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Post by 71se3834v on Oct 17, 2014 21:05:54 GMT -5
Basically if the car is too heavy and if its got tall gears, combined with an automatic it will bog off the line. It will lose too much engine vacuum initially. Now having said that, the OP did mention that his is a 4 speed car, which helps. But it is still heavy and has a stock engine. I assume it has stock gearing as well. I would not buy a mechanical sec. carburetor for this application. The Carter AVS is a different animal than a Holley double-pumper. Are you talking mechanical vs. vacumn in a Holley carb? Sorry don't know much about Holleys. I assume I have a heavier car. '71 SE (extra trim), big block engine, auto trans, 3.23 open diff, air, lots of options. It came from the factory with a mechanical secondary Carter AVS. Did this carb perform well enough for the engineers to use it?
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Post by robertsrt on Oct 18, 2014 8:40:48 GMT -5
Yes the Chargers are what, like 3700 - 3800 lbs? A pretty heavy car especially with options. If it left the factory with that carb then its definitely the right one. The factory engineers were always pretty conservative. I don't know much about it, maybe some others can chime in, but I would think the AVS is a smaller carb like maybe its a 600 - 650? And even if it has mechanical secondaires, I believe its a hybrid type where it uses a vacuum signal to open up secondary flaps on top as well? Not too sure on those. But for sure its much different than a Holley's design where if its a big CFM unit, dumping open the entire carb at once would not be a good idea unless certain conditions are met like I said...
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Post by 71se3834v on Oct 18, 2014 22:57:50 GMT -5
Yes I believe the AVS is in the 600-650 range. The carburetor date codes to Sept. 30 1970 (I think) with a build date of Nov. 70 on my car and I remember looking up the tag number and it puts it right there for my application. Everything about my car has been original that I've found so far and I've read of others with a Carter AVS on their 383's. Yes it has a spring loaded flap in the air horn over the secondaries that is slow to open when you dump the throttle wide open. I don't know if it opens at the loss of vacumm or if the incoming air pressure pushes it open. This would give the secondaries a much richer mixture along w/the accelerator pump to eliminate a bog. I do know the spring pressure is adjustable. My car had no bog if I jumped on it from a stop and would easily squeal the tire (open diff). ;)I could stomp on it at 40 mph and with no hesitation I was up to 70 in no time. My engine was quite worn out (cylinders, pistons and rings) and had no valve seals left and I thought it ran good before the cam went out. It is my first big block (383 HP engine). It will be interesting to see how it runs after it's been rebuilt and whether the carburetor will have to be tweaked. I know I have some play in the throttle shafts. Working on putting it back together now. Hopefully I will have it done before the snow flys.
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Post by robertsrt on Oct 18, 2014 23:32:17 GMT -5
I had a totally stock 383 auto '71 Road Runner. It probably had 3.23 rear gears. Boy would it roast the tires! All my buddies raved what a great, fast, crisp car it was. The factory carburetor on it was a Holley 3310 series or basically a 780 vac. sec. I guess it was tuned right, or one of those Wednesday cars or something, because for that car, the 780 was perfect. And I know it was a totally stock car. I believe the secondaries on your AVS are vacuum-related somehow. Gosh its been a while! But they open relative to vacuum, so no bog! If it runs crisp its a keeper.
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Post by 71se3834v on Oct 24, 2014 9:30:55 GMT -5
If it was vacuum operated I wouldn't be able to operate it on the bench w/o vacuum. The choke has linkage to lock out the secondaries until the engine is warmed up. Once open the choke linkage drops allowing the linkage to the secondaries to open the plates above approx. 1/2 throttle.
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Archie
Forum Regular
Posts: 258
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Post by Archie on Oct 24, 2014 22:55:53 GMT -5
This is how I understand it. The linkage is mechanical, however the opening of the secondary air door is controlled by both a tension spring and the dashpot which is connected to engine vacuum. The dashpot slow's the opening of the door enough to prevent a lean bog condition. The spring and dashpot work together so the door open gradually to maintain a decent air/fuel ratio.
Jeff, I know I've asked you this before., it's a 6125 correct?
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Post by 71se3834v on Oct 31, 2014 19:10:38 GMT -5
Yep. 6125S
I thought the dash pot was strictly the choke pull off. That's the way it's shown in the rebuild kit. I have the FSM. Maybe I should read up on it. Yeah, right after I get my rebuilt engine up & running!
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brock
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MEGA KILOVOLT-MASTER
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Post by brock on Nov 4, 2014 23:10:26 GMT -5
I love the Carter AVS! Wish I could learn the secrets of the Thermoquad.
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Post by odzking on Nov 5, 2014 11:50:18 GMT -5
I love the Carter AVS! Wish I could learn the secrets of the Thermoquad. You and me both
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Post by 72sunroof on Nov 12, 2014 22:09:25 GMT -5
Well, as long as we're on the subject of carbs, anyone here a ThermoQuad expert or close? My question is this. If I go strictly by the set up manual for adjusting the carburetor, it just doesn't seem right. But I think it is still running to rich the way I have set it up. Here is an abbreviated version of Service Manual for the carburetor and what I did: Base idle mixture setting: screw both idle mixture screws in (clockwise) until they seat *lightly* in their holes. Back both screws out (counter-clockwise) 2.5 turns each. One manual said 1.5 turns and the other said 2, but that didn't seem right. Then I continued until it seemed to peak the RPM's and I turned it back 1/4 turn. I have had a couple people say that is fine, and some say it is not and to follow the Service Manual to the "T" or it will not be right. I have 2 on my site and BOTH are very confusing to me. I would very much like to keep it original but if anyone that can shed some light on this before I convert it back to square bore and forget about it, I'd appreciate it. www.retrorarities.com/images/CarterThermoquadInformation.pdfwww.retrorarities.com/images/Thermo-QuadReferenceBook.pdfThermoquads must be set up in the order the manual states being that they all interact with each other, ie the choke pull off keeps the secondaries closed until wot happens it helps the door spring slow the opening. Have you checked the primaries well seals for leakage. The epoxy hates ethanol and will need to be re-epoxied because a lot of richness can come from that. The mixture screws should be from 20/32 to 24/33 from the base to the tip depending on your tune(aggressive is higher) the tree is really easy all the way down until it stops descending then raise it up 1 1/2 turns. The float level also affects the richness start at 28/32 and the highest is 1 inch The throttle shaft can wear causing unmetered air in can be rebushed using an eBay kit for 25 bucks. Search for vaanth thermoquad guide about Jets and rods. By the way cfm limitations are stories Chevy guys made up. I have a 850 cfm on a 340 runs fine. 340 six pack 1200 cfm! .
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