New 964 Cams in SC surging idle when cold after rebuild

[SCWDP911]

Originally Posted by Soukus

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

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

[otto in norway]

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!

[Peter Seow]

Originally Posted by otto in norway

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?

[911st]

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.

[aigel]

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