Dealing with a high idle

[mustangfool]

3521 (with 3541 harness) installed on a Ford Racing 302
Chip # 3522
M-6250-E303 roller cam with 220 degree duration at .050” and lobe separation of 110 degrees
GT-40X heads (1.94 intake and 1.54 exhaust)

After a few years of finding spare time to get everything installed, I finally was able to fire the engine up for long enough to fully warm it up and tune.

Base timing is confirmed at 10 deg BTDC
With idle control off, I can only get down to around 1100rpm. I have turned the stop screw all the way out so the butterflies are fully closed and confirmed the TPS is at 13 degrees. Timing baselined at around 30 deg BTDC and by taking out 12 deg (to 18 deg BTDC) I could get the RPMs down to around 900, but at the cost of lower vacuum and a rougher idle. I had about 12 in of vacuum and after modifying the idle fuel level I was able to get back up to a somewhat smooth idle and about 14 in of vacuum. With my light cam, I am thinking the vacuum should be higher and I should be able to have a low rpm with a very smooth idle.

According the instructions, I am to turn idle control back on and then set the target idle to match the 900rpm I was able to obtain with control off. I can manual lower the target idle to a respectable number like around 700, but I am not sure I should be forcing the idle that low if I can’t get there without idle control. With an automatic, a idle of 900 rpm will kill my leg at stoplights just trying to keep the thing stopped solid.

What am I doing wrong? What am I missing? The fact that the screw is fully backed out and the butterflies closed and I still have such a high idle can’t be good… any ideas on what I should try next.

[S. Armish]

14" at 850-900 RPM is probably pretty good for that cam. 220 degrees is not huge for duration but the 110 deg LSA will effectively increase the overlap creating a somewhat rough idle. I'd just continue on with the tuning process and see how things go. Our base cals typically have a lot of spark advance at idle. It wouldn't surprise me if you had to pull a little spark out of idle to hit your target.

[mustangfool]

Thanks for the quick response...

I will keep playing with it I guess. When I set the target idle, do I have to match the speed I was able to get with idle control off, or can I lower the target speed down from the 900rpm?

Any clue as to why I can back the idle set screw all the way out and close the butterflies. With this occuring, all I can do is take away spark and fuel but to adjust the idle speed. No matter how much I took out, I never got a good idle speed with fair quality. The only good quality idle was at the higher rpm, which can't work.

[mustangfool]

Also can you help me read the cam specs. According to Ford, the intake lobe center is 110 deg and the exhaust is also at 110 degress. Does that mean the seperation is 110 deg or am I reading their data incorrectly. I assumed the idle would be a bit rougher than a stock 302 but this is the lowest performance cam they included with the engine.