Merc 9.8 go-fast mods

[Scott Danforth]

starting to get ready to put my 1984 9.8 back together for the wooster hellion project.

its a stock 9.8 electric start with the WMB-1 carb. compression is good. I had the motor running just to make sure it ran just prior to pulling the power head off and stripping the motor down.

Now is the time for me to start to tinker.

I know that early Mark 7 and Mark 20's were used for racing. Just wondering what can be done to this little motor (short of bolting on a bigger motor).

I spent a bit of time looking on the internet and got nadda. Any help will be helpful.

I already plan on playing with prop venting.

 

[scamp12]

Bosen reeds. Rering it. Mill head. Bigger main jet .

 

[Scott Danforth]

Boyesen doesnt list reads on their website for the 9.8 (at least nothing after 1978). have an email into them already.

Compression is good at about 130/135 per cylinder, so I am not sure I would need to re-ring. From what I have read, not much gain in changing the jet, that the whole carb should be increased in size, however the motor then looses most of its low end.

I believe the motor is only 11 cubic inches.

 

[Chinewalker]

That's it. 10hp is all you're going to get out of it. The 11 cubers started life as 6hp, so it's already pretty well wrung out. It's a fishing motor and was never intended for high performance. That said, do what you can to optimize setup. Prop, motor height, motor angle, hull light, hull straight, etc.

No head to mill. And putting a bigger jet in it will only make it run rich. A bigger carb won't help either, as the through flow has to increase. What goes in has to get out.

 

[Scott Danforth]

That's it. 10hp is all you're going to get out of it. The 11 cubers started life as 6hp, so it's already pretty well wrung out. It's a fishing motor and was never intended for high performance. That said, do what you can to optimize setup. Prop, motor height, motor angle, hull light, hull straight, etc.

No head to mill. And putting a bigger jet in it will only make it run rich. A bigger carb won't help either, as the through flow has to increase. What goes in has to get out.

not helping my illusions of grandeur here.:facepalm:

Hull is 7' long total, about 6' in the water. as straight as can be for the last 4'. boat weighs about 120#, motor 65# and I weigh just a tad bit more than both together

 

[Faztbullet]

Find ya a minisport master off a 7.5J racing motor along with drive shaft adapter. You will have to make a adapter to bolt it to midsection but your 1:1 ratio now...

 

[Mi duckdown]

my 63and 64 merc 10 hp two blades are the the best motors I own. it blows away my 80 10 hp three blade on the same boat

 

[Scott Danforth]

Find ya a minisport master off a 7.5J racing motor along with drive shaft adapter. You will have to make a adapter to bolt it to midsection but your 1:1 ratio now...

most of the power head of teh 7.5J is the same as the 7.5/9.8 based on part number break downs. Cowl is the same, carb is same as the 7.5 with a different jet.

the magic appears to be the driveshaft houseing (mid section) and gear housing (1:1 ratio, no reverse)

 

[Chinewalker]

The 75J never really took off as a valid racing motor. And they're rare as hens' teeth. I've only ever come across a couple of them in collections over the years. Never seen one in use. If you were to find one - and could afford to buy it, and something in the mid or lower end broke, you'd never find parts for it. I have a hard enough time affording to find parts for the racing motors I have, and most of them were relatively common.

 

[Scott Danforth]

I thing my current plan is to get the motor back together, get the motor on the boat and start playing with the prop.

I am running down a lead on a guy that used to convert the 9.8 into racing cart motors. however that was over 30 years ago.