'94 25hp Johnson stutter

Hard to describe the issue but I’ll try.

First outing month ago or so engine fired up, cruised around up river for a bit to make sure all was good, and was, so flew downstream and on way back I couldn’t throttle motor past quarter way. Seemed like it was skipping or stuttering, whole thing would shake hard. Almost thunk thunk thunk like. Would run fine barely above idle, just couldn’t go past that certain point and would thunk thunk.

So I made my way back up in the slower current and after 5min?, thing just took off and ran fine up to i left, another half hour. Haven’t been out since.

I didn’t treat my gas for winter, only seafoam added and with all the posts on such treatments seems it can gel up, is this a possible answer?

Got the motor last year so am replacing impeller, plugs, wires, gas tank line, and will start with fresh gas too.

Anything else I should check or just replace?

You just described my 94′ mariner 15hp when I would run it with ethanol based gas. The motor ran flawless with ethanol-free, high octane fuel.

Pulling air in somewhere??

Come to think of it i may have filled up with regular gas last time, typically I use high.

I wonder if that’s an indication that something else ain’t right though?

rkd-jim wrote:

Pulling air in somewhere??

Checked the bulb and was fine, tank was vented, fittings seemed solid with no leaks. Tank and motor connections have a lil wiggle to em, but will probably replace those as well while I’m at it. Be nice to have some spares in the boat anyways if they’re OK. They didn’t have any effect on situation when I was playing with in boat.

nhamm wrote:

Hard to describe the issue but I’ll try.

First outing month ago or so engine fired up, cruised around up river for a bit to make sure all was good, and was, so flew downstream and on way back I couldn’t throttle motor past quarter way. Seemed like it was skipping or stuttering, whole thing would shake hard. Almost thunk thunk thunk like. Would run fine barely above idle, just couldn’t go past that certain point and would thunk thunk.

So I made my way back up in the slower current and after 5min?, thing just took off and ran fine up to i left, another half hour. Haven’t been out since.

I didn’t treat my gas for winter, only seafoam added and with all the posts on such treatments seems it can gel up, is this a possible answer?

Got the motor last year so am replacing impeller, plugs, wires, gas tank line, and will start with fresh gas too.

Anything else I should check or just replace?

I would also clean the carb.Sounds like something may have blocked the hi-speed jet.Does not take much to do that,a small piece of debris floating around in carb fuel bowl.

I had a Johnson 10 hp that had an air leak under the hood where the fuel line hooks up to the engine. There was a little square plastic filter screen that was held on by a single screw through the center. That plastic piece was cracked and caused that motor to cough and sputter if I went past 1/4 throttle.

milemark_714 wrote:

<div class=”d4p-bbt-quote-title”>nhamm wrote:</div>
Hard to describe the issue but I’ll try.

First outing month ago or so engine fired up, cruised around up river for a bit to make sure all was good, and was, so flew downstream and on way back I couldn’t throttle motor past quarter way. Seemed like it was skipping or stuttering, whole thing would shake hard. Almost thunk thunk thunk like. Would run fine barely above idle, just couldn’t go past that certain point and would thunk thunk.

So I made my way back up in the slower current and after 5min?, thing just took off and ran fine up to i left, another half hour. Haven’t been out since.

I didn’t treat my gas for winter, only seafoam added and with all the posts on such treatments seems it can gel up, is this a possible answer?

Got the motor last year so am replacing impeller, plugs, wires, gas tank line, and will start with fresh gas too.

Anything else I should check or just replace?

I would also clean the carb.Sounds like something may have blocked the hi-speed jet.Does not take much to do that,a small piece of debris floating around in carb fuel bowl.

That’s what I was thinking too. Probably just some oil resedue that dried out in the jet or on the needle blocking it just enough to make it run bad. Then finally enough fuel flowed through to unblock it.

So is it running fine now?

If yes, I’d test run it on the garden hose to make sure.

An air leak is also possible, especilly at high speed. If the problem is still happening, the easiest way to dagnose a hose or dip tube leak is to change out the hose and the tank with another that you know is leak free.

Running ethanol blend or non ethanol blend or this octane or that octane has no effect on small engines. As long as the fuel is fresh and not contaminated with water or particles, these engines will burn it. Fresh is the key, run your engines and burn up your fuel without letting it sitting around for a year and you’ll seldom have a problem.

All carbed engines benefit from a good carb clean and then a rejettin now and then.

Grouse

Running ethanol blend or non ethanol blend or this octane or that octane has no effect on small engines.

If you run ethanol blend fuel through hoses not designed for it the hose will brake down on the inside and bits of that rubber can plug the filter and carb.

Stan Jenson wrote:

Running ethanol blend or non ethanol blend or this octane or that octane has no effect on small engines.

If you run ethanol blend fuel through hoses not designed for it the hose will brake down on the inside and bits of that rubber can plug the filter and carb.

This is true, but all hoses since the late 1980s have been made of ethanol resistant materials. If you’re running a 20+ year old hose, it’s time to replace it anyway.

I was referring to the fact that ethanol blends don’t make engines run any differently.

Grouse

had a 20hp on my duck boat do the exact thing it had a pull out primer assembly on the front of motor and it turns out that it was bad put a new primer assembly in and ran great. just something else to check

rkd-jim….”I had a Johnson 10 hp that had an air leak under the hood where the fuel line hooks up to the engine. There was a little square plastic filter screen that was held on by a single screw through the center. That plastic piece was cracked and caused that motor to cough and sputter if I went past 1/4 throttle.”

My 25 had this issue and acted exactly as described.

I’d bet it’s your power pack. Learned from experience. Look it up…

cp

I will 2nd the bad power pack comment above, I had a 30 hp Johnson do just as you are describing. It would “kick-in” after warming up a bit. Take it to a mechanic and save yourself some headaches of messing with the fuel system.

I checked that fuel filter and all was good. The whole system is original as far as I know, so did find a small crack in the line fitting, I’ve experienced air, or loss of pressure, getting in the system before and this particular issue wasn’t close to that.
Going to replace hose, and tank fittings, tank fitting might be fine but again be nice to have spare in boat.
Mechanic at Robb. Marine said most likely bad gas, he was able to hear my wonderful sound effects. I tend to agree since it ran fine for an hour before, then had issue, then fine again for another half hour at least. I’ll keep that power pack in mind and hope for the best. Got a big trolling battery in case it turns out to be something else.

Anyone out in NW metro that wants 5 gallons of sketchy fuel its all yours.