2002 Johnson Low Compression???

[rboarderi]

I recently purchased a 1974 15 foot Columbian Tri-Hull with a 2002 30HP Johnson J30ELSNF. I replaced the spark plugs when I bought it and the bottom plug that I removed was a little milky, I thought it might have just been bad gas in my old fuel line as after installing new plugs I didn't have any milkyness. I have taken the boat out twice now but at WOT it only gets up to 4000RPM which appears to be low from the service manual which states that it should be at 5200-5800. It also only reaches about 16-18MPH, I would assume that I should be going a bit faster than this. After doing some searching on the forums today it sounds like you guys like to have a compression reading before troubleshooting so when I got home I tested each cylinder. I have about 80PSI on the top cylinder and 85-90PSI on the bottom cylinder, I assume this is low? If so, would I attribute the low WOT RPM to the low compression? I tested and I do have good spark so I know it is firing on both cylinders. The spark plugs do not look very good after only 2 trips out and about 5 gallons of gas through the motor, I have attached a few pics of the plugs. Could someone please lead me in the right direction as to where I should start looking from here or what might need to be replaced. I am fairly confident fixing small 2 stroke engines in mopeds/scooters/mowers so hopefully this motor won't be too difficult. If anyone has suggestions please let me know!

 

[rboarderi]

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[sutor623]

Compression is ok between both cylinders. Could be the gauge reading low.

a 1974 15ft. tri-hull is a HEAVY boat. I would also guess that there is some water in the hull and possible water soaked foam under the floor. Either way, a 30HP on a boat of this size is pretty underpowered. I would guess a 50HP at the very least on a boat this age and size.

1. All that being said, you will probably get the 5200RPMs that you seek by going down a few pitch sizes in your prop. What pitch prop are you running?

2. Are those spark plug pics the old plugs? Please tell me they arent the new ones..........

3. How does the motor idle in gear? If it does well then I would not attribute this issue to compression problems.

 

[rboarderi]

Thanks for the quick response!!! Its a brand new compression gauge that I just bought over at harbor freight today, I might be able to get my hands on another gauge and test though.

As for the hull, it should be dry, the floor was completely removed and replaced 3 years back and all the old foam was replaced at that time as well.

1. The prop is the appears to be the stock 10 x 13 Johnson prop, everything looks original on the motor.

2. Those are the new spark plugs, why would they looks so terrible after just two trips out?

3. The motor seems to idle and shift well. I idled out in the bay for about 5 minutes yesterday, after that time it sputtered for a few seconds when I put it in gear and started to take off again.

 

[rboarderi]

No Title

Attached is a picture of the boat.

 

[sutor623]

You may just need to do a decarb on the motor. It may just be carbon breaking down in the cylinder that you are seeing on the spark plug.

Carbon on the rings will also be a cause of low compression. The fact that the cylinders match compression numbers and the motor isles well makes me not worry about the compression.
A decarb consists of a full can of seafoam mixed in 1-3 gallons of gas and oil mix. Then run through the motor for 10-15 mins, and let it set10-15 mins. Do that a few times and then run it all through the motor. You will be blowing all kinds of smoke and crud out of the exhaust. Then check compression again, and out in new spark plugs. I bet they won't look like that again!!!!

There is no such thing as a "stock prop". Props are matched to the boat/motor combo. There should be a tag on the boat that tells what the max hp is for the rig. I think that since that motor is underpowered, you would have to drop pitch sizes down to maybe a 9 pitch to get that motor to rev properly. You will then get a few more mph but I wouldn expect to be traveling over 20mph.

These are my opinions, take em for what they're worth

 

[rboarderi]

I'll give the decarb a try, would you run that mixture through the motor on muffs or out in the water under load?

Also, for information, my boat was originally made in CT back in the day and I just got an email back with more info from the owners son. He told me the hull weight is approximately 900 lbs, Hp rating is 80 hp but the boat ran best with 70 hp 3 cyl Evinrude or Johnson because of 3 cyl weight.

 

[sutor623]

Good info!!! You can decarb in the driveway, just keep the revs 1500 or under. Don't hemmer on it in the driveway. 900lbs before batteries, gas, people, coolers, etc is a LOT of weight to push with that 30hp. If the boat calls for an80hp you have just found your issue.

You have two options here. Get a lower pitched prop (7 or a 9) or sell the 30 and get a bigger motor. You NEED to get those rpms up or you will hurt the motor (eventually).

If I were you I'd do a decarb, see how she runs after, and check compression again. If that does ok then I'd invest $100-150 in a new prop and get the revs up. Likely you will hit 22mph or so on a good day. If that's not good enough for you you should sell and get something different.

So to answer your main question, the issue does not have to do with compression, but rather a mismatched boat/motor combo. Get it as good as ya can and smile!!!

 

[sutor623]

One last thing. Good rule of thumb is for every pitch size you drop, rpms raise 200. You are getting 4000rpms, so in order to raise up to 5200roms, you need to go up 6 pitch sizes. I say this because a 7 pitch is the smallest prop you can get for this motor. ( this is all prop theory) You just will not hit 5800rpms with this motor on the boat, and honestly should be happy if you can even hit 5000rpms.
Also, play with the trim height too and see what you can get. You are tucked in pretty hard the pic which creates drag, but you may need this to plane out (if you even can)

 

[rboarderi]

So, I have done a few things since I posted the other day. First, I did Dunks decarb. I ran it through the engine five or six times for 15 minutes each. I also got a new compression tester. I think I also properly tested the compression this time. Previously, I was leaving the bottom spark plug in when testing the top cylinder and leaving the top spark plug in when testing the bottom cylinder which was when I got the low numbers. Now when I tested the compression the proper way with both spark plugs out I got exactly 120psi on both cylinders . After the decarb the spark plugs didn't have all the crud on them although they were wet which leads me to my next problem.
Like I said previously I noticed that bottom spark plug was a little milky when I first bought the motor and boat. Today after I finished everything up I took the spark plugs back out and shined a flashlight in the cylinders just to take a look at what I could see. The top cylinder looked fine although the spark plug was a little wet but the bottom cylinder had a little bit of a milky substance in it and the top of the piston was milky looking as well. I'm guessing water is leaking in some how? Should I create a new post for this or can you guys help me here? Thanks!

 

[metriccrescentwrench]

...... He told me the hull weight is approximately 900 lbs, Hp rating is 80 hp but the boat ran best with 70 hp 3 cyl Evinrude or Johnson because of 3 cyl weight.

I think you'll be much happier if you sell the 30 and get an engine much closer to the maximum rating for that hull, that boat's seriously underpowered. You don't have to use full throttle if you just want to putt along, but it's sure nice to have more speed when you want to outrun a storm or you don't want to tick off your wife by being late for dinner.

It's hard to say for sure if you've got water in the cylinders (though that milky substance certainly is fishy). However, if there's a decent amount of humidity in the air it's possible to get condensation in the carb throat that could do that. Typically if there's water getting in a cylinder the spark plug will look like it's been steam-cleaned. It might not hurt to pull the head to see if anything looks suspicious, then reinstall it with a new head gasket.

 

[David Young]

Hate to tell you but 16-18MPH might be all you should expect to have with a 30HP motor. I have a 70HP on my 15 foot tri-hull and only get 35MPH.

 

[rboarderi]

I guess I'm fine with it being under powered for now. I only paid $1,000 for the whole boat, motor and trailer combo, this is my first boat and I'm fine with speed for now. I am just trying to go though the motor and make sure that everything is working as it should and am a little concerned that water might be getting into the bottom cylinder somehow. Anything that I could check to confirm my suspicions?

 

[sutor623]

Well you know water is getting in. First thing I'd do is pull the exhaust plate because water can leak into the cylinders from there. Change the gasket and reinstall. Next run it and see if problem is solved. If that doesn't fix it you can change out the head gasket. Thy can leak water into the cylinder also. If that doesn't fix it you may have a cracked block. But let's not assume that.....

If you are keeping this combo make sure that you change props as I have suggested. Lugging that motor is bad!!

 

[rboarderi]

Where exactly is the exhaust plate? I was unable to find anything called the exhaust plate in my service manual. Would you happen to know the part # for the gasket?

 

[rboarderi]

No Title

I'm guessing the plate is #20 and the gasket I should replace is either #22, #73 or both?

#20 - Exhaust Cover
#22 - Outer Exhaust Cover Gasket
#41 - Exhaust Cover Inner
#73 - Inner Exhaust Cover Gasket

 

[jakedaawg]

Take off #20. Inspect 41. Replace 22 and 73. Good luck. not easy surfaces to get cleaned in my humble experience

 

[rboarderi]

OK, so I didn't do anything about the moisture in the bottom cylinder yet but now I am having more issues. Last time I went out the boat seemed to drive well, not perfect but well. I went out again today with the hopes of fiddling with the idle and getting it perfect. Turns out the boat didn't really want to drive. As soon as I put the boat in gear it would barely go. I could only go a certain amount with the throttle which was equal to 1000-1500RPM before it stalled out. I tried over and over and it would start every single time but I had to get a little above to 1000RPM to not stall out and just barely putt along at a couple miles an hours, anything over that 1500RPM mark would stall out as well. Like I said, last time I took the boat out it drove well, now I am having problems. The only things that I have changed since the last time I went out was my spark plugs, did a Dunks Decarb X4 and installed a new Michigan Vortex XHS 10.5x9 to replace the 10x13 that was there to begin with. Anyone want to help me troubleshoot my new problems?

Another note, when I got home I hooked it up to some muffs to flush it out and it idled just fine, I even revved it up once to see if it would cut out and it did not...

Last note...the fuel is good and mixed correctly just today, spark plugs are properly gaped and the carb does appear to open fully when when I took the motor cover off when I got home.

 

[jakedaawg]

I believe you still need to address the water in
The cylinder

 

[rboarderi]

I understand that and was planning to replace the exhaust gaskets over the winter. I wouldn't think the moisture in the cylinder would be effecting acceleration when it drove well last time I was out. Just checked compression again, still at 120psi for both cylinders.