6hp Johnson Seahorse power issue

[gsh]

This past March I took my 6 hp 1976 Johnson Seahorse motor into a shop to have it checked over before the season started. I figured I had just bought the motor the previous summer and didn't really know what sort of condition it was in, so I'd be smart and have a pro check it out. Also, by going in early, I could avoid any delays by beating the spring rush.

A week later they called and said they were done. They said they put in new plugs (they did) and they replaced the water pump because it was pumping weakly.

A couple months later when the ice finally broke and I took it out. I was NOT a happy camper.

The previous year I had it out on my 14' aluminum boat and it was pushing me along about 10 to 11 mph according to my Garmin fish finder.

Now, I was barely able to get it over 6 mph with the same load in the boat. I took it back and they put a carb kit on it for no charge. Still ran like crap at the lake and the shop I took it to said they didn't do anything to it and old motors couldn't push my boat to 10 mph. Basically they called me a liar to my face.

Last night I put a Tiny Tach on it and today I tested it at the lake. I got 6.4 mph and 3,750 rpms. I did several runs with the wind, against the wind and across the wind. it was a very consistent 3,750 rpm.

According to the manual I should be running between 4,500 and 5000 rpm under load as the motor makes 6 hp at 4,500 rpm.

I checked the spark and it's good. I'm guessing the compression is still good since it was fine last year. However, I'm planning on testing that tomorrow.

That leaves my fuel air mix.

Right now my motor won't idle when set to slow, so I know I need to adjust that. But what can I do about the low top rpm? I'm running the same prop as last year and it's the factory, 3 blade, 8x7 prop from 1976.

Any suggestions?

 

[Willyclay]

Since I have the 1973 version of that same motor, I read your post trying to learn something new. I also read your previous thread from 2017 when you reported the boat ran "about 7.5MPH" and was curious how you determined that speed?

 

[boobie]

Do a cylinder drop test on it when you have it in the lake.

 

[gsh]

I got that speed from my Garmin as well.

My boat is rated for 600 lbs max load. I weigh 225, my buddy weighs about 170 and his girlfriend weighs about the same.

Considering that our combined weight is 565 lbs and our fishing gear and cooler probably put that to right about 600 lbs, doing 7.5 mph is pretty decent.

However, last year, with just my 60 lb daughter and I in the boat (max weight of 285), and her driving for better weight distribution, we got it to 11 mph. By myself, with a tiller extension I made from PVC pipe so I could better distribute the weight, I was going 10 mph.

Last week I was by myself and sitting in the stern and only got 6.5 mph.

Today, with my 200 lb brother-in-law (max weight of 425 lbs), we only got to 6.4 mph at 3,750 rpm according to the newly installed tiny tach.

 

[gsh]

Do a cylinder drop test on it when you have it in the lake.

I did that today as well. The motor kept running, but ran rough.

 

[boobie]

If I read you right the motor is running on both cylinders then ??

 

[gsh]

If I read you right the motor is running on both cylinders then ??

Yes, it is. It runs rough on one cylinder, but it does run.

 

[gsh]

I did a compression test tonight and both cylinders are within a couple pounds of 80 PSI.

I'm pretty sure that is not the problem.

 

[boobie]

What would you say is rough ?? Shaking, jerky or what ??

 

[gsh]

The kind of rough you get when a 2 cylinder motor is only running on one cylinder.

How else can you explain it other than rough?

Disconnect top cylinder and it runs rough.

Reconnect and it runs smooth.

Disconnect bottom cylinder and it runs rough.

Reconnect and it runs smooth.

 

[boobie]

After all this I think you need a bigger motor !!!!

 

[gsh]

After all this I think you need a bigger motor !!!!

It still doesn't explain why it's running poorly. Not to mention that my boat is only rated for 8hp max and those 2 hp would add 30 more pounds to the transom.

No, the guys at that shop changed something and I just need to find out what.

 

[tomhath]

Are you running with fresh fuel? What was left over from last year is no good, use only fresh mix with a measured amount of oil (no guessing).

Have you fine tuned the carburetor low and high speed settings? The mechanic would have set it to the default, but you always need to tweak it from there once you have it out on the water.

But I'm inclined to agree with the others - a 6hp motor should plane a lightly loaded 14' aluminum boat. But with three people onboard you have no chance.

 

[racerone]

This motor left the factory with a fixed high speed jet. ----No adjustment.

 

[gsh]

Are you running with fresh fuel? What was left over from last year is no good, use only fresh mix with a measured amount of oil (no guessing).

Have you fine tuned the carburetor low and high speed settings? The mechanic would have set it to the default, but you always need to tweak it from there once you have it out on the water.

But I'm inclined to agree with the others - a 6hp motor should plane a lightly loaded 14' aluminum boat. But with three people onboard you have no chance.

I mixed up a fresh tank of fuel this spring using the manufacturers recommended ratio and regular, non-ethanol gas.

Last year I had 3 people in the boat and was loaded to the max weight. I managed 7.5 mph.

This year, after taking it in, with just myself in the boat, I only get 6.5 mph.

I used to get at least 10 mph and there are numerous videos showing old 6hp motors on 14 ft jon boats going 14 mph or faster. I understand my boat is not a completely flat bottom like a jon boat, but it isn't far from it.

There is no high speed setting, only low speed.

I have good compression.

Something changed. I have no idea what it is and the shop claims they did nothing except change the gear oil and the waterpump.

 

[racerone]

What did the old water pump impeller look like ?-----And how old was the impeller that they took out ?-------Does spark jump a gap of 5/16" or more , yes or no ?

 

[Wkd179]

Just a thought...has the tilt pin been moved?

 

[gsh]

Just a thought...has the tilt pin been moved?

Tilt pin?

The motor has a manual trim adjustment. Is that what you mean?

 

[gsh]

What did the old water pump impeller look like ?-----And how old was the impeller that they took out ?-------Does spark jump a gap of 5/16" or more , yes or no ?

I have no clue what the old impeller looked like. And I don't know if the spark jumps that far.

I was going to look at the ignition, but it's under the flywheel cover and I'm not sure how to get it off.

 

[racerone]

The testing needs to be done.----You need a puller that uses those 3 threaded holes in top of the flywheel.-----The threads are 1/4-20 NC