1977 Johnson 70 HP Lack of Power

[jasonlepak]

Earlier this year I aquired a boat from my parents.
It's a 1981 Checkmate 17' with a 1977 Johnson 70EL77S.

The first time I took it out it flew and jumped on plane almost immediately.
The boat itself will usually get on plane between 15-20mph.

After a little while it started having power and running problems.
First time out each day it would run fine for a few minutes, then bog down badly.

So I put new OEM Champion plugs in it, completely went through the carbs with carb cleaner and compressed air and put a float/needle kit in it.
I also put in a new water pump and lower unit oil change.

I checked spark with a gap checker and the spark was bright blue and jumped a 7/16" - 1/2" gap.
The boat has a 16 gallon aluminum gas tank mounted towards the front of the boat and it has an inline high volume new fuel pump with all new fuel line.
It also has a new high volume fuel filter.
The pressure on the fuel pump is about 3-4psi, maybe a little higher.
The fuel pump on the motor was also tested to be working as it should.
I also removed the gas tank and made sure it was clean and that there were new restrictions. There is also a breather on the gas tank that does not have any check valves so I know it's not a tank pressure issue.

I also did a compression test when I changed the spark plugs and all cylinders are around 120psi.

After all I've done to it, it still lacks power.
When I take it out on the lake and give it full throttle it will max out at 3000 rpm.
When I walk to the front of the boat while someone else is driving it, it will plane out and the RPM's will reach about 4000-4500 and I will eventually get up to about 25mph.

I am pretty sure that this motor is set up for a 20" transom, but the boat has a 24" transom assuming we're measuring from the very bottom of the back of the boat to the top of the back of the boat where the motor mounts to.
I've attached a picture of how the motor sits on the boat for reference.

I'm an auto mechanic so I know a lot about engines, but not everything about 2 cycle.
I know it's getting spark, it's getting air and it's getting fuel (50:1 ratio) and I've gone through quite a few tanks of fuel already because I take it out often.

Would anyone happen to know if I'm missing something?
I also redid the throttle linkages and choke linkages and timing which at full throttle I set between 17-20 degrees according to the markings on the flywheel.

 

[jasonlepak]

Forgot about those pictures.

 

[boobie]

Do a cylinder drop test on it and see if you come up with anything.

 

[jasonlepak]

I will do that tomorrow while I'm at work. Should one or more cylinders fail, what would be the cause since this engine doesn't necessarily have "valves"?

 

[jasonlepak]

Well, yesterday I had it at work and pulled the plugs... All three looked the same. All indicating that the engine is burning each cylinder the same and running as it should.
I did not do a cylinder drop test, but I did do a cylinder leak test and at 100psi they were all less than 10% which is better than most cars I do that test on.
I took it out on the lake with just my daughter and I in the boat. It took off right away like it should and I topped about 30mph and got the rpms around 4500-5000 rpm.

It seems to lack power mostly when there are more people in the boat. It still sounds like it's running good but has a very hard time planing.
The seating in this boat is more towards the rear than most boats so I'm thinking that it's mostly just the boat since if I put extra weight in the front it planes out nicely.
It's just a very light boat and the seats are very far back, which makes it not want to drop the front end as easily.

So thanks for the help.

I did find another crack in the transom in the bottom of the boat, and I'm thinking that I don't want to deal with it anymore so I might cut my losses and strip the boat and scrap it since I've been looking at a few other boats with inboard engines and even one that has a 150 Chrysler outboard I think.

 

[jakedaawg]

I would hazard a guess that the boat is waterlogged and now weighs two to three times what it should.

 

[fireman57]

If you are considering a Chrysler you must have tons of cash. You could be dropping a cylinder when it heats up under a bigger load than two people but I doubt it. Has it always planed at 15-20 with four or more people on it or is this a recent development. The transom is not that bad to fix. It does take some time and patience but selling a boat that could have a major problem is kind of a crappy thing to do unless you disclose it.

 

[jasonlepak]

If you are considering a Chrysler you must have tons of cash. You could be dropping a cylinder when it heats up under a bigger load than two people but I doubt it. Has it always planed at 15-20 with four or more people on it or is this a recent development. The transom is not that bad to fix. It does take some time and patience but selling a boat that could have a major problem is kind of a crappy thing to do unless you disclose it.

I never said I was going to sell the boat to anyone... I said I was going to strip everything off of it and scrap the boat. Meaning the hull is going to the dump or wherever I can roll it off the trailer.

It never ran different when it warmed up. It also ran the same regardless if I floored it a minute after starting or 30 minutes after cruising around. Which leads me to believe that the engine is running as it should and that the back of the boat is just too heavy to plane on its own.

 

[fireman57]

You might consider repropping with a lower pitch. If you are running a 21 pitch prop you might try a 17 or so. More hole shot a little less top end but your rpms will increase

 

[jakedaawg]

A 70 hp seems quite a bit underpowered for a checkmate. They looked fast but were incredibly heavy in those years. Especialy the bowrider models. If, upon inspection the hull is good and dry you may consider putting a 125 or so on it.