Hi,

We can not seem to get our 1973 911T (MFI) to run well despite repeated tune-ups. The car seems to run well immediately after each tune-up but quickly degrades to a back firing, hard to warm up beast. In addition, when it is hot it sometimes will not restart; after waiting several hours it starts but runs horribly until warm. Once warmed up it seems to run pretty well but not as well as it should.

The car has 90,060 miles (original) with the following maintenance history:

83,285 top end job
85,676 adjust valves, set timing, set fuel mixture (car running rich)
89,000 replace plugs, replace dist rotor, fuel/air adjustment, new cold start valve
89,787 replace thermal time switch

In the mids 80s the stock exhaust was replace with a 4 pipe Ansa system. Our current mechanic (Joe Schneider of Schneider Autohaus in Santa Barbara) has stated that this exhaust system while popular in the late 70s and 80s made the car very hard to tune. The car has run rough for many years while it was owned by my dad (the orig owner), although in the 90s it was not driven very much

Is it likely that the exhaust system could be the root of these problems?

Unfortunately when the car was repainted the exhaust exit hole in the rear bumper valance was filled in making it a major undertaking to put a stock exhaust back on the car (hole would need to be cut out and the area repainted). If the exhaust is the root of the problem, is there another exhaust system that exits below the valence so that we do not have to undertake body work as well?

I look forward to the incredible wisdom offered by this list!

...Roger

'73 911T
'93 RS America
ex-74 911 (featured in Leffingwell's latest Buyers guide)
ex-60 356B Rodaster

p.s. I am going to become a paid rennlist member today!!

 

I have heard that the Ansa Exhaust system does not play well with the Carb and MFI Children.

Soooo, I think it may be worth a try. What is your wrench recommending?

 

Well, the fact that it runs good immediately after a tuneup tells me that something happens to the freshly installed spark plugs. Car may run rich, glazing the plugs nicely in carbon? How do the plugs look? This may be due to the MFI messing up or worse that exhaust problem? So, it isn't great to start with, but once the plugs are on the edge it will really give you trouble. A thick starting mixture will be harder to ignite than a warmed up engine.

Just my two cents,

George

 

bad exhaust gaskets caused my car to backfire like a **** and run rich as hell.
changed them and cleaned up all the surfaces and runs soooo much better now.
just food for thought.... do you have any exhaust leaks at all?

 

From what I've read here and on the Pelican Part BBS, MFI systems run worse with aftermarket exhausts. The preferred systems are the stock exhaust and the factory sport exhaust. I believe that the sport exhaust should fit without cutting your valence (need to verify this!).

My further recollection on this topic is that the aftermarket exhausts would tend to cause a flat spot in the range of 2,500-3,000 rpm. This doesn't sound like what you've described as your symptoms. Have you gone through the procedure "Check, Measure Adjust" yet? It's the factory procedure for tuning MFI engines. You can find it at http://www.scatliff.mb.ca/pelican/MF...ure_Adjust.pdf
I hope you're not on a dial-up connection because this is a big big file.