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  1. #1
    Join Date
    Sep 2009
    Location
    Nashville, TN, USA
    Posts
    6

    Default "Hello all. New to this forum,

    "Hello all. New to this forum, Hopeing for some help here. I have a 7.4L magnum 330 on my second time out this year it started poping back throught the carb when the rpm reach over 2000rpm. I have drained the gas and replaced with fresh pump gas. Changed the fuel seperator/filter. Replaced the sparkplugs, checked the distributor cap and rotor,(I did notice some rust on the pickup but was told that is normal), checked the timming and rechecked the timeing and still have the same problem. This engine was replaced in 2005 with a jasper engine and has less than 50hrs on it. It has a quadrajet carb and thunderbolt ignition. I sure would apprediate any ideas on what may be wrong. Thanks David"

  2. #2
    Join Date
    Mar 2009
    Location
    Lowell, MA
    Posts
    1,217

    Default How old are the Spark plug wir

    How old are the Spark plug wires ?

    Could also be a dirty Dist cap where the spark is jumping to the wrong electrode.

  3. #3
    Join Date
    Sep 2009
    Location
    Nashville, TN, USA
    Posts
    6

    Default Thanks brian. The sparkplug wi

    Thanks brian. The sparkplug wires were replaced in 2005 with the engine. I ran the engine at night looking for spark jumping and didnt see any although it is kinda hard to see the way the wires are run. but there was nothing at the distributor cap.

  4. #4
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    6,824

    Default "Sounds like the infamous "

    "Sounds like the infamous "Lean Pop"....

    Does it do it all the time over 2000 RPM or only when accelerating? if accel only, fast & hard accel or gradual acceleration?

    Have you checked the fuel pressure at the carb inlet?"

  5. #5
    Join Date
    Oct 2006
    Location
    Sherwood Park, Alberta, Canada
    Posts
    405

    Default David Kingsbury Do a compress

    David Kingsbury
    Do a compression test to rule out a flakey valve before replacing any parts.
    Bert

  6. #6
    Join Date
    Sep 2009
    Location
    Nashville, TN, USA
    Posts
    6

    Default "Thanks for the remarks guys.

    "Thanks for the remarks guys. It does it consistantly over 2000 rpm not necesarily accelerating, but it does get more frequent the higher the rpms. The fuel pump was replaced last year. I will test the fuel presure and the compresion tomorrow and let yall know what I find."

  7. #7
    Join Date
    Dec 2006
    Location
    Chaumont Bay, N.y., USA
    Posts
    5,218

    Default "Ayuh,... Sounds like the Car

    "Ayuh,... Sounds like the Carb is Beat to me...
    You might try Rebuilding it..."
    Any Grease is Better,... Than No grease at All....

  8. #8
    Join Date
    Sep 2009
    Location
    Nashville, TN, USA
    Posts
    6

    Default "Ok guys here is what I found

    "Ok guys here is what I found today. On the compresion I had no compresion (at all) on 4,6,7, cylynders. Thinking back I replaced the manifolds and risers last year. When I did that I took a look at the lifters and noticed several of the lifters had a lot of slop in them. So I adjusted them like the manuel says tdc you asjust 8 and 180 out you adjust 8 snug them then put one full turn. When I found the low compresion today I figured I must adjusted the wrong ones tdc and vice versa. So I loosend all of the rocker arms to were the barely pushed the valves at all. compresion on every cylender 145 to 150. So I got the book out and repeated the valve adjustment procedure 2 more times this time taking the distributor cap off to varify I was at top dead center. Same thing no compresion on all but 3 of the cylenders. So I backed off the rockers to just snuged instead of putting the additional full turn after snug. Checked compresion again 120 to 140 on all the cylenders. I also noticed when I checked some of the rocker arms after testing the compresion they had gotten tighter. Im puzzeled does this sound like some of the lifters have collapsed."

  9. #9
    Join Date
    Oct 2006
    Location
    Sherwood Park, Alberta, Canada
    Posts
    405

    Default "David Kingsbury Try adjustin

    "David Kingsbury
    Try adjusting them hot with the engine running at idle, if possible. Kinda messy but I find it more suitable.
    Bert"

  10. #10
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    6,824

    Default "No, I don't believe the l

    "No, I don't believe the lifters have collapsed. I think you missed the zero lash point and wound up tightening the rockers too much.

    The lifters will bleed down once the engine is cut off. They will also fill up with enough oil pressure. I think that explains why some felt 'snugger' after the cranking during the test.

    I'd suggest you use the "engine running" adjustment procedure. Make sure you follow it regarding the 1/4 turn at a time. You can actually quit after the first 1/4 turn cycle, if desired. (Only drawback is they may be a little noiser at startup than what was normal.)

    Having the valve train too tight (the zero compression scenario) isn't going to let you get full performmance. If its been there a while, you might want to consider pulling the heads over the winter and making sure the valves seat. Having either open during combustion will burn the sealing surfaces in short order, mostly due to lack of cooling because they don't seat fully."

  11. #11
    Join Date
    Sep 2009
    Location
    Nashville, TN, USA
    Posts
    6

    Default thanks yall. it was only run f

    thanks yall. it was only run for a short time with the valves tight once i notice it popping through the carb i idled back to the marina. Ill let yall know how it works out. thanks again!!!!

  12. #12
    Join Date
    Sep 2009
    Location
    Nashville, TN, USA
    Posts
    6

    Default "Ok yall! I hate when the solu

    "Ok yall! I hate when the solution to the problem isnt posted. So here I am apparently what happend when I changed the risers and manifolds, I also checked the valves (it being a new motor wanted to be sure they didnt loosen up durring the break in)I found several that had play in them and adjusted according to the cold procedure. They apparently were not loose at all just bled down over the time it took me to replace the risers and manifolds. Consequently they ended up to tight. I cut the tops out of some old big block valve covers and adjusted the valves accroding to the running procedure. I ran it for about an hour reving the motor produced none of the popping through the carb that I was having."

  13. #13
    Join Date
    Jul 2008
    Location
    Northboro, Ma, USA
    Posts
    4,788

    Default Thanks Good info for others

    Thanks

    Good info for others !!!!!!
    Jack
    Northborough Ma
    Helping others sometimes with an attitude....haha..

  14. #14
    Join Date
    Aug 2009
    Location
    Sydney, NSW, Australia
    Posts
    208

    Default Adjusting whilst running is th

    Adjusting whilst running is the only way to go..

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