Need more power.....6hp

[Tarner]

Hi guys, first time posting here. I have a 1969 Johnson 6 horse outboard on a 16' X 4' flat bottom fishing boat. I'd like to get a little more power out of this little motor. My top speed in this boat is a screaming 4 MPH up stream and 7 MPH down stream. I just rebuilt the carb, and set the points and found that I have a weak coil. This coil had a pinched wire. I still got a spark with the spark plug but nothing on the spark tester set at .250". I couldn't get anything out of it till the gap was under .125" on the spark tester and it looked weak. I went to the boat shop and got a new coil, points and condensers.

Now for my questions:
1. Will replacing this weak coil make a noticeable difference in power?
2. Should both coils be replaced or just the one that's bad?
3. Is there a mark or spot that needs to be lined up to set the points? I've just used the key on the crank shaft and lied it up with the cam follower of the point. Seems to work fine. But if there is a right/better way I'd rather do it like that.

Any help I can get is greatly appreciated.

Thanks,
Tom

 

[kenmyfam]

Re: Need more power.....6hp

Hi guys, first time posting here. I have a 1969 Johnson 6 horse outboard on a 16' X 4' flat bottom fishing boat. I'd like to get a little more power out of this little motor. My top speed in this boat is a screaming 4 MPH up stream and 7 MPH down stream. I just rebuilt the carb, and set the points and found that I have a weak coil. This coil had a pinched wire. I still got a spark with the spark plug but nothing on the spark tester set at .250". I couldn't get anything out of it till the gap was under .125" on the spark tester and it looked weak. I went to the boat shop and got a new coil, points and condensers.

Now for my questions:
1. Will replacing this weak coil make a noticeable difference in power?
2. Should both coils be replaced or just the one that's bad?
3. Is there a mark or spot that needs to be lined up to set the points? I've just used the key on the crank shaft and lied it up with the cam follower of the point. Seems to work fine. But if there is a right/better way I'd rather do it like that.

Any help I can get is greatly appreciated.

Thanks,
Tom

Weak spark will certainly not help. Yes replace both and have a spare if the other one is still serviceable. An OEM manual would help you greatly for set up purposes. Did you do a compression check ?? beyond that the payload on board and weight distribution are vital with your outfit. Then the only option is a bigger motor. What is your boat rated for ???

 

[kenmyfam]

Re: Need more power.....6hp

Almost forgot, welcome to iboats

 

[River Boy]

Re: Need more power.....6hp

Now for my questions:
1. Will replacing this weak coil make a noticeable difference in power?
2. Should both coils be replaced or just the one that's bad?
3. Is there a mark or spot that needs to be lined up to set the points? I've just used the key on the crank shaft and lied it up with the cam follower of the point. Seems to work fine. But if there is a right/better way I'd rather do it like that.

Any help I can get is greatly appreciated.

Thanks,
Tom

1. Doutful. If it is already firing on two cylinders you will not see any
appreciative gain in power.

2. You don't have to, just the bad one.

3. On some flywheels there is. You can't go wrong setting the points gap
at the apex of the cam lobe. .020.

That's a big Jon for a 6 to be pushing. I see a lot of 10 and 15s on those 16'x48"s and they do alright though. A 6 is a 6.

 

[ezeke]

Re: Need more power.....6hp

If you have not already done it, raise the motor until the A/V plate (the horizontal plate above the propeller) is even with or slightly higher and parallel to the bottom of the boat.

You should also be using a propeller with the lowest available pitch., which is probably 7. The OEM #386955 or Michigan Wheel 012008 are both 7 pitch. There are several discounted ones on Ebay, but this one has the best price: http://cgi.ebay.com/ebaymotors/OMC-...8711174QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

 

[wbeaton]

Re: Need more power.....6hp

That's all good advice.

I second only replacing what's broken. I wouldn't change out a good coil. The way you set the points is fine.

 

[Tarner]

Re: Need more power.....6hp

Thanks for the advice/welcome guys.

I went ahead and replaced the bad coil and I also replaced the points and condensers. I now have a nice strong spark on both sides. I Took it out for a run and the only thing that I can notice is it seems to run a little smoother.

I raised the motor up (2 1/2 inches) as far as I can get it and the plate is now about inch below the bottom of the boat. I'll see what that dose for me tomorrow when I put the boat back in the water.

The boat is rated for a 25HP. I can get a 9.9 from my brother in law and see how that runs with that. Will the prop from the 6 work on the 9.9? The 9.9 has a bad prop the slips around its rubber bushing. The prop on my 6 is a 7 1/4 X 8. (That number is pitch X Diameter right?)

What about adding weight to the front of the boat to help plane out or level it more to the water?

I might just go ahead and look for a 25. But right now I'm watching my money till work picks back up. It's been a long slow year for construction workers like myself with only six weeks of work logged into the books so far for 2009.

Thanks again for the input

 

[River Boy]

Re: Need more power.....6hp

I might just go ahead and look for a 25. But right now I'm watching my money till work picks back up. It's been a long slow year for construction workers like myself with only six weeks of work logged into the books so far for 2009.

Thanks again for the input

Hey, run what you got. It gets you on the water and you don't have to paddle. The first motor I had I had to help with a paddle when I went up a fast shoal. Fish don't care how big that motor is.

 

[Willyclay]

Re: Need more power.....6hp

I'm surprised that you did not experience increased power at WOT after you replaced the bad coil and tuned it up. I really think it was only running on one cylinder when under load. However, if you want some help adjusting the motor heighth, check out the link below:

http://www.dillon-racing.com/jackplate/index.htm

Good luck!

 

[kenmyfam]

Re: Need more power.....6hp

In reality you should have at least a 15hp on there for optimum performance and preferably 18hp to 25hp. A 6hp will be marginal at best.

 

[ezeke]

Re: Need more power.....6hp

You need to get the motor high enough to have the A/V plate even with the bottom of the boat. Look at the jackleg rig I made for my little boat for one way to raise it.

Pitch on your propeller is the 8; the first number is the diameter.

Pitch is number of theoretical inches that the propeller would move the boat for each revolution of the propeller. The heavier load and higher drag requires lowering the pitch to get more thrust; this lowers the potential top end, but can help to get the boat on plane so that the motor can run at optimum RPM.

The 9.9HP motors are through-hub exhaust so they have a totally different type of propeller.

 

[wbeaton]

Re: Need more power.....6hp

You need to get the motor high enough to have the A/V plate even with the bottom of the boat. Look at the jackleg rig I made for my little boat for one way to raise it.

Pitch on your propeller is the 8; the first number is the diameter.

Pitch is number of theoretical inches that the propeller would move the boat for each revolution of the propeller. The heavier load and higher drag requires lowering the pitch to get more thrust; this lowers the potential top end, but can help to get the boat on plane so that the motor can run at optimum RPM.

The 6 hp and 9.9 hp use completely different props. It won't work.

 

[crxess]

Re: Need more power.....6hp

ezeke hit the nail on the head.

Get an 8x7p and give it a try. Should increase your speed on that boat.

I had to go to a 8.25x5p just to get the rpm's in proper operating range. Boosted upstream speed from 2.5 to 8 mph

 

[ezeke]

Re: Need more power.....6hp

If you can find a 5 or 6 pitch prop for that motor, use that. I could not locate one.

The 5 pitch OEM prop was #386,587. Maybe you can get one from sea-way.com.

 

[Tarner]

Re: Need more power.....6hp

That jack plate looks like a good project to outfit my boat with. Thanks for the link.


Just to make sure I'm thinking about this the right way:

Getting a lower pitch prop will help me go faster? I understand what pitch is and know that the lower pitch will get me out of the hole quicker but my top speed would suffer.

But, if my motor doesn't have the power to get my boat up on plane with the higher pitch prop I'll just be pushing more water. Thus creating more drag and make me go slower.

If I went to a lower pitch prop I would plane out and the boat would be on top of the water where there is less drag and I'd go faster by allowing the motor to wind out to a higher rpm.

I use to work as a Tool and Die / Mold maker and worked on motor cycles on the side. Changing out props to with different pitches is a lot like changing out sprockets on a bike to change the drive gear ratio.

Right?

 

[ezeke]

Re: Need more power.....6hp

Let's say that you have a gearcase with a ratio of 15:26 or that for every 26 revolutions of the engine, the propeller turns 15 revolutions. 15/26 = .58

If you have a prop with a pitch of 5" you have to convert that to the number of turns at some desired engine RPM.

So, if your engine is running at 4,000 RPM, 4,000 X .58 = 2,320 RPM.

Now, 2,320 X 5" = 11,600" per minute so divide that by 12 to convert to feet = 966.67' per minute.

Take that to 60 minutes: 966.67 X 60 = 58,000 feet

To covert that to MPH divide 58,000'/5280 = 10.98.

Now 10.98 MPH is a theoretical maximum speed possible for that set of variables.

To reach the theoretical maximum you would need to have no drag, so anything else is called slip.

Once you get on plane, the drag is from the planing surface of the craft and the motor itself, so a clean, smooth planing surface on plane and the least amount of motor below the water is the best that you can do.

I believe that your motor is rated for 4,000 to 5,000 RPM Full Throttle Operating Range, so the closer you run to the upper end of that range at WOT, the better. Running at WOT while not on plane prevents reaching the range, lugs the engine and eventually ruins it.

 

[Tarner]

Re: Need more power.....6hp

Gotcha....Clear as mud. LOL

 

[ezeke]

Re: Need more power.....6hp

You won't go faster until you overcome the drag; the numbers you have now are like climbing a steep mountain with your car in high gear. For your boat, each reduction in pitch will give you more thrust but lowers your theretical potential top end.

 

[Tarner]

Re: Need more power.....6hp

Thanks for the jack plate idea guys. I just made mine today. I haven't had a chance to get out on the river for a test run though. The DNR closed the river do to the flooding.


Does this look like it's about the right height?

 

[kenmyfam]

Re: Need more power.....6hp

Looks O.K. for a "dry" check. You need to see where it sits when you are in the water with you in the driving position. You could end up with too much motor in the water at that point. Need another person to help out with this.