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Have a Honda GX 390 (GCBCT- QA2). Pulled it apart to put new piston rings in it. Got everything back together. When I went to start it it runs for a 5 seconds and dies. Will start right back up, but dies again. Will only start with the choke on. I don't even have time to turn the choke of before it dies. Can keep it running somewhat with starter fluid. Had a new carb on it so I pulled it back off and replaced with the same one that came with it- same result. Here is what I've done so far-
1. Changed carbs and checked all gaskets
2. Owner told me he had trouble with governor before so I changed governor gear and shaft. 3. Cleaned out fuel tank and put new fuel line on. Got good flow all the way thru the bowl on carb.
4. Checked valve lash several times. Engine starts really easy, just will not stay running. I can kinda keep it running if I hold the carb shut at idle but cannot open the choke. Seems like I'm getting to much air but it has new gaskets and insulators.
Can the coil be doing this? I'm not to familiar with these 4 post coils I believe they are called. Its the coil with the plug on it. I started it with the switch plugged in , then i unplugged the switch ( checking the switch)and put the red and black wire together. Would not start unless I did this. Oil sensor wire is unplugged. The engine is on my lift do I need to ground the block somehow with this different coil? I'm lost any help would be appreciated. Thank you

Some of these engines have a low oil shut down switch which grounds the spark if the oil level gets too low
So disconnect all the wires at the plug and see if the problem goes away.
Occasionally the kill switch wires can make a ground contact with steel blower cover when the engine is running and shoving wind around

bertsmobile1 said:

Some of these engines have a low oil shut down switch which grounds the spark if the oil level gets too low
So disconnect all the wires at the plug and see if the problem goes away.
Occasionally the kill switch wires can make a ground contact with steel blower cover when the engine is running and shoving wind around

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Ool sensor wire is unhooked already. If the kill wire is shorting out wouldn't it keep the engine from firing at all? In the current condition I can restart it as soon as it dies. I will pull the wires tomorrow and see if I can find any bad spots. Thank you

My go to for problems like this is a red in line spark tester.
Hook it up them watch weather it flashes all the way to the bitter end which indicates the spark is fine
next is a can of carb cleaner with very short shots down the carb throat.
If I can keep the engine running for at least a minute this way then I know it has a fuel problem.
I like to do both as they compliment each other.
Yes, kill wires kill but in order to do that they have to be touching a ground point some where

bertsmobile1 said:

My go to for problems like this is a red in line spark tester.
Hook it up them watch weather it flashes all the way to the bitter end which indicates the spark is fine
next is a can of carb cleaner with very short shots down the carb throat.
If I can keep the engine running for at least a minute this way then I know it has a fuel problem.
I like to do both as they compliment each other.
Yes, kill wires kill but in order to do that they have to be touching a ground point some where

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I understand how all that works. That's the same test I do. As stated before I can keep it running with starter fluid all be it badly. I have plenty of fuel going to the carb. Tank is clean and a new fuel line. Tank is gravity fed and I can open the drain on the float bowl and fuel is running just fine. With or without the fuel cap on. I have put on 2 different carburetors one new one used and have the same exact problem with both. If I hold my hand over the air filter opening it tries to run. Carb has brand new gaskets and insulator. In fact tank and hose came off a unit that is currently running. As soon as the governor tries to open engine dies. That's why I was wondering about the coil. I have spark all the way until engine stops turning but I'm wondering if the on/off switch needs to be grounded somewhere? My lift has a wood top and the engine is just sitting on it. But, if I have a ground problem why is it starting at all. Engine can die and I can start it right back up.

Ground means grounded to a metal part of the engine .
So now it is clear, you have a fuel supply problem
Can be as simple as the gasket blocking off the vent hole .
If air can not get in fuel can not get out.
Could also be a bd inlet valve not opening enough
And can be a governor hooked up wrong
People often get this backwards.
The Government puts a brake on you and prevents you doing what you want to do
The engine governor puts a brake on the engine and prevents it over reving .

The throttle cable pulls on a spring that is connected to the governor arm holding it WFO.
The governor arm is connected to the throttle butterfly via the throttle shaft
The governor mechanism works against the spring and tries to CLOSE the throttle
The faster the engine goes the harder the governor should be trying to CLOSE the throttle
Thus at rest the throttle should be wide open but as soon as the engine starts to turn the governor should move in the closed direction.

So check the governor adjustment
With the engine off push he governor arm to WFO
Slacken the governor clamp and rotate the governor shaft to the fully open direction as far as it can go then tighten the clamp

bertsmobile1 said:

Ground means grounded to a metal part of the engine .
So now it is clear, you have a fuel supply problem
Can be as simple as the gasket blocking off the vent hole .
If air can not get in fuel can not get out.
Could also be a bd inlet valve not opening enough
And can be a governor hooked up wrong
People often get this backwards.
The Government puts a brake on you and prevents you doing what you want to do
The engine governor puts a brake on the engine and prevents it over reving .

The throttle cable pulls on a spring that is connected to the governor arm holding it WFO.
The governor arm is connected to the throttle butterfly via the throttle shaft
The governor mechanism works against the spring and tries to CLOSE the throttle
The faster the engine goes the harder the governor should be trying to CLOSE the throttle
Thus at rest the throttle should be wide open but as soon as the engine starts to turn the governor should move in the closed direction.

So check the governor adjustment
With the engine off push he governor arm to WFO
Slacken the governor clamp and rotate the governor shaft to the fully open direction as far as it can go then tighten the clamp

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So.... governor is adjusted correctly. Carb has good gas flow from the tank to the float bowl. And, I have tried 3 different carbs. Double and triple checked gaskets and insulator installation. Same result. It will only run on starter fluid. Beginning to wonder if I didn't get the head in right and I'm sucking air thru the head gasket. Or if I'm one tooth off in the cam because it gets kinda tough to pull when the choke is open? I don't know guess I will tear it down again.

Not to be insulting, but you did verify the carb gasket holes are aligned for the airflow needed to have the carb run properly? It's easy to mess those up.

HurstGN said:

Not to be insulting, but you did verify the carb gasket holes are aligned for the airflow needed to have the carb run properly? It's easy to mess those up.

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Yes sir I have checked a few dozen times. Lol

have you tried using an alternate fuel tank/supply?

Well it has to be the carb or the throttle/governor set up
When the engine is at rest the governor should have the throttle wide open and as soon as the engine starts to run it should CLOSE down the throttle .
Remove the throttle link to the carb at the governor end & start the engine
See if you can work the throttle by hand.
Next silly question is do you have the linkages on the right butterfly ?
throttle is closest to the engine and choke is closest to the air filter.
Not sure if you can cross them over on your engine but I have had some come in with the control rods connected to the wrong butterflys so the governor turned the choke on.
And on some the throttle &/or choke shafts have 2 hook up points so they can be either pushed open or pulled open and again I have seen them set up backwards.

Most GX390s have the plastic handle that actuates the choke, right above the fuel cutoff.

Scrubcadet10 said:

Most GX390s have the plastic handle that actuates the choke, right above the fuel cutoff.

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Depends if it has the remote control kit fitted which is why the "not sure" was put in there .
Same with the throttle
Some have 2 holes so they can be hooked up backwards and some only one hole.

The last option would be an obstruction in the muffler causing the engine to suffocate on it's own exhaust.

Mr Wilson mentioned he replace the piston which you can do without touching the cam so I would expect that the valve timing is correct .
He also says he has checked the valve lash so unless he is adjusting it on the exhaust stroke one would assume that is correct.
And I am fairly sure there is not enough adjustment in the rockers to get clearance on the exhaust rocker at TDC exhaust stroke .

Scrubcadet10 said:

have you tried using an alternate fuel tank/supply?

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I pulled the current tank off a running unit but let me try another one. I'm willing to try anything now. Thanks

bertsmobile1 said:

Depends if it has the remote control kit fitted which is why the "not sure" was put in there .
Same with the throttle
Some have 2 holes so they can be hooked up backwards and some only one hole.

The last option would be an obstruction in the muffler causing the engine to suffocate on it's own exhaust.

Mr Wilson mentioned he replace the piston which you can do without touching the cam so I would expect that the valve timing is correct .
He also says he has checked the valve lash so unless he is adjusting it on the exhaust stroke one would assume that is correct.
And I am fairly sure there is not enough adjustment in the rockers to get clearance on the exhaust rocker at TDC exhaust stroke .

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I did take the engine all the way down when I replaced the rings just to make sure there was no other damage. I really watched when I put the balance rod and the cam back in to line up the timing marks. ( Maybe I need to check again) Have not checked compression but I know it tries to blow my thumb off of the hole on the compression stroke. Valves are set at .006/.008. Its got a brand new head and valves. Tonight I found that if I hold my hand over the intake it tries to stay running with the choke closed but as soon as I move my hand it dies. I pulled the head again tonight to make sure I haven't messed up the head gasket. It still looks new. Hard to diagnose when it won't run long enough to even grab the choke lever. This engine originally came in for a broken governor gear and that's when we found the bad rings. I've even swapped governors again. Starts really easy on choke but pulled kind hard and will not start when the choke is open. That's why I'm starting to lean towards maybe I got the cam off or something . I will check the exhaust out again tomorrow. Thanks to everyone for the help. I know its something dumb but I will find it.

I have repaired a few hondas with your problem and the fix was simple. I cleaned a carb few times on first as it would crank then die like it was fuel starved did some forum research and read that the ngk plugs are not made in japan anymore and are prone to premature failure and as yours is doin crank and die, sure enough i changed out the plug and it ran perfect. Since then i have diagnosed a few more. Not sure if this was coincidence or what but the honda engine w ngk plugs were all on woodsplitters. G'luck

I don't know if anyone mentioned this. I have seen new insulators with cracks in them. Check it closely, or use the old one. You're definitely getting air from somewhere.

Ok here is a update- pulled the cover off double checked the cam timing which was good. Tried to start it a couple of times and it yanked the rope out if my hand. Swapped out flywheel and key. Original key might be bent... have to measure and see. Ran the valves again which were spot on. Still won't run. Unhooked fuel tank and hooked my remote test tank up. No change. Swapped exhaust. No change. Swapped to a 3rd carburetor- now I could hold it against idle and it would run for a bit. Noticed fuel spitting out of carb where the airbox should be- shut the gas off and pulled the rope and it started then ran out of gas. Opened gas back up and it fired right up but, the rpms went thru the roof. So I held the throttle partly shut and let it run for a little bit. Ran out of time to mess with it so when I get back on it I will put the airbox and air filter back on and see if I can figure out why the governor seems to not be working. When I set it with the throttle rod pushed to the right counterclockwise so I was at WOT and turned the governor shaft counterclockwise as far as I could since the governor gear it in the back cover. Will dabble some more tonight or in the morning.

Oh yeah swapped out the insulator and gasket it too before the carb swap .... no change. Thank you guys for all the help!!!! Don't know if I'm there yet but at least it runs somewhat now.

So.... I have the held all the way to the right if you are facing the engine at WOT. I loosen the jam nut and turn the governor shaft counterclockwise as far as it can go. Tighten jam nut back down. Start engine up and it revs to the moon. I can push the throttle shaft shut it will idle down but if I let it go it revs right back up. The manual throttle handle is in the idle position and there is zero tension on the governor spring. Any ideas? Governor worked just fine before I tore the engine down.