1978 Johnson Stinger 70 hp runs slow

[dlammon]

I have recently aqcuired a 70 HP Johnson and the previous owner says that boat will do 30-35 mph with same prop and gear. The boat sat for a year without running but when I got the boat it started up first time. I was really happy with the easy starting and ran it on the trailer. It runs up top 3-4000 rpm with no load. Sounds good like its hittin on all 3. <br />When you put the boat in the water it only gets up to about 1500-2000 rpm. Sounds fine but no revs. Boat only does about 10 mph. I checked throttle linkage and it opens butterflies wide open. Choke is closed and not stuck. Checked for blockages in fuel line, sqeezed fuel bulb, no difference.<br />What to do? where should I look next. I though about timing or water in gas. How to check this?<br />The prop is the same prop that was doing 30 mph so I'm sure its not over-propped.<br />Water pressure OK, temp seems ok. <br />When you rev the engine at idle it screams, but in gear its a slug..

 

[Uncle Fred]

Re: 1978 Johnson Stinger 70 hp runs slow

Krave,<br /><br /> Go back to 5/10 and check my note. <br /> I didn't have a repair manual but have<br /> one en-route. I want to sell the boat <br /> as I have a new Triton SF-19 with a 200<br /> OPTIMAX due in at the dealers by the end <br /> of this week or early next week. <br /> By the way, the dealer told me that Merc<br /> went to the 250 cu inch block for the 200<br /> Optimax. I fired a note to Merc and got <br /> back the complete list of engines with the<br /> 185 cu.inch highlighted. Guess the dealer was in error.<br /> Anyone know different?<br /><br /> Uncle Fred

 

[dlammon]

Re: 1978 Johnson Stinger 70 hp runs slow

Sounds interesting. Were your gears dragging when you pulled the inspection. I was thinking that something may be dragging as well.

 

[thmlrmg]

Re: 1978 Johnson Stinger 70 hp runs slow

i was just about to post almost the exact same problem when i found this one. i have the exact same problem with my 70hp johnson. any help would be greatly appreciated.

 

[dlammon]

Re: 1978 Johnson Stinger 70 hp runs slow

Was your problem gears or something else...

 

[JB]

Re: 1978 Johnson Stinger 70 hp runs slow

Hi, Krave. <br /><br />You are probably running on 2 cylinders. How it sounds and revs without a load means nothing. You may be missing spark on one, but it is more likely to be a gummed up carb.<br /><br />Check each cylinder for power by shorting/disconnecting each plug in turn. If there is not a drop in rpm that cylinder is not making power. If you have spark on that one overhaul that carb.<br /><br />Good luck.

 

[dlammon]

Re: 1978 Johnson Stinger 70 hp runs slow

Thats good info JB. I think you are on to it. I'm going to go home tonight and try shorting out the wires on each. I had also planned on taking the bowl off the carbs and wash them out, look for water, floats etc.<br />Anything special to watch out for?<br />Also is there a jet adjustment on each carb? I dont see one like on a convention engine.<br />I was also looking at the filter where it connects to the engine. Strange looking device. Has like reed valves in it or something. Probably eliminates pulsations in the line?<br />The filter itself consisted on a wire mesh, that was it. Should there be more in there? Also can anything go wrong with the reed assembly.<br /><br />This is a great web site. I've got more help in a matter of hours..<br /><br />We must love our boats, we work all day at our regular job then we go home and dink around till dark so we can be back out fishing ( or beer drinking ) by the weekend.<br /><br /><br /><br />DL

 

[waz57]

Re: 1978 Johnson Stinger 70 hp runs slow

Sounds like the exact problem with my 78 70hp Johno but I found whilst running mine at home the top cycl wasnt firing so I tapped the coil plate and it seemed to break into 3 cycl, I am not sure but I think it may have been causes by a bad earth with this plate, so I took the rubber boots underneath the plate off and bolted driectly to the motor.
So far so good it runs on all 3 and it VERY responsive. Maybe this plate is a common fault in all the 70hp ?? as it made a HUGE difference to my motor.

 

[iwombat]

Re: 1978 Johnson Stinger 70 hp runs slow

If you keep running it up to 4k RPM w/ no load, a cylinder dropping out will be the least of your worries.

2k Max unless its in the lake or in a real test tank.