Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

I have a strange problem with my Honda GX390, 13HP engine.  I replaced the carb. with a working one it did not help.  The engine seems to run fine in low RPM.  Once I open the throttle it dies.  No problem to restart.  Maybe there is an air leak but I can't figure it out where can it happen other than the carb.  The gasket between the carb. and engine is in good condition.  What else can I try?

Thanks

Bob

Does it  have a fuel filter? If so, is the filter clogged? Sounds somewhat consistent with, as jrtrebor was asking, getting a small flow of fuel to the carb. Maybe it's enough at idle, but not enough to keep up with the demand as you raise the throttle.

Out of curiosity, could you add a load to the engine with it at idle? Something that will let it call for more gas, without raising the engine's speed (which slightly complicates the troubleshooting situation)? If it dies immediately on raising the throttle, however, that might not point just to a slow fuel delivery, since it would have still needed to burn through what was in the carb bowl, and that would take some time (my 8hp Tecumseh will run for 1-2 minutes on what's in the bowl).

Is the air filter clean, and not restricted? When it dies, does it smoke or otherwise do anything different? Will it run at low RPM indefinitely? Please forgive the question, but when you changed the carb, you're sure the governor & throttle linkages went back together properly?

Edit: Also, does it have a low-oil shutoff? I assume the oil level is OK? Have you tried unscrewing the gas cap, in case the vent is somewhat plugged, and it's not able to let air into the tank quickly enough? When it dies, does it gradually start to sputter, etc, or is sudden, like you suddenly killed the ignition?

Was the engine acting the same way with the old carb.?  If it was, then it's probably not a carb. problem.
What position is the choke in when it dies.  I was also curious about this.
"The engine seems to run fine in low RPM".  As in, at an idle?  Or does it seem to run fine a little above an idle?  Just not when you really crack open the throttle.
If it only runs at an idle with the choke on.  But dies as soon as you start to open the throttle.  It's usually a fuel problem.

 This is my first post. Thank you for everyone who answered.  I appreciate the support. 

The engine acts the same way with the old carb or the new carb.  No fuel filter and I ran it without the air filter.  The fuel comes out of the fuel line quickly(not restricted) when discounted from the carb.

It runs pretty smooth in low RMP, not  just in idle position.  I can increase the RMP slightly without any problem.   I will double check tomorrow but I think I ran it slightly chocked (full chock to start) in low RMP. Once I open throttle fully It dies. 

I will check the spark plug first and adjust the valves.  If that does not help I will remove the exhaust.  Does it sound like a good plan?

Sounds like you might have two dirty carbs.  Unless the new carb was on an engine that was running fine just before you did the swap.
What you're describing   "It runs pretty smooth in low RMP, not  just in idle position.  I can increase the RMP slightly without any problem.  I think I ran it slightly chocked (full chock to start) in low RMP. Once I open throttle fully It dies.  Sounds like it's starving for fuel, clogged main jet, stuck float etc.  You might also pull apart the fuel shut off valve on the carb.  Sometimes you can get a
restriction in the fuel flow through the valve because the thick rubber gasket starts to come a part.  It's got holes in it that let fuel pass when the handle/arm is in the open position.  You just take out the two screws on top of the valve holding down the plate.  But watch it there is a spring in there.   The part is just below part number #26 in the diagram I guess it's part number 29   .          Honda parts

The carb, I replaced came from my working pressure washer,  It works fine before I removed it and works fine after I put it back.  It has to be something else.

I will check the valve and spark plug tonight.

Check that the gas tanks cap is venting.  I once placed the plastic cap on the hot muffler, it melted the vent hole closed, and produced exactly the same problem you describe.   A clogged fuel filter will do the same thing.

If the air can't get into the gas tank fast enough the gas won't be coming out of the gas tank. 

Does it just cut out or chug a little before stalling??...if u had a inline spark tester u could hook it up and make sure your not loosing spark when it stalls...given that its not likely a carb issue its most likely a spark problem..u could disconnect the kill switch and the low oil switch see if it still stalls.. a spark tester would be of help...u could put a new spark plug just to rule that out.

niper99 wrote:

I came in from garage and saw this post.  It is exactly what happened.  I discounted the oil level switch and the engine ran great,   Do you think it is the oil alert diode or the switch.  I honest don't know what does the diode do.  I connected an ome meter from the switch to the ground.  It showed open in low RPM and jumped around when I increased the throttle. Looking at the ebay the part costs around $20.  I am glad I got this problem solved. 

I'm glad you made some headway on figuring it out. First things first, the oil level is OK, correct?

I had a Generac engine with a low-oil switch that had failed. It would start, run 5-10 seconds until the control board enabled the low-oil shutoff, then it would die as the control board killed the ignition. I took the switch off and found that the switch would electrically open/close (I think to ground; it only had 1 wire) if you pushed  inside the switch with a screwdriver. So the switch was free to move, etc. But it would not detect oil pressure, you had to actually push on it with something solid to make it register "pressure". I had a few guesses as to why, but never tried to cut it open. Replaced the bad switch, and it's run fine since.

But I'm admittedly unsure of why your switch would see *lower* pressure as you raise the throttle. I'm assuming that the oil level isn't low, and that the engine is on a level surface. If it has an oil filter, could you try replacing that? Maybe there's some kind of restriction in it? I'm sure someone with more experience can help shed some light on this. But I might not just leap to replacing the switch, based on the symptoms. Unless someone can explain why the switch would perceive lower pressure at high throttle, when I'd assume it should be higher.

The GX390 just has a standard oil slinger type lubrication.  It's not a pressure system.  As the rpm increases so does the agitation of the oil in the crankcase.  If the level is low it can
cause the switch to activate. Had the same thing happen on my Honda powered generator one time.  If the crankcase is showing full.  Try bypassing the little metal box the crankcase switch plugs into.
Or you can disconnect the switch all together and just check the oil every once in a while the way it was done for decades.