Welcome to, The home of the PCCA => General Help- Ask the Experts... => Topic started by: yhalkeeiron on August 10, 2014, 10:03:48 PM

Title: Carburetor
Post by: yhalkeeiron on August 10, 2014, 10:03:48 PM
I have the stock carburetor on my 73 2.0 pinto and sometimes it seems like there's a dead spot in between the barrels like at half throttle I was wondering if it's supposed to be that way? And also I was wondering what this thing is in the picture that goes from the intake manifold to the carburetor?(

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Title: Re: Carburetor
Post by: amc49 on August 14, 2014, 12:17:31 AM
No, it's not supposed to have a dead spot and that is an early incarnation of a decel valve, it richens slightly at decel to prevent afterburning bad enough to make audible backfire, which sells no cars.
Title: Re: Carburetor
Post by: Wittsend on August 14, 2014, 12:53:26 PM
The Datsun 510's etc. had a similar version built into the carburetor.  They called it Boost Controlled De-acceleration Device (BCDD). The reason they stated was to enrich the lean mixture under de-acceleration.  Apparently the lean mixture would not completely burn and that increased HC levels. By adding more fuel the burn was more complete and the HC levels dropped.  Back in the day I found it strange that one would add fuel to decrease HC levels. I is smarter now than I be then. LOL
Title: Re: Carburetor
Post by: jeremysdad on August 14, 2014, 07:37:36 PM
It's a huge vacuum leak. I plugged mine with a random pipe thread plug that I found in my Dad's shed.

Also known as a 'Deceleration valve'. You can get them from Rock Auto.
Title: Re: Carburetor
Post by: yhalkeeiron on August 14, 2014, 09:17:44 PM
Ok thanks I'm going to take it off and see if that fixes the dead spot.  And I'm going to also put in a larger air filter to try and increase the flow

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Title: Re: Carburetor
Post by: amc49 on August 16, 2014, 05:30:56 AM
You'll be wanting to look at power valve operation and the second barrel starting position before it opens, if dead full shut it can make a slight dead spot when it just cracks open.

I've seen engines correct lean OR rich to control decel emissions, it just seems to be the quirk of that particular motor that shows which way to go. If the engine sucks too hard on the idle circuits at decel then those adjust leaner like some Hondas do. They use a separate add-on idle air bleed that opens only at decel time. The Fords ran too lean and why the decel adds fuel there as well. Rich leads to high HC obviously, but lean does too when the combustion becomes incomplete enough. Bad enough and you get the popping noise in pipe from incomplete combustion still afterburning in the pipe. If lumpy enough instead of smooth burn it goes off all at once. Like when you used to take old car, cut the key off, pump gas 2-3 times and then turn key back on to explode the unburned stuff you just put in the pipe. We blew off a muffler on a '57 Ford wagon one night doing that. Fun.