140 high revving issues

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tipsydaisy
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Joined: Wed Jun 17, 2020 4:24 am

140 high revving issues

Post by tipsydaisy »

I have a '63 Corvan and a '67 convertible. A few years back I swapped a 140 motor into my '63. This van is my "rat rod" so I make modifications as needed. Due to various issues I've always had a tricky throttle, but for the most part it runs great and I put on a few thousand miles each driving season.

This week, for no apparent reason, when I start it, it will immediately rev super high (as in starting to float lifters) with my foot off the gas pedal entirely. Obviously, it acts like something is catching, holding the throttle open. However,
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I've gone through from my pedal all the back through the firewall, then through all the carb linkage (and for those of you with 140s, you know!) to find a hold up, but with no success. Sometimes it will just start and idle as it should. Then every once in a while it does this. I'm baffled. Any ideas?

Thanks! Tiffany
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bbodie52
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Re: 140 high revving issues

Post by bbodie52 »

With engine off, try disconnecting each carburetor from the throttle linkage and then manually open/close each throttle butterfly on each carburetor to check for any binding or movement restriction that could result in a throttle butterfly sticking open. Also monitor the motion of the disconnected linkage while an assistant opens and closes the disconnected throttle linkage. Again, look for any binding or sticking of the mechanism.

The only other thing I can think of that might cause an RPM increase would be timing advance (centrifugal or vacuum) if that part of the distributor was sticking. I don't know if this would cause a runaway engine speed though.
Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible

66vairguy
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Re: 140 high revving issues

Post by 66vairguy »

Hard to say what your problem is, but the secondary carburetors DO NOT HAVE A POSITIVE LINKAGE TO CLOSE THEM LIKE THE PRIMARIES. The only thing that closes a secondary is the accelerator pump spring and linkage. During the 65 model run Chevrolet was getting complaints of throttles that "would not close". A LONG story, but the dealers were instructed to remove the accelerator pump CUPS (leave the pump mechanism intact) to eliminate fuel squirting in when the secondary is opened and could cause "gummy/sticking" throttle plates. Of course inept drivers then complained about stalling when jamming the gas pedal to the floor when the engine was still cold (something that never should be done). Chevy responded by installing a complex linkage to block the secondary carburetors from opening UNTIL the primary carburetor chokes opened (called the 1966 setup) It worked until corrosion, dirt, wear and tear made the linkage bind and caused other issues.

Recently a club buddy came to me with an issue of a secondary staying open -- made for exciting driving!! The carburetor had those awful after market rubber "O" rings, with washers and springs to block air leakage on a worn throttle shaft. The "O" rings had turned to a sticky gooey mess causing the throttle to stick open. Supposedly the local Corvair "expert" had rebuilt them. I took the secondary apart and dumped the "O" ring crap and worn throttle shaft into the trash. Installed a new throttle shaft and used the Clark's secondary shaft return spring. Secondary carburetor now snaps shut like a mouse trap.

Not saying any of this is your issue, but something to check.

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