1977 911 start up an idle issue

[raw122]

i picked up a 77 911s an having a priblem when i start it cold or hot when i start the car it starts idles up a little the dies right away,if i give it throttle will starting then when it starts slow let off throttle then it idles fine right at 900rpms,shouldnt i be able to turn the key let the fuel pump start then start it and it should idles fine,any ideas?

thanks for any help

[john walker's workshop]

first you need to set the specs. could be a lot of things, but do timing, idle mixture and idle speed for starters. then see if it's any better. if it's still cranky, a fuel pressure check with the proper CIS gauge set is next. if the cold control pressure is too high, or too low, the cold running and warmup suffers. somewhere around 1 to 1.2bar usually works. maybe also check the airbox to see if it's blown (separated at the seam). they never run well with a leaking airbox. look for any screws inside under the paper element that are missing or 1/2 way out for your first clue.

[Flat6pac]

The 77 has a connector on the airflow sensor that wont let the fuel pump pump until it starts running lifting the air flow sensor
Bruce

[steinec]

Is your fuel filter fairly new ? If you don't know, put in a new one. Then check for air inleakage-hoses & gasketing. You can do this by getting the car started and idling normal, then spray little shots of carb cleaner at hose connections & gasket areas-you will hear a noticeable change in the engine rpm (drop) if/when you find inleakage. If all is good here, make sure your cold start valve is functioning (verify voltage to it as well as actuating when you turn the key to start).

[john walker's workshop]

the big brake booster vacuum hose can develop severe cracking down where it wraps over the rear torsion bar housing. clamp it off up in the engine area where you can get at it, by #2 and #3 intake, and see if the idle improves.

[T77911S]

hey jw,
i pulled a vacuum on my line with a hand pump and it held, is that a good way to check it?

[john walker's workshop]

that works. you wouldn't pull any vacuum if the hose was split.

[raw122]

ok great ill get checking on them things this weekend,and could it still be the cold vavle even if it does the same thing even when its warm,the fuel filter does look new but ill replace it this weekend ,and the only thing that was throwing me off is after i start it and let off the throttle slowly ,then it will idle all day no miss or anything right at 900rpms,and i did notice the gasket on the gas cap is half gone i know on newer cars it could cause problems

thanks for info and let me know if any more ideas while i get started everything else

[T77911S]

Originally Posted by john walker's workshop

that works. you wouldn't pull any vacuum if the hose was split.

thanks, thought it would be an easy way to check, that hose is tough to get to.

[john walker's workshop]

that's why i suggested clamping it off.