MFI, thottle valve zero angel???

[Ceder2.4S]

I’m the tired owner of a 1973 2.4 S. I bought it February last year. The engine had covered 2000 km after a major overhaul. But it ran like sh… The Porsche dealer had apparently given up and I bought it cheap, depending on my own skills as an engineer to make it run ok. At first I got it to run ok. But the TB’s where worn around the valves, and the idle was stumbling. I found another set of TB’s, not worn around the valves. But I found out that they were major worn in the bushings. So I fiddle with the MFI all through the summer last year. Eventually I found another set of TB and have just installed them. They seem all right and there is good response when adjusting the part- and idle load. But…. When running rich on the part load, the car pulls very strong, but runs uneven when coasting. When I lean out the part load it starts to run good when coasting, but seems to loose power. Also, the idle adjustment make good response when the part load is rich, but seems to loose its effect when going against lean on the part load. I have read all about MFI, that I have come across and have done a lot of search. The thottlevalves and the pumplever is resting on their stops and all the linkage seems to be correct according to “Check, measure, adjust”. 114 +-0.2 mm, synchro. at 2500 rpm, etc… So here comes the big question.

What is “zero degrees” for the throttle valves? Or what would be the correct length of the two pushrods going from the cross shaft to the TB’s?

Lee Rice says 149,5mm. “Check, measure, adjust” (which I believe the most) says that when thottlevalves and pump lever are on their stops, and you got the pump pushrod at 114mm, the length of the two rods going to the TB’s will be given. I suspect that the “zero degrees” on my throttle valves is not the correct “Zero degrees”, and therefore have to be closed more. But I do not want to mess with it, if it’s not absolutely necessary. I hope you can understand my English.

Carsten

[brokke]

Hi,

Last winter I cleaned the throttle bodies of my 2.4S. After reassembling the TB I closed the throttle valve completely (the disc closes the port). Then I opened the throttle disc a half turn with the set srew. That is for me the throttle valve zero angle.

Peter
911 2.4S

[Ceder2.4S]

Hi Peter

I have done exactly the same thing. And I thought that would be "The zero" angel as well. But my main concern is, that Porsche (check, measure, adjust) describes the relationship between the throttle valves angel and the pump lever angel to be very precise with only 0.5° tolerance. On my car the length of the two pushrods going from the cross shaft to the TB’s side seems long (155mm). According to Lee Rice they should be close to 149,5 mm. That equals up to 28° offset (calculated with a 40mm long valve lever. I’m on work and can’t remember the exact length). That’s way off the tolerance. That could be a good reason why I can’t adjust the car to run prop-erly. As it is now, it runs very rich and with a compromise between having power and coasting ability.

Today I got the phone number of a guy, who is supposed to now something about MFI. He con-firmed that the two pushrods going from the cross shaft to the TB’s has to have a certain length. He has a tech manual where it is stated. But he could not remember the length, and seemed re-luctant to look it up. Maybe he wants to make some money on me… Therefore I had set my hope up that some one on this board could confirm the 149,5 mm that Lee Rice says. It would be interesting knowledge for all struggling to get their MFI cars to run properly.

Carsten