72 911T running rich - fuel in exhaust?

[mgoldleaf]

posted 06-03-2002 09:57
--------------------------------------------------------------------------------
I just purchased a 1972 911T w/ MFI and have enjoyed 450 mile weekend which included 2 runs on an autocross circuit. BUT...

1. I believe the car to be running rich

2. I see what looks like unburned fuel in the exhaust (under power)

3. I smell a small fuel leak in the car.

How hard is it to set the mixture and what special tools will I need. Any thoughts on the exhaust effluent? Is this fuel smell common?

Any help is appreciated. I am a recent convert from the Church of Lotus...so I am mechanically inclined and used to frequent maintenance.

Thanks for your time.

marc

[pwd72s]

http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm

Should answer your questions...

[304065]

Welcome to the Porsche Community. Please deposit your oil-soaked coveralls in the fireproof canister and help yourself to a Beck's: it will ease the transition to understanding MFI, the best technology to be cross-pollinated from the diesel truck world this side of direct cylinder injection. . .

1. Everybody complains that their MFI runs rich. The factory could get the system to idle and smoothly accelerate with very low CO settings (like 3%) but most folks prefer 5-6% on idle and 7-8% at 3000 rpm. Read the material that pwd72s turned you on to. Then read it again. . . it only took me about 10 runs through to begin to understand how the system works. . . but totally worth it, becuase there is a major shortage of competent mechanics who understand the system- my mechanic actually went to Bosch School on MFI in the late '70's!

You can order the tools from Pelican for adjustment of MFI. But be sure to follow "religiously" the "Check Measure Adjust" procedure, it is the only way to reliably diagnose and tune the MFI system.

2. Unburned fuel escaping from the exhaust: I had this problem when I disconnected the ground strap from the factory CDI box (the PO had convered to and MSD Unit). Evidently the trans ground strap under the motor was insufficient to ground the plugs for a good spark on all 6 cyl's- so I got lumpy running and raw fuel. Check the ground straps.

3. Fuel leak in car. . . I would take this one very seriously. The first place to start is at the tank up front- check the gasket for the fuel level sender, then jack up and properly support the front end, remove the metal cover over the steering rack, and inspect the fuel lines that exit the tank and go over the steering rack and into the tunnel. If you are satisfied that there are no leaks there, then go inside, remove the carpet over the tunnel, and look inside the oval opening for leaks. Then remove the cover over the shift coupling at the rear of the tunnel and check there. Then elevate and properly support the rear of the car and check the supply and return lines where they exit the tunnel and go over to the port side of the car to the fuel pump. Then follow the lines up to where they enter the engine compartment at port rear.

Good luck! Welcome to the MFIWDP.

[Rot 911]

pwd72S got you off to a good start, but having worked with the MFI in the past I think you should not make any adjustments until you have a good CO tester to find out what your adjustments are doing. This thread should get you started: Product Review: Gunson Gas Tester

[pwd72s]

Gas smell. Another thing to check is the "gas box" (What I call it for lack of a better name). This is part of the wonderful (?) factory fuel evaporative system. The box itself is up high, under the dash, driver's side. Visable from the trunk if you remove carpet and the cardboard barrier (10mm socket on a 1/4" drive ratchet works well). Look for two clear plastic lines running from the box. The box itself is sealed, supposed to catch the fumes. But if one of the lines came loose, or the box is cracked, you'll smell gas. Especially right after filling the tank...How do I know this?

[IAN]

Another thing to check is the cold start system. I have a 72 and when I got it all of the 5.0mm fuel lines that go into the velocity stacks were leaking.

Everytime I would start the car it would dump fuel. Pretty easy to take care of.....

Good luck.