Engine reving to full throttle on start

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StevieMac

StevieMac

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Belton, SC
I have a 70 Duster with a 340 in it. Was running great without any issues, and when driving home the other day, the engine RPMs just started to sky rocket like at full throttle. I turned off the ignition and coasted to the side of the road to check it out. I check for stuck linkage, and looked for any missing/disconnected vacuum lines, but found nothing out of place. Needless to say, I had to push it onto a trailer to get it home.

When I start it, it immediately tries to sky rocket the RPMs, so I can't really spray carb cleaner around the base of the carb to see if the leak is there.

Any ideas on what else to look at? It is a 340 running MSD ignition and a Holley Slayer series carb.

Thanks
 
Carb must be dumping fuel.
Secondary stuck open?
 
well it's getting air from somewhere , look down the carb and look at the throttle blades , like said check the secondary blades closely . does your car have power breaks ? if so maybe the booster sprung a leak . pull the vacume hose off and plug the carb port and try . just a few things to check first .
 
Make sure a vacuum hose hasn't fallen off or some other air leak somewhere.

When there is no load on the engine it doesn't take much of an air leak to make them RPM through the roof.
 
Thanks everyone, after getting it off the trailer and re looking in the carb, I found the below piece in the passenger side secondary. After getting it out by taking off the carb, I can run the car without it trying to grenade itself. I didn't put the piece back on the carb yet, but it is now running around 3200 rpm. Should I put this back in as it seems to provide air. I also sprayed carb cleaner around the base to make sure there were no leaks...


Again, thank you everyone for your help!



yellow rose said:
Make sure a vacuum hose hasn't fallen off or some other air leak somewhere.

When there is no load on the engine it doesn't take much of an air leak to make them RPM through the roof.
Click to expand...

IMG_20190721_102403.jpg
 
WOW are you lucky! that is a vent tube and could have gone all the way down to the piston, good call on pulling the carb.
 
bugger the end a little and tap back in, that's just the bowl vent. I hand an AFB with no air cleaner lose its threaded stud and wedge the secondary open on the freeway. I took foot off gas and it kept accelerating, strange sensation! Killed motor and coasted to stop on side of freeway. You are fortunate that that didnt make its way down further and stick a valve. I dont think that would have made it past the valve at .20 second slowest opening time...anyone?
 
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