Ecomod my gx390
 

I have an old Honda gx390 from the early 2000s that burns a lot of oil.
When I got this engine the air filter o ring was gone so the engine ate steady diet of dirt for who knows how long and the carb was running stupid rich. Both conditions are bad for bore and ring wear.
Looks like time for a rebuild.
Rebuild kits range from re-ring with seals for like $29 to 80% new engine kits with a new 88mm block, crank, rod, main bearings and piston for up to $200 or so, pretty much only reusing the head, pushrods, cam, side plate, gas tank, carb, coil, flywheel, muffler, ect.
If the bore is tapered (worn at the top) I'll at least upgrade the block to a starter block and look for a 420cc block, which is the same block other than a bigger bore and takes a 90mm piston. Preliminary search doesn't show a good supply of 420cc, 90mm blocks.
Before that it's got 2 big piles of brush to turn to wood chips.

Nobody makes oversize pistons anymore?

If I can reuse the block with out boring, I will. Oversize pistons are available but the problem is getting someone to bore it where I am. If I have to take off work so I can drive 70 to 100 miles each way to bore the block then I'm definitely over the cost of a new block at that point.

I did find a 90mm bore block with electric start for $180.
https://www.nrracing.com/product-p/12000-z5t-90mm.htm
A 90mm block with the original crank will make it a about a 409cc and if the crank is worn I'll try to find a predator crank and rod to make it a 420cc.

Back in the day....... cylinder hone with new stones in a drill press can oversize .005 pretty reliably if you stay concentrated at the bottom and only occasionally go hone the full bore. Or you can mount the worn stones upside down. Light source for quick checking using new piston loaded from top. Slow, but faster and cheaper than a shop doing it.

Smallest over sized pistons I have seen for gx390 are +0.01''.
If it's got a bad cylinder taper, which I bet it does between breathing dirt and running eye burning rich. It will take more than 10 thousands to flatten out that taper
Since I can get an electric start 420cc predator for about $450 after state sales extortion I'm going to say my budget should be $350, because I could get some sucker to buy that old gx390 for $50 and I should be able to save a little money.
An electric starter kit is like $90, unless yanmar starters are the same as honda. In that case I have one.

I pulled the gx390 from service as it just burns too much oil.
I'll at least get the cylinder head removed this weekend and know if I'll need a block or not. I'll be shocked if I don't.
Hopefully I'll break it down enough to know if I need new crank, rod, roller bearings or not.

Oh yeah its a full rebuild.
Got a standard bore block, file fit rings, a 0.01'' head gasket.
The standard head gasket is 0.45''.
Thinner head gasket should bump the compression about a half number, from 8.2:1 up to 8.7:1 and I think I'll get a 2 and 4 degree timing advance keyway.
Since it will be a pressure washer, or just a pump it's perfectly testable. Hold the throttel wide open and more RPMs means more PSI and more GPM. PSI x GPM is power.

Got the short block reassembled.
It's a 42 to 48 state legal build.
Still nee to get the 2 and 4 degree advance/retard keys.

Compared the old rings to the new ones, there's almost nothing left of them. The oil rings were so worn they had almost zero tension. The other rings had a fair bit gone from them too.
Went ahead and got a carb with the air bleed screw, probably 0 state legal now.

I got it put back together.
Using a Chinese knockoff F188 piston it looks like my compression ratio is more like 10:1, as I'm getting 120psi on my compression tester. Not the expected 8.7:1.

https://youtu.be/ca-e5MrVbVU
https://m.youtube.com/watch?v=ca-e5M...ature=youtu.be
10:1 compassion, adjustable carb a 0 state legal build for sure.

I got it to fire up on the first pull. I get a little smoke on throttle up.
The new carb came with a big unmarked main jet, at least a #100 and running rich. I grabbed one I figured would be too small, a #78. Didn't want to run, it would sputter and die if throttled up, went to an #86 too lean would die if I throttled it up too fast. Then I went to a 95# and it started raining.