1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

[Laser Boat]

New to boating, bought a 18ft Laser Bass Boat with 1978 Mercury 150 V6 Blackmax Thunderbolt, has 21" prop. Bought this last year as a fixer upper. got it running, smooth iddling and rev up - runs smooth, makes no difference. sounds good, no issues on the hose either just using the iddle throttle or even putting in gear. But Take it out on the water cant hardly go over 15mph either slow increase or hammer down, the prop feels like slips into nuetral and motor revs up. Last year was told prop was spun, took it to a prop shop, said my 19" was chewed up & outta balance, they gave me a old 21" replacing mine for the same cost of the rehub they were doing for me, so only spent $60,. I didnt get to take it back out until this year. My gear oil is good, took it back out with the 21" prop with new hub. i get same results. So now few people telling me they think my engine coupler is bad. Im open for any other sugguestions and also ive tried looking up engine coupler or finding out where its at, ive had no luck, if you can give input about the engine coupler, maybe a diagram of where its at, what it looks like,, etc. is it a part i can replace without replacing gears, etc.. Thanks for anything you can give.

 

[Laser Boat]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

also just fyi old throttle cables were cracking & splitting, cables coming out the sides of the lines. I just put new cables on.

 

[wired247]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

Engine coupler? On an outboard Merc V6? There is no such thing.

Have you got the right washer in front. Leaking exhaust onto the prop will ventilate it.

 

[Laser Boat]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

OK, word from Jestor. On myused replacement prop. They did put in a new hub. but it didnt occur to me that it could be bad or spun also, ill try to mark my hub and prop to rest its not spun also.

 

[Laser Boat]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

well glad to hear i dont have engine coupler, heard was expensive to replace, and also explains why i couldnt find one in my searches

 

[Laser Boat]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

Wired, 1st thank you. When the shop gave me the 21# prop, they said had a new hub in it, gave me the washers, nuts etc with it to use. and the rear push plate or big washer (goes on first behind prop). as my old orignal prop that i took in, they was was old style pressed in hub, just fyi. but i have not marked it to test this new one for being spun. Ill try get out the next few days for test

 

[wired247]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

You just want to make sure you dont have a gap between the prop and the prop housing. If the washer is not the right one or the prop is not right for the housing and there is a good sized gap,the exhaust gases get into the prop blades more than they should and the prop will just wizz right up there without the boat going anywhere. Same thing happens if your prop is one that is prone to ventilation and your motor is mounted too high. I have one prop that LOVES to ventilate unless I drop the motor to its middle holes.

 

[Laser Boat]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

reporting back, the replacement prop i was given, with a new hub. was in fact bad too. Spun. Ive had this boat for 1 year now, never had it above 10-15mph haha. Any way i came across a SS prop, not new but i got it to test out my boat, so put it on, its a 22.5 pitch. i locked it down tight, tested the boat and for the first time, i was cruising.. maxed out about 40-45mph (just by old boat gauge) its the only working gauge on the boat, so i dont have tach working yet, cant tell you my rpms but i really thought should be going faster. At full throttle though did feel the boat kinda slipping a little in power or prop power. It would run smooth wide open and then kinda could feel it go back n forth to full throttle, then little miss, then back.. but if i backed off just a fraction cruised no issues at all like at a smooth 40mph. I did look at where my prop is vs bottom of boat, it is below bottom, or the spindle and half the prop are all the way below. also will say i had to keep the motor trim'd all the way down, almost tucked up underneath the boat to keep it cruising or if i tried to trim it up some it would then like loose all power kinda like shutting down, but id tuck it back down and off id go. one other thing, when i got this SS prop 22.5 pitch, it didnt have new thrust plate or washer with it, i used my old one, felt and looked snug. I almost think it might have been getting some of the escaping exhuast gases mention before causing the full throttle missing. but ????

 

[Chris1956]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

So to sum up your issue. She runs smooth up to about 40MPH, with motor trimmed all the way down. If you trim her up, the prop ventilates? If that is it, the motor may be mounted too high.


40MPH with the motor all the way down is not too bad. Trimming is where you can add some significant speed.

BTW there should be no gap between the gearcase and the prop hub. No gap around the hub to the gearcase nor from the inner end of the prop to the gearcase.

 

[Laser Boat]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

Thanks Chris, you helped me with this boat lastyear when i bought it , getting some of the wiring issues lined out. I didnt get to work on it much after i got it running and the prop was spinning, got the different prop with new hub at end of last year and kept spinning. also it would die soon as increased throttle. This spring i pulled carbs off and found 1 of the six floats, didnt have a wire on it tying to the needle, got that replaced and now finally trying to get it boating.. thanks again for everything. Ill double check the spaces and gaps, i do have one question about a WHALE or TAIL FIN flaps , would that benefit me to help push water down on the prop more??

 

[Chris1956]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

I do not think a whale tale would help with a ventilation issue. It could very well add drag, so I cannot recommend it.

Get a friend to drive. Get her on plane, look over the transom and see where the water is, in relation to the antiventilation plate. If the A-V plate is any more than an inch or so above the water level can cause ventilation issues. Normal performance boats usually run with the A-V plate right on the surface. Your boat is a higher performance boat, so you should be running the A-V plate out of the water a bit, but not too much. Of course a lot depends on what kind of prop you are using. Some ventilate more than others. Make sure the top water intake hole is not sucking air as well.

 

[wired247]

Re: 1978 Mercury Blackmax Thunderbolt Ign. 150 V6- Engine Coupler ??

I do not concur. Normal high performance boats run the AV plate about 4-5 inches or more out of the water when the boat is on plane. The Mercury prop centerline is about 9 inches from the AV plate and there is no good reason to run the prop centerline more than 5 inches from the lowest point at the back of the hull. The AV plate is not supposed to be anywhere near the water when the boat is on plane. The AV plates only purpose in life is to keep air from sucking into the prop off idle until the boat gets on plane. The whale tales push the stern up quickly and then get out of the water once the bow drops . If you are running your motor so low that the AV plate is within 2 inches of the surface when you are on plane then you might as well just throw the anchor out while you are running because once you trim it up that AV plan will act just like an anchor when it drags through the water at an angle.

I just got out of the water. With a 17 pitch prop that ventilates unless the prop centerline is at least 5 inches below the lowest point on my hull I was running 55 MPH at 7000 RPM with a 2.0 liter 150 HP Merc. That put my AV plate at more than 4 inches out of the water while running. A 4 blade 21 pitch prop runs 67 MPH at 6000 same boat. If I drop the motor two inches I lose 3 MPH with any prop on there.