911 SC misfire

[Aetto]

Hi all. Just bought a 78 911SC with cis fuel injection and MSD ign box and I have some questions. The cars distributor has two vacuum ports none are hooked up, or have hose's running to them.
When I put vacuum to it the car starts to breakup at 4000 rpm. What is the vacuum source for the distributor advance? Does it sound like the distributor advance weights are frozen or do I have a fuel problem.
I changed the plugs but it made no difference the car does not breakup with vacuum disconnected other than a high idle ( 1300RPM) and slow acceleration performance.
Thanks:

[JohnJL]

You might get more response in the regular "Technical Forum."

You say it "breaks down" about 4000 when you apply vacuum. How does the car run when you dont hook up vacuum to the distributor? A high idle and slow acceleration could point to vacuum leaks.

Vacuum source can be anywhere in the intake past the throttle.

You might check what the PO did with the vacuum hoses that were connected to the distributor and make sure any open hoses are plugged.

[Aetto]

Thanks for the reply, The car runs okay and does not breakdown with the hose disconnected, but it just doesn't seem to pull real good .
I will check for leaks and see if I can see if the PO plugged the lines. I do not hear any sucking or vacuum leak noise but will check further.
There has to be a reason why the PO left the distributor disconnected .
thats why I was thinking advance weights frozen.

[Shadetree930]

I am going to move this thread to the 911 Technical Forum .

You will get more eyeballs on your issue from owners that are more familiar with your specific engine and the idiosyncrasies of same.

[T77911S]

if you removed the vacuum and the RPM's went up, that is the vac retard diaphram, the other diaphram is the vacuum advance. you need the vac advance connected, you can live without the retard as many here prefer to not use it.
so now it seems you know which one is retard and what one is advance. now you have to figure out which one is connected to what port on the throttle body(TB).
i dont know which port on the TB is which one, but the retard will have vac at idle and the advance will not.
you also need to check you mechanical advance. you can do that with a timing light and both vac ports not connected.

[Bob Kontak]

Vacuum advance is the port on the distributor that points away from the distributor cap Retard points towards the cap. Advance is the outside one.

The vacuum advance connects behind the throttle body up near the bellows that connects the air sensor and the throttle body. If you are at the back of the car - say the closest part of the throttle body is six o'clock - the vac advance port is at one o'clock. Vacuum advance is orange.

Retard is a blue hose. Facing the engine, right behind the upper left hand corner of where the rectangular air cleaner cover sits is the port for the advance. It's on the air box side but you get the idea. It's not connected to the air box or air cleaner cover - just right behind it.

You can check if the weights are frozen by pulling the cap and seeing if you can twist the rotor. It should move some degrees and spring back.

Edit - This is assuming your 78 has the same vacuum port config as my 81.

[Bob Kontak]

Highly doubt it is fuel. However, if you don't know, change out the fuel filter and the gas tank filter screen this winter.

Keep you plug gap under 40 thousandths until you are sure you have all secondary ignition wire spark jumping problems identified and corrected (i.e., tired wires). I would not go over 45 thousandths. Also do the late night - with no lights on - test for a fireworks display. Give your eyes five minutes to adjust and don't wear a necktie.

Check your timing. The PO may have "heard" of some idea on where to set timing other than factory. There may be some benefit to messing with the timing for track use but for the street, factory is fine. FWIW, you can set the cam timing to improve top OR bottom end but not both, It requires a boat load of work. Bruce Anderson's book has info on that and surely it is talked about in one of Wayne's books or in this forum.

[Aetto]

Thanks for all the good info. I went out and checked a couple of things on the throttle body . 1st of all the PO removed all the vacuum lines going to the distributor. The vacuum source at idle is what I used on the advance so just to confirm I should look for one a source with no vacuum at idle. I did find a small port on the throttle body with no hose on it. It was about 1/8 inch diameter and was next to a port with a cap on it about a 1/4 of inch. I put a hose on it and at idle no vacuum could this be it? I plugged and test drove the car really no diffference.
I also checked the rotor and it does have some spring to it. I will check advance with timing light and let you guys know. Also the TB was a little dirty is it ok to clean with intake cleaner?
Thanks

[Bob Kontak]

You should connect the vacuum advance port on the distributor to a port that has vacuum at idle. That advances the distributor timing at idle. As you give it "gas" (actually air) with your foot, the motor spins the rotor faster and the weights in the mechanical timing system take over. Since the throttle is open more than when at idle the intake chamber does not have as high a vacuum as when at idle. The vacuum advance function then tapers off in function as you go faster. I am not sure but I think that the mechanical advance overrides the vacuum advance once you get going. Someone will know for sure.

Edit - Forgive my rambling if you know all this stuff.

[Flat6pac]

78SC is nt suposed to have 2 ports, just advance. Advance is taken from above the butterfly so you need to find that tap on the throttle body.
Bruce

[psalt]

Bob wrote,

You should connect the vacuum advance port on the distributor to a port that has vacuum at idle. That advances the distributor timing at idle.

Bob,

The vacuum advance uses ported vacuum, with no vacuum at idle. Vacuum advance is a fuel economy device, no extra advance is required at idle. Additional advance is desireable at low load part throttle cruise, where the vacuum advance is additive to the mechanical advance. Vacuum retard is an emission device, designed to reduce HC emissions at idle and uses manifold vacuum, full vacuum at idle. If you hook up the vacuum advance to a manifold source , the two devices would conflict and alter the spec idle timing.

My advice to the PO would be to leave both vacuum lines disconnected and plugged and concentrate on the mechanical advance, the spec is 25 BTDC. Vacuum advance is irrelevent to measured performance, hooking up the lines will not make it "pull harder" . The 911SC was detuned to run on unleaded regular with max advance octane limited to 25 BTDC, instead of the more optimum 35-38 BTDC of earlier engines tuned for high octane leaded fuel.

[Bob Kontak]

Originally Posted by psalt

Bob,

The vacuum advance uses ported vacuum, with no vacuum at idle.

Gulp - That's embarrassing. Thanks for the "tune up" and taking the time to explain, Paul. My advice made me a deserving target for a double barreled eight gauge to the chest.

I did check the vacuum ports just now. I simply screwed up and made assumptions.

Sorry about the bad data Aetto.

The port that my 81 vacuum advance hose hooks to is on the back of the throttle body up near the top (above the throttle plate as flat6pac mentioned) and just a little to the passenger side from center. I can get a pic with a mirror if that would help. I looked (felt) for the capped port you mentioned but did not find it.

Advise on the pic.

[Aetto]

No Worries guys !! I used to own a shop and told my customers I can't fix them over the phone. I know how hard it is if not impossible to try to diagnose a problem from a distance. It all makes sense now. Here is a update, I put a timing light on the engine and found the timing to be retarded at idle,not sure how much because I can,t see the Z mark but the mark on the flywheel was the furthest to the right when the strobe flashed.
I advanced it to where the fan housing mark is in the middle of the 2 crankshaft marks.
Anyway the idle increased so I adjusted the idle down and discovered it will not idle smooth below 1250 rpm, ( it hunts and lopes) regardless of where the timing is. However in the advanced position the top end was MUCH BETTER also noticed with the vacuum connected to the distributor ( the one with no vac at idle) the car breaks down at 5k instead of 4 k.
Were gaining on it!!
Anyway I think I have a fuel problem now because of the will not idle issue.
So again where to start injectors, fuel distributor Etc?
Guys I really appreciate all the help and your advice this site is awesome!!

[Paulporsche]

Idle hunting w/ CIS cars is often caused by a too rich mixture. Have you checked the CO?

[Gunter]

Aetto,

A 1978 SC should have a '78-'79 distributor with 1 vacuum connection, not 2.
I think that someone may have installed a distributor from a later SC (1980-83)

You want the right distributor to match your engine because the weights/springs are different for the right curve.

Numbers tell the story. If you give me the Bosch number on your distributor, I can tell what you have.

It also sounds like a service wouldn't hurt. Aside from sluggish weights, most of the time the bearing in the advance plate freezes and needs to be freed up.

Distributor service (Clean and lube) real easy without removing the pinion gear! - Pelican Parts Technical BBS

The ignition distributor is the heart of the engine; you want the right one and you want it functioning correctly.

[Aetto]

Appreciate all the good advice.
I am trying to put everything back to original specs and then go from there.
I will try get the distributor # and post it but I agree that it probably has been replaced.
I checked the pressure regulator with a gauge. I have 50 with the valve closed power disconected. 20 lbs with valve open and power disconnected and 20lbs with the power connected. So I think I have a bad pressure reg because it does nothing with power applied.
And from what I see It can cause the problem I have just hate to throw money and guess. Thoughts.........
Also tapped on the aar valve and the idle went all ovwer the palce , I also sprayed some pb blaster in it not much difference.
Thanks all.

[Aetto]

ok Gunter I have the distributor # 02370 016 or 018 kinda hard to see. underneath that # is PGFU6 0r 8. Motor # 6281269.
Thanks again.

[Gunter]

Sounds like you have Bosch 0 237 304 016 (Vac advance and retard) which is for US SC 1980-83 with Lambda.

For your '78, you'll need Bosch 0 237 306 001 (Vac advance only) which is for US SC 1978-79 non-Lambda.

The right distributor makes a difference in performance.

IMHO, you have 2 choices:

Get the right 001 making sure it's serviced or,
Keep the 016 and have it recurved to run without vacuum.

There are reasons why ignition distributors are matched to the specs of an engine.

Just disconnecting the vacuum and hoping for the best isn't going to do it because the weights and springs are different between the years.

It's up to you.

[Aetto]

Thanks Gunter, any idea where I can get the correct distributor or who can recurve the one I have?
I also have adjusted the valves, set the timing and replaced the coil with a MSD 8222 ( car has a MSD 6al box) and adjusted the decel valve. Car is running alot better but still hunts at idle and I get a small breakdown at 5k . I'm going to change the wires next.
Will keep u posted again thx for the helpful info.

[masterdave]

I had a misfire issue a while back, i replaced the distributor cap, rotor, and spark plugs, it was cheap to do and solved the issue. I realize this sounds like a vacuum issue, but a cap and rotor were less than 20$ and made a huge difference.

Best
Dave