79 sc idle problem, any ideas?

[renopor]

hi form dublin!
i have a 79 911 sc euro spec. the problem i have is when i rev her a idle, say up to 5000 rpm, when she come back down to idle it cuts out. i can start her up and it will idle fine. it does this when hot or cold. i checked the timing and it seems fine at idle, there is a vac advance on the side of the dizzy but it is capped off. the timing is set a 5 BTDC but when i rev her it seems to move to about 20 ATDC? is this normal, could this be part of the problem? any help would be great. thanks

[Walter_Middie]

renopor,

Did you put a distributor in from a different car? Your distributor should rotate counterclockwise and you cannot replace it from an earlier car.

Are you sure the timing is changing toward retard with increasing RPM? You should set your timing at high RPM. The idle setting is just to get it started, so you can set it at high RPM.

Here is the timing from a 1980 to give you an idea - yours should be similar.

[ossiblue]

Walter has given you the basic information on timing but your symptoms could also be from a faulty/plugged Decel valve. Have you checked out the Decel valve or have you checked to see if it has been plugged by a PO? Sometimes the valve is plugged because of the slowness of the RPM returning to idle, but an overly rich mixture can result and possibly cause a stall. It's something to check.

[renopor]

thanks walter, that explains it perfect. going by those pictures my timing is perfect, well give or take 2/3 degrees, does that mean my dizzy is working? ossi what is a decel value? it does smell a bit rich, and if i rev her hard from idle it sometimes gives a bit of black smoke.

[ossiblue]

The decel valve looks like this: (It's the metal canister thing on the right, with the hoses connected to it)



On your 79 I believe it's on the left side of the throttle body, as you look at the engine from the rear. In this photo, it's behind the throttle body and you cannot see it easily from the rear of the car.

[SP2]

There is some screw type thing on the left hand side of the engine on the top (when you are facing the engine) that you can adjust. I forgot the name of it. John Walker showed it to me once. My idle was too high so I turned it down and the idle was better at first, but it looks like I overdid it and the engine would cut out after the RPM's decelerated, just like you described. So I readjusted it in the other direction and the engine stopped cutting out.

Sorry for the very untechnical description, but thought it might help.

[renopor]

yeah i seen that ossi, do they get blocked? should i take it off and clean it?

[Walter_Middie]

yeah i seen that ossi, do they get blocked? should i take it off and clean it?

Just make sure the decel valve is connected. Vacuum on the top should complete the connection between "A" and "B". The decel valve will keep your RPMs from dropping too quickly when the throttle is closed.

The engine dying after the throttle is closed, is a sign of running too rich. You say you smell raw fuel - has anyone changed the mixture setting lately? Sometimes when vacuum leaks develop, people will richen the mixture rather than fixing the real problem. This will make the car run OK when the throttle is open, but run rich closer to idle.

If neither of these helps, it may be that your WUR is running rich, but I'd make sure everything else is OK before blaming the WUR.

[T77911S]

walter caught the statement that caught my eye. i assume the timing is correct and not retarding as the RPM's increase. nice diagram walt, and nice pic of the decel valve, i saved that one.

if the car will normally idle other than dropping from higher RPM's, the decel valve (DV) is not making it cut off. the only thing the DV can do is open and increase, or hold RPM's above idle.

blip the throttle just a little. try reving to different RPM's. do the RPM's drop down below idle then come back up? if you let it idle for say m 5 minutes or so, does the idle start to go up and down? if yes to either of these, you are way too rich.

[vash]

i had a car that did that...the mixture was way off.

[renopor]

yeah that all sounds good, ive just put the dizzy back in, took it out and gave it a good clean, wasnt to bad to be honest, just going to set the timing again, ill set it at 6000 and 25. but i now think your right that its the mixture. ill tell you all what i have done to get me to this point, i had a couple of head bolts broke in old engine, so while i rebuild it, i put an engine i had here in, i change all the inlet manifold and injection over from the bad engine, new gaskets etc. the last engine was running fine, but had a couple of broken valve springs to so i said id rebuild it. so here i am at this stage. i hadnt touched anything on the injection system adjustment etc because it ran fine on the last engine. everything is hooked up and in its place etc and the car is idling while, i drove it and it was fine till you come to stop then she cuts out. smells really rich and i suppose it could have been set richer for the other engine etc.

[T77911S]

you likley fixed some air leaks.

blip the throttle, if the rpm dips, lean it out, keep doing this until it does not do it, this will get you close. make small changes, about 1/8 turn.

[renopor]

all sorted lads, thank a million for all the good advice, mixture was way off, but is running fine now!! thanks again