Jet boat RPM Problem
+ Reply to Thread
Page 1 of 2 1 2 LastLast
Results 1 to 14 of 18

Thread:
Jet boat RPM Problem

  1. #1
    Junior Member
    Join Date
    May 2014
    Posts
    6

    Default Jet boat RPM Problem

    I recently bought a 1979 Crusader Tunnel Hull with a Berkley 12JG "B" cut impeller. Engine runs great but I can only turn 5000 RPMs. The engine only has about 15 hours on it and has amazing amount of power. I can floor it and it immediately hits 5,000 and just stays there. I was told the way this engine was built I shouldn't have any problems turning 6,000 RPMs. The guy I bought the boat from had a mild 454 in the boat before he had this one built and he said that the old enigne turned 5,900 rpms. I am thinking it might be a fuel problem but not for sure. I would appreciate any help. Thank you.

    The engine is
    GM 454, bored .03 over
    Pro Comp Aluminum Heads;
    • Intake Port - 320cc (rectangle)
    • Combustion Chamber - 115cc
    • Intake Valve Size - 2.25"
    • Exhuast Valve Size - 1.88"
    • Valve Steam Dia. - 11/32"
    • Intake Port Size - 1.8" W x 1.95" T
    • Exhaust Port Size - 1.9" x 1.65" T
    • Spring Pocket Dia. - 1.730
    • Valve Material Stainless Steel
    • Valve Length - +.250" intake, +.100 exhaust
    • Valve spring Dia. - 1.525", Single
    • Spring Pressure closed - 140lbs @ 1.900"
    • Spring Pressure open - 380lbs @ 1.200"

    Camshaft
    • Herbert Racing
    • Adv Dur I 316 E 332
    • @ .05 I 244 E 254
    • Lift .596 .625
    • Valve Lash I 002 E .024
    • @.05 I open 29 closed 72
    • E open 74 closed 35

    Forged Pistons (10.5:1)
    Forged GM Crankshaft
    Roller rockers
    Pro Comp H-Beam Connecting rods
    Main and head stud kit
    6AL MSD Igniton
    Tunnel Ram intake with two Holley 660 Carbs.

  2. Remove Advertisements
    PerformanceBoats.com
    Advertisements
     

  3. #2
    Senior Member jetboatperformance's Avatar
    Join Date
    Dec 2007
    Location
    Atascadero Calif
    Posts
    7,464

    Default

    Where does this engine make power and how much per the builder @ 6K ?, what jet and what impeller & what altitude , is the jet fresh ?

  4. #3
    Junior Member
    Join Date
    May 2014
    Posts
    6

    Default

    To be honest I don't honestly know a lot about the pump just that it is a few years old and is a berkley 12JG with a stainless B cup impeller, stage 3 loader grate, ride plate, wedge, droop snoot and diverter. It has tons of power but when From a stop it shoots up to 5000 and wont turn any more RPMs. I am new on jet boats, so I really don't know anything about them.
    Last edited by awhite10; 05-12-2014 at 01:03 PM.

  5. Remove Advertisements
    PerformanceBoats.com
    Advertisements
     

  6. #4
    Senior Member jetboatperformance's Avatar
    Join Date
    Dec 2007
    Location
    Atascadero Calif
    Posts
    7,464

    Default

    the jet with B impeller @ 5K is suggesting your making a bit under 400 HP , the motor build description sounds like it should make quite a bit more ..


    4900 361.02
    5000 383.58
    5100 407.05

  7. #5
    Junior Member
    Join Date
    May 2014
    Posts
    6

    Default

    I have built many truck engines but I haven't messed with any kind of engine like this. When I'm and full throttle the engine seems to be producing the same amount of power. The carburetor secondaries are 1 to 1 as for as I know. I am running about 4 psi on the fuel too.

  8. #6
    Senior Member BGMAN's Avatar
    Join Date
    Jan 2008
    Posts
    568

    Default

    You didn't mention what exhaust you have on the engine. headers? logs? What about your ignition timing? Fuel pressure at 4psi seems a little on the low side. I would bump it up to the 6-8 range if possible.

  9. #7
    Junior Member
    Join Date
    May 2014
    Posts
    6

    Default

    It has Bassett Headers and the timing is at 34 degrees. It does have a fuel regulator. I just wanted to make sure it was okay to bump it up a little.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	01717_jU6FDrq9gu4_600x450.jpg 
Views:	107 
Size:	22.4 KB 
ID:	533249  

  10. #8
    Senior Member jetboatperformance's Avatar
    Join Date
    Dec 2007
    Location
    Atascadero Calif
    Posts
    7,464

    Default

    Quote Originally Posted by awhite10 View Post
    I have built many truck engines but I haven't messed with any kind of engine like this. When I'm and full throttle the engine seems to be producing the same amount of power. The carburetor secondaries are 1 to 1 as for as I know. I am running about 4 psi on the fuel too.
    Assuming its running a Holley, 4 is not enough need 6 to 7 , be interesting to see the cam card from Herbert (rpm range)

  11. #9
    Senior Member BGMAN's Avatar
    Join Date
    Jan 2008
    Posts
    568

    Default

    Try bumping up the timing a few degrees, maybe try 36 total. 7 psi on the fuel seems to be what most everyone tries to run. I have seen some guys get away with 9 to try and fix a off idle stumble. I know this is going to sound stupid, but try turning your scoop backwards and see if you pull more rpm. There was a guy a while back that was having a similar rpm issue and figured out the scoop was disrupting the air flow to the carbs, too much ramming.

    If nothing improves, then you need to start looking a the engines vitals. Leak down, compression test, etc. It's also possible that the motor doesn't have what the previous owner has told you and you really have a 385-400 hp motor. It's also possible the pump doesn't have a B cut impeller.

    Lots of variables and a big mystery when buying used.

  12. #10
    Junior Member
    Join Date
    May 2014
    Posts
    6

    Default

    I actually have receipts to everything for the engine, so I do believe him on the engine. The pump I do not have any kind of documentation. How can you tell what size of impeller that I have? Ill try to post a video to show how it runs.

  13. #11
    Senior Member Chuck Wagon's Avatar
    Join Date
    Apr 2008
    Location
    Palm Desert
    Posts
    258

    Default

    5000 rpm pill in the MSD?

  14. #12
    Junior Member
    Join Date
    May 2014
    Posts
    6

    Default

    That was one of the first things I checked and it has a 6500 RPM limiter in it.

  15. #13
    "BOATLESS" blender over's Avatar
    Join Date
    Jan 2008
    Location
    Wildomar/Big River/OcotilloWells
    Posts
    3,468

    Default

    Quote Originally Posted by Chuck Wagon View Post
    5000 rpm pill in the MSD?
    That's what I would check first!, or quite possibly it has bigger cut impeller then a B, especially if it pulls real hard out of the hole then flattens out, or its just a 400hp motor.

  16. #14
    "BOATLESS" blender over's Avatar
    Join Date
    Jan 2008
    Location
    Wildomar/Big River/OcotilloWells
    Posts
    3,468

    Default

    Quote Originally Posted by jetboatperformance View Post
    the jet with B impeller @ 5K is suggesting your making a bit under 400 HP , the motor build description sounds like it should make quite a bit more ..


    4900 361.02
    5000 383.58
    5100 407.05
    Tom, what's the estimated hp rating @5k rpm on a A or AA impeller?

+ Reply to Thread
Page 1 of 2 1 2 LastLast

Quick Reply Quick Reply

Register Now

Please enter the name by which you would like to log-in and be known on this site.

Please enter a password for your user account. Note that passwords are case-sensitive.

Please enter a valid email address for yourself.

Please select your insurance company (Optional)

Log-in


Tags for this Thread

Digg This Thread

Posting Permissions

  • You may post new threads
  • You may post replies
  • You may not post attachments
  • You may not edit your posts
  •  
 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95