I read in the MM&FF (Ask Bernie) that you can drill a 1/8" hole in the TB butterfly to help a car with proglems idling after a cam install (i.e. surges and stalls). Is this for real? How does this help the idle. I have a 95 Cobra. Thx. Peace
When I first fired up the car, it would not hold a steady consistent idle. It would cut off when I came to a stop. I adjusted the screw on the throttle linkage (not sure of exact name) and adjusted the TPS and while the car was idling I adjusted the idle screw for the IAC. It idles now at about 850 - 900 rpms and every so often it idles up to 1000 rpms for about 2 seconds and goes back down to 900 rpm. Before those adjustments the car was at stock idle but it acted like it was going to cut off when I came to a complete stop. Now after the adjustments, at idle, the TPS reads 1.1 volts. Should I adjust it back to between .5 and 1 volt or is it ok at 1.1 volt? To let you know I modified the car by adding AFR 165s, mild lunati cam 218/226 duration and edelbrock performer rpm intake, 75mm MAF, 70mm TB. I pm'd you about the tweecer and I am looking a purchasing one soon but I don't have a laptop computer. Thanks for any help
That’s an OLD shade-tree mechanic’s trick from back in the good ole days when men were men and cars had carburetors on them!
It was designed to get more air into the engine at idle when using a “big” cam with lots of overlap. It actually worked in many cases. The reason it worked is because there were idle transfer slots in the throat of the carb that were spanned by the butterfly valves in the primaries (considering its a four-barrel carb). As the butterfly valve passed and eventually cleared the idle transfer slots, the carb transitioned from its idle system to its power system (main jetting) to control the air/fuel mixture.
You needed a lot more air at idle to keep the big cammed engine running. You could get that increase in air by turning in on the idle stop screw and opening up the primary butterfly valves. However, you typically needed to open them up so far that they were nearly past the idle transfer slots at idle! Very bad. The idle system was thereby nearly rendered useless and the engine would stumble coming off idle when trying to transition from idle to power system for air/fuel mixture control.
On an engine controlled by computer, doing this will just screw things up IMHO. Adaptive control will try to learn its way around the extra air it senses coming in at idle. I suspect that it will try to use the IAC (Idle Air Control) valve to try to compensate. This will just make it worse.
You can adjust your TPS, it should be just under 1 volt. Like .980 - .996. You might need to open the bolt holes up a bit to get it adjusted right. If you haven't heard 94-95's don't take well to some cams, but I read in Ask Bernie, Yo Ken that the cobra's use a different tune in their pcm's, so they aren't really suseptable to that problem. You might want to get a custom burned chip, especially to get the most out of your mods. That should clear up any driveabbilty problems.