What would cause a motor to only get up to 2300 RPM at full throttle (1988 Johnson)

[jhouser]

First thank everyone who has helped me on this forum. This forum is invaluable! I have been able to fix a few problems on my boat for free and learn more about boats, which I love.

Now I have a new problem that I hope I can also fix. I have a 1988 Johnson 88 HP SPL. The last two times I have taken the boat out it has done two distinct things:

1. It has a very hard time starting, takes about 10 minutes! It used to take just a couple of minutes.
2. Now, most of the time when I get it out on the water it will only get up to about 2300 RPMs at full throttle, and will only go about 10 miles per hour. It used to do about 4500 RPMs with no problem and up to 30 miles per hour, just hit it and go. Now it struggles to get to 10 miles per hour.

This happened the last time I took it out last year in November and the first time I took it out this spring. Thinking it was a spark plug issue, I replaced the spark plugs. But this last weekend it was very hard to get started and again only got up to 2300 RPMs.

Does this motor have points or something like that that I could replace? What could it be?

Thanks,

jhouser

 

[ondarvr]

Check the compression. If all it could do is 4500 RPMs, that could kill it right there.

 

[nabiul]

Did you check the spark before replacing spark plugs? It sounds like you're running on only one cylinder.

 

[emdsapmgr]

Agree, you may be running with only 3 cyls firing. After you do a compression test, perform a cyl-drop test to see if all 4 are firing.

 

[racerone]

If it takes a couple of minutes to start that motor there is something wrong !

 

[jhouser]

Thank you everyone for your responses. I will check the compression and spark per cylinder. Here is one other weird thing, that I wanted to not say initially because it might have been too much information all at once and might have confused the issue. Sometimes if the motor is struggling to get up to full RPMs, if I put it in neutral, then use the idle bar to make the engine rev very high, then put it back into gear, put it to full throttle it will indeed get up to full RPM and speed.

Why would it work sometimes but not others?

 

[ondarvr]

And clean the carbs.

 

[jakedaawg]

And make sure timer base moves freely and butterfly's are indeed opening completely, but compression check is first step absolutely

 

[jhouser]

Bad news, I got a compression checker, one of them reads zero PSI. The other three are at 110 or so. So I pulled the head. Even worse news, something got in the piston and beat it up. The ring is shot (piston wiggles around) and there are some scores on the piston wall that I can feel with my fingers. Is there any amount of piston scoring that is acceptable?

 

[interalian]

Sounds like a broken ring. Turn the motor over until the zero compression piston is at the top of stroke and shine a flashlight down the side of the piston. Move the piston around to see the top ring.

If you can catch a nail on the scoring it's probably too much.

 

[boobie]

Depends on the owner but really none.

 

[jhouser]

I think the piston and the ring at the very least are shot. Is it possible to replace the piston and ring without taking the motor apart?

I think that the cylinder is OK, it is not terrible. I cannot make my nail catch on it. How can I replace the piston and ring?

 

[interalian]

No, crank case needs to be removed and split to change a piston. And if you're in that deep, you're probably looking at re-ringing and honing the 3 good holes and one new piston with an overbore. How handy are you and how much are you willing to tackle?

 

[jimmbo]

Is it possible to replace the piston and ring without taking the motor apart?

I saw an engine where someone did that, rather I saw what was remaining of an engine after someone did that. It started, the first time it was revved up, it grenaded

 

[interalian]

This is as close as you'll get to the conrods without splitting the case.

 

[emdsapmgr]

Before you get too far into an overhaul, you'll want to do an autopsy on the block to determine why the ring let go. No point in rebuilding without knowing what the problem was. Did you pull the carb apart that feeds the bad cyl?. Was one of the jets plugged? Did the engine overheat on that cyl?-what shape are the rubber water diverters in the block? are they in correct position? Are they plugged up with sediment? Saltwater engine? Was one of the thermostats stuck shut? Do the pressure relief valves open in the stat housing?

 

[interalian]

BTW, "piston wiggles around" is normal. Which piston let go?

This little clip shows the one that let go on my 140 donor block. All the others moved about the same. After rebuild, the fresh pistons moved but not quite as much.