Help can't figure it out?

[scottbeef]

Bought a new to me 1986 winner zz880 17.5 bass boat with a 1977 johnson140hp V4. Can not get the boat over 14 mph. giving it throttle it will lift the nose of the boat up and plow water slowly gaining speed until it gets up to about 12-14 mph and then the engine feels like it free spins. and a little rooster tail will shoot up about 15 feet behind the boat. Do not have a tach on the motor yet but it doesn't sound like it is going to high maybe 2000-2500 rpms. if we put weight in the front of the boat it will wash out sooner. If one of us sits back by the motor we can plow water a lot longer. We checked compression and it is across the board at 105. A little on the low side but all the same so it should be ok. We checked spark and found 1 coil pack that is not working. Swapped it with one of the good ones and got spark so the brain box part is working. (picked up a new coil tonight to install soon) The motor starts like a car. you just barely touch the key and she fires off. Even with 1 cylinder not firing a 140 should push this 950lb boat like crazy. It seems like we plow water until we create our own lack of water and the prop cavitates. We thought spun hub so we brought that to the prop shop today to have a new hub put in. 384241 13X19 ss. while there we picked up a used 13X21 SS and tried that same result. no idea if it was just a used prop and hub is bad or if they replaced (tested) hub before selling. Maybe 2 bad hubs but not likely? Tried advancing the timing a little bit while on the water. No difference. Tried choking it under load and it slowed right down as you would expect. At a lose as to what to try next. motor seems to run great. Very smooth and starts better than most. Even down 1 cylinder. Shifts into forward and reverse smooth and quietly. Not grinding or clunking. Bottom of boat looks like new. Bought it from a guy who bought it from a guy and so on. No idea if this boat has ever been run with this motor. I can't be the first person with this problem. Any ideas of what to try next? or possible causes/ Solutions.


Thanks for your time
Scott

 

[5150abf]

Re: Help can't figure it out?

Does this engine have tilt trim?

Sounds like you have the engine tilted way too much, try to bring the lower closer to the back of the boat and see if that helps, after you get the engine right of coarse.

105 is good for compression and I doubt you have to spun props, maybe a combination of the wrong trim and the engine not running right.

 

[emdsapmgr]

Re: Help can't figure it out?

How high is the engine mounted on the transom? If it is too high, it won't plane out properly and will tend to blow out just as it is ready to plane. A high rake stainless prop will compensate somewhat for engines that are mounted too high. When new, this engine had 130+ compression, but it should run with 105.

 

[tashasdaddy]

Re: Help can't figure it out?

are you sure that, that 1986 hull is not water logged.

 

[a70eliminator]

Re: Help can't figure it out?

Look at the cavitation plate on the motor, it's that horizontal flat fin just above the prop, with the motor down in running position the plate shouldn't be much above the lowest part of the transom. Do you have trim control?

 

[kenmyfam]

Re: Help can't figure it out?

Post photo of the motor, stern of boat and motor position in your normal "set off" mode. Other option is to weigh the outfit, do a little math and see if it is water sodden in there. A 140hp should bring quite an amount of weight on plane.
You did remove the plug and raise the bow on the trailer ?? could be full of rainwater that just needs draining.

 

[scottbeef]

Re: Help can't figure it out?

Motor is a long shaft and mounted as low as it can go. on the very top hole. Cavitation plate is right where it should be. Tilt n trim is not strong enough to lift it at speed but we have tried the old fashioned way with the pin to adjust height from very bottom to very highest setting. As far as being water logged. The boat is on a trailer made for a much larger boat but you can move it around very easily by hand so I do not think it is carrying a ton of extra weight. I've had boats that were full of rain water below the floor and they were HEAVY. It is almost as if we plow water long enough to create our own air pocket behind the boat. No matter how bad a 140 runs it should be scary crazy on a 950lb boat. I am sure we are missing something rediculously simple. I have no doubt my 25 merc would get this boat up and on plane and over 15 mph. Floor is solid and all seats are like new still even though they are 23 yrs old. looking down through any cubbys doesn't look water logged.

 

[scottbeef]

Re: Help can't figure it out?

could it be something in carberation not letting it get over a certain rpm?

 

[bustermin]

Re: Help can't figure it out?

Do not have a tach on the motor yet but it doesn't sound like it is going to high maybe 2000-2500 rpms.

I may be confusing this motor with another, but I thought "WOT" on this was around 4800rpm. If that is the case then you are only getting half power out of the motor. I would correct the coil problem and see how it runs before doing anymore troubleshooting.

 

[emdsapmgr]

Re: Help can't figure it out?

When the boat is moving, visually check the water coming out from under the boat at the transom. Check it vs the engine's antiventilation plate, just above the prop. The water from the hull should flow just under the antiventilation plate.

 

[scottbeef]

Re: Help can't figure it out?

I think that applies once up on plane. I never get up on plane. Right now it tilts the bow up in the air and pushes the stern down. Plowing water. The water just bubbles up half way up the mid section of the motor. The motor is mounted as low as it can be. If anything it may be to low. But cavitation plate is even with bottom of boat so it should be ok. O

 

[tmcalavy]

Re: Help can't figure it out?

I wouldn't apply any more fixes until you get a tach and figure out if the motor is reaching WOT rpms, or at least in the ballpark. If it's not reaching the power curve it won't plane your boat.