engine tuning

[fricker]

hey, guys
I require some advice about tuning my 1970 911e with mfi engine.
It idles at 1100, it revs up nice and fast and smooth except for a short burst of back pressure through the throttle bodies!
Is my engine running too lean? any suggestions as to what it is and what adjustment might help.

Thanks
Chris

[stlrj]

Too lean? That's very unusual since they all tend to run very rich.

[Rot 911]

MFI's are difficult to get into perfect adjustments. Your's sure sounds close enough that I am not sure I would fool with it. A bit of popping in the stacks does indeed indicate a slight lean condition. Look at your plugs and see what they may be telling you. Your idle is also high. Need to get it down in the 750-800 rpm range.

Before you do anything to your MFI go to this link and read up! http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm

[Grady Clay]

Chris,
Welcome to the Forum.
You will find lots of help here.

Basically you want to do an extended Check, Measure, Adjust (CMA), focusing on the Check and Measure. There is a lot about this in this Forum’s archives. I recommend you search on the keywords ‘MFI’, ‘CMA’, ‘CMA2’ and more. Edit and save the results in your own personal MFI file (both electronic and paper). You will need it.

With your engine running reasonably well, now is the time to measure the cylinder leakage, cranking compression, ignition timing, MFI pump timing, cam timing, fuel pressure & flow and do a valve adjust. A regular ‘tune up’ (spark plugs, plug connectors, wires, cap, rotor, filters, oil change, etc.) are in order. Basically all the items in CMA and more. Lastly is to measure the mixture under the varying conditions (idle, part throttle, full load). This will establish a baseline for any possible future adjustments.

Adjusting the MFI makes sense only if you absolutely know that all the underlying machinery and state of tune is correct.

Best,
Grady

[Flieger]

I have a 1971 911S with a 2.7 liter MFI RS spec engine.

I have no thermostat connection and I think my secondary warm-up injectors do not work. (not a problem in Southern California) I also removed my plenum chamber.

Mine makes lots of pops through the intake, even on the nice warm days. It will backfire occasionally when cold but it pops in many combinations of temperature. I only noticed the poping when I removed the plenum.

The poping mainly happens after I am on the throttle hard and let the revs get into the 6000 range (right when the S cams start to really work), then lift off and compression brake. The car seems to like the high rpm blasts because it will start to go "pop-pop-tick-pop-(repeat)" when at low revs. I think this is just a normal symptom of the high-overlap S cams suffering from intake flow reversion at low revs. The ram air and short breaths at speed clean it up.

It doesn't seem to harm anything and it breaths well at the high, hard revs. I think the pop-ticking is kind of fun. It sounds more like a mean race car.

[fricker]

Thanks for the info. I've done a compression test-OK, valve check-OK, mfi timing check- OK. New plugs... I'll read the info from here to check the rest. I only get the popping when I rev the engine hard, its fine when i smoothly hit the pedal.

Chris