Help w/ poor running 73T

[rpmooreii]

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 ******** 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

[bigchillcar]

A couple of things to consider. What is the condition of the points? Dirty points that are starting to close will certainly cause mucho backfiring and poor running conditions; eventually the car simply won't start at all when they've closed completely. Points are good for about 10,000 miles in most cases.

Hard to restart when warm...could be an accumulator issue, not holding fuel pressure at shutdown. I don't know if your fuel pump has the internal check valve my '74 has, but if so, if this check valve goes you've got 'hot start' issues. Sounds like you've replced the thermotime switch and the cold start valve, so it should start cold easily, from there a properly functioning WUR regulator handles fuel pressure management. Sometimes these units fail.

Clogged injectors could give you problems I suppose. I would also consider spraying some type of carb cleaner in the engine bay around the hoses to check for vacuum leaks, but that may be more of a CIS concern as in my car. Hope these ideas help.
Ryan

[rpmooreii]

Thanks for the reply Ryan.

I should be using that 007 icon!

...Roger

Roger Moore

[Rot 911]

Bigchill, he has MFI not CIS injection. There is no fuel accumulator.

[Tim Walsh]

I also have an MFI 73T and I think If I had your problem I would change my exhaust to stock Heat exchangers and a sport exhaust. The factory sport exhaust has two exits that run under the valence like the ansa exhaust. The factory sport exhaust has a slight backpressure that the MFI seems to like.
The next thing to do is to download check, measure, adjust and follow the steps outlined. It will work wonders for your car.

check measure adjust

[rpmooreii]

l33t9eek,

Thanks for the reply. Two questions:

1. Any idea where I could source the heat exchangers and sport exhaust? Did we lose the stock heat exchangers when the Ansa was installed?

2. What do yo mean by: "download check, measure, adjust and follow the steps outlined"??

Thanks,

...Roger

[Tim Walsh]

I have no idea about the heat exchangers but I'm thinking what you have are stock HE's. You can get the factory sport muffler from out host.

download the link I"ve provided. The link is to the factory MFI tuneup document. If you follow it step by step in the proper order it will set your whole engine up like a well tuned piano, music to your ears

[rpmooreii]

Thanks Tim. Just after I made the last post I realized that you provide a link to the MFI guide. I have downloaded all 49 pages although some of this is probably beyond my limited mechanic skills!

I noticed on the Pelican Parts site that they sell a factory sport exhaust but it requires "mods to the rear valance"

...Roger

[Tim Walsh]

I have not seen the PP version of the factory sport muffler but if it is the same as the factory muffler I don't think you'll have a problem. I'll try and get some pics of the muffler to show you

[Tim Walsh]

[304065]

The exhaust system should not be causing THAT much of a problem. MFI cars will run on race headers and open megaphones. Having said that, a set of SSIs and a factory sport exhaust will give your T much needed torque and a pleasant exhaust tone.

If you want to do it yourself, get a set of the MFI adjustment tools, and a Gunson Gas Tester (see the archives).

The skills to tune the MFI system are rapidly dying out, so with some reading and practice, YOU could become an "Expert." I can't emphasize enough, read "Check Measure Adjust" over and over and over and over and NEVER touch the MFI pump. 9 times out of 10, the MFI pump does NOT need to be rebuilt.

Good luck! May the sputtering and black smoke soon give way to the chainsaw-like rip of six straight stacks!

[Tim Walsh]

Roger,
John's entirely right there aren't a whole lot of us out there who can work on these injection systems. I've done a decent amount of this but not all. I have synced my throttle bodies and helped with points and dwell. I was working threw it when I ran into a dead cylinder so I had to tear down my engine and find out why. Long story short I just had a tired engine that needed a rebuild.

[HarryD]

Originally posted by john_cramer
The exhaust system should not be causing THAT much of a problem. MFI cars will run on race headers and open megaphones. Having said that, a set of SSIs and a factory sport exhaust will give your T much needed torque and a pleasant exhaust tone.

If you want to do it yourself, get a set of the MFI adjustment tools, and a Gunson Gas Tester (see the archives).

The skills to tune the MFI system are rapidly dying out, so with some reading and practice, YOU could become an "Expert." I can't emphasize enough, read "Check Measure Adjust" over and over and over and over and NEVER touch the MFI pump. 9 times out of 10, the MFI pump does NOT need to be rebuilt.

Good luck! May the sputtering and black smoke soon give way to the chainsaw-like rip of six straight stacks!

John,

Why would an early (pre 74) car need SSI's if it has the factory heat exchangers? I thought the early factory ones and the SSI's are fundemenatally the same.

As far as the Ansa muffler, I have heard several times that the system is not friendly for Carb'ed and MFI'ed engines due to loss of power and torque. I would think that the sport muffler would be a positive improvement.

Also, I second your advice of carefully doing all the simple stuff first. I believe we all have a tendency to try to find some complicated solution instead of spending time to carefully diagnose the problem. 90+% of the time, a good diagnosis will reveal a simple solution. In my work life, I know every hour I spend doing careful design will save me 10 hours of construction and mucho dollars in unwanted change orders.

[David E. Clark]

Because of your modified exhaust system, you probably don't have the air hose from the left heat exchanger to the thermostat. Without this hose sending hot air from the heat exchanger to the thermostat the engine will take a very long time to lean out the mixture. If you have improvised a hot air hose then check the condition of your thermostat. When was the last time you cleaned it? A dirty thermostat contributes greatly to a rich running condition. There are many good threads on this; search "Clean MFI Thermostat." These suggestions may solve your rich running problems and your long warm up problems. I second all the earlier suggestions -- Always follow Check, Measure, Adjust!!!

As far as the other problems, when was the last time you checked the compression/leakage? A problem in this area contributes greatly to all other problems. Finally, you might want to check the distributor ground strap (is it still connected) and test the spark plug wires as these can sometimes fail and cause problems like some of those you describe. Good luck.