Wanna go faster

[mforino]

My son presently races a johnson 10hp converted to a 15hp motor on a 10ft
hydroplane. We are looking to refresh this motor in the hope that we can increase the rpm and speed of the motor. I am not very familiar with rebuilding
these motors. Any guidance would be appreciated. The motor is a 1986 year motor and the only internal change has been the reeds were changed.
Thank you in advance.

 

[tashasdaddy]

Re: Wanna go faster

is it propped correctly, is the engine height correct, before you go messing with the motor? trim angle also makes a difference.

 

[mforino]

Re: Wanna go faster

We are going testing next week and will try to zero in on all those ideas. I know we can do better and gain a couple of mph. We are 5-7mph slower than the competion and figured a fresher motor may help to gain us an advantage. How big of a deal is it to rebuild a motor like this as a beginner?
Thanks for your input.

 

[tashasdaddy]

Re: Wanna go faster

you need a Johnson service manual, http://www.outboardbooks.com/ what are your compression #'s? pull the rope till the guage quits going up.

 

[steelespike]

Re: Wanna go faster

Engine height is critical.Next you need a speed prop.A prop specifically designed to go fast on a light boat.Next best would be a stainless that gets you to that 7000 rpm.These props will help you reach the maximum height.If it is truly a 15 it has a max rpm of 7,000
You need a tach A tinytach is reasonable easy to install and will work on any similar motor without regard to stator poles.

 

[mthieme]

Re: Wanna go faster

I assume you are running "A" class (?) - stock?
How close, or how much over the 345 # are you?
Before you tear the motor apart, what have you done with the prop(s)?

 

[mforino]

Re: Wanna go faster

We do run A stock Hydro. I weighed in at 375 last weekend. I got around 52 mph out of the motor. We currently have gotten props from Dewald. My dad tested the same motor and boat on the same set-up and got only 43 mph he is an additional 40 pounds heavier than me. The fastest guys in the class are supposedly doing around 60. We know we can get a little more with a better set-up and testing with more props. Any other suggestions.

 

[Joe Reeves]

Re: Wanna go faster

DO NOT attempt to file any area of the steel cylinder sleeve ports as that would change the port timing. Now, that being said.......

The ports (holes) in the alumnium casting surrounding those steel cylinder sleeves are at a sharp 90? angle with the intake and exhaust flow. Some improvement in fuel and exhaust flow may ocurr if that sharp right angle in the surrounding alumium port entrance/exit is carefully filed down, rounded off somewhat. This would be done ONLY the exterior BOTTOM portion of the alumium port casting....... visualize the fuel going up and in, and the exhaust going down and out. This was a common practice in years past.

 

[mthieme]

Re: Wanna go faster

I'm not familiar with Dewald props, can you describe it? Has it be reworked in any way?
60 is darn good - I'm doing 60 in an 11'6" with a stock 25 Merc on the back, but I'm probably pushing 500#.

 

[mforino]

Re: Wanna go faster

We have racing lower units on the motor not the original factory lower units.
The props we use are smaller than the factory props. They are 3-blade and called a cleaver type prop.

 

[Chinewalker]

Re: Wanna go faster

Hi Mforino,
Well, since you can't actually modify the motor (what Joe described may give you more power, but will get you tossed if you ever get inspected), here's what you CAN do. Has the motor been rebuilt? If this is a fishing motor that was converted to a 15A, then you probably still have a stock bore block. Punch it out to .030" oversize. Also, work with Craig on props. If the one you have isn't working, send it back. But, send it back with data - tach readings, speed readings, conditions, etc. He will work with you. Play with your set-up. You can't go higher than the height limit, but you can mess with in/out, etc.
If you really want to make it work, send me a PM and I'll put you in contact with a friend of mine, and former Nationals winner in A-Runabout. He didn't race this year but he could put his boat in the water any race in the country and be the fastest one out there.
What are you running for a boat? Round or flat turn? Is the bottom absolutely true? No beach burrs on your turn fin?
Scott (semi-retired C-Stock Runabout driver)