New 964 Cams in SC surging idle when cold after rebuild
 

Hi,

I just got my SC fixed up for the next 30 years with a top end rebuild, new valve guides, rings, rod bearings and new 964 cams.
My mechanic took it for a boot and said it runs great but surges a bit at idle when cold, but idles fine when warm.

I've read about this before with the 79's (bigger ports) with 964 cams and SSI's, but I can't find the article.

My mechanics going to run through everything on the mixture, idle, air, etc. I just thought I would post here to see if anyone had the same issues.

Cheers, and looking forward to feeling the new cams in the ols girl :D

Check the WUR. Hook the gauges up and see what pressures you are getting. I'm running 2.5 bar which is a bit lower than spec but it works for me.

I just finished a top overhaul which included JE 9.8:1 pistons, 964 cams along with SSI's and a Dansk Sport muffler. She running sooooooo sweet.

Ed Garcia
1977 Carrera 3.0

Thanks,

I just spoke to my mechaninc and apprantly it does it hot or cold so I don't think it's the WUR. I will get the car tomorrow and take her for a drive to see if I can figure it out, and I'll ask them what they went through to troubleshoot.

Any more thoughts, anyone?....

Cheers

Surprised the mechanic didn't adjust the mixture, timing and idle all before sending you home with the new engine?

I had no hunting idle on mine but my mixture was all off when I installed my 964 cams and SSI (done at the same time).

George

+1

may just be mixture since hot and cold are doing it.

George, Was this on a 79?, As well what were the final numbers on afm, idle, etc?

Thanks so much

edgarcia,
This is weird I just posted on an issue starting with exactly the same set up apart from the Dansk, mine are custom headers and a 2 in 2 out Powerflow. I have not altered any CIS settings since re-build which is still as it was was when removed from the engine.

In speaking with the mechanic, he has gone over idle, mixture and timing, but wants me to break in the engine first before a re-adjust. He said there is some resistance until the pistons/rings seat, then he can adjust in the final state.

I pick up the car tomorrow so I'll have a better feeling as to what the issue is, I'll post my findings.

Bob

Sorry stealthn, it has been too long for me to remember . I used my innovate afm meter to set it up. Idle was stock 950 and I was conservative on the mixture (richer side) as well as the advance (not many degrees over stock). I had an 81, so it is a little different with the lambda sensor.

George

BS on the adjustment after breakin. It can be tuned as is. Get it up to operating temp then make tuning adjustments. With new cams you'll need to adjust idle air bypass, mixture screw and timing. Even just an mixture screw adjust will be good for it n

Souk is right. There isn't any drag or resistance in an engine that is being broken in. At least none that would influence mixture, idle and ignition advance. I have built my share of engines and none ever needed re-tuning after the initial set up.

George

Thanks guys, just picked it up and took it for a drive, it stalled 11 times. It was not up to temp yet, but what happens is when I slow for traffic light, and push in the clutch RPM drops to zero and doesn't recover/stalls. I tired coming up to a light slow in low RPM, but same thing.

I was able to keep it from stalling at a light by kepping the RPM's up to 1100, if I took it out of gear and let off the clutch it would surge between 650-950 RPM.

I called the shop and told them this is a BIG issue and it is not safe. I will be taking the car back. The wrench said he did adjust the air mixture screw and I assume he did the timing, but I wish I had true numbers to give him for advance, AFM, Idle, etc.

This is a mechanic that I trust, to an extent and he knows he stuff, but my safety is more important.

Any more thoughts or real numbers would really help.

Thanks

A good mechanic isn't always a good CIS tuner. Plenty of good shops and good techs out there can fix just about anything and fail at CIS.

If he just turned the idle mixture screw and nothing else, he's asking for trouble.

He has to check and make sure everything is inplace and all hoses connected. Not know how much he changed the idle mixture screw, I'd say he has to start as if it is all out of whack. He's going to have to spend a good bit of time timing this thing. I hope it's part of his fee (already paid for or agree upon for the rebuild).

Agreed, but this one knows CIS.

Update so I took it back to the shop after work, and we checked all the hoses and connections, now warm it would have issues starting.
The mechanic then increased idle to 1050 RPM to prevent stalling, then adjusted the mixture to lean it out a little (about 1/16 of a turn), then he checked timing and adjusted it about 2 degrees.
He then set idle back to normal and the car seemed much better. I took it for a rat around the block and it was very close to correct, but not perfect.

He said to keep driving for a while, and when everything "finds its' place" he will adjust again.

It's not perfect but very close. I know people have different opinions on breaking in, but I do agree with his thinking on this; CIS is very finicky with not much in the way of help such as O2 sensors or computers (in my year) and introducing something new to the mix (the 964 cams) along with all the new parts requires the engine to seat, find it median mark, then you can adjust from there. Everyone may think different, but I like this thinking.

Anyway thank you for all your help and once I get a few miles on her, we'll get it adjusted perfect and I can post the exact match.

Cheers,

Bob

Bob,

What did he set the mixture at?

I don't think it is a matter of opinion. It is a fact that there is nothing getting seated in your engine that would affect your fuel injection system and timing settings. Your engine should run like a well oiled machine on the trip home after a rebuild. It should run better and have more power than before you dropped it off. There was no required re-adjustment of the system after factory delivery either .... If you do not trust Souk's and my opinion you should go ask in the engine rebuild forum.

It is good that you are patient, but don't let the mechanic string you along long enough where it is not his responsibility any more to get the engine running right. I sincerely hope you'll get this resolved soon so you can enjoy your new flat six.

George

George,

Your post comes off as kind of defensive?

I didn't say I don't trust anyone...

I'm not sure what the final AFM was set at I'll ask. Like I said the engine is very close I'll check the AFM after I get it up to good temp on a long drive.

I appreciate everyone's advice and I take it under consideration the same way I take a Porsche certified mechanic that has built hundreds of engines, runs his own shop and whom I've know for years. Everyone gets their input then I make the choice.

Bob

Bob,

Just trying to help as I think this thing is time sensitive. Not trying to argue or get defensive. Sounds like you are sure he's going to make it right - that's something that wasn't that obvious before you were starting to defend your mechanic. ;)

Good Luck and keep us posted!

Cheers, George

Hi Bob, you forgot the most important thing: How does it feel? Can you feel any difference?

I have my 964 cams on the way here now, and I can't wait to get them in the car.

Thanks,

Johan

Johan,

I did 964 cams and SSI at the same time. It was a very noticeable difference. My car would walk away from any chipped but otherwise stock 3.2 on the track (straights).

George

If i am getting this right?

I have a euro 83 sc if i can the cams for 964 cam alone, will get more more power using the same CIS system?

What adustment did you need to do? Do i need to get an adustable WUR?

I am taking off my engine this week am keen to hear about the 964 cams, shoukd change it any other things needed?

Good to see you are still out there Souk...SmileWavy

It's not rocket-science to adjust these engines.
The idle should be between 850-950RPM, with 3,5% CO, and 5 degrees advancement on the ignition. Then it should take care of itself.
I did this to my 3 liter after after rebuild, and it was a no-brainer... Worked like a charm!!
But, if it is difficult to adjust, you could have a air leak in the intake. When a leak occurs, the air/fuel mix is messed up, and leads to erratic engine performance.
Try the start-gas experiment. When the engine is at idle, spray some start-gas onto different areas on the intake manifold joints, but not in the air intake itself.
If the idle RPM suddenly increases when spraying a joint, you've found your leak.
Good luck!

3.5 % co means its a bit high... will the car pass the emission inspection?

Was reading one of the cam threads and it was said that one of the cam experts noted that the 964 cam needs more compression than 8.5/1 to work well.

Thus, the later SC, euro, or 9.5/1 replacement pistons should probably be a must when going this direction.

I second the 3.0-3.5% CO. I think 3.5% is about 13.5/1 AFR. Most SC's have a lean surge off idle as the motor gulps air when the throttle plate opens and the metering plate has not yet moved enough to correct for the change in air flow. This helps overcome that. Also peak TQ is found at around 13/1 AFR.

I would look closely at the timing side of things. I am not an SC expert but I think it has vacuum retard at idle. Check with a timing light that timing jumps to 10deg or so when blipping the throttle. Or after timing is set, pull the vac line (s) to the pot and see if timing jumps to about 10 deg or so.

t looks like you have SSI's which is good.

Just a thought, I would probably not run stock heat exchangers with these cams ether. CIS makes for enough restriction on the intake side. We can counter this on the exhaust side to some degree with good headers and sport muffler. This will reduce back pressure and reduce reversion at low rpm for less exhaust dilution of the incoming air and fuel. 8.5/1 pistons can not push out as much exhaust as say a 9.5/1 piston to make room for the incomeing mix.

Just a thought.

You guys bring up a good point. When I researched cams, the 964 was only recommended for the Euro or later SC compression. For the earlier SC with the 8.5:1 one should go with a milder profile (less overlap bleeding off less compression). That said, I doubt running 964 profile with 8.5:1 compression would give the erratic idle issues the OP describes. The engine just would not reach its full power capability.

George