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Technical Rochester 2 jet troubles

Discussion in 'The Hokey Ass Message Board' started by SlowBoat, May 9, 2019.

  1. SlowBoat
    Joined: May 9, 2019
    Posts: 3

    SlowBoat

    Hello all, and thanks for everyone who posts on this forum. I've found much good help and humor here for years now trolling from a distance... but after exhausting the archives for what I could I finally had to make an account and see if I could figure this little problem of mine out.

    I have a Rochester 2GV divorced choke, large bore 1 & 11/16", large spread bolt pattern close to what I've seen referred to on here as a "rectangle pattern", that came on the car I believe based on the serial (1969 impala 327/t350) with the matching intake.

    I bought a new carburetor since the Amazon deal seemed right at 90 bucks vs 40 for a rebuild kit and float and the time to rebuild it. Regret the decision a bit now, but since I am down the rabbit hole already here I am... I digress so I'll get to the question.

    What in the frig is this hole here, maybe EGR?And can I plug them with jb weld and use the new carb and use what appears to be the equivalent vacuum system for what I believe to be an idle enrichment circuit of some sort? I will also attach some photos of the differences between the two carbs. Hoping I can at least Frankenstein the two together and have a well working carb between them.

    Hopefully this all made enough sense with the photos.
    . 20190509_141214.jpg 20190509_141347.jpg 20190509_141329.jpg 20190509_142808.jpg 20190509_142747.jpg 20190509_142701.jpg 20190509_142719.jpg 20190509_142450.jpg 20190509_142458.jpg
     

    Attached Files:

  2. demian5
    Joined: Apr 22, 2013
    Posts: 43

    demian5
    Member

    Those look like two totally different carbs. Id be rebuilding the OG one.

    BTW that's an exhaust crossover to help with warm ups and to get the choke to open up. There is a valve on the exhaust between the manifold and the pipe that is shut when the vehicle is cold to force hot exhaust under the carb (usually on the passenger side). It has a spring that opens the valve with heat, like the divorced choke. I think its called an EFE valve (Early Fuel Evaporation).
     
  3. saltflats
    Joined: Aug 14, 2007
    Posts: 10,317

    saltflats
    Member
    from Missouri

    If the throttle shaft is not to loose, maybe you could change the throttle plate.
     
    Last edited: May 9, 2019
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  4. fordflambe
    Joined: Apr 9, 2007
    Posts: 516

    fordflambe
    Member

    The two carb tops you are showing represent two of many ways Rochester vented the fuel bowl over the years. The one with the flapper is an earlier version while the one with the tube coming out is a later carb and I believe that port was connected to a vapor recovery system.

    Charlie Price (Vintage Speed) has several YouTube videos of rebuilding Rochester 2bbl as well as Stromberg and Holley 2bbl. Here is one link to a Rochester video. There are more, check them out.

     
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  5. dan griffin
    Joined: Dec 25, 2009
    Posts: 474

    dan griffin
    Member

    The new carb is a Al Gore approved 71-up smog carb. It will work but it will run very lean. take the main jets out of the 69 carb and put them in the new carb.Set the float at 69-70 specs. Its going to idle to lean you may have to put a small copper wire in the air bleed. bend the wire in a horse shoe shape so it will stay in place.






















    al
     
  6. Bay City carbs came out at a later date. Bad deal , by some vendor who probably didn't have a clue.
    Your 69 should be a 1.25 venturi. The 69-70 350 had the 1.375 v . That's about the biggest 2GC there was. Jet Carbs sells those for the limited roundy-round racers.
    Stick with the 69 carb.
    Yes, the passages are for exhaust heat. You could make a plate to block it off and use the later carb, depending on where you live. The BC carb is probably leaner. What jets are in both carbs?
     
  7. SlowBoat
    Joined: May 9, 2019
    Posts: 3

    SlowBoat

    Demian5 - The new carburetor is a Rochester 2GC based on a Pontiac early 70's design. Was told by the company it would be a direct swap. The original carb is a Rochester 2GV, made for the '69 Chevy 327. Same basic model of carb, but quite different for sure.

    Saltflats - I did pop the throttle body plates off to check if they were a direct swap but they appear to have different idle functions based on the difference in holes and slots. And I was worried about this affecting the vacuum or fuel delivery for the carb to function well. I thought the extra holes are used to adjust the fuel mixture under light load at idle, such as freeway driving.

    Does anyone see any issues blocking off the exhaust crossover besides maybe a little more difficulty starting up? And then running the new carb as it is?
     

    Attached Files:

  8. SlowBoat
    Joined: May 9, 2019
    Posts: 3

    SlowBoat

    Fordflambe - thanks for defining this difference for me about the fuel bowl vent system. Any idea where would I route the vapor recovery system outlet to in place of the flapper if used? And thanks for the video link. I think I might end up rebuilding the original equipment in the end but it's great to increase my understanding.

    Dangriffen - would swapping out the whole venturi assembly have the same effect as changing the jets? Sounds more and more like I should just rebuild the old carb, tuning with wires doesn't seem to be the most reliable method for a good idle mix.

    Mark yak - you are correct about the old carb having 1.25 venturi and I havent measured the new carb as I'm starting to think I should return it instead of pull it apart further. How would that plate affect operation if I was to install it and use the venturi out of the old carb in the "bay city" model.


    Thanks everyone for the quick and helpful responses!!
     
  9. If you're talking about the venturi cluster, or the boosters, then no, it won't help you any. Might not even interchange.
    What is the problem with the 69 carb? Those kits, jets are available all over the internet

    Edit : Again, what jets are in both carbs?
     
  10. Flathead Dave
    Joined: Mar 21, 2014
    Posts: 2,987

    Flathead Dave
    ALLIANCE MEMBER
    from So. Cal.

  11. rmorris
    Joined: Jun 3, 2017
    Posts: 102

    rmorris
    Member





    Sent from my iPhone using H.A.M.B.
     
  12. rmorris
    Joined: Jun 3, 2017
    Posts: 102

    rmorris
    Member

    I rebuilt my Rochester 2 Barrel small bore, 3 times and never got it right. After losing 3 months of cruising, I put on a new Edelbrock intake and 600 carburetor for carefree cruising and peace of mind.


    Sent from my iPhone using H.A.M.B.
     
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  13. dan griffin
    Joined: Dec 25, 2009
    Posts: 474

    dan griffin
    Member

    I was GM tune up mechanic in the 60s-70s and in 71 the new smog laws came into affect. A lean engine makes less smog but runs like shit. Over time I came up with ways richen up the mixture and make the car drivable. In your case I would rebuild the old carb.
     
    Deuces, Truck64 and saltflats like this.
  14. They're not a very complex or troublesome carb. Just sayin'...
     
  15. Heavy Old Steel
    Joined: Feb 1, 2019
    Posts: 58

    Heavy Old Steel
    Member

    You need to run a spacer or plate that will block the two outer heat holes, plug the vacuum port unless you need it for something and don't know about that particular type of bowl vent, but it needs to have atmospheric pressure on it no vacuum or blocking it off.
     
  16. carbking
    Joined: Dec 20, 2008
    Posts: 2,871

    carbking
    Member

    Wish posters would list carburetor ID numbers, it makes answering questions so much easier.

    Assuming you have an original 1969 Chevrolet 2-Jet, and an original 1971 or newer Pontiac 2-Jet (if the new carb is a modern clone, ignore the rest of this post, and just rebuild the original):

    The carbs, while extremely similar externally, have totally different internal calibrations. Without checking individual specifications (cannot anyway, no carburetor numbers), would expect the Pontiac to have MUCH larger idle jets (pressed into the cluster) than the Chevrolet. There are other differences as well.

    Assuming that you can figure out the throttle body differences, I would expect poor idle and off-idle, but run very well at higher RPM.

    As others have suggested, my vote would be rebuilding the original carb. One of the easiest carbs to rebuild that has ever been produced, and virtually bullet-proof.

    As far as the price of the new carb, check the second line in my signature block ;)

    Jon.
     
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  17. I use a lot of the Rochester two bbl carbs. The one on the 305 I just installed in my wrecker. Its one of those that only has two bolts holding it to the intake manifold. And I suspect a slight vacuum leak. I plan to swap the intake to one from a 60,s 283 and use a small bore carb. and I will get a filler tube in the front! Any road my advice is to rebuild your original carb.
     
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  18. fordflambe
    Joined: Apr 9, 2007
    Posts: 516

    fordflambe
    Member

    If you watch the video, Charlie Price tells you he strips all the vent mechanisms and plugs the vent holes. You should be able to do the same to the late model top. I have 6x2's on a Pontiac that i built under his guidance and it works great with the vent holes plugged.

    You would be miles ahead understanding the various functions of the pieces of the Rochester 2g's if you watch his videos. While you will find some variations in your carbs from the ones Charlie is rebuilding (because they used that design for so many years), the fundamentals of that carb are the same.
     
    Last edited: May 11, 2019
  19. David Chandler
    Joined: Jan 27, 2007
    Posts: 1,101

    David Chandler
    Member

    If those manifold heat tubes bother you, you can do what I did. I tapped them and screwed a threaded plug into them, that's slightly below the mounting surface. To be sure I drilled in from the side, and installed two small set screws to lock them in place. That way you don't need the original "insulated" gasket between the carb and the intake. In my case, I was installing an adapter, and really didn't want that gasket in there. Later intakes, but not all of them, have those holes filled in from the factory. Think later 305's and 350's. But with them plugged you will still get ample heat but not quite as high up in the system. They work fine even when it's below zero out.
     
  20. David Chandler
    Joined: Jan 27, 2007
    Posts: 1,101

    David Chandler
    Member

    That area on the back side, by the throttle plates is where your PCV hooks up. I wouldn't bother with that. You also have a vacuum fitting that goes to your distributor, that feeds from that area, if I remember it correctly. So don't block that off.
    As for the bowl vent, you could vent it into the air cleaner. The old ones vented beside the choke plate, so somewhere close to there would work. Or better yet, swap that part with your old carb. That way it still vents the same way. As far as I recall that's the upper casting, and it will interchange, as far as I know.
    That large cast in horse shaped area in front of the throttle plates, is for exhaust heat, being fed up from those heat riser tubes. Your intake shows some carbon left behind from that. The later carbs eliminated that area, and sealed the riser tubes, as I mentioned above. .
    I think it could be made to work, with a little effort. Those carbs were used on 350's and 400's, as well as 305's, so it should have ample capacity for a 327. I got my first one from a 400, and it worked fine on a 350. Later I used it on a 305. Again, no problems, and no fiddling with restricting anything, or jet changes. The theory was under load, sometimes an engine wouldn't pull enough vacuum to operate the secondary circuit. So in many trucks, and station wagons they came on larger engines. Cars used them on base model engines. I've had three of those over the years, and they worked well, with no unusual problems. All were the older style, though.
     
    Last edited: May 11, 2019

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