So something that I randomly thought about yesterday was
@mwebfj60 post on the secondary system for the carb. This got me thinking about my high idle issue and the fact that my secondary slow cut valve on my OEM stock carb has always had a slight leak.
After the last rebuild I did the mod where I put the white holley lighter spring in the secondary diaphragm to allow the secondary to open easier. However, I am wondering if for whatever reason my secondary slow cut system is now completely open to vacuum which is causing the system to constantly draw fuel in through the secondary system, thus creating the high idle. The momentary blip of the gas pedal would cause a drop in the vacuum that could potentially get the leak in the secondary system to then go away which allows me to go back to a "normal" idle that I have set.
@mwebfj60 @OSS @Spike Strip @FJ40Jim @CaptClose What do you guys think? Does my thought process make sense?
That's some nice out of the box thinking. I've mulled this around since you tagged me and the blip of the throttle is what I keep coming back to. To me it seems either mechanical or vacuum related. Like Jim mentioned above, could be a butterfly hanging up and staying open just a little. There is fore to aft movement on the shaft the butterfly valves are screwed to and really what keeps them centered in the bore is the butterfly itself. Can you see any marring or anything on the bore where maybe one is hanging up? When the high idle is happening maybe try moving the butterfly shaft fore or aft by hand and see if it drops the idle. If you just sit and let it idle high does it eventually settle back down by itself?
On the secondary side, the butterfly valves would have to be partially open as well to draw enough vacuum to pull gas through the slow cut valve and if it is just a leak, it doesn't seem like there would be enough fuel supplied with the air to bump your idle up unless that valve is totally shot and fuel flows freely. Maybe an issue with the secondary diaphragm not returning back down after it is engaged, that would cause the secondary butterflies to stay open a little. This still wouldn't account for enough fuel through the slow cut valve though. Have you tried manually pushing the diaphragm rod back down by hand when the high idle is happening? Maybe pop the slow cut valve off and check how much flow it has when closed with some low pressure compressed air?
Then the power valve keeps popping in my head. Could be a power valve sticking open and letting excess fuel flow but then again, you need the airflow. This one is kind of counterintuitive because the power valve opens under low vacuum like WOT. A couple of my carbs had very sticky power pistons that had to be "honed' a bit to get them to slide in the bore smoothly.
Then vacuum comes to mind as it's when the motor is warmed up and various vacuum scenarios change with the HIC and BVSVs. But your issue only happens with your carb so that pretty much rules those things out.
The only way I see extra fuel causing an increase in idle without the excess airflow through the butterflies is if you are running lean at idle or you have a vacuum leak. If you're running lean at idle, it would seem backing out the IM screw would give you increases in RPM and I'm sure you've been through the lean drop a few times and would have found that by now. Or...a different scenario, if you are running overly rich and after driving, the fuel level in the bowl is dropping and causing the mix to lean out and increasing the idle, then with the little shot of fuel from the pump, richens it back up and drops it down...again, you probably would have caught this on the lean drop and sometimes bumping the throttle works and sometimes it doesn't.
A test for test's sake may be to pull the vacuum line off the choke opener, back the high idle screw out, then see what happens when pulling the choke cable when it's doing it. Just to see what happens.
That's all I got bud. Wish you were closer, this is the type of tinkering I enjoy...as frustrating as it is, still fun for me.
