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Diagnose a Quadrajet carburetor problem / Custom Tuning on Weekends
« Last post by Cliff Ruggles on April 19, 2021, 03:36:44 AM »
For several years now I've been opening up a couple of Saturdays a month for custom tuning.  Been having a great time with it and have met some really nice folks and had quite a few "troubled" carbs and entire vehicles brought here to custom tune.

I thought I'd start sharing a few of the stories as we get to see some pretty cool vehicles and a lot of different combinations.  I'll not mention any names, just going to talk about the what I did to fix the issues.

Last Saturday had a later 70's Corvette driven here with a pretty "fresh" 383 stroker engine in it.  It was topped with Pro Comp heads, about 10 to 1 compression and Comp XE256 cam.  I know it's a pretty small cam for that engine combo but we were able to get it sorted out pretty good.

When it showed up I checked the timing and it was at 26 degrees initial, 52 total and another 11 from the vacuum advance for 63 degrees at cruise.  The curve was pretty quick but not adding any timing at idle speed.  With that much timing in it at idle backing the speed screw clear off it was still running WAY too fast at idle for my liking, probably around 850-900 rpms.

I moved the timing to 10 initial, which provided 36 total plus 11 VA for 47 at cruise.  This dropped manifold vacuum at idle to about 18" and I could lower the idle speed to about 700rpm's, so good to go there.

I dove into the carb next, the owner had no idea who, what or when it was worked on last.  It had a good NAPA blue pump in it but recent "soft" blue seal which was already swelled up and failing (sticking in the bore).  The N/S assembly was the typical small windowed variety showing up in all the kits from the parts store and on-line sources. 

The power piston hanger arms were purposely bend down about 3/8" and the tips of the 54K metering rods had been removed.  Not quite sure what was up with all that but the jets were stamped #71 but drilled to just past .080".  I scratched my head on that one for a few moments then moved on.

The carb had been converted to electric choke but the vacuum was still open to the choke housing so it was sucking dirty air thru the engine.  Blocked that off, left the bypass air alone and ultra-sonic cleaned all the parts and started putting it back together.  I installed 75 main jets and my full tapered 44 custom rods with .026 tips.  Removed the plug from the airhorn for APT access and installed a 3/8 set screw in it.  Installed a solid high-flow N/S assembly, new accl pump, orange PP spring, and new secondary spring.  Replaced the CH rods with DA rods.

Since it was making PLENTY of vacuum at idle with only 10 degrees timing I left the idle tubes and DCR's stock, and all airbleeds idle and main at the stock sizes.

The choke pull-off had no restriction in it so I replaced it with one that I modified with a tiny hole" hole in the inlet for a clean/quick/smooth release to prevent hesitation/stumble/bog going quickly to full throttle.

....continued
92
Lobby / Re: Unicase th350 rebuild
« Last post by Cliff Ruggles on April 19, 2021, 03:05:53 AM »
Thanks....
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Diagnose a Quadrajet carburetor problem / Re: No Vacuum at Carb Ports
« Last post by Cliff Ruggles on April 19, 2021, 03:04:37 AM »
I see the WRONG gasket used between the main casting and baseplate on those divorced Chevy all the time.  There was a change made in the early 1970's when EGR started showing up so they changed the baseplate some and the gasket is different.  Since "generic" rebuild kits come with a bunch of gaskets not needed for your carb number it's pretty common to see the wrong one used. 

Of course when mixing/matching main castings and baseplates you have to know which gasket to use for the parts you have selected or there will not be any vacuum at the port coming out on the choke side........
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Diagnose a Quadrajet carburetor problem / Re: No Vacuum at Carb Ports
« Last post by Kenth on April 18, 2021, 01:16:37 PM »
Sorry for my mixup on port-#´s, but still the ported source should provide vacuum at slightly opened throttle blades IF the throttle plate is correct for the application.

Looking closer to your pic it seem like you have a "frankencarb" put together with parts from several different units.
F.e. Carter © air horn with Rochester Parts RP choke blade never happened from factory, two completely different plants.
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Diagnose a Quadrajet carburetor problem / Re: No Vacuum at Carb Ports
« Last post by Chevynation on April 18, 2021, 11:18:38 AM »
#1 port should be manifold vacuum and #3 should be ported. I don’t have vacuum to either. I do have vacuum at the port for the choke pull off and the pcv.
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Diagnose a Quadrajet carburetor problem / Re: No Vacuum at Carb Ports
« Last post by Kenth on April 18, 2021, 10:26:31 AM »
#1 port is "timed"/"ported" and as such it will provide vacuum with throttle blades opened some.
#3 cause of no vacuum here is most likely due to incorrect gasket between throtlle plate and float bowl.
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Diagnose a Quadrajet carburetor problem / Re: No Vacuum at Carb Ports
« Last post by Chevynation on April 18, 2021, 08:21:34 AM »
The large port in the front for the pcv has vacuum and the choke pull off has vacuum. There is not a large port in the rear for power brakes. I use a fitting on the manifold for power brakes. Numbers 1 and 3 do not have vacuum.

https://cimg9.ibsrv.net/gimg/www.corvetteforum.com-vbulletin/1600x1200/carbs_l1600_83f94708a78c1e0d7a24b60152618a279819e0fe.jpg
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Diagnose a Quadrajet carburetor problem / Re: No Vacuum at Carb Ports
« Last post by Cliff Ruggles on April 18, 2021, 03:38:07 AM »
The vacuum port just to the right of the fuel filter housing in the main casting is the distributor port.  It is ported vacuum and will work fine for the vacuum advance.

What is the application and engine specs?  Most engines are fine with a good ported vacuum source designed for the distributor.......Cliff
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Quadrajet Parts and Numbers / Re: Throttle Plate ID for a 7040273
« Last post by Kenth on April 17, 2021, 11:55:39 PM »
I would say yours is correct, comparing it to my original 7040270 plate.
Your spare shaft is from another carb obviously.

 
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Quadrajet Parts and Numbers / Throttle Plate ID for a 7040273
« Last post by lintmann on April 17, 2021, 06:33:12 PM »
Hey Guys I was wondering if someone could help me ID this throttle body assy and the levers. I'm pretty sure it is original to my 7040273 carb but just wanted to be sure.

The casting is 7037174 which I think is correct however it was also used on other quadrajets of the era so that is only a clue. Hopefully it will have some other features that someone could use to confirm it's the correct throttle body assy.

My other question was about the primary throttle shaft/levers. Which one would be correct for this carb, the one that is in the body or the one beside it)? The one that is installed is the one that I believe to be correct. If you look closely at the small levers on the choke side, you will see that it is more pointed that the one that is not installed. Also, the radius of this one where the plates attach in the bore is quite sharp where as the one (green one) that is not installed has an obvious radius. Not that this radius should matter but it might help to ID the correct shaft.

Any help is appreciated as I'm about to rebuild this carb and want to have it as correct as possible.
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