OEM carb swap 1983 / 1986

69hurstolds

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Jan 2, 2006
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It appears it's a 5204 carb, and under that stamping is the production date code of what looks like 1205, which is the 120th day of 1985. April 30, 1985 to be exact. A Tuesday.

As stated, it probably has the same calibrations so it's likely it's not a big deal.
 

Clone TIE Pilot

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Aug 14, 2011
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It appears it's a 5204 carb, and under that stamping is the production date code of what looks like 1205, which is the 120th day of 1985. April 30, 1985 to be exact. A Tuesday.

As stated, it probably has the same calibrations so it's likely it's not a big deal.
As already stated they do not. Chevy 305 baseplates have idle air bypass installed, the Olds 307 baseplates do not. Also the Olds 307 baseplates have additional timed vacuum slots right above the throttle plates that Chevy 305 baseplates lack.

Idle bypass air is what the name implies. Its a calibrated amount of air that bypasses the throttle plates at idle. Its purpose is to allow the throttle plate opening angle to be reduced at idle to prevent the dreaded main nozzle drip. Thus the presence or lack of idle bypass air has a large effect on the idle system calibration. As Cliff Ruggles states in his book about Qjets, the idle air bypass is instrumental in controlling primary throttle angle at idle speed even with sufficient idle fuel. Not all Qjets used idle air bypass but most applications did, especially HO applications.

Thinking about it, it makes sense that Olds 307s didn't use idle bypass air since the 307's low flow swirl port heads and low compression probably limited idle airflow to the point that bypass wasn't need. Meanwhile, Chevy 305s used in G bodies never had swirl ports and often had higher compression than 307s. In this case the 305s needed bypass air to meet their higher flow demands.
I highly suggest to the OP to purchase and study some books about Qjets such as the Doug Roe and Cliff Ruggles books before proceding further with his car. Hopefully I haven't too aggravating but this is something that shouldvnot be overlooked.
U
 
Last edited:

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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As already stated they do not. Chevy 305 baseplates have idle air bypass installed, the Olds 307 baseplates do not. Also the Olds 307 baseplates have additional timed vacuum slots right above the throttle plates that Chevy 305 baseplates lack.

Idle bypass air is what the name implies. Its a calibrated amount of air that bypasses the throttle plates at idle. Its purpose is to allow the throttle plate opening angle to be reduced at idle to prevent the dreaded main nozzle drip. Thus the presence or lack of idle bypass air has a large effect on the idle system calibration. As Cliff Ruggles states in his book about Qjets, the idle air bypass is instrumental in controlling primary throttle angle at idle speed even with sufficient idle fuel. Not all Qjets used idle air bypass but most applications did, especially HO applications.

Thinking about it, it makes sense that Olds 307s didn't use idle bypass air since the 307's low flow swirl port heads and low compression probably limited idle airflow to the point that bypass wasn't need. Meanwhile, Chevy 305s used in G bodies never had swirl ports and often had higher compression than 307s. In this case the 305s needed bypass air to meet their higher flow demands.
I highly suggest to the OP to purchase and study some books about Qjets such as the Doug Roe and Cliff Ruggles books before proceding further with his car. Hopefully I haven't too aggravating but this is something that shouldvnot be overlooked.
U
To clarify- I was talking about a 5204 and a 3204 carb possibly having the same calibrations. So I meant that if that's the case, a 5204 or 3204 carb shouldn't perform any differently.
 

Clone TIE Pilot

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Just a little update, I figured out what the extra off idle ports are for in the Olds baseplate. They are the discharge ports for the PCV. The Chevy baseplate has a totally different PCV discharge setup far under the throttle plates rather than just above like with the Olds basrplate. Inmother words, Olds uses ported vacuum for their PCV vs manifold vacuum for PCV like Chevy does. The PCV barb is at the same location for both baseplates, just the internal routing is different.
 

69hurstolds

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Jan 2, 2006
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Just a little update, I figured out what the extra off idle ports are for in the Olds baseplate. They are the discharge ports for the PCV. The Chevy baseplate has a totally different PCV discharge setup far under the throttle plates rather than just above like with the Olds basrplate. Inmother words, Olds uses ported vacuum for their PCV vs manifold vacuum for PCV like Chevy does. The PCV barb is at the same location for both baseplates, just the internal routing is different.
Believe me, I know how confusing it can get chasing ports on these carbs. But I think you're getting the Canister Vent Valve (CCV) and the PCV port mixed up on the Olds. Olds uses full-time vacuum for the PCV.

PCV comes through the 1/4" vent tube on the passenger side of the baseplate and vacuum is from the common area under the primary fuel well plugs that is open via rectangular "windows" cut into the baseplate for full-ride manifold vacuum underneath the throttle blades. This also provides the vacuum for the "Tee" port for the primary choke pulloff and air cleaner dampers. When the baseplate is bolted on, you cannot see the PCV porting route very easily.

The driver side has a slightly smaller tube on the baseplate that administers ported vacuum from either side to vent the Canister Control Valve. This porting is on the bottom of the baseplate. These are two small bleed holes above the throttle blades with a common channel "connecting" all 3. There's one more ported vacuum port on the front of the carburetor float bowl and that's for the EGR. It's not on the baseplate, however, but the end of that channel is and is respresented as a horizontal slot just above the throttle plate on the driver side.

Olds idle channels have an upper idle air bleed in the air horn as well as IABV air porting, a lower idle air bleed under the venturies along with a transition slot at the throttle blades. And of course, the mixture screw ports below the throttle blades.

These pics may shed a bit more light on this.

Olds Baseplate Vacuum.jpg


Olds Baseplate Vacuum 1.jpg
 
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69hurstolds

Geezer
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Jan 2, 2006
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Oh, and on that 2nd picture, you'll notice a snaky groove cut into the top of the baseplate on the right where this is for the full time vacuum source for the hot air choke. This port is not used nor desired on E4ME. There's a plastic restrictor in this port behind the choke housing on hot air chokes. Easy to plug that plastic restrictor if you want to convert from hot air to electric choke, with the ability to return to a hot air choke system should you desire. A decent blob of glue in that restrictor's tube and it's done.
 

Clone TIE Pilot

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Yeah, its the CCV and not the PCV, I was mistaken. Moreover, on the Chevy baseplate the CCV circuit is not drilled like the Olds. Instead Chevy cut channels right to the bottom of the front barrels so the CCV discharge is under the throttle plates.
Carb153.jpg


You can also see Olds still used the older channel type idle bypass baseplate despite not being drilled while Chevy used the newer tap style idle air bypass baseplates with the bypass holes drilled. The baseplates serm to have much greater differences than the airhorns. Its difficult to drill bypass air into a baseplate that originally didn't have it.
 
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69hurstolds

Geezer
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Jan 2, 2006
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I dragged it out. This one is an odd duck. I have no earthly idea what car this fits. Maybe even a motorhome? Heck, I dunno.

It sorta mimics an Oldsmobile baseplate in many ways. It has a dead nuts same Rochester part number primary lever as an Olds E-jet carb. Same with everything else on it. The only thing that I can discern different is the little port on the passenger side front corner that has full-time vacuum on it from a port under the primary blade on the passenger side. Also those "grooves to nowhere" on the baseplate in front of the primary blades on the bottom aren't there. But they were useless on the Olds baseplate anyway.

Now, can you use this baseplate on a 4MC Olds VIN Y or 9? After studying the porting paths and what would need modification, it appears so. The only thing required would be to simply plug that corner port. The hot air choke port is there already. Interesting.

Weird Olds Qjet Baseplate.JPG


Weird Olds Qjet Baseplate 1.JPG

Weird Olds Qjet Baseplate 2.JPG
 
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Clone TIE Pilot

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Aug 14, 2011
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That is an odd one. Chevy changed the vacuum emission hose routing almost every year with the Monte Carlos, even adding an extra vacuum port on the carb in 1986. Perhaps Olds did something similar? Changing their vacuum routing and adding another port at s8me point?
 

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