83' Merc 70hp stalling

[Darren44]

HI , this is my first post here. I just bought a 20ft bass buggy pontoon boat w/ 1983 70hp merc. I had it out for the first time today. The problem: It starts and idles fine,but when I go to gas it , it bogs then stalls. If I rev it up abit in neutral and slam it into forward and floor it, it takes off. I cruised for about 5 minute and then slowed and came to an idle, but it wont take off unless I rev it and slam it to forward (I know it s not good to do that) The compression is 110 psi in all 3 cyl. It has the weirest spark plugs I have ever seen, gapless flat, they look clean.

Any guesses to what this could be? It runs good in my driveway with the muffs on. I live an hr away from the lake and would like to fix this before I go out again. Any good merc mechanics in Colorado Springs CO? I have rebuilt motorcycles and cars, but this is my first outboard and I am sorta clueless on this.

Thank you

Darren

 

[WillyBWright]

Re: 83' Merc 70hp stalling

Welcome to iboats.

Take the air box cover off of the front of the powerhead and you'll see the throats of the carbs. The low speed mixture screws are at 3 O'clock. Open them (unscrew) 1/4 turn to richen the mixture. If that doesn't do it, the carbs might have to come off for a thorough cleaning. (I'm assuming your gas is fresh. It'll bog if the gas gets old.)

 

[Darren44]

Re: 83' Merc 70hp stalling

Willy the boat came from sea level, Now its at 6000' Wouldn't it be a rich condition I am experiencing now with the elevation change? Gas is new


"Open them (unscrew) 1/4 turn to richen the mixture. "

 

[Darren44]

Re: 83' Merc 70hp stalling

My motor's serial # is 5883132 Do you know what yr motor this is?

Thanks in advance
Darren

 

[Darren44]

Re: 83' Merc 70hp stalling

Thanks again Willy,turning those screws out 1/4 turn was the trick. I had it out yest and it didnt stall when I gassed it. The 20' aluminum pontoon only did 15 mph (per gps) does that sound alittle slow? just me and my lady on the boat.

 

[jebeebe]

Re: 83' Merc 70hp stalling

Hi,
I've got one just like it with a 60hp and that is pretty close to max. They aren't a speed boat. Mine is propped so low it will turn 6k in a short time. They are better off with more power and less speed as they can be loaded heavy and can be lugged around a lot. I have trouble with trimming the motor up. Just a little trim and the prop blows out. (cavitates). Mine is a suntracker DL. party barge. I rebuilt it up from the tubes. Motor included. Good luck................Jerry

By the way........those are the right plugs. (fire ring)

 

[Darren44]

Re: 83' Merc 70hp stalling

Thanks Jerry

 

[Darren44]

Re: 83' Merc 70hp stalling

Jerry what prop pitch are you using? My motor is only going to 3500-3700 rpm's WOT, I question if the tach is correct. The original motor was a 80hp johnson or everude i think, dont know if it was a 3 cyl or 4 cyl motor. If it was a 4cyl motor the boat came with , the tach would think the motor now is a 4 cyl and I now have a 3 cyl.

Thanks

 

[jebeebe]

Re: 83' Merc 70hp stalling

Hi,
I had the pontoon out monday and checked it with the gps at 18 mph. That was at 5500 and 6000. Higher rpms got no more speed. I'm not sure what pitch the prop is. I'll check it and get back to you.
We had some issues while loading and unloading . The motor dropped down and the ss prop got into a piece of tubeing going across the trailer and sounded real nasty. Fortunatly didn't hurt anything and when loading the motor got down again and came up against the tubeing with the skeg, again not hurting anything. I bought a new piece of tubeing today and cut it to length made holes in the bottom for drainage and will weld it in front of the motor and cut out the one behind the motor. Why they would put one there is beyond me. Other than that and the warning horn we had a good day.
I would think your tach is off. There is a setting on the back of some tachs that you can change from 6-4-3 cyl motors. If you have choices, ask someone on this site what it should be set at. I know I have seen it discussed here. ......................Jerry

ps...........just checked the prop. It's a "9"p