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Thread: power loss

  1. #1
    I have an 18 foot flat bottom v drive with a 468 chevy. Tunnel ram set up with 2 660s on top. Took it out for the first time this weekend and power throughout the rpm range is great, however, at WOT (around 6000 rpms) I can hear the motor cutting out. If I back out of the throttle just a hair, the boat picks up more speed. Why is it lacking at WOT? I am also running a mag and tried to adjust timing to get rid of the problem but this didn't seem to help. Could I be running out of spark? Thanks in advance for your responses.

  2. #2
    If changing throttle position fixes it and you gain RPMs, it is in the fuel delivery system. If it hapens cold, but not hot, your intake velocity may be slowing down enough to drop gas out of suspension... At this point we are guessing.
    What do you have for carberation? Does it have a choke? Manual, electric or tempiture? Have you checked for leaks in the intake manifold?

  3. #3
    To add to BOFH's questions, do you run a fuel pressure gauge, and if so what's it reading when the problems start? You should have at least 4-5 pounds pressure or you need to upgrade the fuel system. Sometimes if a carb is WAY oversized, it'll do what you say. When you back off the throttle a little you're reducing the opening and the velocity picks back up.

  4. #4
    I am running two 660 holley center squirters on a tunnel ram manifold. 130 gph mechanical fuel pump with 1/2" aluminum fuel line. No choke and no leaks on the manifold. Fuel pressure is regulated at 7 psi. I am kind of thinking that I may need to change jet sizes.

  5. #5
    1320 CFM on a tunnel ram? Damn! I think we found your problem. At 6000 RPM a 468 with a volumetric efficiency of %85 is sucking 690.625 CFM, or about HALF of your carberation! You are droping a bunch of still air into a tunnel ram, and bogging like crazy. You need air flow to pull gas out of the carb, and you got none. My bet is if you replaced it all with a single hot Holly 750 you would actually have more power.
    FYI - You current carb setup is good for an engine that winds to 11,500 RPM. You arn't spinning it that high are you? :-)

  6. #6
    69 Elim
    I would say the 2 four barrels are killing ya at WOT. I agree that you are not pulling the required air to keep those carbs happy. I have the exact same problem. Sure I could put one carb on and have more power and better drivability, but I like the look of the tunnel ram. Really makes the boat look good. I just put a stop on the throttle so it doesn't go WOT.

  7. #7
    Not too much carburation, just not enough engine...J/K The 660s should work good on your engine but everything will have to be checked, like fuel pressure at WOT, pressure should not drop at all. ANY loss of f/p will cause the float level to drop. 1 pound of pressure loss can cause as much as 1/8" lower float level. Do not mess with jetting until this is checked and confirmed. One last thing, 660s usually have a single fuel inlet and a spindley little transfer tube to the secondary bowl. The sec. bowl should be drilled and tapped for its own fuel inlet fitting and set up with a 1/2" i.d. 4 outlet fuel log.

  8. #8
    Thank you all for your replies. One other thing I think I need to mention. The elevation where I run is appx. 4200 feet. Makes a big difference in performance applications. 69 elim I have to agree with you that the boat looks great with the tunnel ram. Next best thing to a blower. Maybe next year.

  9. #9
    058, I think I'll take your advice on the second inlet on my 660. Good idea.

  10. #10
    There is one thing not perviously addressed that may help.My friend had a LS7 454 chev with 2 850 cfm holleys and experienced the same problem.The motor would turn about 6000 rpm with a stainless B impeller.It would run better with the throttle blades partially closed rather than wide open.He was using a scoop and the air turbulance was causing the problem. He installed a set of stainless mesh flame arresters and cured the problem.They straightened out the airflow. He was also running a vertex mag. Try a couple of flame arresters and see if this helps.

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