Gheenoe Mud Motor: 6.5hp Beavertail mod thread... This should be a fun project!

Discussion in 'Outboard Maintenance' started by TomFL, Jan 16, 2010.

  1. TomFL

    TomFL Well-Known Member

    A while back I picked up a 6.5hp beavertail mud motor. I really like this motor and think it's the ultimate layout/sneak boat motor. At just 68lbs, it's easy to take on and off, and when hung on even the little 13 footer it's light enough the boat doesn't squat at all.

    The great thing is it'll push a fully loaded gheenoe and one man through anything. The only downside to it is that, well at 6.5hp you can guess: It's no speed demon.

    On the classic hull with just me I'm getting 10.4mph max; fully loaded with hunting gear and 5 doz dekes I'm getting 7.4. I will post new/accurate speed numbers on the 13 now that the bottom is coated.

    These are open-water numbers, and I think for comparison's sake we'll stay with those. If you hit some really thick matted vegetation obviously it'll slow a bit.

    Keep in mind that the beauty of this motor isn't the speed. The motor's light weight and it's ability to get into places with a sneaky little boat that no other boats can. Far runs are not it's strong suit. But, it's always nice to get to the spots quicker, and more speed broadens your range.

    So, as an old-school hot rodder jonesing for more speed (Give it all she's got Scotty!) I searched the web, and found a reputable company with a super-helpful honda motor wizard at

    After some discussion with Shane, it seems this little honda can be brought up to the 12-14hp range without adversly effecting the reliability (only thing slower than a 6.5hp mud motor is paddling ;) ).

    I wound up going with the "best bang for the buck kit" for the honda GX200. This kit consists of a 2 stage header, high flow filter with adaptor, #92 main jet, 18lb valve springs, billet rod with bearings, aluminum flywheel, .265" lift cam with 236 duration on the intake and exhaust lobes, chrome moly pushrods & a 14cc cylinder head. I've also added to that list a larger GX390 carb, 1.2 rockers and some grade 8 engine bolts. I'll also be removing the rev limiter to be able to gain a few more RPM's beyond the factory's limit of 3,600.

    As soon as the kit is in, I'll begin the install and keep this thread updated with some pics. After completion I'll detail the findings in speed and see where we're at.

    I'm also assuming I'll have to change props, but we'll cover that ground when we get there.

    Here's some pics of the starting point:




    I'm also going to try to convert this slide throttle into something that will snap back when released; maybe based on a motorcycle clutch/brake lever setup. I think with the addition of more power, it'd be safer to be able to control the throttle quickly and this stock slide setup is more of a set-it-and-forget-it type of adjuster:

    On paper this seems like a great setup; retain the light weight of the small honda motor, and gain some top-end speed and load capacity to boot.

    Should be fun

  2. gheenoe

    gheenoe Got snook?

    When I was racing lawnmowers I got all my stuff from here:

    Sound like you got all the right parts with a billet flywheel and rod you wont have any problems spinning 6000 rpm.

  3. tom_in_orl

    tom_in_orl Founder of Microskiff, Member of the Gheenoe Army

    Very interesting. I will be following your mods closely. It may be something I try this summer.
  4. TomFL

    TomFL Well-Known Member

    Cool. Maybe we'll have gheenoe mud boat drags at the next rally...

  5. noeettica

    noeettica Well-Known Member

    I've Got a ton of Honda stuff ...

    Now If I can get the rest ... to build into a "mud Motor"
  6. TomFL

    TomFL Well-Known Member

    Parts should be here today or tomorrow so I hung the motor on the baby ducker for a good baseline.

    10.4mph is it. Doesn't really matter if you're running through thick grass/weeds 1' from the shoreline, or down the middle of the canal.

    She looks good on the boat. The nice thing about the beavertail is it swings 360 degrees so you can put the tail in the boat for easy trailering:


    In the water:
  7. Frank_Sebastian

    Frank_Sebastian Well-Known Member

    Fascinating project! I have often thought about building one, or at least about how hard it would be.

  8. TomFL

    TomFL Well-Known Member

    Well I got a box of shiny new parts in today. Everything except the carb, which I forgot to add to the order. Shane from Affordable Go Karts got on it ASAP and sent one out to me today. These guys are great.

    I'll post up some pics of the upgrades as I get into it this weekend.

    It's amazing how small the parts are! The cam fits in the palm of my hand.

    I might get the larger carb tomorrow or Saturday at best, so keep your fingers crossed.
  9. TomFL

    TomFL Well-Known Member

    Well I got some time tonight and cracked the seal on the motor.

    I took a few shots as kind of a "before and after" sorta thing.

    Drive setup:

    Exhaust side:

    Pull-start side:

    Drive side:

    Other side:

    As you can see, nothing fancy there!

    Now for the fun stuff!

    Here's a shot of the new exhaust pipe flange on the left, and the stock one on the right.

    Here's a close-up of how mis-matched the exhaust port is with the stock muffler: [​IMG]

    You can see in the above pic that the new header will need a touch up with a dremel to smooth out that lip and make it match the flange and exhaust port. For all you non-hot-rodders out there the idea is to not have any "lips" or blockages that will interrupt or block exhaust flow. This was definately NOT the case with the stock muffler!

    More cool stuff!:

    Stock rocker setup:

    Stock head on right, new head on left. Notice the difference in the size of the chamber:

    Stock valve springs vs new valve springs:

    Stock crank w/piston on it, and the billet rod on the right:

    Close-up of the oil passage on the long protrusion of the billet rod. This is a neat piece of engineering as they've essentially made the protrusion a scoop for the oil to send oil directly to the rod bearings:

    I should be able to finish this thing off tomorrow night, and if lady luck is on my side the carb will show tomorrow to complete it.

    Then it's time for a new spin and see what kinda difference the mods made. I'm pretty sure a change in props will be in order, but we'll see. May need to get a tiny tach to see what's going on as well.

  10. cal1320

    cal1320 Well-Known Member

    The new head and rod look nice. If you really want some power, throw some nitro in it. [smiley=carcrash.gif]
  11. FSUfisher

    FSUfisher Well-Known Member

    Cool build! Got the new numbers yet??
  12. superslack

    superslack I Love!


    did you ever start a new thread to report on the performance increase? I'm going to build something similar next year and I'm wondering how much more performance you got from the mods performed. Thanks.
  13. AfterHours2

    AfterHours2 Stripper in my own Mind!

    I had an 11 HP I did the same thing too a year ago. My whole priority was gaining speed like yourself, but unfortunately these motors don't work like that. After many hours of work and plenty of $ on new parts, in the end these motors are built for low end torque and that you get. If your happy however only gaining 2-3 mph with the upgrade then you will be satisfied. Myself on the other hand would have went with the bigger motor to make up for the speed. Hope this mod works for you though! Good luck :)
  14. TomFL

    TomFL Well-Known Member

    This setup is for sale in the classified sections now!

    I will sell the motor separate for you go-cart freaks, the mud motor frame separate, or the whole shebang together.