valves adjusted...idle characteristics changed?

[bigchillcar]

well,
another twist for me to figure out...think i'll cheat and ask for opinions. just got my '74 911 back from the wrench after having had the valves adjusted. notice that my idle characteristics have changed. it used to that the idle speed on the car was generally higher when cold or warm. in particular, the idle speed always seemed to increase a little as the engine temp. increased...usually on the order of 1000 up to a max. of about 1500 when very warm. i've been told that air-cooled engines are like this. anyway, now the idle seems to stay pretty much around 1000 no matter what. when shifting, as i let the rpm's build, say to 3-4000, then engage the clutch to shift, the rpm's seem to 'drop' quicker than before until i release the clutch, making the transition lto the next higher gear somehow less fluid. the hand throttle seems to have a smaller effective range during warm-up. what's going on here? maybe the better question should be...what should my idle speed be cold and warm? thanks for your feedback.

side note...is it possible to remove all 'tapping' by doing a vavlve adjustment? mine are clearly better, but as it reaches operating temp., i'm sure i can still perceive that damn tap a little!


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[Jdub]

Yes, there is a direct connection between idle consistency and valve jobs, but as to "pulling" idle down, etc. that is another matter I would think.

Tight valves give you a lumpy idle die to the inconsistent clearances between each cylinder's valve set. Perfect adjustment gives you either a smooth idle, or just removes one possibility of *why* your idle is bad...could be many other things especially when we are talking CIS.

Now as to the idle pulling down fast between revs: what else was done to the car? I was able to stop a "floating" idle by working the idle decleration valve down. Any chance a wrench had a go at that? Did they unstick a sticky distributor? A bit of oil on that felt pad can do wonders in that respect.

So no, I don't think the valve job had anything to do with the idle pull. That's my humble yet esteemed opinion.

John

[Tyson Schmidt]

CIS is very sensitive to manifold pressure. Valve lash has an effect on manifold pressure, as does ignition timing. This is why you need to reset ignition timing and fuel mixture in kind of a back-and-forth manner.
You will likely need to reset timing, fuel mixture, and idle speed.

[bigchillcar]

tyson &jdub,

thanks for your replies on this...just spoke to wrench. he says he did adjust the idle as necessary, agreeing about the effect of valve lash on mainfold pressure and timing, etc. he says the book mentions a range of 750-950, depending on whether air is installed; i have air, so he set it at about 1000. at any rate, the idle is definitely smooth...guess it wasn't idling right due to the valves being so loose before, as he observed when he got inside it.

jdub, what do you mean exactly by a 'floating idle'? is that what i was talking about? the spped at which the rpm's drop when the clutch is engaged between shifts? it would seem that if it drops too fast, performance would be adversely affected...not fast enough and it would 'surge' a little. guess there is an otimum rate for the engine speed to drop at in between shifts? how is that determined?

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[Jdub]

I am going to defer to Tyson on this matter because he knows loads more than I do on the matter - in fact I just learned something reading his post that explains my over-rich condition (the car, not me!).

By a "floating" idle this is what happened to me. I would find that, after highway miles, I would pull off the exit ramp and find that the idle would remain high, around 1600 rpm. I'd engage 1st to stall the engine a bit and pull the revs down (very embarassing). With the help of this 'board I discovered an emissions device known as the Deceleration valve. Located directly behind the throttle body, just to the left of the air filter cover. Three hoses in, pinch the top when high idle and note change, if any. It pulled my idle down, so I went ahead and unscrewed the lock bolt on the top of this device and turned the threaded post in (clockwise). Result was that car would pull nice and smartly down to idle after highway miles, between stop signs, etc.

However, I'd leave that puppy alone and set your base as per Tyson's humble yet esteemed opinion. Get it set to basics and you know you've removed questionables from the equation.

John

[Early_S_Man]

Hey, chill!

The '74 spec book says 900 rpm +/- 50 rpm for manuals, and 950 rpm +/- 50 rpm for sportomatics ... nothing about A/C ...

BTW, 'REAL QUIET' valves are an invitation to burnt valves!

[bigchillcar]

early_s,

glad to hear that about the valves...now i can tell my brother to shut up ("i think i can still hear some tapping"). this, of course, after i've spent $250 on the job...gonna tell him 'they're supposed to sound this way'...anyway, its much nicer, at least 1 75% improvement over the clatter i used to hear.

didn't mean to misquote the manual...wrench was generalizing; not speaking from the book.

jdub,
thanks for sharing that piece...all part of the process of 'getting in tune with your car', eh early_s?

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[Jdub]

No problem. I guess you have figured out the "reverse engineering" part of decoding my post: that you would have had to unscrew the decel valve to free up the float of the idle.

Without a doubt the happy sound of clicking valves was something I was taught from day one; from a CBX out to the classic Mercedes straight six 2.8 (an easier valve adjustment I have never performed, except maybe a BMW /5 boxer engine). Those Mercedes engines, God bless 'em, have the greatest sound when all is well; clacking little feet tapping like an octopus in Fred Astaire's shoes.

However, when I got the Pcar I was told silence was golden. I still hear that nowadays.

Why don't you collect all the bits and pieces you will need for your next valve adjustment as they go on sale and do this yourself? Lost of information via the Search button. It is a very rewarding experience!

John

[bigchillcar]

john,

yeah, i want to give it a try. i've got a friend in town with a '76 who does his own and plan to get together with him soon to look and learn. since the car is my daily driver, i gotta be sure of what i'm doing before i find myself bumming rides until the mess is sorted out, though!

silence is golden...that's kind of what i thought as well, but sounds like, from early_s, that too much may be 'too much'. says that that can lead to burnt valves; maybe that's from making them too tight. guess it would be nice to hear exactly how the car is supposed to sound; being 28 years old, i gotta believe that some chatter, even a little, should be expected, short of a complete engine overhaul to specs. it is enjoyable, i think, to own a classic car in spite of the extra planning involved in their operation. people without the patience for quirks and occasional unpredictability need not apply!

ryan

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