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7.4 Rochester Carb Choke Issue

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by Oscarlyy, Mar 28, 2016.

  1. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    image.jpg image.jpg

    Hello CSR!

    Hoping somebody would be able to help me with this...
    I have a choke issue I believe in my twin 1989 7.4 454's with I think Rochester carbs. If you look at the photo I'm pretty sure the choke flap is supposed to be completely closed not half way like mine is. Is there a adjustment? Photos are while engines are cold and haven't been ran in a week. Throttles were just pumped before the photo. Harder to start the port engine than the stbd and the port is running a bit rough for the first 4 mins or so until it's warm. The rpm has to stay above 1300 for it to stay running and not stall when cold. the port engines choke flap is the one pictured, the stbd choke flap is more closed and runs great cold. Any help at all is appreciated! And the engines were fully rebuilt 40 hrs ago. Engines run perfectly with no hiccups when warm. Thank you in advance!
     
    Last edited: Mar 28, 2016
  2. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    Also, are there any aftermarket carbs for this engine that would make it more fuel efficient? Any smart upgrades I should do to these engines to make them more reliable/efficient, I do not need more power in any way. Just little things to make this engine last longer?
    Thanks!
     
  3. jthatcher

    jthatcher Member

    307
    Jul 27, 2014
    Nashville, TN
    1990 310 Sundancer
    7.4L V Drive
    9.0 BTC Westerbeke
    Google quadrajet divorced choke and you will find several videos on how to adjust. I have recently replaced my carbs and they needed to be readjusted to the new linkage.
     
  4. BillK2632

    BillK2632 Well-Known Member GOLD Sponsor

    Jun 25, 2009
    Lake Norman, NC
    1999 185 Bowrider,
    Mercruiser 4.3, Alpha I
    In general terms a cold engine with the choke set, the choke butterfly should not be completely closed, it should be opened approx 1/8" (specs vary by carburetor). That one looks to be opened too far. Once you start the engine, as it warms the choke should open fully in about 4-5min. The adjustment is usually bending the choke arm slightly to adjust the takeoff gap. Like mentioned above, if google, should be able to find exact procedure for your carburetor.
     
  5. dwna1a

    dwna1a Well-Known Member PLATINUM Sponsor

    Apr 23, 2012
    James River
    88 Weekender 300 "Seahorse"
    Twins 350
    Have you manually manipulated the choke as the engine warms up? When was the last time the carbs were cleaned and rebuilt?
     
  6. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    No I haven't looked at any of it while the engines are running, is that safe to do? To have the hatch and flame arrestor off while running the engines? Not sure when the carbs were rebuilt, but I'm guessing the carbs were rebuilt when the engines were rebuilt, 1 year ago. (Only had the boat 6 months)
     
  7. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    okay, thank you. Am I able to start the engine without the flame arrestor and the hatches open? Sorry, I'm still learning :smt024
     
  8. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    Thankyou, I looked up a video and I'm gonna try and see how they run.
     
  9. dwna1a

    dwna1a Well-Known Member PLATINUM Sponsor

    Apr 23, 2012
    James River
    88 Weekender 300 "Seahorse"
    Twins 350
    Yes, it is safe to run the engine with the hatch open and the arrester off. But just for tuneup, not for a run down the river. Remember to cut your blowers on first just as you would at any startup. While the engine is running never look straight down the carb, if she does backfire you don't want it in the face.

    As for the carb rebuild. It's not nessasarly true that when the engine was rebuilt the carb was. Sometimes the choke spring gets weak, mine disconnected itself when the clip holding the arm came off.
     
  10. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    Okay, I'll do that. I am at my boat right now looking at it and what is happening is the choke lever is hitting the arm that opens the secondaries. I looked around a bit on the Internet and the choke lever was bent back on a lot of the photos I saw so I tried it. Doing some maintenance so I'll start the engines in about a hour or so and see how it is. Also, my choke butterfly has a lot of play in it. The first linkage coming off of the choke butterfly is very loose. The butterfly has a lot of play in it back and fourth from the first linkage. Is that normal? Thanks. You've been a great help.
     
  11. julietbravomike

    julietbravomike New Member

    56
    Aug 7, 2014
    Dallas & Lake Texoma
    1989 340 Sundancer &
    2007 Yamaha Waverunner FX140HO
    7.4l Merc MIE's w/Borg Warner 72c V-Drives and Onan 6.5 MCCK
    This isn't exactly an answer to your question, but I'll just add that I would recommend you watch several YouTube videos and study the appropriate service manual to get familiar with how the Quadrajet works. It's not complicated, but it does take some time to figure out how everything works. There are lots of variations - mostly automotive variations - so don't be surprised if what you see on the internet doesn't exactly match what you have. You should be able to find the correct Merc manual via Google.
     
  12. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    thankyou, I know. I have tried looking through YouTube and have mostly found automotive.
     
  13. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    okay, started the Port engine, (rough when cold) and it started easier because the choke was closed but immediately within 3 seconds the choke flap opened fully? And the engine obviously was Still running rough for the 10 seconds We had it running. I don't know if it changes anything but the engines do not have thermostats.
     
  14. dwna1a

    dwna1a Well-Known Member PLATINUM Sponsor

    Apr 23, 2012
    James River
    88 Weekender 300 "Seahorse"
    Twins 350
    I am in agreement with Mike. You've got to get a feel for how the Qjet works. They are a very simple carb but one stupid little thing can upset them.

    The choke shouldn't open fully that fast unless the engine was already up to temp. The thermostate or the lack of will not have that much of an effect. It's the heat that the motor puts out into the area around the choke spring that makes the choke spring expand and open the choke plates. Are the covers still on the springs?

    If you've never worked on a carb it may be time to call in a expert.
     
  15. Oscarlyy

    Oscarlyy New Member

    65
    Aug 25, 2015
    Canada
    1989 345 Sedan Bridge
    1989, rebuilt Mercruiser 454, 100 hours
    okay, well I think this is becoming too much for me. I'm going to try and find a Carburetor expert to have a look at it and might bight the bullet and get it rebuilt as well because it's very dirty. And don't know last time it was done. Thanks for your help!
     
  16. julietbravomike

    julietbravomike New Member

    56
    Aug 7, 2014
    Dallas & Lake Texoma
    1989 340 Sundancer &
    2007 Yamaha Waverunner FX140HO
    7.4l Merc MIE's w/Borg Warner 72c V-Drives and Onan 6.5 MCCK
    [​IMG]

    This pic is from an automotive application, but should be close to what you have. The spring that holds your choke closed is inside that square looking thing attached to the intake manifold (the red thing). It's a bimetallic spring that uncoils as the engine heats up. Yours may be broken or unhooked or even dirty. This is what the spring looks like:

    [​IMG]
     
  17. Pirate Lady

    Pirate Lady Active Member

    323
    Jun 2, 2020
    Chesapeake Bay, Middle River
    1991 250 Sundancer & Goplus 7.5' / 2.6hp outboard.
    Previous: 180BR, 220DA, 268DA, 250DA, 268DA
    7.4 Bravo 1
    See on this picture how the mounting screw is facing away from the carb? Well, mine, see picture attached, has the mounting screw facing toward the carb, and every one i find on the 'net is like the pic above (toward the carb). Can anyone explain this? Cause i need a new one and cant find one like mine anywhere. (454 mercruiser, rochester carb)
     

    Attached Files:

  18. hpcrank

    hpcrank Member

    130
    Feb 4, 2016
    Col. Spgs, Co-Lake Pueblo State Park
    1981 260 Sundancer with rebuilt 454/Mercruiser330-I/O. w/ alum. hard top with front and side wdos
    One rebuilt 330 hp. 454 Chevy
    The Rochester Q-Jet is the finest general use carburetor of all time (IMO)! There are racing and special application carbs that may work better in unique applications but you have the best general service carb ever built. You probably (as most owners ) could pick up a efficiency by getting the jets dialed in which is easy to do on a q-jet. I know nearly everyone that reads this part of my post will disagree but do not run your fuel pressure more than 3.5 psi., if you do you will run rich and hurt your fuel economy. Prove it to your self with a fuel pressure gage and an adjustable regulator. This is not a big issue if you run non-oxyigenated fuel.

    VR.
    hpcrank
     

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