convert 6hp to 8hp

[kiwiguy]

Can I mod a 1986 6hp up to a 8hp.<br />I have been told they use the same powerhead and by changing the reed valve and carby I can boost the hp's?<br />Any help would be greatly appreciated.

 

[mikeyzx2]

Re: convert 6hp to 8hp

You really won't notice the 2hp difference. Sometimes there's more to it than just swapping the carb and intake.

 

[Paul Moir]

Re: convert 6hp to 8hp

On that one, it's pretty near impossible. The big (only I think?) difference is the intake & reeds. Unfortunately you need to replace the front half of the crankcase to fit the new intake & reeds. And since the crankcase is bored with the cylinder side in place, if you replace one you need to replace the other to match.<br />So to make a long story short, to make a 6hp an 8hp you have to change the block. Even though the crank/bearings/pistons/porting/etc/etc is the same.<br /><br />The 6hp is a pretty stout engine as is though.

 

[kiwiguy]

Re: convert 6hp to 8hp

Thanks guys,<br /><br />I may just have to stick with the kids complaining about the lack of power. The motor is fitted to a 8' tinnie and with two 10 year olds it does about 4 knots (via gps). It just wont rev out !!!!

 

[tommays]

Re: convert 6hp to 8hp

i would look at the motors condition and if the compression and tune-up are good look at the prop<br /><br />i have the same motor and it pushed a 24 ft sailboat at 6 KNOTS +<br /><br />tomays

 

[Paul Moir]

Re: convert 6hp to 8hp

Some shorter tinnys can be real bears to plane. Do you know if the boat is really supposed to go? <br />If it's not reving up past say 1/2 way, you may have engine trouble. Like loosing spark on one of the cylinders.<br /><br />By way of reference, I've run a 6hp Johnny on a 14' tinney with 2 adults + 1 child, not ideally loaded. I got 14-15 knots by GPS.

 

[mikeyzx2]

Re: convert 6hp to 8hp

Well that'll sum it up. I have an 8 horse Johnny, that was pushing a 14 foot jonboat. It'll plane it, pushing around 10-15mph, with just me and my fishing tackle. If you're barely chugging along, then something's out of whack. <br /><br />Hit us with some plug and spark conditions, and we'll see if we can make your kids happy!

 

[kiwiguy]

Re: convert 6hp to 8hp

Ok.. now for an update. plugs are a nice tan colour with no signs of carbon or oil. Have checked points and timing and are all to specs. Engine seems to drop a cylinder at about 1300 rpm's. may be bad coil ????. Kids are over my shoulder asking when is someone going to reply (lol). Wife is giving me the "hurry up and buy another motor look".

 

[Paul Moir]

Re: convert 6hp to 8hp

So it seems to hit a wall just as you're opening up the throttle?<br /><br />Start with a couple simple tests. Check to see if spark will jump a 7/16" (1 cm) gap from each spark plug to the block. Use insulated pliers when performing this test or the wife.<br />If that doesn't turn up something, try firing up the engine with one spark plug lead grounded and see how it performs. Then try the other.<br /><br />If it seems to be the same on both cylinders, then most likely there's a fuel problem.

 

[lark2004]

Re: convert 6hp to 8hp

The tinnie....it's not an old De-Havalind Topper Cub is it?<br /><br />I have one of these, it's 8ft, and with the mariner 3.5, it only seems to get about half throttle. Btw with mine, It doesn't seem to matter how many people are it, me by myself, or with my two boys (8 and 10).

 

[Tommo]

Re: convert 6hp to 8hp

Hi Lark, <br />I got the same De-Haviland 8ft, I cant get it to plane with two people onboard and thats with a new 6hp, Whats with that? I cant get above 10mph. <br />So maybe it is the hull not the motor.<br /><br />P.S <br />Lark, Thanks for the wave washer I got It few weeks ago though I have yet to use it. Waiting for some piston rings.

 

[kiwiguy]

Re: convert 6hp to 8hp

I have tried a force 3hp on the same tinnie and get the same speed as the hp.. go figure

 

[lark2004]

Re: convert 6hp to 8hp

I wonder if we may have a hull problem, I'm going check the bottom of mine with a straight edge.....