12' jon boat won't plane with 9.8..... bigger prop??

[Arrowhead74]

I have a 12' jon boat with a Merc 9.8. With just me (220lbs) and a trolling motor and small battery it barely won't plane out. If I move forward just a couple feet, the boat will almost plane . Would a hydrofoil of a different prop do the trick? It has the factory prop, anybody know what the pitch may be? Any suggestions would be appreciated.

 

[steelespike]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

Is the motor trimmed in, prop closer to the boat? Are you sure it is running on both cylinders.
Pull one plug wire at a time and test run it. Is the stator and carb advancing as they should.

 

[Mi duckdown]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

What steelespike said.
That boat with 9.8 should run like a raped ape. the motor is most likley an issue.
In fact you are probably over powered for that boat.

 

[Sportdog7]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

I agree it's motor issues more than likely, it should really zoom with a motor that size.

 

[Arrowhead74]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

Thanks for the help guys. The motor is trimmed properly, I played around with that. The boat is rated for a 10hp. I just checked for spark on both cylinders. It's getting spark on both when I pull it over. Is it possible to loose the spark on one cylinder at high speed? I believe the stator has a high and low coil. The carb and stator are advancing fine. My father has a 9.9 on a 14' fiberglass boat, it goes twice as fast as this one. I did a comp test, both cylinders have 125lbs.

 

[steelespike]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

Pull one plug wire at a time and ground it.See if it will run on the other cylinder.
Spark should be blue with an audible snap.If spark is weak it may fail under a load.
Being half as fast as the 9.9 may be a clue.
Could be a carb issue.See if a little choke helps and/or try squeezing the primer bulb.

 

[Texasmark]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

I ran a 7.5 Merc tiller steer on a 10' Loweline Lake John which is a very large john for the dimensions, and rated for a 10 hp also. I weighed over 200# and usually had it loaded with gear. I put the 3 gallon tank aft of, and against, the front seat and the rest of the gear was between the seats (2). It ran like a scalded ape (extremely well considering) with a stock OEM prop.

We only know what you told us. You could have numerous problems besides the ones the guys put you onto.....like what are you using for fuel, is the fuel oil mix accurate,is the prop a standard 9.8 hp OEM in good shape? Does the engine rev but you go no where in trying to come out of the hole.......slipping prop hub??? Is the hull straight and free of crud like barnacles and slime? Is the engine the right shaft length for your transom height and is it mounted correctly.....anti-vent plate more or less in line with the bottom of the boat, not 5" above or below it?

Come back on this and you will probably get your problem solved....lots of talent on here.

HTH,

Mark

 

[Arrowhead74]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

Everything on the boat is set up properly. The prop is factory and is in good shape. The hub does not seem to be slipping. My hobby is rebuilding and porting chainsaws, so I know some about 2 strokes.... although outboards are much much more complicated. I understand the virtues of good gas, ethanol is a killer! It's fresh premium pump gas mixed 50:1. I really appreciate the help! I'll be here till we get this problem figured out.

This was a yardsale purchase, so I don't know the history.

Starting with a simple question, what is the proper spark plug for this engine? It has NGK BUHW plugs in it now. I'm not sure if that's even the proper plug. I tried these plugs in my 6hp Johnson (which I know are not correct). It idled great, but fell on it's face at wot. These plugs are flat bottomed, no gap adjusting.

 

[Arrowhead74]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

I ran the motor in a test drum. At high idle it will not run on only 1 cylinder.

 

[Arrowhead74]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

After checking NGK's site, the plugs I listed are not for this engine. They are for the older 110 9.8's. The proper plug is B9HS 10. I'm heading out to get a couple and try.

 

[steelespike]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

Probably not the plugs but give it a try. If it won't run on one cylinder check for spark.
Should be blue and make an audible snap.
This does seem to be a motor issue; Post in the Merc section just tell them we suggested it.

 

[Arrowhead74]

Re: 12' jon boat won't plane with 9.8..... bigger prop??

Probably not the plugs but give it a try. If it won't run on one cylinder check for spark.
Should be blue and make an audible snap.
This does seem to be a motor issue; Post in the Merc section just tell them we suggested it.

The plugs made little to no difference. Thanks, I'll post in the Merc section.