 911 or not.

I'm going to check it with a dwell meter tomorrow morning. Have you guys got any other suggestings?

Oh yah, it isn't pinging at all and it runs strong.

[rickeolis]

I have the same engine and when I time it, I use the marks on the pulley that show up to the right of the TDC marks. I think they correspond to about 35 degrees.
It runs fine there, and then I fine tune from there.

[unclebilly]

So what you're saing is that you set it to fire before top dead center?

There are 3 marks going from left to right - 15° - 5° - Z1 which I beleive is TDC. You use something right of these?

[Paulporsche]

Unclebilly,

If you have a meter rather than a feeler gauge, set the dwell @ 35-36 deg w/ new points and change @40-41.

Rickeolis,

Is that 35 deg @ max advance or idle?

[audi2.7t]

Originally posted by unclebilly
Hi Guys,

I searched the archives, looked in both of Wayne's books and the Haynes Manual.


My 77 911S has a sticker that says the timing it to be set at 15° after TDC.

Shouldn't that be BEFORE TDC ?? ,if ignition occurs at or after TDC you have already started losing compression as the piston would be on the down stroke......then again ,I've been staring at my 911 from every angle for 3 weeks trying to get familiar with this engineering and if you told me ignition occured at BDC I might believe you.........
Dohh ,you're talking valve timing ,right ?

[john70t]

The total advance(spec is at 6000rpm but it may reach that before) is more important than idle. That's closer to where the engine spends its time.
There may need to be distributor vac advance lines plugged etc. and make sure the centrificle advance mechanism/plate is clean, lubed and working in its total range and that your using the correct timing marks.
The backfiring could be mixture, exhaust leaks or valves if the timing is correct.

[alan911]

About timing marks........somebody figure this one out.

I set it @ TDC, never any problems and runs great but I've never seen one like it on this board. Why is mine @ TDC ??



It's on my "77S non-catalyst 2.7.

[unclebilly]

That makes more sense... I can't find my digital camera (wanted to post a picture of it yesterday). Mine definately says " Ignition Timing: 15° +/- 2° ATDC ". It won't start there tho...

My car is also certified for any altitude and has CATS. Another difference is my idle speed is 1000RPM +/- 50

[Randy Webb]

I think the ATDC specs are smog related and that BTDC specs were for older pre-smog cars.

[rickeolis]

Yeah, that would be 35 degrees before TDC. this is done at idle, with the vacuum port capped off.

[unclebilly]

OK, now it's 35°..?

Here's what my car is, 1977 S with the air pump removed. Still has the cats in place, they might be hollow - not sure, it has a bursch muffler (not that it matters).

I have it set to 0° right now and it runs OK. It wouldn't start for me at 5° this morning when I went to leave the parts store. It idles much higher with more advance tho. Capping the vacuum line makes no difference tho because the line is connected to port vacuum so there is no vacuum at idle. Once it revs up, there's plenty of vacuum on that line. Should this be connected to manifold vacuum?

As soon as I find my digital camera I'll take pictures of the sticker on my fan shroud.

Should I set it at the 35° mark or leave it close to 0°? I'm pretty much at a loss on this one.

The biggest problem with my car is the number of Chev mechanics who seem to have had their paws on it over the years...

[jstobo]

Your car is a CA spec 911S. For 1977 the spec timing on CA cars was 15 degrees ATDC +- 2. Idle RPM 1000 +-50. 49 state was TDC +-2degrees. Idle 950 +- 50. 15 degress ATDC is to the left of Z1. As stated the best way to time a 911 to rev it to 6000RPM and set the timing at 30-35 degrees BTDC(mark to the right of Z1.) On my 69, 4 degresss ATDC at idle gave me 30 degrees BTDC at 6000 RPM. But the advance curves on the distributer may be different because of wear. The retarded timing at idle for the CA cars was for emissions. You probably don't have to worry about it in Canada.

[rickeolis]

My method works to get things going, but I have to play with it a couple of times on the road to get a more exact setting once it's running decent.
Also, I'm at 7000 feet above sea level, so yours and mine will be different, but as I said before, you should have a notch about and inch and a half to the right of Z1, and this should be a good place to start. The final sweet spot should not be far from there.

I agree with jstobo that these cars are setup for Cal. emissions, and the vacuum port actually retards it for emissions, so I have capped mine off permenantly.

Good luck.

[unclebilly]

I tried about 35° at 6000 RPM and it ran ok, actually seemed to surge a bit and popped like a Camaro on deceleration. This lookded to be about 7° BTDC at idle. I set it back slightly closer to the Z1 mark (probably about 4° BTDC), we'll try it there tomorrow.

Scott.

[Paulporsche]

Scott,
What is your mixture setting?

[unclebilly]

I have't measured it yet but it should be 3%. I did richen it up a bit (1/4 turn) to get rid of the bouncing idle.