Tired motor.....run or rebuild?

[dhawk428]

Hi everyone,
Newb here. Great forum!
Since I picked up 88 Ev 9.9 and a '78 Johnson 15 in the last month, I did a few searches and here I am!

Yep - I have a question.. did a few searches, but didn't find exactly what I am looking for.

I wanted a little more speed for my little fishing rig (currently has the 88 Ev 9.9 on it)

I picked up a 78 Johnson 15 HP electric start thinking it would get me a little more speed, even if it was tired and needed a ring job.

Stuck it on the boat and she runs exactly the same top speed as the 9.9 Ev. Tried a couple different props, didn't help - it's propped right.

Bottom line - it's a 78 and it's probably tired - checked compression - 75 pounds both cylinders... I haven't got my manuals yet (just got the motor last week) - I know being the same is good, but isn't 75 a little weak?

Reading through the forums here, I see all of you talking about decarbing and the engine tune......

Is there any sense in trying, or could it cause problems by knocking deposits that could get stuck in the cylinder?

No problems with the motor - cranks first hit, very responsive..... just a dog compared to what I was hoping for a 15 (on a little Gheenoe)....

Do I just run her til I am ready to throw rings in, or give decarb/tune a shot?

 

[freddyray21]

Re: Tired motor.....run or rebuild?

75lbs in both cylinders is not bad. I would do a decarb on it first. Are you sure you have the cavitation plate in the right spot? A motor that runs too deep will rob speed and power.

 

[dhawk428]

Re: Tired motor.....run or rebuild?

75lbs in both cylinders is not bad. I would do a decarb on it first. Are you sure you have the cavitation plate in the right spot? A motor that runs too deep will rob speed and power.

Hey, thanks for the reply.

It probably is running a little deeper than I would like.
I plan on putting a jack plate later to get the motor up a little. There's no real way to get the motor up any further on these little Gheenoes except with a jack plate.

It planes perfect and quickly (less than a couple seconds), it just doesn't get the same top speed as other boats rigged similar, but they are running new motors.

Most of the people running the same set up are getting about 22-25 MPH, mine gets 18 with a tail wind downstream. Tried a couple different props and they all performed worse... The other rigs I refer t also run short shafts with no jack plate, so I don't think that is it, although it would help

 

[tashasdaddy]

Re: Tired motor.....run or rebuild?

if you are runnng a longshaft on a short shaft transom, that is most of your problem. also decarb the motor, probably never done.

Decarb, take a can of seafoam put 3/4 of it in the gas tank, with only 1 gallon of premixed gas. put the rest in a spray bottle. start the engine, and let it come up to temperature. then remove plugs, and them some real good shot of seafoam into the cylinders, replace plugs, let sit 15 minutes. restart, and spray the rest of the seafoam into the carbs, so the the motor almost stalls, wait and repeat until the seafoam is gone.then take for a wide open spin. then put in new plugs, ad premixed gas to the tank, and take it for a wide open throttle spin. it is going to smoke like a house on fire, during this process.

afterwards compression.recheck

 

[dhawk428]

Re: Tired motor.....run or rebuild?

if you are runnng a longshaft on a short shaft transom, that is most of your problem. also decarb the motor, probably never done.

Decarb, take a can of seafoam put 3/4 of it in the gas tank, with only 1 gallon of premixed gas. put the rest in a spray bottle. start the engine, and let it come up to temperature. then remove plugs, and them some real good shot of seafoam into the cylinders, replace plugs, let sit 15 minutes. restart, and spray the rest of the seafoam into the carbs, so the the motor almost stalls, wait and repeat until the seafoam is gone.then take for a wide open spin. then put in new plugs, ad premixed gas to the tank, and take it for a wide open throttle spin. it is going to smoke like a house on fire, during this process.

afterwards compression.recheck

It's a short shaft engine - think I forgot to say that. Boat is a 15" transom. Doing a decarb this weekend and see what happens. Thanks for all the advice

 

[CATransplant]

Re: Tired motor.....run or rebuild?

The decarb might help. But...remember the 9.9 and 15 are essentially the same engine. The 15 has some minor differences that let it run at a higher RPM. That's it.

If you're not cranking up enough RPMs, then it could be running just like a 9.9. There's one other thing to think about on that '78. There are a lot of them out there with a different powerhead or different carburetor than they came with. Guys are forever swapping parts between the 9.9 and the 15 to keep things running. I don't think you can tell the difference on either by looking at it. Check the welch plug on the engine itself and see if it matches the serial number on the plate on the transom mount.

Given the age of that '78, it could actually be a 9.9 in disguise. Heck, some folks even swap their 15hp cowling for a 9.9 cowling to pretend to be under 10 hp. On ebay, 9.9 cowlings go for a pretty penny. 15 hp cowlings...not so much.

In any case, you could be running a 9.9 powerhead or a 15 with a 9.9 carb, and not even know it. Seriously.

 

[mchin]

Re: Tired motor.....run or rebuild?

I run a gheenoe as well and with mine it runs a lot better with a hydrofoil like a doelfin, without it, it tended to porpoise and squat, especially with a heavy motor. Which noe do you have? The 15.4 classic has a narrow stern and not a lot of planning area, so the fins help get you on plane faster and probably gain some speed. Have you seen the site

http://customgheenoe.com/


A lot of good information on that site, especially in the forum.

 

[dhawk428]

Re: Tired motor.....run or rebuild?

I run a gheenoe as well and with mine it runs a lot better with a hydrofoil like a doelfin, without it, it tended to porpoise and squat, especially with a heavy motor. Which noe do you have? The 15.4 classic has a narrow stern and not a lot of planning area, so the fins help get you on plane faster and probably gain some speed. Have you seen the site

http://customgheenoe.com/


A lot of good information on that site, especially in the forum.

Mchin - I visit that site every day -username dhawk on there......(this rig is a 96 Highsider)

There is absolutley no problem with the way the boat planes or rides. Bow sits just right as long as my TM batt is up front - never has porpoised. Hole shot is about 3 seconds to plane........... just not as much top end as I hoped....
I have to get a tach to see what my rpms are actually - if it has a 9.9 carb on it, that would explain everything.

Catransplant - I was thinking EXACTLY what you are........I wonder if someone robbed the 15 HP carb to upgrade a 9.9 to a 15, and my 15 got stuck with a 9.9 carb........ I don't have a mic to measure the throat on the carb - but that is the main difference in the 9.9 vs 15 form what I read, plus different exhaust on the later models from 87 up or something like that

 

[CATransplant]

Re: Tired motor.....run or rebuild?

Actually, I don't know how you'd tell the difference between the carbs. Maybe someone else here will.

The reason I thought of that is that I watch ebay for small hp johnnyrude parts all the time. I see a LOT of 9.9 carbs, and NO 15 carbs. So many folks want to put a 15hp carb on their 9.9, and there are lots more of the 9.9s. It wouldn't surprise me at all.

Another way to tell would be to run both the 9.9 and the 15 on the same day, and compare the sound of them running at WOT. The 15 runs quite a bit faster. If they're running at the same RPM, I'll bet you've found your problem.

Of course, finding a 15hp carb ain't all that easy.

 

[wbeaton]

Re: Tired motor.....run or rebuild?

Here's my opinion...I doubt you will see a big difference between either at top speed. As I understand the earliest 15 hp models actually out put closer to 13 hp where as the 1988 9.9 hp is actually 10 hp. The difference will likely only be seen when the boat is loaded heavy. Your 15 hp might fall into the period where the only real difference between the 9.9 and 15 was the carb, which is when the 15 hp was underpowered. It was only a few years in the 1970's, but that is more likely in my mind then mis-matched parts.

 

[dhawk428]

Re: Tired motor.....run or rebuild?

Here's my opinion...I doubt you will see a big difference between either at top speed. As I understand the earliest 15 hp models actually out put closer to 13 hp where as the 1988 9.9 hp is actually 10 hp. The difference will likely only be seen when the boat is loaded heavy. Your 15 hp might fall into the period where the only real difference between the 9.9 and 15 was the carb, which is when the 15 hp was underpowered. It was only a few years in the 1970's, but that is more likely in my mind then mis-matched parts.

I think you are right Wayne.
There just isn't that much difference between a 10 and 13 that I would be able to even tell......I'm just going to run the 9.9 for now since it is so much newer.