MFI tuning (again)
 

Gents,

I set my timing today with a strobe, for the first time. I did this with the vacuum hoses attached, warm engine, and twisted the distributor a few degrees to get ATDC as specified in Chilton's Tune-up manual. This increased my idle speed to about 1200rpm.

When I removed the vacuum hose, the speed increased to 2000rpm (sorry, I didnt check the new timing). When I put the hose back, it stayed at 2000!!

Goes back to 1200 after restarting the engine. If I give it some gas, it goes to 2000 and stays at 2000!! Goes back to 1200 after restarting the engine.

What's the problem???? :confused:

BTW. Fuel-pump timing mark spot on, air intakes cleaned and synchronized, fuel pump thermometer hot (with help of heat gun because the heat exchanger heat outputs are straight to open air so the air getting to this thermostat is not very hot), thermostat clean and tested. Fuel pump control rack moves freely (biro check). Butterfly flaps adjusted until just closed then turned screws in 3/4 turn, control rods adjusted to leave flaps resting on stops, pump rod adjusted to 114mm (interestingly, this pushed the pump lever of its stop by about 2mm but does not increase engine speed). Air screws and air screw channels. Vacuum hoses checked and vacuum ports on throttle bodies cleaned.

Best wishes, Dean

1973 911T 2.4 MFI Targa

What's your timing at 6,000 rpm with vac hose disconnected? I forget the spec on a T, maybe 32?

Howdy RStarga, I dont really want to take it to 6000 until this problem is fixed. What did you have in mind when you suggested this? Best wishes, Dean

Timing is 5 degrees After top dead center with the hose attached. Once the throttle is cracked the vacuum goes away and advances quickly. Makes no difference if the hose is attached or unattached at anything other than idle. The vacuum hose is there to give you 5 additional degrees of advance by starting you out 5 degrees retarded at idle.

Start at 5 ATDC with the hose attached and see how close it comes optimal advance as you increase RPM's. By 6,000 RPM's you should see a set of markings with the timing light indicating the max advance.

Chris

73 911 E