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It's fully smogged (stock) with both the primary (ported) and secondary (HAC) vac advance hooked up.assuming your carb is running desmogged and with the vac advance hooked up, I'd suggest plugging the ports to the vac advance cans and get a baseline with mechanical advance only and see if you still have the flat spot...(if gone, then vac advance canisters could be source of vac leak at 1800 revs...)
OK. Thanks Pin_Head.Fuel seepage is not unusual.
Throttle shaft bushing wear is not common for Aisan carbs like it is for Q-jets.
Is there any slop noticible?
Spray some carb cleaner around the shaft while idling and see if the idle speed increases. This would indicate that there is some vacuum leakage.
Yeah, my vac at idle is terrific, but at idle the fuel is being drawn through the 1st Slow circuit (1st Throttle Plate is closed). So what I'm thinking is once that 1st throttle plate moves off idle, air flowing down the primary barrel might also draw air in through that sloppy primary throttle shaft...(Bernoulli Effect)...Throttle bushing wear usually causes a vacuum leak at idle, but you don't seem to have a problem with this.
Right, and then once the secondary kicks in it improves a little, but still not what it should be...A stumble at 1,800 sounds like you might need a little larger primary main jet.
A stumble at 1,800 sounds like you might need a little larger primary main jet.
Like the Secondary Diaphragm?The hollow bolt provides manifold vacuum to the carb to run the power valve among other things.
The hollow bolt provides manifold vacuum to the carb to run the power valve among other things.
Ha Ha! Sorry Pin_Head...I'm just trying to make sense of this...Oy, so many questions I might forget a few.
Ok. So this means that setting the Idle Mixture screw to 4 turns out will not address fuel starvation issues I might be having at 1,800 RPM. Instead, making sure the Power Valve is working correctly will and/or increasing the 1st Main Jet size.1. Off idle fuel comes from the transition slot just above the throttle plate that is fed from the idle circuit. When people have a manifold vacuum leak, they increase the idle speed to compensate, pulling the fuel from the transition slot and causing off idle bogging down.
Yeah, and looking at the cross-section image of the carb above, it shows a vac circuit from which the 2nd diaphragm draws from that runs up the middle of the carb.4. The vacuum secondary does not run on manifold vacuum. It gets vacuum from high up in the primary and secondary venturis. You need vacuum in both bores, which is why it is important that the linkage kicks the secondary open enough to get some vacuum in the secondary venturi.
Thanks for the great tips! Right now I'm thinking unless I have a solid reason to pull it, I'll just leave it. I used the Hygrade Kit, so I'm not sure there was a new gasket that came with the kit for that hollow bolt.6. The hollow bolt can be a pain. If you got it out once, chances are it will come out again. Use a jewlers file to repair the slot or cut a new slot 90 degrees to the other one.
3. Manifold vacuum keeps the power valve closed and when it drops below about 3 inches of water, it opens and richens the primary. I don't really know how to test it other than to verify that the piston moves freely.
5. The power piston operates the power valve. Manifold vacuum retracts the pistion which keeps the valve closed by its spring.
I'm going to go out on a limb ans step on some toes...the power valve is only active under WOT(less than 3" vacuum); won't fix the off-idle bog.
I remember reading a thread once recommending stretching the Power Piston spring about 1/4" during rebuild (new springs don't come in the kits) to give it a little more 'springiness'.I don't really know how to test it other than to verify that the piston moves freely.
I'm good on the Idle Speed screw setting...it is just in from the contact point on the linkage, so I'm pretty sure that I'm not using the throttle (fuel through the 1st nozzle instead of the 1st slow circuit) in order to idle (compensate for a vac leak, for example)more important to make certain that you aren't pulling from the transition slot at idle speeds(back off the idle speed set screw...)
I haven't tried this. What's the best way to do this?have you tried bypassing the egr valve?