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Old 10-09-2017, 01:38 PM
garza7585 garza7585 is offline
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Default GX390 Mudmotor build

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 bit 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
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Old 10-09-2017, 01:43 PM
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take the black plastic idle speed screw adjuster off of the carb, get some needle nose pliers and pull the black tab oout of that the screw was covering. This thing is the idle jet. It has a very tiny hole in the bottom of it, that you can poke the crud that is stuck in there out with a single brush wire from a wire brush.

If you have messed with the idle mixture screw, that could also be the culprit.

I have your exact setup, just different exhaust, and it starts 1-2 pulls.

Might just want to make sure your carb is clean too.
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Old 10-09-2017, 01:52 PM
garza7585 garza7585 is offline
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KartFab,

Good to hear you have a similar set-up. What kind of kit are you running?

My first thought was replacing the idle jet but I haven't been able to find a place locally to source it. I'll have to order one from NR is I need a larger one, did you leave your idle jet stock?
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Old 10-09-2017, 09:38 PM
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my idle jet is stock. One problem that i was having, was that I was running so freaking lean. I had to drill out to a bigger jet, and i still think im a tad lean, but that was after porting the head and shaving the head to get a decent compression ratio (i want to say 75 thousandths was shaved off). you might want to just start the engine, let it idle for 5 mins, pull the plug and see if its white, black or brown to see if you need a bigger idle jet. i doubt that you do though, again, might just be carb cleaning is needed.

Whenever i start my engine, i have to do full throttle with choke on, and it starts, or at least crack the throttle halfway with choke on.


The problem i was having is that since its a stock carb, it leans out on the top end, so i had to jet to i forget now but i want to say .046"? I may have gone up just a tiny bit more, i cant remember. It runs great now. So here is what i have

stock everything but:
  • timing set to 26 DBTDC w/stock electric start flywheel
  • ported intake and exhaust, mild port job
  • shaved .075" off head
  • cut to length push rods (just because i couldn't run the stock ones because i shaved the head)
  • header is just a custom one using 1" flexible header pipe with a yamaha R1 muffler, port matched.
  • intake is just the arc high flow intake for the gx390 carb, and im running a little 4" K&N filter with air filter oil on it and a dust cover.
  • governor removed, custom throttle linkage fabbed up to work with governor arm cut, but still in block
  • gotta run it on premium pump gas i want to say its anywhere from 10.2:1 to 10.5:1
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Old 10-10-2017, 11:40 AM
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That's what i'll do, i'm gonna pull the carb and spray everything down with some carb cleaner and blow it all out. Hopefully that solves the issue.

If not, i'll let it idle like you said and pull the plug. Maybe i'll do the same at higher RPM's to see what my top end is like. I may have to redo both jets.

Jose
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Old 10-10-2017, 02:15 PM
mckutzy mckutzy is online now
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I don't know fully if it had something to do with your issues... But if you took out the gov and upgraded the springs... The rod will soon to break... If you already haven't upgraded to a billet rod...
I'm assuming it would also make things harder to start as the valve train is stiffer...
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Old 10-10-2017, 07:11 PM
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I watched a video on the tube where a dude running a mud-motor boat thing said the engines worked fine until modified, then nothing but issues. Was that you?
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Old 10-10-2017, 10:28 PM
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No sir, that was not me. Most videos of “built” mud motors have just a stage one type kit on them. The guys that run V-twins build em up pretty regularly.

Tonight I tore the carb down and cleaned it as thoroughly as I could. When I did the build I put in an e3 plug as thats all I could find. So I got a few NGKs and threw one in just to rule that out. Gaskets were shot so im replacing them. Ill give it a go tomorrow.
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Old 10-11-2017, 01:46 AM
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Governors hold the throttle open when starting and then close it after it's started. You have to manually do that after removing the governor.

Is the carb float bowl more or less level?
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Old 10-11-2017, 10:37 AM
garza7585 garza7585 is offline
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Bob,

Yes, the float is about level, I took the carb all apart last night and cleaned it and put it back together.

I talked to a gentleman at the local go kart shop and he thinks its a timing issue, but I don't believe it is, it will start if I give it gas and pull the cord.
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Old 10-17-2017, 02:45 PM
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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/idle jet, maybe that will help.
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