GX390 mud motor build

garza7585

New member
Hello everyone, new to the forum and i'm looking for advice. I know you guys specialize in go-karts, but I figured this was my best bet at solving an issue i'm having with a recent build that I took on.

So I'm running a GX390 on a mud motor frame. It has been a very reliable motor in its stock form, starts on the first pull and gets me where I need to go. But, when adding a load to my boat, it gets sluggish really quick. By myself with all my equipment it jumps on plane and will run a good 18 mph, but as soon as I add a person it takes a pit longer to plane out and slows down to about 12 mph. Add more equipment or a dog and it just plows.

What I did for this season is I pulled the air box and replaced it with an open air filter, rejetted the carb with a size 100 from a 92 (going by the tiny number on the jet), added a Robertson's boat loop, installed an NR Racing Torque Cam, HD springs and a 4* timing key.

The good thing is, the torque I needed to get out of the hole is great. The motor will flat snap your neck when you punch it, and it gets out the nasty stuff quick, something my motor struggled with stock.

The bad, it's a pain in the rear to start. I went from choking it, one pull and its running to now, I can't get it to start without pulling it a million times. The engine will either not start at all or sputter for a few seconds and die. I messed with the idle screw a bit and still nothing.

The solution, give it some gas, choke it, pull, then turn the choke off, throttle open and pull, it will start and rev up real high and I have to back off the throttle and then let it run a bit.

My question is, is that just how this thing is now with the mods? Or did I miss something? When i'm out in the duck marsh, I want to make sure my motor starts when I want it to, the way it used to. I don't want to constantly have to rev it up on starting to get it going. If I can get this thing to start like it used to, it will be perfect. It now has the torque needed to carry a load, but its lacking the reliable start up that it used to have.

Any idea of what is goin on?

Thank,

Jose
 
The fact you have to make all those carb movements would lead me to the intake tract. Air leaks or carb issues.
Obviously check valve lash and ignition timing.
 
I will check the valve lash again, when I replaced the springs I set the lash at the factory spec .006-.008. I will stop at the parts store today and I am going to get new gaskets and replace them.
 
Run great once it's started?
No idle issues?
Or other symptoms.
Idle jet may be plugged but then it would need about half choke to idle.
Air filter as in racing k& n type?
Need a bigger idle jet.
 
My first thought is you need a larger idle jet. Maybe a .025 or so. It needs to be drilled to that size. However your description of what needs to be done to start it sounds like a few pulls floods it because choke off and WOT is used to start it, so a larger jet wont help but hurt if that's the case. Your problem is in the ratio of air to fuel when your starting. All your mods messed that up. Figure out if its way too rich or lean and correct it. Is using starting fluid, just a squirt on the filter, an option? does that help? Many mods to help it run at WOT hurt at other rpms like starting.
 
Run great once it's started? Yes, runs like a scalded dog.
No idle issues? Once it starts, it tends to idle ok, except I have the idle set at like 2k rpms, I would like to drop it down to the recommended 1400 +/- 200
Or other symptoms. No
Idle jet may be plugged but then it would need about half choke to idle.
Air filter as in racing k& n type? Yes
Need a bigger idle jet.

That was my thoughts, I'm going to tear down the carb an make sure its clean, replace gaskets, and run everything again. I'm thinking I need a larger idle jet.
 
jsstump,

I agree, that is my thoughts, could I just not be getting enough fuel on the low end causing it not to start? I haven't run any starting fluid, once I figured out a way to start it I just went with it, but its kinda pain.
 
So just a quick update.

I had a hard time being able to do anything over the last week because of weather and family commitments. Finally had a bit of time and did a few things and started with the carb. I cleaned it out as best I could and I also swapped out the spark plug. I was running an E3 and put in an NGK.

After that, I feel like the motor is starting easier, enough that if this is as far as I get, I am content. I was able to start it much easier than before.

One thing I noticed is once I start it, I played with the idle screw, I can get it to idle really low once its running, I don't have a tach to know what the rpms are, but what I did is backed the idle screw out till it started sputtering and then screw it back in a hair and left it. Problem with that, it won't start with the idle screw at that setting. I either have to open the trottle and start it, or I put the idle screw in further and just idle higher.

Again, I can live with it how it is, but just to try it out I did order a larger pilot jet, maybe that will help.
 
The typical way to adjust the carb is adjust the idle circuit until you get the highest idle RPM and then adjust the throttle stop screw until you get the RPM you want.
 
It does not matter how many electrodes you put on the plug the current will arc the easiest path and it is done, it will not take several paths. Marketing at its'
best.
 
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