How smooth does your MFI engine run?
 

How smooth does your MFI engine run.

My engine pulls strong etc. It has maybe 10,000 miles on a professional rebuild, never fails to start, never dies, but at idle it will miss every couple of seconds. It has optical ignition and is timed properly etc, but as this is the only 911 and only MFI car I've ever had, I don't know what normal is. Or maybe I do and this is it - which is what I suspect. Am I expecting too much that it never miss - what with it being 42 years old and is mechanically injected to boot???

A very nebulous/non-scientific question I know - but I'm curious for your seat-of-the-pants experience. Thanks

Owned a MFI for 12 years and it always ran like a clock. Never missed, surged or otherwise misbehaved. What is happening is not normal. Don't have the experience to diagnose though. Good luck finding a solution and enjoying a well running. MFI car

Mfi
 

Hi Daepp.

My 73 T, MFI Is very smooth, powerful, fun to drive. No missing or popping. During the restoration, I had the MFI pump rebuilt to E spec by Gus @ Pacific Injection. I had Matt Blast in Boston rebuild my T throttle bodies to E spec. I found a set of E stacks here on Pelican. It's been rock solid.

1973 911 T MFI Coupe, Aubergine

Stevehttp://forums.pelicanparts.com/uploa...1402889798.jpg

Mine runs butter smooth. Sounds like faulty fuel injector. Did you check or replace yours?

Are you running a stock Bosch CDI box?

What everyone says above, mine will idle <400 RPM and you can just about hear every cylinder fire, 42 years old and never been touched. Amazing.

If your butterflies are org. then it's going to pop/fart once in a while. Also, if you have a franken motor where one has different cams, cylinders, etc then it's tough to get just right.

What is a "fraken motor" never heard that term!

'franken' motor, as in Frankenstein.

This Franken Motor starts like a street car, purrs like a kitten at idle, and screams like a demon when it come on cam.

http://i327.photobucket.com/albums/k...7a2899d396.jpg

"lightning illuminated the object and discovered its shape plainly to me; its gigantic stature, and the deformity of its aspect, more hideous than belongs to humanity, instantly informed me that it was the wretch, the filthy demon to whom I had given life."

that 'splains it.........I just did not know what "Fraken" might be.

IS THIS A "Fraken" motor?
 

this might be a Fraken motor.

It has an overhead fanshaft!!

http://forums.pelicanparts.com/uploa...1402942543.jpg

My car is not like the ones above, but if you take solex cams, rs pistons, T stacks and heads, E pump, mate it to a 75 tranny with an exhaust you built yourself, you my friend have what is termed a franken-motor. :p

Maybe you're referring to "Franken" as in Frankenstein rather than "fraken? Autocorrect isn't going to help with this one.

Whooops, you are correct my friend, corrections have been made, reading glasses used.

Love your car, Steve!

My old aubergine 73E w/ 2.7 RS-spec engine was smooth at idle, as well. My throttle bodies, stack and linkage were all rebuild to S/RS specs by Matt Blast. I ran a Pertronix ignitor with an MSD box. My dizzy was new, too.

Thanks for the input.

The car has the numbers matching case, original T MFI and original TB's - not rebuilt. Pump has never been into. It case DC60 cams, JE 9.0 pistons and used S cylinders. It was professionally put together by Supertec. And AFAIK the injectors are original and "unrestored" or simply removed and re installed.

It has always run strong, rich, and misses every once in a while, but not all the time like a bad wire/plug. It has plenty of power.

Henry offered to rebuild the entire MFI but I was running out of $$. Since the optical ignition was installed it has been so trouble free that I have just lived with it, but it has always had this "miss". When it's happening, it might miss once every 15 seconds. Perhaps it IS an injector - can they be serviced? And what tells them to open?

Is it miss, or is it a mini back fire through one of the stacks, kind of sounds like it spits?

If only I could get my Webers to run that good...

On decel in say second gear I'll get an occasional pop that I always believed was from worn TB's, but this is a miss at idle - like at a stop light. They are not rhythmic - just every so often, 3 or 4 times a minute.

How old are your plug wires? Mine did this just tad, decreased timing and this fixed it.

Daepp, I would try to rule out any possible ignition issues. Over the years, I have had some funny idle and part throttle misses from one brand or another of spark plugs, so I might do a spark plug change. Also, I would check the resistance of all your plug and coil wires from terminal to terminal. A couple of years ago a friend of mine had a funny miss on his 2.7, turned out to be a bad connection at one of the spark plug terminals.

Pressure opens the injector, the mfi cam pushes the fuel out in time with the cams.

Could be running too rich at idle and low speed. They like to be rich to help get rid of the flat spot at about 1/4-1/2 throttle. Rich at idle and above 6% c/o will cause a very definate miss. Have a shop sniff it with an exhaust gas analiser to see how rich it is. Lean it down as needed but the flat spot may show up. Also make sure the thermostat on the pump is getting hot air and shutting off. Mark Kinninger