Honda gx390 engine ticks, burns oil, 72psi compression.

Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
Already did the value lash. Ain't the valves. The engine is old, like 20 years old, has the old style cyclone air cleaner. Air filter was improperly installed and was sucking in some dirt when I bought it for $50.
It was an air compressor most of that time. Then I got a hold of it gave it a good tune up, set valve lash, found a seal missing in the carb, put in a smaller air bleed plug and main jet, took the main jet from a 104, to a 100 and then to 95.
I put it on a stump grinder and it was a stump grinder for a few years, now it's a wood chipper. It will probably end up as a pressure washer eventually.

What do you think.
Is the piston rattling around in the bore?
Excessive rod bearing clearance?
Edit: there is no rod bearing, just aluminum.
A new one like this is about $800.
A predator 459cc was about $500 two years ago, after tax with a 20% coupon.
 
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jkrubi12

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B2601/LA435/QA54"/BH70/B8160box/BB1254/PFL1242/SGC0554/WC-68 Chipper
Sep 24, 2012
397
289
63
right coast
I've got a Honda snowblower and the engine makes all kinds of extra noise; I think the engine is a GX390. Every season I wonder if it's going to come apart but it just keeps on ripping on....The oil looks fine as well, never figured out what the source of the noise is.
 

PoTreeBoy

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L35 Ford 3930
Mar 24, 2020
2,446
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Already did the value lash. Ain't the valves. The engine is old, like 20 years old, has the old style cyclone air cleaner. Air filter was improperly installed and was sucking in some dirt when I bought it for $50.
It was an air compressor most of that time. Then I got a hold of it gave it a good tune up, set valve lash, found a seal missing in the carb, put in a smaller air bleed plug and main jet, took the main jet from a 104, to a 100 and then to 95.
I put it on a stump grinder and it was a stump grinder for a few years, now it's a wood chipper. It will probably end up as a pressure washer eventually.

What do you think.
Is the piston rattling around in the bore?
Excessive rod bearing clearance?
Edit: there is no rod bearing, just aluminum.
A new one like this is about $800.
A predator 459cc was about $500 two years ago, after tax with a 20% coupon.
It also has a gear-driven balance shaft that could be contributing.
 

L35

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Equipment
L35/TL720/BT900/York rake/Valby chipper
Jun 13, 2010
435
260
63
CT
My Kawasaki 2 cyl 23hp on the zero turn ticks like sloppy valves. Adjusted the valves and they were fine. Been doing it since new, I guess it’s just how it sounds.
 

Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
It also burns oil. I figured before it was burning oil because it was a stump grinder and I was tilting the engine too much. Nope it just burns oil constantly. Worse at idle, not much when it's running hard.
 
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Runs With Scissors

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L2501 TLB , Grappel, Brush Hog, Box Blade, Ballast box, Forks, Tiller, PH digger
Jan 25, 2023
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I have had 2 Honda engines in the past, both of them needed valves and valve adjustments so often, I was hoping someone made a "Auto-valve shim-button". I never did find one.

This attachment fixed them both though.

1685009226315.png

Then I started buying Yamaha....Sorry, I couldn't resist....LOL 🍺
 
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Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
I did a compression test, it has about 72psi cold and about 78psi hot.
Well, it's easy to pull start.
 

#40Fan

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Jul 21, 2022
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USA
I'd be happy that it has lasted this long and just keep on filling it with oil and run it.
 

Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
I filler up with oil and I get about an hour of run time before the low oil shutoff kills the engine.
Burns a lot of oil.
 

#40Fan

Active member
Jul 21, 2022
264
148
43
USA
Ah, one of those, check the gas, fill it up with oil engines.

Rip it open and check the bore, piston and rings. See if anything can be replaced to bring it back into tolerances.
 

Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
I took it out of service and replaced it with a predator 459cc from China freight for about $560 out the door.
This weekend I'll at least get the head off the old gx390 and see how bad the bore taper is. If the weather is nice I'll pull it apart and see what else it needs. Side bearings, connecting rod, crank.
It would make a great pressure washer engine.
I'm thinking if the block needs bored just get a 90mm electric start engine block and 90mm piston kit. The original engine is 88mm, so going to 90mm would make it about a 408cc.
If the stock 64mm crank is worn out put a 66mm predator 420cc crank and rod in it.
 

Matt Ellerbee

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MX6000
Jun 27, 2019
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I took it out of service and replaced it with a predator 459cc from China freight for about $560 out the door.
This weekend I'll at least get the head off the old gx390 and see how bad the bore taper is. If the weather is nice I'll pull it apart and see what else it needs. Side bearings, connecting rod, crank.
It would make a great pressure washer engine.
I'm thinking if the block needs bored just get a 90mm electric start engine block and 90mm piston kit. The original engine is 88mm, so going to 90mm would make it about a 408cc.
If the stock 64mm crank is worn out put a 66mm predator 420cc crank and rod in it.
It’s jealous of your tractor, turbo it.
 

Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
The old Gx390 had plenty of power for a wood chipper.
Before it was on the wood chipper it was on my stump grinder and it was scarry. The 459cc powered stump grinder is unstoppable.
 

GreensvilleJay

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BX23-S,57 A-C D-14,58 A-C D-14, 57 A-C D-14,tiller,cults,Millcreek 25G spreader,
Apr 2, 2019
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Greensville,Ontario,Canada
OS piston and ring kit will be $200-$250, few gaskets and such.. more than 1/2 way to a new engine,even when not on sale. Not sure if I'd fix it. You could have 300 into it and next day, some other $$$$$ part would fail !
 

Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
I got it apart. The bore has some taper, and the rings were severely worn, the oil rings had odd wear and virtually no tension.
Couldn't find my plasti-gauge but the rod seems to rattle around on the crank quite a bit. Crank seems fine.
The roller bearings are great, nice and tight no wobble.
One of the lifters has a bad wear pattern so it will be replaced.
If I was po I might throw new rings on and run it.

If you have small engines, make sure they have a gasket or O-ring under the filter and don't run them so rich it will make your neighbors eyes bleed.
 

Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
Not so cheap.
I got an electric start std bore block and two .25mm/0.01'' thick head gaskets from a go kart site for $153.
New cam followers for $14.
Std piston and ring kit with other seals for $41
+.25mm over size rings from another go kart site for $23 to use as file fit rings.
Then if I want it later on there's electric start kits for like $80. It's going to be a pressure washer engine which are normally pretty easy to start.
 
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Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
De-CARB'ed engine.
I also picked up a 0.01'' head gasket, normal head gasket is 0.45'' that should take the compression from 8:1 up to about 8.5:1, file fit rings for even more compression and 2 and 4 degree ignition timing advance/retard key ways.
Make this engine what it always should have been.
The epa declared NOx the source of all evil. Running so rich it makes your neighbors eyes bleed is fine, but a little bit of NOx is the end of the world?
Since its going to be a pressure washer or just a complicated pump, I'll test the different ignition settings by putting on an overly restrictive nozzle and holding the throttle wide open. More RPMs means the pump is pumping more water and at higher pressure. Whichever keyway (advanced, stock or retarded) makes more RPMs wins. Easy.
To run the pump at peak performance would take at least a 16hp engine, to make 4gpm at 4,000psi while turning 3,600rpm.
 
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Oil pan 4

Active member

Equipment
L185 turbo
Sep 21, 2017
412
107
43
NM
Conveniently the std bore block and +.01'' rings showed up first. I'm thinking bust out the ring grinder, which means put my dermal in a vise and gap them. Probably do 0.015''.