Johnsnon 25r70 speed issues

I have a Johnson 25r70 on a 1542 flat bottom. I bought boat, motor, and trailer for a steal and had to have new lower end seals. First time I took it out I could tell the carb needed cleaning as it would not idle and would only go about 7 mph. I had the carb cleaned and rebuilt and new plugs and it will only go about 10 according to my gps with 2 guys weighing about 400 pounds. It runs alot better and sounds strong. Anyone have any idea as to why it would be so slow?

Re: Johnsnon 25r70 speed issues

welcome to iboats, could it be a short shaft motor on a long shaft boat? measure transom from where motor sits to center bottom. measure motor from where it sits, to the flat plate just above the propeller. what are the measurements???

Re: Johnsnon 25r70 speed issues

I believe its a short shaft, I think 15 inches or so.

Re: Johnsnon 25r70 speed issues

make sure. do you have spark both cylinders, and what is the compression on both cylinders, is it electric start or recoil?

Re: Johnsnon 25r70 speed issues

I will get the correct measurements in a bit. I have spark to both cylinders, I don't know about compression though have not run a test. it is Electric with recoil backup.

One thing that I have noticed and this might be the problem.

I noticed that on my carb that on the top front there is a needle valve adjustment. I noticed that in the exact spot on the bottom there is not a adjustment rod coming out but a bolt. Correct me if I'm wrong but I would think something besides a bolt should be there, as a hole is in the front cover that once looked like it was some kind of adjustment knob. I told my mechanic about this however he is 98 years old and has his grandson doing most of the work. When I talked with him about this he didn't seem to understand and when I picked it up the bolt it was still in there but the old man said it was running like it was new.

Re: Johnsnon 25r70 speed issues

Correct measurements are - 15" boat transom and 18" motor shaft. I thought they were 15",20", and 25" but I measured like you said and I came up with 18"

Re: Johnsnon 25r70 speed issues

That motor has a fixed high speed jet deep in the hole behind the "bolt"...should actually be a short hex head plug with a slotted head.
Sounds like the linkage to the slow speed needle is missing. It should have a link leading to a knob at that hole.

See it all at http://shop2.evinrude.com

Re: Johnsnon 25r70 speed issues

your motor is a 1970, the legs were longer back then do to the placement of the water pump. you could slip a 1x2 under the transom mount, that will bring it up 3/4 of an inch. which will improve performance. like the pic below.

it sounds like who ever did you carb rebuild did not remove the high speed jet, and clean it, or it has become clogged again. ( don't run it like that, lack of lubricant).

in this link you will see the necessary tools and steps to remove the high speed jet, and clean it. http://forums.iboats.com/showthread.php?t=244013

you can adjust the low speed jet, with out the linkage to the knob, buy turning the stem, once adjusted it rarely needs attention.

(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.




any question just ask. good luck.

Re: Johnsnon 25r70 speed issues

Thanks so much I will check all of those things out. I figured the motor might be low as it puts of the wake of a skiboat! I will make adjustments and let you know how it does.