Page 4 of 15 FirstFirst ... 2345614 ... LastLast
Results 31 to 40 of 144

Thread: MFI troubleshooting - high idle, pops and bucks at part throttle

  1. #31
    Sorry Arne, I knew that,,,,just didn't proof read my post well enough.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  2. #32
    Serial old car rescuer Arne's Avatar
    Join Date
    Aug 2011
    Location
    Eugene, OR
    Posts
    690
    Quote Originally Posted by edmayo View Post
    Sorry Arne, I knew that,,,,just didn't proof read my post well enough.
    No harm, Ed, just lost a little time. And I've got plenty of that.

    Printed this on a card to keep with the tools:

    Screen Shot 2019-03-31 at 12.54.50 PM.jpg
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  3. #33
    I'm not trying to encourage more rabbit holes, but sometimes the thermostat has to shimmed differently. The thermostat consists of 25(?) wafer pairs and then various shims to adjust the cold running characteristics. It could be possible you have to go into that, but for now I'd try to just work on the warm running adjustments. By the way the thermostat housing has to get very warm,,,,I assume yours does?
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

  4. #34
    Senior Member beh911's Avatar
    Join Date
    Aug 2009
    Location
    Atlanta
    Posts
    3,360
    Quote Originally Posted by Arne View Post
    No harm, Ed, just lost a little time. And I've got plenty of that.

    Printed this on a card to keep with the tools:

    Screen Shot 2019-03-31 at 12.54.50 PM.jpg
    Unless it's a 69 one year only pump
    1969 S Coupe #761
    Early S Registry #1624

  5. #35
    Senior Member beh911's Avatar
    Join Date
    Aug 2009
    Location
    Atlanta
    Posts
    3,360
    Your throttle bodies are worn. This summer with fully warmed car your idle will run closer to 2k
    1969 S Coupe #761
    Early S Registry #1624

  6. #36
    Senior Member 2.5MFI's Avatar
    Join Date
    Sep 2006
    Location
    Bend, Oregon
    Posts
    351
    Quote Originally Posted by beh911 View Post
    Unless it's a 69 one year only pump
    It's the same for all the MFI pumps.
    Mark Jung
    MFI Werks
    Early 911S Registry #972
    Carrera T w/LWB, MT, RWS, PCCB
    72 T 66 x 100 MFI Twin Plug Coupe
    R Gruppe #686

  7. #37
    Senior Member 2.5MFI's Avatar
    Join Date
    Sep 2006
    Location
    Bend, Oregon
    Posts
    351
    Thermostat: On the end of the thermostat rod, after the 25 pair of thermo disc's, are various sized shims (from .10mm to 2.0mm) that make the adjustment for how rich the fuel will be at cold starts. One .30mm shim can make a 10 to 15% difference in fuel delivery. When below 32 degrees this will move the main rack for 35% to 45% more fuel for the cold start. Adding shims will lean out the cold start fuel, removing shims will richen up the cold start fuel. Once the thermostats 25 disc's warm up and expand to the max, about 105 to 110 degrees F, it has reached the warmed up condition and stays there till you shut off the engine and it cools way down.
    Mark Jung
    MFI Werks
    Early 911S Registry #972
    Carrera T w/LWB, MT, RWS, PCCB
    72 T 66 x 100 MFI Twin Plug Coupe
    R Gruppe #686

  8. #38
    Serial old car rescuer Arne's Avatar
    Join Date
    Aug 2011
    Location
    Eugene, OR
    Posts
    690
    Quote Originally Posted by beh911 View Post
    Your throttle bodies are worn. This summer with fully warmed car your idle will run closer to 2k
    Thanks for sharing your thoughts on this. As noted in post #10 of this thread, that was the first thing I suspected and checked. There are no apparent vacuum leaks there.

    Quote Originally Posted by edmayo View Post
    I'm not trying to encourage more rabbit holes, but sometimes the thermostat has to shimmed differently. The thermostat consists of 25(?) wafer pairs and then various shims to adjust the cold running characteristics. It could be possible you have to go into that, but for now I'd try to just work on the warm running adjustments. By the way the thermostat housing has to get very warm,,,,I assume yours does?
    Yes, the thermostat does get hot. It is plumbed to the HE, although it is currently connected with a big rubber hose rather than the double paper hose. But the evidence is that the thermostat is working fine. While the enrichment is present, the part load behavior is very good. Once the thermostat gets warm and enrichment ends is when it starts running badly. So the thermostat function is not in question.

    Quote Originally Posted by 2.5MFI View Post
    Thermostat: On the end of the thermostat rod, after the 25 pair of thermo disc's, are various sized shims (from .10mm to 2.0mm) that make the adjustment for how rich the fuel will be at cold starts. One .30mm shim can make a 10 to 15% difference in fuel delivery. When below 32 degrees this will move the main rack for 35% to 45% more fuel for the cold start. Adding shims will lean out the cold start fuel, removing shims will richen up the cold start fuel. Once the thermostats 25 disc's warm up and expand to the max, about 105 to 110 degrees F, it has reached the warmed up condition and stays there till you shut off the engine and it cools way down.
    Thanks for this explanation, Mark. So there is really no way to know just how much the thermostat is moving the main rack without disassembly and analysis. I don't think I'll go there now.

    The key still seems to be that part load behavior is very good while the 'stat is doing its thing. So that still points to the main rack being lean when at normal temp.

    As noted above, I'm going to recheck base settings of the throttle plates today, before making any other changes. I've got this feeling that I may have them all too tightly closed, and that may be the cause of the very low idle, as well as the perceived richness at idle.
    - Arne

    In Progress
    - 1972 911T coupe, born (and someday will be again) Silver
    Sold - 1984 911 Carrera coupe, Chiffon white; 1973 914 2.0, Saturn Yellow; 1984 944, Silver Metallic

  9. #39
    Senior Member beh911's Avatar
    Join Date
    Aug 2009
    Location
    Atlanta
    Posts
    3,360
    Quote Originally Posted by 2.5MFI View Post
    It's the same for all the MFI pumps.
    Indeed, my bad. It was the number of adjustment clicks that is different in 69. Thanks
    1969 S Coupe #761
    Early S Registry #1624

  10. #40
    Arne, the air-bypass screws should be able to adjust to the idle speed you want. It is possible that their passages are coked up, but in addition to removing the adjustment screw and cleaning that passage way, there is also a counterbore that comes in from above the throttle plates that can only be accessed after removing the throttle stacks. Another diagnostic tool you might try is a infra-red heat gun. While the engine is running aim the gun at each exhaust pipe as it exits the head and compare exhaust temps. They will be in the 200-275 range. What you are looking for is a cylinder, or cylinders, noticeably different, like maybe 125 when all the rest are higher. That would be a cylinder not working well, maybe a clogged injector. If you find such you can swap it to another cylinder and see if the problem goes with the injector. Given the history of these parts that is a real possibility.
    Early S Registry member #90
    R Gruppe member #138
    Fort Worth Tx.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Message Board Disclaimer
This is a public forum. Messages posted here can be viewed by the public. The Early 911S Registry is not responsible for messages posted in its online forums, and any message will express the views of the author and not the Early 911S Registry. Use of online forums shall constitute the agreement of the user not to post anything of religious or political content, false and defamatory, inaccurate, abusive, vulgar, hateful, harassing, obscene, profane, sexually oriented, threatening, invasive of a person's privacy, or otherwise to violate the law and the further agreement of the user to be solely responsible for and hold the Early 911S Registry harmless in the event of any claim based on their message. Any viewer who finds a message objectionable should contact us immediately by email. The Early 911S Registry has the ability to remove objectionable messages and we will make every effort to do so, within a reasonable time frame, if we determine that removal is necessary.