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| mixture screw = nothing https://www.slantsix.org/forum/viewtopic.php?t=66722 |
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| Author: | 69moredoor [ Thu Aug 11, 2022 3:38 pm ] |
| Post subject: | mixture screw = nothing |
Scratching my head on this slant with a 1920. I was attempting to adjust the mixture screw and noticed turning it did…nothing All the way in fully seated? It kept running. All the way in, all the way out…no change. I’m trying to fix a high idle Condition and tried changing the mixture to see if that affected it…but nope. No change What could cause the mixture screw to be ineffective It’s does run great at 1100 rpm’s. 
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| Author: | kesteb [ Thu Aug 11, 2022 4:00 pm ] |
| Post subject: | Re: mixture screw = nothing |
A vacuum leak or worn out throttle shafts. Both can be discovered with spraying carb cleaner around as a test. If the rpms go up, you found the problem. |
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| Author: | 69moredoor [ Thu Aug 11, 2022 4:27 pm ] |
| Post subject: | Re: mixture screw = nothing |
That’s what I thought too…but I sprayed carb cleaner, wd40, starting fluid over everything. Then tried a small propane torch (unlit). Everything. Disconnected all vac ports on carb. Capped them. Same. No leak detected… Hmmmm |
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| Author: | SlantSixDan [ Thu Aug 11, 2022 5:03 pm ] |
| Post subject: | Re: mixture screw = nothing |
Plug off the PCV (pull the valve and put some sturdy tape over the end, or pull the hose off the valve and stuff something of suitable diameter in it). Now does the idle mixture screw work? Also, your screen name suggests you have a 1969 car. That means it was built with this throttle plate anti-ice system. Whether it's intact or in pieces, it's a likely and easily-overlooked source of a vacuum leak. It is thoroughly unnecessary with today's gasoline formulations. All it does is heat up the carburetor like a teakettle and cause a rip-roaring case of percolation: fuel boils in the carb bowl, overboils through the nozzles into the intake tract, and floods the engine. Bypass it by removing the metal pipe from the carburetor throttle body, installing an appropriately-sized invert-flare/hose barb fitting in its place (I think it's 1/4" IV-flare, but could be 3/16"), and connect the rubber hose directly from the nipple on the underside of the air cleaner baseplate to the new nipple you just installed on the carburetor throttle body. |
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| Author: | 69moredoor [ Thu Aug 11, 2022 5:32 pm ] |
| Post subject: | Re: mixture screw = nothing |
Dan, thanks for the info It’s actually a 78 peanut 225. (In a 1980 aspen) So it doesn’t have the de-icer You didn’t say if sealing the pcv port , and the mixture screw works, what that means? This a.m. I had the pcv hose plugged at the carb. It could suck air through the valve cover causing a leak? I won’t be able to tinker with it until Monday , so please stay tuned as I’ll need the help 
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| Author: | SlantSixDan [ Thu Aug 11, 2022 9:43 pm ] |
| Post subject: | Re: mixture screw = nothing |
If you plug the PCV off and then you can adjust the idle with the mixture screw, it could mean you are getting too much air via the PCV (faulty PCV valve) or too much combustible stuff via the PCV (a faulty fuel pump or a whipped engine). |
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| Author: | wjajr [ Fri Aug 12, 2022 6:15 am ] |
| Post subject: | Re: mixture screw = nothing |
one other thing to consider, if the base of the carburetor is not flat or loose one can not squirt enough carb cleaner, water, bunny farts to get engine to change rpm. Too much total area or linear crack sucking air to be fully covered with said spray of above test compounds. I suspect engine could be sucking air from some other source aside from the carburetor/ intake interface such as a bad power brake booster if equipped, or head to intake gasket contact failure, or a vacuum line up stream from the carburetor is cracked or disconnected. A quick test for bad vacuum lines is to cap off all vacuum taps on the carburetor, and see if that results in the mixture screw causing a change in rpm. |
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| Author: | 69moredoor [ Wed Aug 24, 2022 4:27 pm ] |
| Post subject: | Re: mixture screw = nothing |
Ok so I got the idle figure out. Believe it or not something was binding allowing the throttle blades to not close all the way. Looked closed when looking down but after manually pulling on the throttle I could feel it bind and then release. Weird but solved. Now only one thing remains. It’s running rich and the mixture screw does nothing. I screw it all the way in and it won’t die. Somethings plugged? Pop it off and clean it? |
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