CIS low rpm flutter?

[Jay Laifman]

On my 73.5 911T, it runs beautifully above 3,000 rpm. It also runs beautifully at all RPMs under any load, including full throttle. But when I'm cruising along in 4th at about 45 mph and 2,900 rpm and below, the engine is not glass smooth. It wants to flutter a little bit. If I push it up to about 3,100 rpm and about 50 mph, it seems to smooth out. Also, 45 mph and 2,900 up any incline (ie load) seems to smooth out too. Idle is pretty decent. It seems to have an extra flutter or beat in there. But nothing overly bad. I wouldn't say oscillating either.

My suspicion is that the CO level is off for idle. I have the long allen key tool to adjust it. I'm thinking of going to my local smog shop and getting a CO reading. As I've read, it seems the early CIS should have a 2.5 reading, while the newer cars are at 1.5.

I could just give the adjustment a slight turn each way and see if I get any improvement. And put it back to where it was if I don't - or then go to the smog shop for the CO reading if I do get improvement. But I have no idea how sensitive the adjustment is.

Does anyone have any other thoughts of what can be causing this?

Also, if it is the CO adjustment, is there any reason with today's gas to go up or down on the 2.5?

The engine is relatively recently rebuilt. I did check the pressures not that long ago. But I don't drive it that much anymore. So I can't swear that this issue hasn't be around for a long time. I did do a valve adjustment, and replace filters, plugs, rotor, cap, etc., not that long ago too. It did not cause or fix it.

Thanks for any thoughts,

Jay

[savaden]

Adjusting the idle mix (CO) on CIS is very easy. First, remove the intake boot and make sure the air flow plate is centered in the opening. You can loosen the 10mm screw and move the plate till it is centered. Next replace the boot and start the engine. At idle adjust the 3mm screw for best idle speed. The tool adds a bit of weight to the arm, so you should compensate for that by removing the tool and make sure the idle speed does not change. You're done!

You could also have vacuum leaks at the injector seals or bad spray pattern from the injectors. You can observe the spray pattern of the injector by placing it into a clear vial or beaker and raising the air flow plate to simulate air flow (with the engine off). Or take them to a shop that can perform the test.

SV

[Frank Beck]

I would not be so quick to assume mixture.
Don't rule out ignition. Points closing up a bit or a good old fashioned misfire are possibilities.
Always start with basics: Dwell, timing, CO.

[Jay Laifman]

Thanks Frank. I'm comfortable that dwell and timing are not factors.

But, I'm similarly NOT comfortable that this adjustment does anything other than strictly idle? However, I understood that this adjustment does change CO - at idle at least.

My mind is used to adjusting my Webers - where the "idle jet" covers the RPM range up to around 2,500 rpm. So I'm sort of thinking this "idle" mixture adjustment would similarly impact more than idle. But if it is only idle, then it's the wrong tree!

It very well could be a leak in a hose somewhere. I am going to try to chase that down too.

[ESully]

From what I read, CO originaly should be 1.5-2.0 for the '73, 1.5- 2.5 shown for the '74.
Interesting question regarding todays gas. With the alcohol in todays gas, I would think initial mixture would need to be slightly richer from old specifications.
Frank makes a good point of checking the basics first. Check to see what the spark plugs look like to see if there is any indication of a problem with a particular cylinder also.
As savadem wrote, especialy since it is a fresh rebuild, make sure all the intake runners and cis injector seals are good.
The interesting part of the 73 and 74 CIS is the throttle valve position sensor. Later CIS used vauum to the wur, but the first couple of years the valve on the throttle body helped adjust the mixture, slightly leaning it out at mid throttle cruising rpm, slightly richer at high rpm. When checking the control pressure this needs to be checked to make sure it is working properly along with the wur.

[Jay Laifman]

Well, I gave it a 1/8th turn clockwise to richen it before driving home. It sure seemed to smooth it all out. I was trying hard to find it flutter. Maybe it did a tiny bit - but I'm not sure. Tomorrow before driving to work, I might give it another 1/8th turn and see what happens. I can always go back an 1/8th if I don't get any better - - - if there is any better to get even!

Thanks everyone.

[Harvey Weidman]

I would not be so quick to assume mixture.
Don't rule out ignition. Points closing up a bit or a good old fashioned misfire are possibilities.
Always start with basics: Dwell, timing, CO.

+1
We would check for closing points then turn 1/8 rich..and repeat if necessary.
If the surging is recent and the points have enough gap then, look for leaks.
Leaking after the flap will cause a lean condition and surging at low speeds.
H

[Jay Laifman]

FWIW, I have had this car for 27 years and have kept it very well maintained (first sold at Neillo), still sporting its original factory Viper Green paint and cork interior. I actually did a full tune up within the last few months. So I'm comfortable that the timing, plugs, dwell, filters, and valve adjustments are all good. The rubber connectors to the air box were all replaced a few years ago. That's no guaranty that they are still good. I DEFINITELY plan on spraying them (and elsewhere) and seeing if I get a kick in rpm. I have tested the fuel pressure from time to time over the years when tracking down issues. At one point I did have a bad WUR. I checked the pressures probably about 2 years ago. I'll do it again. When the WUR went bad, I did focus on the TPS too. If I recall correctly (though it's been a while), I think I learned at the time that the "idea" of the TPS is one thing, but the way it worked was not all that nuanced - but really more open and closed. But, again, I could be forgetting that one, and I will check it again.

[RSTarga]

"comfortable" is not the same as certain. Things happen. Slap a dwell meter on it, then timing light before going to mixture.

[Frank Beck]

I do muse at these threads as it reminds me of what we battle constantly in the biz. We had a call the other day from a guy who wanted to know what the oil leak repair was going to cost on his 993. No other info; that was it. I told my office staff to give him an estimate of $10k for a topend rebuild and we'll work down from there if that isn't necessary. (Since they're much more diplomatic than me they simply scheduled an inspection/estimate appointment. Turned out to be chain housing and valve cover gaskets.)

My point is how do you "fix" anything without seeing it or listening to it? I would venture to a say a "flutter" could mean 100 different things to 10 different people. Without knowing a bunch of data points this is an exercise akin to one hand clapping or pissing up a rope. Personally I enjoy both those activities but the lesson here is the more precise language one can use along with industry accepted definitions (maybe your "flutter" is actually a mid range hesitation?), the better the chances are that we could hit a target at such a great distance during pitch-black darkness. What would be even better is to fill in some of the boxes on a proper troubleshooting procedure.

In any event I'm curious to hear what the final fix is. Good luck.