Idle bounce/hunting

[ClickClickBoom]

Hello,
I have a 1975 911S Calif spec. Since acquiring the beast I have been chasing small gremlins away with wrenches and voodoo.
My latest gremlin became evident when I replaced my stock airbox with a Webb Industries SS airbox and intake gaskets as well as the rubber couplers. The idle was steady when cold, but after warming up became bouncy/hunting at aprox 1000RPMs +/- 500RPMs and devolving to a stall if the throttle wasn't blipped.
I did the starting fluid trick, looking for the elusive vacuum leak. I couldn't get so much as a single RPM using most of a can of starting ether. I thought it was water, a quick spritz into the intake proved that theory wrong, real stuff. I checked the decell valve on the bench, it is fluid and air tight. I reset it to open and add air at aprox 20" of vacuum. I was getting puzzled, I have tuned motorcycles, airplanes and autos, both diesel and gas. I thought I was being tricked by Mr Bosch and those Teutonic wizards with black magic and evil spells.
I thought about it for awhile and scratched my head, I went back to basics, air/fuel, and spark all in the right proportions with a little timing and the sucker should be ready for the Autobahn right?
I back tracked to the last things done, airbox, intake runner gaskets, and rubber couplers all air tight, right!
This process started long before I got the car. The folks who had the car prior to me did the basics, but deferred anything beyond easy(cheap) fixes. The car is in pretty good condition for a 35+ year old car, few rust bubbles and the engine had been basically left alone for the last 15 years. The couple did take it to their local mechanic for "tune-ups" and basics. I am assuming that after replacing the plugs and oil, the mechanic put the CO detector up the tail pipe and adjusted the mixture to get the proper mfg spec. It would seem all those loose vacuum hoses and gaskets added a little extra air every year, and the mechanic dutifully adjusted the CO accordingly.
Here comes Dufus(me), who decides that the Webb Airbox was too cool for school and must be installed.
The SSIs I had installed last winter worked wonderfully along with the Leistritz muffler 2 in, one 3" out. The gremlin became more profound as the weather heated up.
The answer to the question of the ages! It would seem that as the mixture richened up over the years the little P337 tool mixture screw had been adjusted way out of spec for an airtight system. I was chasing in the wrong direction. Thanks to a previous poster, in another post replied that its too rich and explained why. I went out and broke out my 3MM allen wrench and schwacked it in the lean direction, almost 1/2 of a turn. I started the car and the idle was at 1500RPMs, now we are talking, even I know that a stoch mix will cause increased RPMs in a static system. I backed the idle bypass back down and gor a rock solid 400 RPM idle, blipped the throttle and sucker came right back to 400RPMs with a slight delay thanks to the decell valve. I adjusted the idle up to 700RPMs and it was schweet.
I took it out to the local hill climb and hit 2nd gear full throttle and got to 6000RPMs and a slight detonation(1/2 sec). Stopped and adjusted the mixture screw about .020", more pressure than any real movement on the allen key. Did another run to 6250 RPMs no detonation, stopped and did another adjust in the rich direction, just till the RPMs dropped 25RPMs or so, more a hearing thing rather than measured. Did another run in 3rd gear, 6250 RPMs and one pissed off 2.7L. The howl was unreal. Did another 2nd gear run and coasted to the shop, pulled #1 plug and it was a nice chocolate mocha brown. I will do an official plug chop when I get some new plugs and do a lap on the HWY 9 F1 course.
I guess this write up is 2 fold, one, these cars/engines are machines, and respond to basics. The second thing is, is it supposto be this simple?
Hola
eric
P.S. I had a rock steady idle at 250RPMs, with oil at operating temp, showing 20PSI. Any harm with the idle at 700RPMs it is so happy there, I don"t want to throw a cam chain or anything stupid(I do have the hydraulic tensioner upgrade)?

[T77911S]

nice. thats the joy of CIS, one adjustment, covers all RPM's. when you get a feel for it, you can adjust it by ear/trial and error.

[d.a.autry]

Okay. . .Here's another one of my stupid questions.

Exactly how does one know when you're getting detonation?? What does it sound/feel like??

[Vincent Hill]

Originally Posted by d.a.autry

Okay. . .Here's another one of my stupid questions.

Exactly how does one know when you're getting detonation?? What does it sound/feel like??

If this is a real question, I will bite. You hear the Pinging in the engine, SOrt of like a Rattle of Hundreds of small Hammers hitting on a Can! What is actually happening is the flame front from the spark plug is meeting other areas of the cylinder where combustion has already occured from things like Carbon glowing or the heating of the piston Dome. A little retard of the timing is the usual way to reduce this. A lot of people "Tune" their engines for the grade of fuel they use byt either advancing or Retarding the timing to avoid knocking.

[d.a.autry]

Originally Posted by Vincent Hill

If this is a real question, I will bite.

Oh. . .it was a real question alright.

I know what detonation is. I know it's a big issue with a charged (turbo, etc.) engine. I know that you can reduce it by higher octanes, retarded timing, even by injecting water in the old radials of past etc., etc.. . .

But I've never heard it. I'd be absolutely clueless unless it was a very detectable, distinct and noticable change in the engines normal sound.

Is there any detectable performace decrease that is noticable when this happens?? IE, power loss in a full throttle pull??

[ClickClickBoom]

Originally Posted by d.a.autry

Oh. . .it was a real question alright.

I know what detonation is. I know it's a big issue with a charged (turbo, etc.) engine. I know that you can reduce it by higher octanes, retarded timing, even by injecting water in the old radials of past etc., etc.. . .

But I've never heard it. I'd be absolutely clueless unless it was a very detectable, distinct and noticable change in the engines normal sound.

Is there any detectable performace decrease that is noticable when this happens?? IE, power loss in a full throttle pull??

Hello,
Once you hear it, the sound will never be missed again. Think like you are under a tin roof, and some one dumps a shovel full of gravel from 10 ft up. As soon as the detonation hits an abrupt loss of power occurs. Light detonation is easy to miss, not much performance degradation but the ensuing wreckage to be is building up.
Detonation is different than preignition, detonation is the fuel/air mixture, well detonating as opposed to a burning over time.
Pre-Ignition is usually from a piece of carbon, metal, or even wrong heat range spark plug.
Google/wiklipedia can explain better than I can.
eric
P.S. Put a couple hundred miles in over the last couple days. It is amazing how well the little 2.7 runs with the proper mixture, its like flying a piston engine a/c at altitude without proper leaning. It seems that every time I do a little thing to bring it back to stock spec, it just keeps getting better.

[ClickClickBoom]

Update,
Got gas and checked mileage 26.8 average over almost 18 gallons, mind you, this is at sea level and driving like it's stolen 10% of the time. I was averaging 22-23 with the same driving mix. I also did the sensor plate lift/depress trick, movement of equal distance in either direction causes an equal drop in RPMs.
Nirvana
Eric

[d.a.autry]

Actually took mine through emissions the other day.

I got a '76 completely backdated on exhaust, no air, no EGR, etc. I attached my vacuum retard and backed the ignition down (I've noticed that with my dist. I have to back off to app. 10 BTDC for proper curve). ANYway, our limits are HC 600ppm and CO 3.5%. I had moved things rich previous and failed my first time for high CO. I pulled out of the bay, brought it lean by about a 1/8 of a turn, took it right back through and hit my numbers. I whis I had kept the original #'s.

You're right about bringing these things into stock tune too. The driveability goes WAY up. You may be able to coax a pony or two out with an advanced timing, different mixture, etc. But end the end it simply runs much better if you keep it in spec.