Ahhh- the GX31- I have several of them- never have gotten happy with them- one of the only engines that has just about whupped me to get power out of. I tried six different carbs before I got the mixture like I wanted it. The stock carb is around 9mm or so- I ended up with a 12.2 mm barrel-style walbro with the dual adjustments on it- idle and main- keep searching- they do make one- just hard to come by- they are hard to find cause' they are non-emissions carbs. Anything you can do to get fuel and air in and out helps- velocity stack on inlet and open exhaust with a short section of .5 inch pipe is good- but, go easy with the porting- ruined one block porting too much in intake- totally killed the rpm capability by somehow messing up the flow- probably developed a turbulent spot that works against flow- but all I did was make it real smooth- I didn't butcher it- I know a little about porting. I have it from confirmed sources that if you move the wristpin- it will help power dramatically across the board. I am like you, I don't quite have the equipment to do the wristpin mod at home. They eff'ed us when it came to the compression and that darned one piece cylinder/head design.


Timing advance- I used a dremel and slotted the holes in the coil itself on one of them- made a adapter plate for the others- helps it a little bit- but not dramatically. I tried also advancing or retarding the cam one tooth- one tooth either way is too much- doesn't run well at either.

As far as fuel, they run worse on race gas than they do on the crappiest 87 pump gas that has been sitting in the garage for three months.

They gave us an engine with alot of latent potential- but they screwed us by using the intake port size/design to provide governing for the engine- and limiting compression by making the head/cylinder all one piece.

Honda corporation is a major tailhole when it comes to anything to do with using their small engines for a hobby/powersports application- I would not doubt it that they designed it like they did on purpose.

Honda released a new engine a while back in Europe but not in USA- it is a 35cc just like the GX25 OHC- why they didn't bring it over I dunno- I have tried in vain to locate one for at least two years-
with no luck- I did find a power equipment dealer in the UK that would ship one- along with the rest of the brushcutter it was on- for about $700- nah, I don't want it that bad.

I love the little engine though- it is a sweet running thing- just down on power- and hard hard hard to pump it up.