Early 911 engine/idle sound.

H.G.P. · 04:54 PM

Well, I'm back to driving 911E like crazy. Only two (possible) kinks to work out.

1. Some drips from the lock nuts on the lower right valve cover. (three of the lowest nuts). But I'm not concerned, as I've had the cover off several times to re-check valve clearances, and have attempted to re-use the only gasket I have. So I figure I need a new gasket, I get next oil change.

2. But here's the problem (maybe just with me). The car, after starting and 5 minute warm-up, idles fine, right at the 900-950. It holds the idle and does not stall, nor does it hesitate. However, it definitely at idle has the acoustics of a miss.

It accelerates fine, shifts nicely through all the gears, and at higher RPMS the acoustics are just a smooth powerful sounding engine with no backfire, no hesitation, and no missing or any backfire on de-acceleration. (best it's been since owning it)

It's only at the idle speed, it sounds accoustically like a miss. A non-Porsche friend at work today noticed the same, without me even mentioning it. He said "why does it sound like it's missing at idle?" Is this a symptom of a problem? Should I take action here to avert a possible future problem? Valves are adjusted, and cams set right at 3:15 L and R. (One other thing, if I try to get going before the 3-5 minute warm-up to reach 900 constant idle, if I try to shift and go it will stall)

(Plugs are new, cap is new, points are new and gapped, rotor is new, spark plug wires are almost new, valves and heads new.)

Thanks

Grady Clay · 03-29-2005, 05:03 PM

It should run perfectly at idle, even when set very lean. The only noise you should hear is the “chirp” of the nozzles. Is it set rich or lean? That said; take it by a shop that has an ignition scope. Perhaps you can isolate it to one cylinder. The next step is to summarily replace the sparkplugs. When the engine acts like this it is usually an ignition issue but can also be a too lean or too rich issue, perhaps only on a single cylinder. Injector nozzle performance is critical here.

How long did it sit? Under what conditions? What preservation did you do prior to its rest? Have you added any fuel additives since it went back into service? Does it have fresh gasoline?

Please report back.

Best,
Grady

ed martin · 03-29-2005, 06:58 PM

Not to second guess Grady, but what about air leaks? I have a 73 CIS and I had problems with missing and power. I replaced all the intake boots, gaskets and fixed a leaking air box and now it performs beautifully. You don't have CIS obviously but an air leak in any system can give you problems.

H.G.P. · 03-30-2005, 04:45 PM

Listened very closely today at idle, and it appears it's not missing at idle now. Just a little missing on the very start off of first. So this leads me to believe I need to go back and check the carb mixtures. (I'm using the plugs you recommended in a past thread Grady) and thus I think the fuel is being more efficiently used and thus the mixture needs some tweeking.

The exciting thing is no smoke from the exhaust and no smell of fuel from the exhaust or the oil cap. What a change after a rebuild.

jluetjen · jluetjen's Garage

Does your car have the original MFI? How tight are the throttle shafts?

H.G.P. · 06:13 PM

Quote:

Originally posted by jluetjen
Does your car have the original MFI? How tight are the throttle shafts?

I have some of the original MFI peripheral hook ups etc.,, but I'm running Zeniths. I actually like the Zeniths and find them very responsive. So I'm hoping a slight more adjusting of the individual mixture screw(s) will solve my problem (if it is one).

jluetjen · jluetjen's Garage

The throttle shaft question still applies -- how tight are they?

Jeff Higgins · 03-31-2005, 06:03 AM

Do Zenith carbs need to be synchronized in the same way as MFI throttle bodies? If so, have you checked them? My MFI set-up missed and popped a bit at idle until I synchronized them as a part of the CMA procedure. It would clear right up as soon as you gave it some throttle, which it sounds like yours is doing.

john walker's workshop · 03-31-2005, 07:12 AM

zeniths can be a bit wierd until you spend a lot of time with them. what size idle jets are in them? the original size is stamped on the end of the jet. g.50, g.55. etc. they can also be drilled larger, so a jet gauge is needed to be sure. could be that the mixture adjustment is still a bit off.

H.G.P. · 03-31-2005, 08:49 AM

Quote:

Originally posted by jluetjen
The throttle shaft question still applies -- how tight are they?

I don't see these labeled on p#61 Haynes, but I assume you mean the long shafts parallel with the carbs. They seem to me firm, not real loose, not too tight. Respond without catching.

Quote:

[i]john walker's workshop zeniths can be a bit wierd until you spend a lot of time with them. what size idle jets are in them? the original size is stamped on the end of the jet. g.50, g.55. etc. they can also be drilled larger, so a jet gauge is needed to be sure. could be that the mixture adjustment is still a bit off. [/B]

I don't see any stamp on the screw head ends. do I need to remove them and look on the other end?

jluetjen · jluetjen's Garage

Actually the trick is to move the ends of the shafts up and down while the car is idling. Does the engine miss or change while you wiggle the shafts? You're trying to identify if there are air-leaks between the shafts and the bushings.