Running Rich

Hi Guys,

If you take a look at the image below you will see I've circled an area of my rear bumper that’s perpetually stained with the soot from unburned fuel; I have to clean it off with a household grade dirt & grease remover every time I wash the car.




It's quite clear the car is running rich, but it's not just after a cold start; the car puffs out dark fuel clouds when I stomp it in either 1st or 2nd. The engine (911/83 MFI 2.7 with 10.5:1 Aries pistons) has been tuned to run on the highest grade fuel available in Australia - Shell Optimax, (98 Octane), and has been like this ever since I bought it.

I treated the engine to a major service 1,000kms ago, and I just assume it's meant to run rich, as no adjustment was made while it was in the shop - the same reputed shop it has been sent to by the previous owner for the last seven years.

What do some of you more technically inclined chaps think? Should I take it back in to have it re-checked, just in case it was overlooked?


Matt Holcomb

Hmmm...you know what, if you've got a 2.7 10.5 C/R MFI car running well and the only worry is a little black soot on your bumper...who cares!!! Seriously though, it is pretty common (normal???) for MFI cars to run rich and especially with your high C/R you do not want to go too lean. Has your MFI pump ever been rebuilt and/or known to be in good working order? Any running problems? Terrible gas mileage?

PS -- any pics of the whole car? Show her off, Matt, we wanna see!

Chris,

Yeah, lean would be bad! This is my first MFI 911, so I wasn't quite sure, even though I've heard these engines do run rich, although HOW rich is rich is the question I wanted answered, which you have done.

As far as I know, the MFI pump was not rebuilt by the previous owner who babied the car for seven years. The owner before him, the second owner, used to race it, so I'm sure it was rebuilt, if needed, while in his care.

It starts fine and idles as smoothly as it can with those pistons, although I must say, the gas mileage is pretty ordinary, certainly compared to my previous CIS 2.7. I'm not sure if that has anything to do with my liberal use of the throttle and those subsequent smoke screens I occasionally leave behind me, or something else, something sinister, such as a dying pump. Wouldn't average gas mileage have to be accompanied by other symptoms to indicate a dying pump?

You want more pics?

Then head on over to:
http://www.holscope.com/74Carrera

Thanks,

Matt H.

Matt - my car does the same thing ('78SC w/CIS) on purpose. The car runs better than good and gets 25mpg @70mph. Rich is not a bad thing unless your mileage is bad or you live in one of the emission prick states. I use bug and tar remover a couple of times a month to clean the areas. Some say that a buildup of carbon will occur on the valves if you run rich, I figure if the "cobs" are blown out every now and then it should be OK.

Rich running can heat up and overwork the CAT. I have a CAT that is fairly new. I'm considering the bypass pipe, but what I'd really like to do is go to the SSI and a cool final exhaust. I'd like to be able to hear my engine better. It runs like a top but is too quiet.

I digress. the point is that today's cars are set up TOO LEAN. This in not for your motor. Your motor would prefer more fuel and will run better and cooler with a richer mixture.

If I had a car with a compression ratio over 10:1, I would definitely set it on the rich side, scared to death of what a lean mixture might do.

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'83 SC

My 74 cis runs a bit rich, and it used to get my bumpers dark like yours. At the end of the tail pipe there is a cylinder that fits in the pipe, this cylinder can move in and out, pull yours out a bit ( with a rubber hammer) and the problem is solved. It worked on mine.

Matt- I think the above posts are correct. MFI systems do seem to run a little rich. If you read all the MFI data on the Pelican tech site you'll see more evidence of this. I have the same tell tale black bumper soot after a while of running occasionally, but the TE's were supposed to run a little 'fat' it seems- yours too probably. You may be able to go a little more lean on the main mixture screw (behind the allen headed bolt, CLOCKWISE) a click or two and see if you get less smoke and smoother running, but like they have said, don't keep it this way if you start to hear pinging or stuttering. Perhaps in your case with the high comp pistons a part load CO test would be worthwhile if you're worried. Check the thread titled 'CO Meter' for more info.

From what research i've done on the MFI pumps so far, they don't often need rebuilding in 9 out of 10 cases. It's sort of a last resort fix when you really have tried everything else.These pumps last. If the pump needs rebuilding they go SO rich that the engine will hardly run, and if the centrifugal governer breaks you can hear it clattering- so I don't think you need concern yourself with it in your case.
One thing worth noting is that sometimes fuel from a worn pump can contaminate the oil supply from leakage within the pump arond the finely machined interface at the plungers. I'm not sure how common this is, but it pays to check your oil every now and then just in case.
I trust your warm up thermostat and related plumbing (black ribbed pipe in engine bay, reinforced hose to heat exchanger underneath)is working correctly? You can check its operation through a small triangular observation window above the thermostat.

Although I'm normally a staunch environmentalist, all that goes out of the window when I put my key into that black plastic round bit and twist for a couple of seconds. Justify these feelings by the fact our cars don't need remanufacturing every ten years- and enjoy the soot.

BTW- still keeping those Ocker ricers at bay to good effect??

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'72 911 TE

Actually, Matt is a paid enviromentalist

Some things you just gotta make an exception for.

Cam

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Cameron Baudinet
1975 911S

Yeah, yeah you got me Cam.... But hey! You've got to earn a living somehow. If only the ocean's bounty extended so far as providing an unlimited supply of early Porsche parts in addition to diverse marine fauna and flora I would be a very happy man indeed........

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'72 911 TE

Thanks guys for your feedback, it seems as though the mixture is as it's supposed to be, post the addition of the high compression pistons. I can certainly put up with soot for it to run smoothly and coolly, which it definitely does; it hardly ever creeps past 185F, and that's after 30-45mins of hard charging while adopting the Porsche tester’s technique of keeping it in a lower gear than would be the norm. Having said that, I’m going to keep my eye on the fuel consumption, which is noticeably worse than my old CIS 2.7, although I have read that Carrera RS 2.7’s do like to drink.

As for my stance on dirty MFI 911's v The Environment, I seem to recall that ALL 911's use something like 11-12 quarts of oil, of which is changed routinely!

Thanks again,

Matt H.

P.S. Matt Smith - shame on you.

Matt H.,

I don't think you have a real 'too rich' problem until you are getting less than 12 mpg around town, or fouling plugs ... whichever affects your wallet sooner!

As far as the environmental issues go, a good friend of mine who has a Phd. in Ecology says a good, strong volcano like Mt. St. Helens or Penitubo (in the Philipines) makes human effects on greenhouse gasses look trivial ... though I have to question what govt.-dictated efflux of sulfuric acid (itself a SERIOUS greenhouse gas ... ask the planet Venus?) has had to accomplish, other than satisfy beaurocratic mandates for 25 years, and of course make smog alert days serious health threats for those suffering from lung diseases ...

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Warren Hall
1973 911S Targa

Hmmm, I guess it is a little ironic that I have a "protect our coast and oceans" license plate on my 911, which I park/maintain/wash a few yards from a storm drain that dumps right into the Santa Monica bay.

Almost makes me run out an buy an electric car(we actually do have a charging station outside of my office building!), or ride my bike everywhere

Naahhhh...

Hi Matt,

Nice homepage, even nicer car. Good to hear that there are more people out there happy with there 2.7!




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Joris
75 911S
http://members.xoom.com/joogie/Joogie.htm