Honda GX390 not starting

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Aug 22, 2022
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I have a GX390 that was left outside. Water leaked into the crankcase from two missing bolts on the exhaust header. It was seized. I flushed out the crankcase and gas tank, rebuilt the carb and it started fine and ran fine in my shop.

I took it out to use it and it won't start. I have had the head off a few times and reset valve clearances. I have spark. With the recoil off, I hand spin the flywheel. I get a spark when the piston is at tdb. I know it should be firing at tdc. Is the timing on these engines that far in advance?
 

jeffreybandel

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Aug 22, 2022
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It doesn't fire on starting fluid. It has spark. I have an in line tester and I can see it sparking.
 
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It doesn't fire on starting fluid. It has spark. I have an in line tester and I can see it sparking.

Seeing it spark in uncompressed air is different than sparking in compressed air. I have experienced this first hand - motorcycle would not run. Pulled plugs, could see spark on each one...but would not run. Put new plugs in, and it fired right up.

Check Compression and ignition timing if you are sure you have strong spark.
 
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Spark should fire about 10 degrees before the piston reaches top dead center. When you have the flywheel air shroud cover off you can turn the engine slowly watching the piston and the magnets on the flywheel passing under the coil at that point. If the flywheel nut was not properly tightened, the key will shear and put the engine out of time.

If timing is correct, try a different spark plug, preferably one that is known good. Plugs can fail that they receive power from outside but don't spark inside. The only good way to diagnose one of these is by substitution.

Temporarily disconnect any kill switches or oil level monitors from the low voltage coil wire.

The compression should feel normal as you pull the starter rope slowly.
 
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jeffreybandel

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Aug 22, 2022
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Spark should fire about 10 degrees before the piston reaches top dead center. When you have the flywheel air shroud cover off you can turn the engine slowly watching the piston and the magnets on the flywheel passing under the coil at that point. If the flywheel nut was not properly tightened, the key will shear and put the engine out of time.

If timing is correct, try a different spark plug, preferably one that is known good. Plugs can fail that they receive power from outside but don't spark inside. The only good way to diagnose one of these is by substitution.

Temporarily disconnect any kill switches or oil level monitors from the low voltage coil wire.

The compression should feel normal as you pull the starter rope slowly.
I think the nit must have slipped. When I trun it by hand it is firing on the bottom of the intake stroke. I'll try a new plug just to confirm.
 

JTK

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Given water in the CC, I was thinking maybe the oil level switch was hanging up, but you say it's got spark....

If it's got spark and compression, it should at least pop if it's getting a fuel source of some kind.
 
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I get a spark when the piston is at tdb. I know it should be firing at tdc. Is the timing on these engines that far in advance?
Did you shear a flywheel key? Spark should still be near TDC. Edit: Also make sure you didn't put the magneto on upside down, the pickup coil usually has to be on the leading pole.
 

jeffreybandel

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Aug 22, 2022
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In the picture you can see the magnet on the trailing edge of the coil relative to the piston position. Now it is firing on the downward stroke. Yesterday I think it was doing it at the bottom of the stroke. Sounds like it is slipping?
 

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The piston looks clean and the cylinder looks like it still has cross hatch marks. That engine probably has a lot of life left in it.
 

jeffreybandel

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New key in and valve job and I am up and running. RPMs seem a little low so I will adjust the governor a bit.
 
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