Why does the idle continue to lower even after initial warmup?
Thread Starter
Race Director
Joined: Nov 2018
Posts: 13,197
Likes: 79
Why does the idle continue to lower even after initial warmup?
Trying to understand some carb theory. I just installed a Holley Brawler carb - basically a clone of a double-pumper. I notice that with this carb as well as the Street Avenger it's replacing once it's warmed up to where the choke plate is full vertical and the fast idle cam is no longer in play, the idle will continue to lower a bit after driving around. Say go from an initial 900-ish RPM to 700-ish.
Why does that happen?
Why does that happen?
Race Director
Joined: Mar 2017
Posts: 11,934
Likes: 4,498
From: Virginia
If that carb has a high-idle similar to a Q-Jet, it is either on, or off.
Continued idle lowering may be due to something else warming up. TVS causing (or more likely eliminating) a vacuum leak, perhaps? Carb gasket only sealing when warm?
Continued idle lowering may be due to something else warming up. TVS causing (or more likely eliminating) a vacuum leak, perhaps? Carb gasket only sealing when warm?
Thread Starter
Race Director
Joined: Nov 2018
Posts: 13,197
Likes: 79
Race Director
Joined: Mar 2017
Posts: 11,934
Likes: 4,498
From: Virginia
Thermo-Vacuum Switch. They can either open or close depending on temperature.
Usually these are in your water neck, and do things like shut your heat riser, enable EGR, etc.
I'd look for a vacuum leak, or change. Is the vacuum advance connected to a manifold vacuum port, with nothing else T-ed into it?
Post some photos!
Usually these are in your water neck, and do things like shut your heat riser, enable EGR, etc.
I'd look for a vacuum leak, or change. Is the vacuum advance connected to a manifold vacuum port, with nothing else T-ed into it?
Post some photos!
Thread Starter
Race Director
Joined: Nov 2018
Posts: 13,197
Likes: 79
Thermo-Vacuum Switch. They can either open or close depending on temperature.
Usually these are in your water neck, and do things like shut your heat riser, enable EGR, etc.
I'd look for a vacuum leak, or change. Is the vacuum advance connected to a manifold vacuum port, with nothing else T-ed into it?
Post some photos!
Usually these are in your water neck, and do things like shut your heat riser, enable EGR, etc.
I'd look for a vacuum leak, or change. Is the vacuum advance connected to a manifold vacuum port, with nothing else T-ed into it?
Post some photos!
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
I have never built or tuned any blower setup with vacuum advance....set a tight curve that pulls like 10-12 degrees and that's it. If it responds better with it hooked up...so be it.....but it has to be below the blower.
Jebby
Thread Starter
Race Director
Joined: Nov 2018
Posts: 13,197
Likes: 79
Disconnect the vacuum advance......it should not be above the blower anyway and certainly not on the ported vacuum nipple. If you think you have to have it.....it should be below the blower like your boost reference power valve.
I have never built or tuned any blower setup with vacuum advance....set a tight curve that pulls like 10-12 degrees and that's it. If it responds better with it hooked up...so be it.....but it has to be below the blower.
Jebby
I have never built or tuned any blower setup with vacuum advance....set a tight curve that pulls like 10-12 degrees and that's it. If it responds better with it hooked up...so be it.....but it has to be below the blower.
Jebby
Maybe you could chime in on my recent carb/ignition tuning thread.
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