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  1. #1
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    1975 stinger 75hp , 78 sea nymph v, 150psi all 3, only 30mph WHAT? 13 3/4 x15 p,HELP

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ID:	326012HI I am new to this sight and hoping for some good advice getting some better speed out of my 16 foot sea nymph. I got the boat a week ago. I drove 500 miles and $1650 and one of my fishing poles. I have my new to me rebuilt 1978 sea-nymph with a stock 1975 75 hp stinger long shaft on the back and a 13 3/4x15 prop. The guy spent a year fixing this boat up new transome,decks,floors,carpet, radio.He did a good job the boat is solid. I know this prop is more for a pontoon of heavy boat and I need to change it. I took the boat out with the old lady and the boat ran only 30 mph and it ran good no missing ect. I checked the compression and it has 150 psi on all three cyl. I also made sure all 3 had spark. I changed the spark pluges and foged the carbs since is sat for 2 or 3 years. I know this is not the right boat for real speed but 30 really? I was shocked gps said 30 BOOO! This is not a heavy boat and it is flat bottom at the back . I am getting a tack saturday and Ill get some pics up when I get a chance. plz HELP Thank you for any help in advance
    Last edited by unreal-inventor; 08-21-2015 at 02:22 PM. Reason: trying to put a pic in

  2. #2
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    I would say you need a 19-21 pitch prop. The tach will tell you. Be careful that thing probably don't have a rev limiter and you could over rev it and blow it.

    You answered your own question. Who runs a stinger on a pontoon?

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    The guy I bought this from put this together with what he could afford. I don't know what the motor was on before this boat,but I found out the prop is for a pontoon or heavy boat.

  4. #4
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    From what I know, it should take off from stall great but the top speed is, according to what you say, awful.

    By raising the pitch to 19 or 21, the boat will be a little slugish on take-off.

    The tach will tell you about the RPM range in Wide Open Throttle ( WOT ). max RPM If my memory recall should be around 5700rpm
    Glastron GT 150 W OMC 140
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    I know two things that are infinite, the Universe and Human's Stupidity, although I am not sure about the first one.

  5. #5
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    Quote Originally Posted by dompie99 View Post
    From what I know, it should take off from stall great but the top speed is, according to what you say, awful.

    By raising the pitch to 19 or 21, the boat will be a little slugish on take-off.

    The tach will tell you about the RPM range in Wide Open Throttle ( WOT ). max RPM If my memory recall should be around 5700rpm
    Thats what I would recommend and raise the motor to at least get the cavitation plate out of the water at wot. Gary
    "12" Super Lite Tunnel (11') "88" 25 Yammy twin carb "BANANA SPLIT"
    "77" Hydrostream Viper "87" 140 Rude heavy modded w/15" mid, Bobs nose and lwp "DANGER ZONE"
    "72" Checkmate MX-13 "80" 75 Rude w/15"mid and Nitro Lu (to be restored)

    "Too much is never enough" Keith Richards " Dreams become reality via hard work and perseverance" G.A.Carbonneau

    "This coming from an old man that strapped two bananas together, hung a motor on it and calls it a boat" XstreamVking

  6. #6
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    The Stinger is a valuable engine if it is healthy....... do a compression test, and sell it to a guy who has a light boat who wants to go fast and use the proceeds to buy a 70 hp fish motor, a few cases of beer and a bunch of bait............... and about 50 gallons of gas.

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    Last edited by flabum1017; 08-21-2015 at 08:36 PM.
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

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    Thank you all for your great information. I raised the motor 1 and 3/8 in so its 1/8 inch above the bottom of the boat almost dead even. I understand going up in pitch makes the boat faster if the rpm of the motor is 5500 to 5800. The confusing part is what size diameter prop do I go with? The prop now is 13 3/4 di x 15 pitch . Ok so do I go to say a 12 di x19 to keep close to the same rpm or do I just go to a 13x19?

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    check the cowling for clogs.

    I bout props and had carb work done before someone noticed a giant paper wasp nest in the cowlings air intake

    on my 50 nissan
    '06 Tracker All-Fish/Tournament V-18 90 Optimax, 46.8 gps Goal one complete reach 45 mph.

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    first thing I would do is pull head and inspect cylinders/piston tops/exhaust ports...then go from there.IMO...a 75/75 is a high ring/highcompression piston motor.=(high failure rate piston.)high horsepower motor.high octane Is MUST, along with a good carb clean/overhaul + compleat synk and link with timing backed off at least 2*(other people might chime in and fill you in with better info.-search this site-I ran my 74/70 short @6300 with almost stock timing back in 74 BUT we had good high test 1974 fuel-- or avgas at races--with double Johnson oil on my FE race boat-no power trim)that was a stock 70. stock1975/75s were supposed to have 84+horse power, in longshaft like yours.high ringed pistons will not last with todays fuel.should b able to twist it to 5500/5700 rpms.search this site for better info-th
    Last edited by mn808gade; 08-22-2015 at 02:03 AM.

  11. #10
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    At 30 mph you're not that far off, my 16'-6" Discovery Marine did 32 mph with a modern 70 hp Johnson, I wasn't impress with the speed so the first thing I did was to put a jack plate on it with a 5" set back, the set back will help carry the front of the boat and will require less trim, I than jacked the motor 4.5" (don't worry you will get a lot of water for the water pump) and changed the prop to a 13.75" x 19 pitch, not only will your speed increase but you will gain another 4.5" before you hit what lurks underneath the water, oh and my boat is now running at 37 mph and the motor is spinning at 5900 rpm.
    Hope this helps.

  12. #11
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    Quote Originally Posted by unreal-inventor View Post
    Thank you all for your great information. I raised the motor 1 and 3/8 in so its 1/8 inch above the bottom of the boat almost dead even. I understand going up in pitch makes the boat faster if the rpm of the motor is 5500 to 5800. The confusing part is what size diameter prop do I go with? The prop now is 13 3/4 di x 15 pitch . Ok so do I go to say a 12 di x19 to keep close to the same rpm or do I just go to a 13x19?
    The smaller diameter,higher pitch prop will allow the motor to be raised more for less drag. The 75 Stinger is a performance motor with a higher gear ratio than the stock 70 and will not perform as well on a heavier boat and I agree with flabum. Gary
    "12" Super Lite Tunnel (11') "88" 25 Yammy twin carb "BANANA SPLIT"
    "77" Hydrostream Viper "87" 140 Rude heavy modded w/15" mid, Bobs nose and lwp "DANGER ZONE"
    "72" Checkmate MX-13 "80" 75 Rude w/15"mid and Nitro Lu (to be restored)

    "Too much is never enough" Keith Richards " Dreams become reality via hard work and perseverance" G.A.Carbonneau

    "This coming from an old man that strapped two bananas together, hung a motor on it and calls it a boat" XstreamVking

  13. #12
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    The gear ratio is only different on the 15" short shaft motor, they have the nitro gearcace, the short shaft motors are what people rave about, the long shaft is just a fishing motor.
    Last edited by sharpeye Mike; 12-05-2020 at 06:31 PM.

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  15. #13
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    Quote Originally Posted by sharpeye Mike View Post
    The gear ratio is only different on the 15" short shaft motor, they have the nitro gearcace, the short shaft motors are what people rave about, the long shaft is just a fishing motor with tall porting.
    Thanks for the correction.....always learning on this site!!! Gary
    "12" Super Lite Tunnel (11') "88" 25 Yammy twin carb "BANANA SPLIT"
    "77" Hydrostream Viper "87" 140 Rude heavy modded w/15" mid, Bobs nose and lwp "DANGER ZONE"
    "72" Checkmate MX-13 "80" 75 Rude w/15"mid and Nitro Lu (to be restored)

    "Too much is never enough" Keith Richards " Dreams become reality via hard work and perseverance" G.A.Carbonneau

    "This coming from an old man that strapped two bananas together, hung a motor on it and calls it a boat" XstreamVking

  16. #14
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    As mentioned, this engine is a high port engine. Made to breathe at high rpm's. At least 5750-5850 max rpm's. The engine only has 49 cubic inches-trying to make 75 hp, so it will never be that good out of the hole. Not a hole-shot engine. But with proper propping, it should scream in the upper rpm ranges. The guys are correct don't reprop till you know your rpm's-once the tach is installed.

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    update on my 1975 75hp stinger

    Quote Originally Posted by unreal-inventor View Post
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ID:	326012HI I am new to this sight and hoping for some good advice getting some better speed out of my 16 foot sea nymph. I got the boat a week ago. I drove 500 miles and $1650 and one of my fishing poles. I have my new to me rebuilt 1978 sea-nymph with a stock 1975 75 hp stinger long shaft on the back and a 13 3/4x15 prop. The guy spent a year fixing this boat up new transome,decks,floors,carpet, radio.He did a good job the boat is solid. I know this prop is more for a pontoon of heavy boat and I need to change it. I took the boat out with the old lady and the boat ran only 30 mph and it ran good no missing ect. I checked the compression and it has 150 psi on all three cyl. I also made sure all 3 had spark. I changed the spark pluges and foged the carbs since is sat for 2 or 3 years. I know this is not the right boat for real speed but 30 really? I was shocked gps said 30 BOOO! This is not a heavy boat and it is flat bottom at the back . I am getting a tack saturday and Ill get some pics up when I get a chance. plz HELP Thank you for any help in advance
    Ok I put new plugs in cleaned the carbs and took it out for a test run. It ran poorly and blew the exhaust gasket and got water in the cylinders but it did not overheat. I took it apart and had the parts machined as they were warped. I put it back together with a 3.25x17 prop and it ran ok until 4700rpm. I contacted a johnson mechanic and he told me I forgot the center seal in the carbs so I took the off and looked and they were instailed correctly. I bought a new fuil pump (external) and reset the floats. I willl try again tomorro ;-( . best speed so far is 36 at 5000 rpm but did not hold it.

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