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. Idle is pretty good too. It seems to have an extra flutter or beat here and there. But nothing overly bad.

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. But I have no idea how sensitive the adjustment is.

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

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.

Thanks for any thoughts,

Jay

[T77911S]

always start with pressures before making adjustments. also check timing and any advance that it has. make sure that is all working.
mkae sure there are no air leaks.

me, i would adjust it, but then i can get it back to where it needs to be. since you were going to take it to a shop anyway, it may not hurt for you to adjust it. it does not take much. use the angle of the allen as a reference and maybe an 1/8 turn. i would try CW (richer) first.

[heliolps2]

Remember that the newer cars have cat's and air pumps to bring down the CO's or AFR's,

[Jay Laifman]

Do you happen to know if this adjustment does anything other than strictly idle?

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!

[Josh D]

Originally Posted by Jay Laifman

Do you happen to know if this adjustment does anything other than strictly idle?

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 does impact more than idle. The fuel curve of CIS is mechanically set by the shape of the metering plate plenum. Adjustments at idle will either lean or richen the A/F at idle and beyond as well. The only way to alter A/F seperate from idle is to alter the control pressure, and that's what the frequency valve and O2 sensor do on the '80-'83 US CIS.

[T77911S]

josh is correct.
this is whats cool about CIS. one adjustment effects the mixture across the range. bosch figured out the hard stuff. set the idle mixture and the rest should be good, assuming everything else is working properly. that is the case for what ever system you have on the car.

[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.

[T77911S]

if you dont check out the things i said above, just richening it up can cover up another problem.
i use to set mine by ear all the time. (still had an LM1 that i could verify the mixture which was always good). after making an adjustment i would blip the throttle and let the RPM's drop. i would make it richer until the RPM's dipped down then came back up to idle. the point just before theh RPM's dip down is where you want it. then i would let it idle for 5 or 10 minutes. as long as the RPM's did not start to surge, the mixture was good.

if you go another 1/8 turn and the idle is good, i would leave it.

[Jay Laifman]

Originally Posted by T77911S

if you dont check out the things i said above, just richening it up can cover up another problem.

Two thoughts. First, I have had this car for 27 years and it is very well maintained. 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 it 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.

Originally Posted by T77911S

i use to set mine by ear all the time. (still had an LM1 that i could verify the mixture which was always good). after making an adjustment i would blip the throttle and let the RPM's drop. i would make it richer until the RPM's dipped down then came back up to idle. the point just before theh RPM's dip down is where you want it. then i would let it idle for 5 or 10 minutes. as long as the RPM's did not start to surge, the mixture was good.

if you go another 1/8 turn and the idle is good, i would leave it.

I guess the question is if merely my initial 1/8th turn does the trick, and all appears great, than that would seem to suggest against a major issue elsewhere. But, I guess if there is a small issue elsewhere that I'm masking, I could have EVEN BETTER running and I will pursue it!

I did turn it another 1/8 before driving to work this morning. I can't say that it changed anything. I did look for the drop in RPM you mentioned. I don't seem to get it for a light blip of the throttle. If I get the RPM up pretty high, like over 3,500, and drop it, I can get a little RPM drop/bounce. But the Decel Valve seems to generally do its job and lets the RPM land smoothly. Is NO bounce correct, or if there is less than 100 rpm (as shown on the gauge), does that still show overrichness?

Thanks for your awesome suggestions and help.

[Jay Laifman]

They say a picture is worth 1,000 words. Here is a shot of my car (that's factory original paint, though changed turnsignals/grilles, mirrors and bumperettes).

[Josh D]

It's not uncommon to have to make seasonal adjustments to CIS. As components age, gas formulations change due to seasons, etc, small adjustments maybe necessary. Key is to make only small adjustments and analyze before making another. Documenting what you've done also helps if you stray to far from your baseline.

Heck, I have a tune for summer, and a seperate tune for the rest of the year where I change ignition timing and make CIS adjustments. I see it as part of the character of the car and I rather like it! It doesn't mean there is something wrong with the car, but having to deal with climate extremes that the CIS doesn't have the ability to compensate for on it's own.

BTW, nice car! Love the color!!

[T77911S]

nice looking car. i love the long hoods.

you are probably ok.

[Jay Laifman]

I have chased this for a long time. And there are more than one thread on it (and thank you to everyone who has helped along the way). Through the path, I have found things to fix that have certainly made improvements. But, in the end, every time, the flutter still seems to be there. I recently gave up and brought it to my mechanic who ran through everything again, and even checked cam timing and leak down, and redid smoke tests and things that I have done. Other than the light flutter, it is running fantastic. There is not a single other issue: cold start, warm start, hot start, idle, acceleration, low rpm through a parking lot without bucking. Just that dang flutter.

So I pulled out all my invoices over the years and now see when this all started: 2004. In 2004, I was getting a lot of smoke on decel. So I had a shop do a top end rebuild AND I got a set of "NOS" pistons and cylinders. It was right after that rebuild that the flutter started.

Since that time, I have done everything that might have been touched during that rebuild, EXCEPT anything about the pistons and cylinders.

But, I'm now remembering there was something off about those "NOS" cylinders. There were some markings in the bores, and they had to be cleaned up.

Given that everything else is good. Is there a possibility that those cylinders have something going on that are causing this flutter? Yes, the leak down was done and it showed great. But when a leak down is done, the piston is in one spot. Is it possible that the cylinders are out of round at some point, but not where the piston would be during a leak down test? And this out of round spot allows a loss of compression while in operation - just enough to create this flutter I keep chasing?

[PeteKz]

Do a compression test too. As you noted, in the leak-down test, the piston stays at the top of the cylinder. At least it goes through its stoke in the compression test.

[Jay Laifman]

Any suggestions on the type of adapter that is best for the 911?

I have a compression tester that is fine for cars with easy to reach spark plugs. But with these down in a hole, I don't know what would work best - and what risks getting stuck in there. Some of those little adapters seem ripe for falling out of the socket and being difficult to fish out.

I do have a magnetic socket for spark plugs. But I don't think that would help here.

Ah, looks like I found it: https://www.pelicanparts.com/More_Info/STOMSKISR031.htm?pn=STOMSKI-SR031&bc=c&SVSVSI=

[Jay Laifman]

FYI, my mechanic suggested instead that I bring it by and he do a horoscope and look at the cylinder walls. Next step...

[PeteKz]

If leaksown and compression tests are okay, the borescope wont reveal much.

[PeteKz]

I have several adapters, but without any adapters, my compression tester screws directly into the Porsche spark plug hole. You may need an adapter.

[Funracer]

Originally Posted by Jay Laifman

I have chased this for a long time.

I’ll say. Your persistence is noted.

Took me 10 years to find a low rpm miss on a 91 Corvette. You’ve got me beat.

Good luck.

[Jay Laifman]

FWIW, I got the Stomski adapter and did the compression test when hot, but admittedly I did not put in any lube. I got very even, but reading low. 102 for 5 cylinders, 105 for 1 cylinder. Yes, this reads low. But as I said it was dry. And on hindsight, my batteries might not be cranking has hard as possible. Still too low?

Note, this all started when I had the top end rebuilt and I replaced the pistons and cylinders with a "new" set I had picked up. This was 20+ years ago. I have since learned there are low compression versions of the P&Cs. I have no idea if I accidentally got one of those lower compression ones and didn't realize it. And I have no idea what the shop that installed them did about measurements. For all I know, they just slapped them on.

I believe higher compression is more likely to cause flutter problems. But I read that low compression can do it. Anyone agree/disagree?

That said, the car is running great otherwise, including hard acceleration all the way up to redline. It may flutter, but it doesn't remotely seem down on power. So I don't know that it really is low compression - and think my testing is not great. Note too that my mechanic recently did a leak down and it was great.

I want to drive that home, other than this flutter, the car is perfect. Hot, cold, starting, slow parking lot driving, hard acceleration, deceleration, above 3,300 rpm smooth, you name it. There isn't another issue. So do I trust my mechanic that says "this is just the way CIS cars are. Just enjoy."? I am frustrated because I know it did not used to do this.

Anyone have a great CIS expert in LA/Orange County area who might drive it and confirm? Or any 73.5 owners in So Cal who want to compare?

What about it being "California gas"? I'm tempted to buy some racing gas and have it shipped to me and seeing if running on that changes anything.