First a look at where the vacuum for the secondary diaphragm comes from..
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- #2
The secondary slow cut circuit can be thought of as serving the same function for the secondary circuit as the idle circuit (primary slow) serves for the primary circuit. The slow cut valve is closed until the throttle linkage mechanically pushes the secondary throttle plate open just a little...about like the primary plates at idle. That is when the slow cut valve should be opening to allow fuel flow. The vacuum port and route is shown in the pics
Thank you for these great carb write ups! I was drawing schematics of every port and then stumbled onto your threads, it’s making me think hard about the operation.
On the first picture of secondary slow cut (cast steel base), you show the port above the throttle plate circled in red and refer to the “vacuum route”. That is actually the passage for the gas to flow from the slow cut valve into the secondary throat. As soon as the primary linkage touches the slow cut lever and there is a pressure differential in the throat of the secondary, gas starts flowing through the secondary slow circuit. The prerequisite is the primary opens enough to create a vacuum on the secondary diaphragm for it to open the secondary throttle plate, and the primary linkage opens the slow cut valve via the external lever.
I’m not totally clear what happens in terms of vacuum after the secondary vacuum diaphragm opens the secondary throttle plate, but I think a pressure differential is created and suction (vacuum?) then pulls gas through the hole in the secondary throat above the throttle plate when it’s partially open.
Hope this doesn’t muck up your clean write up but it’s giving me a chance to learn more. Below is the picture you posted that I’m referring to.
On the first picture of secondary slow cut (cast steel base), you show the port above the throttle plate circled in red and refer to the “vacuum route”. That is actually the passage for the gas to flow from the slow cut valve into the secondary throat. As soon as the primary linkage touches the slow cut lever and there is a pressure differential in the throat of the secondary, gas starts flowing through the secondary slow circuit. The prerequisite is the primary opens enough to create a vacuum on the secondary diaphragm for it to open the secondary throttle plate, and the primary linkage opens the slow cut valve via the external lever.
I’m not totally clear what happens in terms of vacuum after the secondary vacuum diaphragm opens the secondary throttle plate, but I think a pressure differential is created and suction (vacuum?) then pulls gas through the hole in the secondary throat above the throttle plate when it’s partially open.
Hope this doesn’t muck up your clean write up but it’s giving me a chance to learn more. Below is the picture you posted that I’m referring to.
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I’m glad you brought this up honestly cuz my carb now isn’t pulling at the secondary. The rod doesn’t move even tho I’ve swapped out for a lighter spring and I made sure the o-ring is correct. I can engage the rod and hear and feel vacuum when I’ve got the diaphragm housing in hand off the carb. When it’s attached to the carb I can push the rod and I can hear the diaphragm move. But the paperclip trick doesn’t move when I go for a drive and have WOT.... So that leaves me to believe I’ve got crud in the bowl or in the slow jet.
Right?
Right?
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- Thread starter
- #5
That's correct, fuel is pulled through those passages like the primary slow circuit. Once the butterfly is opened further, vacuum through the secondary bore will start to pull fuel through the secondary circuit...just like going from idle to primary on the primary side. The reason the secondary cut valve is there is to only allow the secondary slow (for lack of a better term) to operate when you are at a point in the throttle position for the secondaries to start engaging.
I don’t think it has to do with crud in your bowl. After the primary kicks the secondary linkage open a tiny bit the secondary can open by vacuum and is separate from the slow cut circuit. Make sure you’re primary linkage is hitting the secondary linkage and the secondary plate is opening the amount called for in the fsm. There’s probably an adjustment that can be made there by bending the tab. I’ll look at it in a minute. Then the diaphragm is operated by vacuum per mweb’s picture that I snagged from above. If it’s not opening, and you’re positive the diaphragm and gasket are good, and I know you have the lighter spring, the only other thing to look at is to make sure the insulator block is oriented the correct way - it seals the vacuum path and directs the vacuum to the diaphragm, shown in the picture below when oriented correctly.
This is the correct orientation. The area I’ve circled in yellow will seal the vacuum port and direct vacuum to the secondary diaphragm.
Edit: check out my pics below to tell if the insulator is flipped, while carb is on truck
Edit: check out my pics below to tell if the insulator is flipped, while carb is on truck
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This is what you want to see on the outside of the carb for correct orientation of the insulator
Incorrect orientation, FCS won’t behave right and vacuum leak to the secondary diaphragm.
I’m 99.9% sure it’s correct but I’ll eyeball it for sure. Dark out here now and nasty weather coming tmrw. Been driving the beater non gas guzzler for now.
Thanks both of you for responding.
Thanks both of you for responding.
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- #11
You can pull the diaghragm off and blow some air through the port...not 150 psi, maybe 20 or so, and see if air moves through the carb and out the port in the bore.
Prob prime the fuel bowl first then try that so I’ll see bubbles? Or I’m thinking opposite? Holy mackerel is thinking making my head hurt!
Looked thru my photos and found proof right away I’ve got that spacer orientated correct.
Tho I do see cracks now in that hose off the carb insulator that I didn’t know were there! Damn.
No, take off the diaphragm and just blow air into the vacuum hole, and feel if it’s coming out the hole in the throat on the secondary side. It’s just the vacuum path your are checking to see if it’s clear. I guess if you spray a little brake cleaner in there and then blow out with air you might be able to see it better. EDIT: I just blew brake cleaner into the vacuum passage where the diaphragm mounts to the carb and the stream comes out two holes located just at the base of the Venturis in the primary and secondary throats. Should be easy to see...
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Worst design in the world!! I’m always afraid of touching it for fear it will crack. I thought you already jb welded it before.
The silicone hose is cracked. Not where it exits the insulator.