SC coldstart/coldrun characteristics - need comparison please

[Flojo]

hi!
two questions:

a)
my 79 SC starts fine and the coldstart mechanism seems to work fine.
well does ist?
rpm 2000 is the coldrun idl.
summer or winter, ist drops within aprox a minute to 950.
but compared to my 10 years of mercedes w123 experiance (230ce and 280ce/1976-1984) with same bosch k-jet system, it seems to me, that my 911 idles to high and takes to long dropping.
symptoms for "dying" WUR?

b)
same I noticed, when starting driving from cold ist doesn't like low rpms in 3rd or 4th (just above 2000), so I rather hold the gears max to 3000 (for about 10km).
and sometimes, when rolling up to a trafficlight I have that jumping "bonanza"-effect.
no difference with bosch CDI or permatune silverbox.
pump, cables, plugs, cap, coil are as new and in good checked condition.
symptomes gone when "hot".

if a) is OK, am I just to lame regarding b)?
what do you say?

[Bob Kontak]

Hi Flo,

If the only thing that happened were the rpm's stayed at 2000 for a minute on start up, I would think no problem.

But you have another issue that is not correct. The car has very poor response when cold at low rpm and it can "buck" like riding a wild horse (bonanza, correct?).

Pretty sure you have a lean condition. Most likely an air leak that allows "false" air to enter the air-box.

[Flojo]

ok. the airbox will be checked soon anyway, as I sceduled my enginedrop end of august, due to the "triangle-of-death" fix up.

[theiceman]

I would say a little more clarification means , when you say when cold .. cold in texas may not mean the same as cold in Finland ..

your first isue is not an issue .. i have a 78 sc and it performs similar .. winter months ( canada cold ) it holds higher a bit longer than what you state but the engine runs and performs fine.

Point B
I would not jump to "fixes" and likely causes just yet. Porsches do NOT like to be "lugged" ( which is low RPM in a high gear ) this is especially bad when the engine is cold. I would say your engine sounds fine. See if you can get with a fellow owner in your area and compare notes . A lot has to do with driving style. I would NEVER be in fourth at 2,000 RPM on a cold engine , i usually run 3,000 or so till it warms up .

Not saying you dont have an issue , just saying do a bit more diagnosis. Sounds to me like it might be just fine.

[Flojo]

@iceman: honestly, your opinion to a) and b) is the guess I had, but wanted to make sure, as there are still componants on my SC that are just old.
that's why I plan to exchange most tubes and o-rings, when the engine is out.

[prebordao]

Have you checked your CIS pressures: cold and warm ?

If you really have a problem, it will most likely be K-jet related - wrong pressures or vac leak...

[OsoMoore]

Behavior A is perfectly normal. My 79 SC has always done this and it is considered standard operating procedure.

I'm not sure about B - mine doesn't do that.

[Bob Kontak]

Originally Posted by theiceman

Porsches do NOT like to be "lugged" ( which is low RPM in a high gear ) this is especially bad when the engine is cold.

+1

I completely missed that. Nice job.

[timmy2]

For part b, your engine is cold and the WUR has it running rich until it warms up. The bucking at low RPM's is due to too much fuel and not enough air with an almost closed throttle body plate.
Even with EFI in my '78 it will do the same thing when cold and lugging the engine at low RPM's.

[Bob Kontak]

Originally Posted by timmy2

The bucking at low RPM's is due to too much fuel and not enough air with an almost closed throttle body plate.

This makes more sense than lean like I said above. If it were lean from an air leak it would be lean when warm too and suspect there would be symptoms that would present themselves.

[Pazuzu]

Originally Posted by timmy2

For part b, your engine is cold and the WUR has it running rich until it warms up. The bucking at low RPM's is due to too much fuel and not enough air with an almost closed throttle body plate.
Even with EFI in my '78 it will do the same thing when cold and lugging the engine at low RPM's.

Agrees, mine bucked with CIS and now with EFI, it's NOT just a CIS problem as some people think.

Parking garage after sitting for 4 hours, 1st gear, just above idle, bucks like mad.

Someone once postulated that it might be some relationship between the throttle linkage and clutch cable, since they basically sit against each other, and you could get some sort of reving/bucking if they got into a vibrational mode somehow.

[timmy2]

I just push in the clutch, select a lower gear, keep the rev's above 2500 and it goes away. These engines like to be revved, they are a sports car after all!

[Pazuzu]

Originally Posted by timmy2

I just push in the clutch, select a lower gear, keep the rev's above 2500 and it goes away. These engines like to be revved, they are a sports car after all!

Yeah, but 2500 rpm in my parking garage at work=every car alarm and dirty looks. Idle is too slow, so I need to just just off idle and modulate the clutch to stop the bucking.

Once I'm out of the garage, the car doesn't go below 3K

[timmy2]

Funny, I set off a few car alarms in the neighborhood last night when I took my Harley for a spin... Gotta love the hills in my area...
I don't lug the bike either!

[Bob Kontak]

Originally Posted by Bob Kontak

This makes more sense than lean like I said above. If it were lean from an air leak it would be lean when warm too and suspect there would be symptoms that would present themselves.

The more I think about it the more I remember an air leak at the base of one of my intake runners getting sealed at exactly the same time my bucking symptoms went away.

However, the bucking occurred shifting into second gear - not third or fourth. It was pretty violent. My car now never bucks at all now, running standard CIS.

Just for fun, squirt some carb cleaner around the base of your intake runners while idling to see if the rpm changes.