User Tag List

Results 1 to 15 of 15
  1. #1
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    OMC Turbojet , Pros & Cons

    G'Day to all , I purchased a sugar sands mirage with omc turbojet drive about 4 years ago , it was a lemon . with multi layered engine issues , its now runs great . just add fuel and go , the road to reliability was long and lonely . so I'd like to offer some encouragement to anyone interested in these great little boats , I"ll post a couple of maintenance tips . and things to steer clear of for durability . Click image for larger version. 

Name:	IMAG0081.jpg 
Views:	172 
Size:	432.6 KB 
ID:	407681 and yes that's a 140 hp FT motor siting on top ,!!!

  2. Likes powerabout liked this post
  3. #2
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    From the other forums I've read . " Overheating " is the big one , Well lets split the cooling system into easy to sort out levels . 1 filtered supply . 2 engine circulation 3 heat generated. lets start at 1 . filtered supply . first remove impeller housing . check orifice in impeller chamber for foreign body . bit of wood, rock; anything look for the hole and make sure its clear all the way to the check valve . unscrew the plug and check it out . the spring should be stainless steel and light . replace if not happy . ag supplies have heaps of them .:relief valve spring in boomspray bits .this valve should blow off at 25 psi "then you must have a suitable filter . one that can stand at least 50 psi. and drain cock is an advantage . also a clear bowl to see is you grounded on mud or sand , I've got a pressure gauge at this point ;;. 2, engine circulation. there's lot of good advice on this forum about crossflow heating precautions . listen to them . the only thing I'll add is the water bleed on the top of the heads . change them , the lower head route to the exhaust muffler . the high side route to a pee vent mounted somewhere you can see it . this does two things . you can see if you have water volume. just like any other outboard plus . when you hold continuous load the exhaust heats up and exceeds the water pressure psi,pushes the water back into to block and creates a air lock in the high side of heads . Yep you heard it hear . you can take it or . leave it . 3, heat generated . once again lots of good info on this site . especially the rubber diverters . because your running slightly higher water pressure . I glue mine in . carefull not to block the bleed hole below the hose . well I hope this helps Click image for larger version. 

Name:	IMAG0075.jpg 
Views:	857 
Size:	263.5 KB 
ID:	408092

  4. #3
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    OK . now the boring stuff is done . lets get into the interesting stuff . PROPULSION . THE jet unit . from a maintenance point off view , unclip the impeller housing and inspect the impeller . there's a couple of things here that are mandatory . check for bearing lash . if . this is not checked on a regular basis . its gunna cost ya . A quick in boat test is . get up to top speed . then slight turn left . then slight turn right . you should notice a slight drop in RPM . if its tight < .5mm any more than this you using wasting fuel and HP . ideally .2 mm is ideal . until you run it to shallow , the impeller bearing unit is another thing . OMC recommend running grease pack . however every other brand runs oil bath . this is my prefered system . its up to you . ok now everything is working well . here's some food for thought . if you pull skiers . biscuits or anything else . the standard turbojet nozzle is fine . but if you want to go fast with a light load . OMC has a factory performance kit . through Turbojet marine . that is a big help . much more punch out of the hole. no cavitation , plus a bit of top end , this is accredited to the different angle of the exit nozzle . more trim . plus if your a petrol head like myself there are different motor options . and gearbox options . OMC were lying dogo on this one . there are five different gear options . and a v6 bolts on .. no mods ..a yam looper bolts on . just burn the fuel enjoy . next rant . jet efficiency . I'm hoping some jetboat guru's might shead some light on this . subject > " IMPELLER PITCH "cheers JIMMY

  5. #4
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Firstly I'm no expert on this subject . and never let THAT get in the way of research .water jet propulsion like most energy transfer systems rely on parameters that when meet in balance produce varied results of efficiency . as far as I can tell , for it to work it needs " Induction " "Impeller " "Exit velocity " I like to start with the impeller . the standard turbojet impeller is 10/20 . the 20 is the exit angle . this sets the engine rpm at full throttle , but as you increase eng power and torque . of course you can increase pitch . we all no dat . but how much ?? what's is the ideal impeller speed . . from what I've seen and read and observed . 7000 RPM is up there . even turbine super river rats . and piston boats do 7000. OMC turbojet is no where near that . its 165 mm diameter impeller and in MHO 6000rpm is plenti . I've repitched to 14/22 . 5400RPM gives best economy @ 40 mph until i'm enlightened that's what i'm working on . Induction . very hard to find info on this subject . there are some aspects of induction that let the impeller maintain a constant pressure at the exit exit nozzle .and that changes . for example . at low speed you need volume , or you cavitate . ideally enough as not to push water. like outboards trying to get on the plane , thats why jet boats blast out of the hole . they consume most water that builds under the transom . as a jet accelerates water velocity increases to the point where the impeller glues the boat to water. causing drag " then the outboards fly past you " and its a fine line for jet boats to sit up on the planing deck and still feed the intake grate , even best scenario . jetboats have more drag the any other . thats the trade off ....35 --45 MPH is the sweet spot for jetboats . correct me if i'm wrong !!! Exit nozzle . mainly diameter is the one . for speed SMALL but you reduce thrust . for thrust LARGE but you reduce speed . you get the picture . Trim nozzles are a good thing . most performance jetboats have these . and jetboat roosters love em .A turbojet at 45 mph ??? hmmm do you need it ? conclusion . finish all you motor mods , load the boat up and see what you got , 1 mm up or down will make a difference if you want to squeeze that last bit of performance out . well that's about it , there's lots more to discuss here , in the end I'll leave it to you . thank's for reading . Regards Jimmy

  6. #5
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    As a fishing boat . it,s very easy going !!!
    Click image for larger version. 

Name:	IMAG0283.jpg 
Views:	828 
Size:	328.7 KB 
ID:	410849
    Click image for larger version. 

Name:	IMAG0285.jpg 
Views:	804 
Size:	293.1 KB 
ID:	410850

  7. #6
    Join Date
    Sep 2018
    Location
    New Jersey
    Posts
    2
    Thanks (Given)
    0
    Thanks (Received)
    1
    Likes (Given)
    0
    Likes (Received)
    0
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Hello from new Jersey..USA ..I want to thank you for the information on overheating on the tubro jet ...I moved the plumbing to the back of boat .an now the water is circulating.. tomorrow will be a test run to see how it does ..thanks again

  8. #7
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    [QUOTE=Tubro jet john;3079591]Hello from new Jersey..USA ..I want to thank you for the information on overheating on the tubro jet ...I moved the plumbing to the back of boat .an now the water is circulating.. tomorrow will be a test run to see how it does ..thanks again[/QUOTE just checking here , the top side head to pee vent , low side head to muffler , this will confirm water circulation , if your still overheating , get back to me , will help as I can , cheers

  9. #8
    Join Date
    Sep 2018
    Location
    New Jersey
    Posts
    2
    Thanks (Given)
    0
    Thanks (Received)
    1
    Likes (Given)
    0
    Likes (Received)
    0
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    [QUOTE=Jimmybar;3079711]
    Quote Originally Posted by Tubro jet john View Post
    Hello from new Jersey..USA ..I want to thank you for the information on overheating on the tubro jet ...I moved the plumbing to the back of boat .an now the water is circulating.. tomorrow will be a test run to see how it does ..thanks again[/QUOTE just checking here , the top side head to pee vent , low side head to muffler , this will confirm water circulation , if your still overheating , get back to me , will help as I can , cheers
    Hello again ., just update on the tube jet .this boat is a four winns fling that had a wire harness melt down .. engine has great compression so I decided to get it going again ...now that I'm passed the over heating problem ..thanks to you !....I had a good day of running it full throttle ..the vro pump had been uninstalled.i installed a non vro pump .mix the gas instead.at full throttle it surges rom up and down ...we installed a electric in line electric fuel pump and problem went away.not sure why the factory non vro pump didn't supply enough fuel... maybe the pulse limitor ?...it would be the one from original vro pump? other than that ..boat is fast .. steering ..is poor ..it has a outboard motor worm cable set up in it .....I also have the same boat you have ..with the sport jet 90 in it ...thanks again ..

  10. Thanks Jimmybar thanked for this post
  11. #9
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    glad to help , here,s a pic of a 270' steering thats still responsive, and very easy in a straight line . also a shot of bat. system. main isolator tto o motor with option to start from both batteries.also carries alt charge. 50amp circuit/ breaker to dash and sounder, 50amp c/b to tolling motor , troll motor draws from both batteries until 12.5v is reached then volt sensitive relay isolates start bat. advantage is you can isolate all power at one point . easy to charge batteries independently .and of course keep all power cables 50mm clear of any fuel source.... cheers Click image for larger version. 

Name:	IMAG0385.jpg 
Views:	685 
Size:	399.1 KB 
ID:	419598Click image for larger version. 

Name:	IMAG0384.jpg 
Views:	620 
Size:	175.0 KB 
ID:	419597

  12. #10
    Join Date
    Dec 2018
    Location
    Shawnigan Lake
    Posts
    6
    Thanks (Given)
    3
    Thanks (Received)
    0
    Likes (Given)
    3
    Likes (Received)
    0
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Tubro jet john View Post
    Hello from new Jersey..USA ..I want to thank you for the information on overheating on the tubro jet ...I moved the plumbing to the back of boat .an now the water is circulating.. tomorrow will be a test run to see how it does ..thanks again
    where did you find your information for the cooling system ?

  13. #11
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Jimmybar View Post
    From the other forums I've read . " Overheating " is the big one , Well lets split the cooling system into easy to sort out levels . 1 filtered supply . 2 engine circulation 3 heat generated. lets start at 1 . filtered supply . first remove impeller housing . check orifice in impeller chamber for foreign body . bit of wood, rock; anything look for the hole and make sure its clear all the way to the check valve . unscrew the plug and check it out . the spring should be stainless steel and light . replace if not happy . ag supplies have heaps of them .:relief valve spring in boomspray bits .this valve should blow off at 25 psi "then you must have a suitable filter . one that can stand at least 50 psi. and drain cock is an advantage . also a clear bowl to see is you grounded on mud or sand , I've got a pressure gauge at this point ;;. 2, engine circulation. there's lot of good advice on this forum about crossflow heating precautions . listen to them . the only thing I'll add is the water bleed on the top of the heads . change them , the lower head route to the exhaust muffler . the high side route to a pee vent mounted somewhere you can see it . this does two things . you can see if you have water volume. just like any other outboard plus . when you hold continuous load the exhaust heats up and exceeds the water pressure psi,pushes the water back into to block and creates a air lock in the high side of heads . Yep you heard it hear . you can take it or . leave it . 3, heat generated . once again lots of good info on this site . especially the rubber diverters . because your running slightly higher water pressure . I glue mine in . carefull not to block the bleed hole below the hose . well I hope this helps Click image for larger version. 

Name:	IMAG0075.jpg 
Views:	857 
Size:	263.5 KB 
ID:	408092
    YOU HEARD IT HERE FIRST. it was a painfull trail of piston grabs until the penny dropped, water pressure builds up in the exhaust stream when the engine is loaded , enough pressure to push the water back up the water bleed and create a air lock in the top head . the solution is but one. its cheap and effective. the real issue is the relief valve and bowl pressure. if your engine is modfied to make more HP . or your impeller has seen better days the bowl pressure will be affected. ofcourse there are other parameters that effect exhaust pressure . like load . fuel the list goes on . so I came up with THIS solution . vent top head water jacket. and voila . it works . most of the time. VERY CLOSE TO MOD 3 ENGINE BUILD will keep you posted

  14. Thanks matress thanked for this post
    Likes matress liked this post
  15. #12
    Join Date
    Dec 2018
    Location
    Shawnigan Lake
    Posts
    6
    Thanks (Given)
    3
    Thanks (Received)
    0
    Likes (Given)
    3
    Likes (Received)
    0
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Jimmybar View Post
    YOU HEARD IT HERE FIRST. it was a painfull trail of piston grabs until the penny dropped, water pressure builds up in the exhaust stream when the engine is loaded , enough pressure to push the water back up the water bleed and create a air lock in the top head . the solution is but one. its cheap and effective. the real issue is the relief valve and bowl pressure. if your engine is modfied to make more HP . or your impeller has seen better days the bowl pressure will be affected. ofcourse there are other parameters that effect exhaust pressure . like load . fuel the list goes on . so I came up with THIS solution . vent top head water jacket. and voila . it works . most of the time. VERY CLOSE TO MOD 3 ENGINE BUILD will keep you posted
    is that a telltale is the engine cover that you are pointing at?

  16. #13
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    yes
    Click image for larger version. 

Name:	IMAG0282.jpg 
Views:	582 
Size:	369.7 KB 
ID:	424912

  17. Thanks matress thanked for this post
    Likes matress liked this post
  18. #14
    Join Date
    Dec 2018
    Location
    Shawnigan Lake
    Posts
    6
    Thanks (Given)
    3
    Thanks (Received)
    0
    Likes (Given)
    3
    Likes (Received)
    0
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Jimmybar View Post
    yes
    Click image for larger version. 

Name:	IMAG0282.jpg 
Views:	582 
Size:	369.7 KB 
ID:	424912
    is that telltale from one cylinder head on top or teed into both?

  19. #15
    Join Date
    Apr 2018
    Location
    Australia
    Posts
    114
    Thanks (Given)
    25
    Thanks (Received)
    10
    Likes (Given)
    21
    Likes (Received)
    24
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Name:  IMAG0081.jpg
Views: 582
Size:  7.5 KByou can see top cylinder bleed reaching across to pee vent. the lower cylinder must feed to muffler. thats important to maintain high speed pulse . for fuel economy. was thinking about running two pee vents , one for each head . then feed muffler from exhaust water jacket. that would have the same effect . depends on how stressed your motor is running, if you have an idea . i'm listening.

Similar Threads

  1. Pros and cons of an opti
    By wottz in forum Four Stroke and Direct Injected Two Stroke Engines
    Replies: 36
    Last Post: 10-23-2013, 07:47 PM
  2. Pros and Cons of this motor?
    By FoFittyFoSS in forum General Boating Discussion
    Replies: 8
    Last Post: 05-13-2013, 08:21 PM
  3. pros and cons
    By JJ QUEST in forum Technical Discussion
    Replies: 4
    Last Post: 07-20-2009, 05:23 PM
  4. Pros and Cons 250 EFI
    By Fish in forum Technical Discussion
    Replies: 8
    Last Post: 07-04-2004, 09:27 PM
  5. pros & cons.......
    By LiquidBoats in forum Technical Discussion
    Replies: 2
    Last Post: 09-08-2002, 11:22 PM

Posting Permissions

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