82 SC Stumble

Daviboy · Daviboy's Garage

Hi guys,
I have been playing around with my air/fuel mixture and am experiencing a stumble/flat spot between, 3,500- 4,000 rpm. Seems like she is loosing power?Have backed off the mix and does not help, have richened up and still does not help. I need to set my timing at 4,000 rpm as I have so far only done it at idle. I believe for the 82 SC ROW the timing should be 26 deg BTDC at 4,000 rpm but please someone tell me if I am wrong. Does anyone think if I set the timing at load then reset idle and mix this may solve the problem?

Daviboy · Daviboy's Garage

Anyone??

psalt · 02-14-2009, 05:21 AM

Davy,

The ignition timing spec for a 82 ROW is "not above 25 BTDC @ 6000 rpm", not 26 BTDC @4000 rpm. You should always check the timing at high speed, idle timing is largely irrelevent and if your distributor springs are wrong, the idle reading may be false and retarded. You do not need to check it under load, which requires a dyno, just plug the vacuum line(s) and have someone manipulate the throttle while you read the light. The advance should be all in way before 6000 rpm. You should put a few drops of oil under the pad in the distributor and snap the rotor against the spring pressure before you start. Your flat spot could be many things, after you set the timing, check that you are really getting full throttle at the TB while someone presses the pedal. Then I would check compression and AFR under load with a wide band O2 sensor.

TroyN · 02-14-2009, 03:18 PM

Yup, AFR check would be good. Does your stumble occur under load, or even if you rev the engine standing still? And assuming that once you're past 4000, the engine runs smoothly?

Has it always run this way or is this a recent development? If recent, anything changed recently?

If not recent, what, if anything, have you tried changing, i.e. replaced plugs, checked for vacuum leaks, replaced fuel filter, etc.?

Need to figure out if it's fueling, ignition, or maybe both.

Daviboy · Daviboy's Garage

Thanks guys, I recently replaced the airbox as it was blown. Basically after that set timing at idle, and adjusted idle speed and mixture no O2 sensor though. The stumble only happens with load at about 3,500 rpm. Weird thing is that if I apply power very rapidly i.e. pedal to the floor, it stumbles then picks up above 4,000 rpm, and I get a bit of popping (too rich?) However if I apply power more smoothly and progressively it does not stumble? I am a bit confused now I thought the timing was checked with vacumm to dizzy disconnected?

sc_rufctr · 02-15-2009, 02:57 AM

With the new Airbox you probably accidental cured some vacuum leaks at the same time.

Which means your running rich now. I wouldn't play with the mixture screw any more. Once the mixture is right you then need to reset the idle speed which mucks up the mixture again

I think your cars idle should be set to 900rpm. Often older Porsches are set much lower than this by habit by mechanics that are used to Japanese compacts.

It may pay to take it to a mechanic with an O2 sensor and have the mixture and idle adjusted to spec. This shouldn't cost you any more than $100. Be fussy and choose a guy who is passionate about Porsches.

Hope it works out OK...

psalt · 02-15-2009, 04:31 AM

I am a bit confused now I thought the timing was checked with vacumm to dizzy disconnected?

Davy,

Yes, ignition timing is set with the vacuum lines disconnected and plugged. Vacuum controls should have zero effect under WOT. The reason your model spec is "not more than 25 BTDC @ 6000 rpm" is to prevent detonation. The 911 is knock limited and 25 is the max Porsche suggests with 98 RON fuel for your 9.8:1 compression. If your stumble is only under WOT, I doubt it is a vacuum leak, vacuum leaks have more effect at idle and cause a lean condition. A stumble under WOT, that does not happen at part throttle, indicates an acceleration enrichment problem. What caused the airbox to blow ( a lean condition ?), and was it corrected ? Did you replace the airbox with an identical part ?

Daviboy · Daviboy's Garage

Psalt,
The airbox was the identical replacement OEM part. I am going to tackle the timing tomorrow under load. Dont know why previous AB blew was PO issue. When I replaced the box the mix was so so rich due to I think the amount of air the motor was seeing. Did lean it right out till car died then richened up. Need to get her to a wrench with O2 sensor or get her dyno tuned, any comments on that?

TroyN · 02-16-2009, 09:28 AM

There's a procedure here [CIS Idle Speed and Mixture Setting Without an Analyzer that walks you through setting mixture w/o a meter, and I can confirm that it works pretty darn well. You can also get an inexpensive Gunson Gastester, I have one of these too.

The only way to check mixture under load is to install a wide-band O2 sensor setup (doubtful your shop will have one to borrow) or take it to a dyno.

I doubt it's a timing issue but you never know.

psalt · 02-16-2009, 09:32 AM

Need to get her to a wrench with O2 sensor or get her dyno tuned, any comments on that?

Davy,

Airboxes blow for a reason, and you can be 99% certain it was not " too rich" that caused it. If you do not have gauges, have someone check the control and system pressure and set the CO %. The factory spec is 1.0-2.0 %.