Brtele Posted February 15, 2022 Share Posted February 15, 2022 I'm getting ready to replace the current Rochester 4GC to a Carter AFB on my 401. - I've got a steel plate shipping from Allstate Carbs. I've also ordered (2) carb base gaskets (haven't determined yet if I'm going to sandwich the steel plate with gaskets. I'm worried about air gaps between the carb and plate, but also understand the heat transfer would be diminished) - The exhaust fitting to the Rochester choke tstat is in a different direction than the Carter. I'm worried about bending/manipulating the existing exhaust tube without breaking it. Should I bend an adapter to connect the existing exhaust pipe fitting to the Carter choke tstat? Is there anything else I should be aware of prior to digging into this project? I've searched the forum and haven't come up with anything additional. Thanks, Brandon Link to comment Share on other sites More sharing options...
EmTee Posted February 16, 2022 Share Posted February 16, 2022 The steel plate just goes between the carb and the gasket. There's no need for two gaskets. Take a look at this: http://www.oldbuickparts.com/product_info.php?products_id=6333. I think that is what you need to connect the choke. 1 Link to comment Share on other sites More sharing options...
jframe Posted February 16, 2022 Share Posted February 16, 2022 I did this conversion last year. All I did with the choke stove pipe was bend it slightly different, and hook it back up. Bought some new sleeving for it, but that was it. Used the thin metal plate on the manifold, and the rest was pretty simple. 2 Link to comment Share on other sites More sharing options...
RivNut Posted February 16, 2022 Share Posted February 16, 2022 You can borrow a tube bender from a local jobber like Advance Auto, AutoZone, NAPA, or O’Reillys and make some non-kinked bends in a new tube. 2 Link to comment Share on other sites More sharing options...
RivVrgn Posted February 16, 2022 Share Posted February 16, 2022 This post is spooky. i'm also contemplating this. I was thinking the same thing. What made you decide this? my rochester is rough, been sitting for a while and the previous owner thought it looked good to paint it gold, Eww. Stuff i've read hear seems carters are the way to go. So, got a line on one and probably gonna do it. Joey Link to comment Share on other sites More sharing options...
telriv Posted February 16, 2022 Share Posted February 16, 2022 (edited) IF no other reason than a performance increase from 475CFM's, 4GC, to 625CFM's,AFB. Like ALL have read, "Nails" LOVE CFM's, the more the better. AND, Joey, the 3921S is the correct carb. for a '65/401. Tom T. Edited February 16, 2022 by telriv (see edit history) 2 Link to comment Share on other sites More sharing options...
Brtele Posted February 16, 2022 Author Share Posted February 16, 2022 39 minutes ago, RivVrgn said: This post is spooky. i'm also contemplating this. I was thinking the same thing. What made you decide this? my rochester is rough, been sitting for a while and the previous owner thought it looked good to paint it gold, Eww. Stuff i've read hear seems carters are the way to go. So, got a line on one and probably gonna do it. Joey The main reason I'm converting is the Rochester 4GC is leaking more fuel than I care to take a chance on. The existing Rochester has been running great outside of leaking fuel between the bowl and throttle plate (driver and passenger side). Link to comment Share on other sites More sharing options...
Brtele Posted February 16, 2022 Author Share Posted February 16, 2022 18 hours ago, EmTee said: The steel plate just goes between the carb and the gasket. There's no need for two gaskets. Take a look at this: http://www.oldbuickparts.com/product_info.php?products_id=6333. I think that is what you need to connect the choke. I agree there shouldn't be a gasket between the steel plate and the carb (if both are flat), but I'm just worried about potential air gaps that a gasket would eliminate. But I also understand the plate-carb gasket would limit heat transfer efficiency. Link to comment Share on other sites More sharing options...
XframeFX Posted February 16, 2022 Share Posted February 16, 2022 18 hours ago, EmTee said: There's no need for two gaskets. But always good to have extras. That is what I did last year and good thing, I remounted my AFB 3X for various reasons. My rough idle must be something else. Still can't put my finger on it. Link to comment Share on other sites More sharing options...
telriv Posted February 16, 2022 Share Posted February 16, 2022 Just plug the holes in the manifold & that plate is NO LONGER needed. We don't drive our cars like when they were new, snow/sleet/cold/hail/extremely adverse weather conditions, so there's little to no reason to actually use this plate. Without the plate your choke set-up will like it too much better than when it goes bad again. Tom T. 1 Link to comment Share on other sites More sharing options...
XframeFX Posted February 16, 2022 Share Posted February 16, 2022 6 minutes ago, telriv said: Just plug the holes in the manifold & that plate is NO LONGER needed. Thanks Tom for getting the word out - again. This comes up quite often and I learned this from you. Wish there was a better way to get the word out. For vacuum leaks, I might still have one in the Power Brake Booster. A quick look, there is zero vacuum after engine shut-down. Check valve is OK. Engine idles a little slower when bypassing the Booster. I'll look at this closer when my Riv leaves it's winter den in the spring. Link to comment Share on other sites More sharing options...
EmTee Posted February 17, 2022 Share Posted February 17, 2022 3 hours ago, XframeFX said: I might still have one in the Power Brake Booster. Get yourself a hand vacuum pump if you don't have one already. I use my $20 HF special way more than I expected I would... Use it to see whether your booster holds vacuum. 1 Link to comment Share on other sites More sharing options...
Brtele Posted February 17, 2022 Author Share Posted February 17, 2022 Got the Rochester off and the Carter on. Bent the choke exhaust pipe up to fit the Carter with no problem and hooked everything else up. Where does this connection (I believe it's a fresh air pipe?) go? I don't see a connection on the Carter for this pipe - should I just cap it off? Link to comment Share on other sites More sharing options...
RivNut Posted February 18, 2022 Share Posted February 18, 2022 The fresh (clean) air pipe at the right rear of the air horn connects with a rubber hose to the bottom of the heat pipe that runs through the right side exhaust manifold. Provides for filtered air for the choke. 1 Link to comment Share on other sites More sharing options...
jframe Posted February 18, 2022 Share Posted February 18, 2022 1 hour ago, Brtele said: Got the Rochester off and the Carter on. Bent the choke exhaust pipe up to fit the Carter with no problem and hooked everything else up. Where does this connection (I believe it's a fresh air pipe?) go? I don't see a connection on the Carter for this pipe - should I just cap it off? I think that connection is for your trans vacuum modulator. Should be a place at the bottom of the carb for it to screw into. 1 Link to comment Share on other sites More sharing options...
1965rivgs Posted February 18, 2022 Share Posted February 18, 2022 14 hours ago, Brtele said: Got the Rochester off and the Carter on. Bent the choke exhaust pipe up to fit the Carter with no problem and hooked everything else up. Where does this connection (I believe it's a fresh air pipe?) go? I don't see a connection on the Carter for this pipe - should I just cap it off? If your replacement AFB is the `66 version it will not have the fitting in the carb base for the vacuum modulator. In `66, Buick tapped directly into a runner on the intake manifold for the trans modulator so eliminated the vacuum fitting on the carb. Tom Mooney 2 Link to comment Share on other sites More sharing options...
RivNut Posted February 18, 2022 Share Posted February 18, 2022 There are two “ports” on the AFB at what I will call the 5:00 position if 12:00 is the front of the carb. A threaded fitting at the very bottom goes to the vacuum modulator on the transmission. A steel tube threads into that. At the top, on the air horn, is a simple tube that you slide a rubber hose on. That’s the one that goes to the heat tube built into the passenger’s side exhaust manifold. The rubber hose goes into the bottom of the heat tube; a steel line wrapped in heat cloth goes from the top of the heat tube to the choke housing. The rubber hose provides filtered air that is heated to the choke. Your car will run without it, but could get debris sucked into the choke housing. When I say filtered air, the air going through it comes through the air filter just like the air going through the carb into your engine. 1 Link to comment Share on other sites More sharing options...
Brtele Posted February 18, 2022 Author Share Posted February 18, 2022 1 hour ago, 1965rivgs said: If your replacement AFB is the `66 version it will not have the fitting in the carb base for the vacuum modulator. In `66, Buick tapped directly into a runner on the intake manifold for the trans modulator so eliminated the vacuum fitting on the carb. Tom Mooney Tom, based on this AFB being a 66, that's exactly where I was heading. Can I simply "tee" or tie into the same intake vacuum port that goes to the HVAC or should the trans vacuum line get a dedicated vacuum source? Thanks for everyone's assistance - I'm ready to fire this B up. Link to comment Share on other sites More sharing options...
1965rivgs Posted February 18, 2022 Share Posted February 18, 2022 43 minutes ago, Brtele said: Tom, based on this AFB being a 66, that's exactly where I was heading. Can I simply "tee" or tie into the same intake vacuum port that goes to the HVAC or should the trans vacuum line get a dedicated vacuum source? Thanks for everyone's assistance - I'm ready to fire this B up. Sure, that`ll work. Tom 1 Link to comment Share on other sites More sharing options...
carbking Posted February 18, 2022 Share Posted February 18, 2022 (edited) The lack of a port is not a problem. Several GM cars used a hollow stud (on Pontiac tripowers, it was the passenger side rear on the center carb) for transmission vacuum source. I know the Pontiac ones (which are probably the same for others) are reproduced. The stud is about 3/8 of an inch longer than normal, and one slips a vacuum hose over the end. Just need to make sure the manifold hole goes completely through. Jon. Edited February 18, 2022 by carbking (see edit history) 1 Link to comment Share on other sites More sharing options...
Bloo Posted February 18, 2022 Share Posted February 18, 2022 The only thing to keep in mind for a transmission is that it needs to be a pure manifold vacuum source. It needs to follow the vacuum level of the engine's intake manifold, just like a vacuum gauge would. You can tee all you want, but the source needs to be manifold vacuum. Don't use any sources on the other side of a check valve, or a vacuum storage system for vacuum powered accessories, or anything like that. No ported (distributor) vacuum. 1 Link to comment Share on other sites More sharing options...
psychostang Posted February 19, 2022 Share Posted February 19, 2022 Brandon, the trans vacuum was tee'd off of the rear port. One part went to the brake booster, the other went to the transmission. 1 Link to comment Share on other sites More sharing options...
Brtele Posted February 20, 2022 Author Share Posted February 20, 2022 Got all the vacuum lines hooked up, adjusted the choke and fast idle cam on the engine dead cold pumped it once and let it rip. Started up right away and ran really good. I believe the carb fell off the fast idle cam properly and I adjusted the idle speed to a good spot. Idle mixture screws were almost dead on at 1.5 turns out. I hate to jinx it but it went really smooth. I even turned it off and then started it up again without any hiccups. Very cool. Thanks for all the guidance. On to the next issue!!! 6 Link to comment Share on other sites More sharing options...
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