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At wide open throttle should both the primary and secondary throttle blades on a Holley double pumper be vertical?
I ask this because I was verifying my accelerator cable adjustment at the carburetor throttle linkage. I noticed that at maximum rotation of the throttle linkage my secondary butterflies do not get to a vertical position.
Upon closer inspection when the primary throttle shaft stops moving I am still able to rotate the secondary throttle shaft to full open. So the secondaries don't seem to have any restriction in their range of movement. They simply aren't being pulled to full open by the time the throttle linkage stops moving.
So I imagine that I might have interference that I never noticed before between my primary throttle plates and a carburetor spacer/heat shield and/or my secondary throttle link is not adjusted properly.
Can't believe I never noticed this. Correcting this will have a dramatic impact I'm sure.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Ensure there isn't any slack in the throttle cable with the engine off and the cable/linkage is nit binding or interfering with anything. The slightest movement in the throttle should squirt gas out of the primary squirters. Having verified or adjusted the cable and the secondaries still aren't opening, consider buying a throttle cable adjustable bracket. Also, try bending the rod the gas pedal is attached to. That solved my problem in getting the secondaries to fully open.
The primaries open the secondaries. The link is L shaped. It can be bent so it is shorter to open them farther. But you also need the 2ndaries fully closed at idle.
The primaries open the secondaries. The link is L shaped. It can be bent so it is shorter to open them farther. But you also need the 2ndaries fully closed at idle.
I think the issue might be that the primary shaft stops moving early, so the secondaries can't open all the way. I might have to pull the carburetor and check if there is interference with the carb spacer and heat shield near the primaries.
For reference, what normally stops the primary shaft from rotating? It would be nice to inspect that to see of I am getting full rotation or if I am stopping short, and if so by how much. Does full open correspond to contact with some stopping criteria that I can inspect on/near the primary shaft?
You are positive that this is a double pumper and not a vacuum secondary carburetor?
Can you post the list# found on the forward facing surface of the choke airhorn?
My carburetor choke tower is stamped with 76650, a Holley Ultra Double Pumper 650 CFM
I think I found the problem. While I thought my heat shield was trimmed to avoid obstructions, it looks like a piece of it is in fact interfering with the opposite side (passenger) of the main throttle shaft.
Whatever this piece of the main throttle shaft is called, it cannot continue below the height of the heat shield. The main throttle shaft stops its rotation early, and neither the primaries or secondaries achieve true WOT.
I never noticed this because it is pretty well hidden underneath the electric choke. Looks like I have some trimming to do...
If anyone knows what the purpose of that piece of metal is, and the screw, I'm curious what it does.
Last edited by Turbotrad; May 21, 2019 at 04:39 PM.
That appears to be an adjustment for the fast idle choke setting. Hard to tell.
That makes sense. What you see in the picture is the result of moving the throttle linkage as far open as it will go. So when released, in an idle position, that screw would in fact interact with the choke and influence the fast idle speed.
So, how does the saying go...measure once, cut twice?
Last edited by Turbotrad; May 21, 2019 at 04:56 PM.
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So, it may be figured out. The above is the 3 throttle settings showing interaction between pri and secondary. The link does not have to hold sec closed as there is a sturdy spring for that job. Both mech and vac carbs have a stout stop setting the pri at vertical. This guy here: full throttle.
Last edited by derekderek; May 21, 2019 at 05:53 PM.
Holley DP WOT throttle blade position
