GX390 Build

Mammoth

Member
Messages
139
Reaction score
5
Location
Durham, CA
Sorry for the late reply I was camping this last week.

I didn't make the gears from scratch. I found gears that were the correct size.
It took me 2-1/2 weeks of searching the web and asking questions before finding the right answer.

As it turns out the vertical Briggs v-twin's were the right size. Around the 18 hp range.

Had to cut the larger camshaft gear off and the smaller crank gear is a press fit

I did send the smaller gear off to be reamed for a precision fit to the new crankshaft.

I can only imagine how much side load there would be on higher HP engines. Then put straight cut gears in an problem solved, free flowing gears. Its gotta be a huge improvement.

Here is a picture for reference.
 

Mammoth

Member
Messages
139
Reaction score
5
Location
Durham, CA
Got some new parts for it. Really trying to get the ratios correct between the ports, valve seats and valves. This head will be a powerhouse.
Went with a 33mm 40mm valve combo with 6mm stems, and I had to go with a larger intake valve seat that has a 1.250 id.


Had to bore out the old valve seat.


I cut the valve seat to size, but when I fit it to the head it was a little too loose so I have to get another one.


I got a stronger pull handle and a Cobra pull cord thats advertised as unbreakable. Its also like 6' feet long.

 

Mammoth

Member
Messages
139
Reaction score
5
Location
Durham, CA
This engine is going on a mini jet boat hull, but i'm going to build the boat a little different by burying a propeller into a tunnel thats built into the stern.
I'm going to try to build an efficient boat to do some back country river exploration.
Jets aren't very efficient at low speeds so I'm going with a propeller. If you run a larger propeller at a lower speed it will be more efficient so I'm going with a 12" propeller run off a 2:1 gear reduction transmission that has forward, neutral and reverse.

Robert over at beaverdammudrunners was able to ship this transmission from the Philippines to me. Its designed for long tail boats and it should work perfectly for what I'm doing.


I had to make an aluminum adaptor to fit a jet ski coupler that will go to the propeller shaft.


Will be working on fitting the valve seat next and I have to unbury my friends dyno for tuning.
 

Karts of Kaos

the many mowers guy
Messages
778
Reaction score
44
Location
Wisconsin
I like the recoil handle, but it might be hard to get your hand in while wearing gloves. or if the engine kicks back it could like break your fingers. also that is a huge carb,it looks kinda funny though, I am also not used to flat slide carbs.
 

Mammoth

Member
Messages
139
Reaction score
5
Location
Durham, CA
Learning as I go with lots of books for reference. Picking up tons of info on boat design. A lifetime could be spent studying propeller shape.

It would be suicidal to the health of your fingers if you put them in the holes of the pull start handle. The plan is just to wrap my fingers around the handle and not through the holes. I learned my lesson before with a high compression 420cc. Snap backs are serious.

The carb is a basic design that has an oval slide and uses Mikuni jets. Should be straight forward with tuning.
 

Gasteffens

Member
Messages
83
Reaction score
20
Location
Morris, IL, USA
As I read through this, I don't have near the patience to attempt something like this. But it will be a work of art and I look forward to hearing it run!!
 

OPmini

Member
Messages
90
Reaction score
11
Location
Phoenix, Arizona, United States
This looks really promising! It reminds me of what my builds could be if I spent the money! haha, that little engine is gonna be a blast once it's assembled! btw I can't believe that you had these parts sitting around since 2013! cool project so far! #bigblockgang
 

Grizzlymi

Wolverine
Messages
105
Reaction score
0
Location
Michigan
The starter handle is great! In a pinch, cut the rope and knock some (add whatever you want here) and reattach. Great build btw, went through the original thread and love it. Is this motor going on a mud boat?
 

Mammoth

Member
Messages
139
Reaction score
5
Location
Durham, CA
It will go in an aluminum mini jet boat hull. Most people use a jet ski engine, but they use 2.5 gallons or more of gas an hour.
Im going to build this boat for long rang. A stock 390 uses just under 1 gallon an hour.
This is what I have in mind.

 

Grizzlymi

Wolverine
Messages
105
Reaction score
0
Location
Michigan
Thats going to be awesome! Ive never seen the mini versions but seen the bigger ones on the rivers in the mountains of Idaho. Thats going to be a blast. Cant wait to see the the project thread for that(hopefully you're going to do one).
 

ThunderKart79

New member
Messages
24
Reaction score
9
Location
N.C.
same displacement longer stroke (smaller bore)
makes a tad more torque though than a shorter stroke (bigger bore)
reason is simply the lever on the crank being bigger
and by ~ the same amount gained in the combustion
the lever's working against you during compression
that's why longer stroke enignes tend to slow down more than short stroke
during compression.. hence short stroke yields higher rpms ;)

the sweet spot is dependend on compression ratio AFAIR..
too long ago to remember correctly
but I have something like "1:1 bore/stroke ratio for a 9:1 compression is about in the right realms" in mind *shrugs*.

Anyways... this is a very interesting project :thumbsup:
I still can't help but wonder how the equivalent of an additional matchstick dragged through some oil could cause a monster like that to run any slower :eek:
the massive inertia should deal with that on it's own (at least in my head ;))
Ah well you definitely look like you perfectly know what you are doing,
so I just get back to my bucket of popcorn and watch you
getting closer and closer towards the 45horses that are hidden
somewhere in a gx390 :D

'sid
All of the top builders of small engines preach every little gain is a gain just my guess
 

Karts of Kaos

the many mowers guy
Messages
778
Reaction score
44
Location
Wisconsin
what about snowmobile engines? would they be as powerful or more efficient? I don't know if they are either of those thing,but compared to this build they are dirt cheap! but definitely can't wait to hear it run! I have also never heard of people putting engines like this in boats like this. I also did not know that jet ski engine used that much gas. very cool!
 

ThunderKart79

New member
Messages
24
Reaction score
9
Location
N.C.
what about snowmobile engines? would they be as powerful or more efficient? I don't know if they are either of those thing,but compared to this build they are dirt cheap! but definitely can't wait to hear it run! I have also never heard of people putting engines like this in boats like this. I also did not know that jet ski engine used that much gas. very cool!
Snowmobile engines are almost just like jet ski engines although some of the new ones are variations of motorcycle engines Yamaha has one pushing 200 ponies out of the box
 

ThunderKart79

New member
Messages
24
Reaction score
9
Location
N.C.
All of the top builders of small engines preach every little gain is a gain just my guess
This comment was meant to go along with the streamline dipper you created I thought that was a was an awesome improvement forget about hp I can’t wait to see how much tourqe that little monster makes and also I would bet that engine has a long life
 

Mammoth

Member
Messages
139
Reaction score
5
Location
Durham, CA
When your racing every gain in percent counts. After a number of laps the first and second place finishers are usually right next to each other. That means the race was won by a fraction of a percent.
Ive never had any negative side effects from thinning the dipper. You can picture how little oil a two stroke uses to lubricate the crank, rod and piston. Not much at all.

All the jet ski engines I researched for fuel consumption were multi cylinder engines, so naturally they will consume more fuel. They do have some old single cylinder jet skis, but they are two stroke, and two stroke engines use around 50% more fuel than four strokes.

I have lots of thoughts about snowmobiles that would improve the overall design. I believe they have some short comings that could be overcome. I've even started building an engine specifically for a snowmobile that should be quite impressive, but I haven't had enough time for that project.

This 390 engine should be just about right. If it only uses one gallon an hour cruising at 20 mph, and it has a 20 gallon gas tank this boat should have a 400 mile range.

I ordered the boat kit today which is a major piece to this puzzle.

The new welder kept popping the breaker so I installed a step up transformer that took my 240 volt single phase service and bumped it up to 480 volts. This should allow the welder to run at full capacity without tripping the breaker and cut the power bill in half.
 
Top