I am not refering to electronic or mechanical advace, but rather to modify the mounting holes on the magneto into slots so that the timing can be adjusted. I would like to experement with bumping the timing up on my gx31 and compensating with a higher octane gas. I will report my results but I would love to know if anyone else has already tried. The thing that kills me on this engine is it's construction with the head integrated with the cylinder preventing milling down the head to increase compression. That I have the machine skills for, moving the wrist pin upward, stroking the crank etc. I do not.

Hi Brat,
You have some neat ideas and it is good to see people actually thinking these days. Some of your points are true and some don't translate to small engines. Small bore engines don't usually require the high octane gas, even with increased compression and advanced ignition. The only ways that I know to increase compression with the GX31 is to build up the piston crown or make a new one. The third idea is to find or make a longer connecting rod. This is possible because engines use standard off the shelf bearings. I looked at the flywheel position on the GX25 and it is retarded about ten degrees over most chain saws and many trimmers, such as Homelite. The GX31 is probably the same. If you used a dremel tool to remove the cat in key in the flywheel, you will be able to advance the wheel on the crankshaft. Use a gear puller to remove it for timing changes as you experiment. Use a scribe to mark the wheel and shaft at the stock position. A little math will tell you how much movement in thousands,mm etc give you the desired advance. The shaft tapper will hold the flywheel position. It may shift if you give it to much and it backfires. When you acheive the desire result you can cut a new keyway with a dremel or file. I use a file myself, even in steel inserts. It is inportant when experimenting to start at the baseline and take thoughtful small steps. Also take notes. Of course you can always buy a CDI ignition and save time and weight.

Dave

Look at this thread: http://spadworld.net/forum/viewtopic.php?t=10569

I didn't post the results of the methonal fuel, and now I can't find the numbers, I do remember when I mixed the 10% methonal, engine temp went down, and rpms gained think it was 200, with straight meth, went up 500, and couldn't get the engine rich enough, need to work the carb to do that. The easiest way to do it is to go electronic ign, or build an adapter plate for the coil. SS

I haven't played with 1/4 scale gas at all, lots of experience with glow, then electric because after 20 years of flying glow it was still a pain and electric had come so far. But I always wanted to build some big birds, since I got into RC back in the late 70's as a kid. I have also always been facinated with the weed eater type conversions, and lately the 4 strokes in particular. I do have a great deal of experience in building high HP car engines, my daily driver is an STI with 463 at the wheels, but only so much of that translates to a 31cc single cylinder. Timing advance on a forced induction engine is a huge factor in output. In fact the majority of tuning is trying to get the timing just right accross the rpm range to max out the timing within the 93 octane's limits. The same engine at it's max can go another 25% just by running 105 octane race gas. I run a small fuel cell in the trunk that is filled with 105 and 93 in the tank. The tank switch is actuated when the ECU map is switched from a lower boost and ignition map to one with more boost and a more aggresive advance. Applying my experience there to this is a challenge. I did find a place with spare pistons for $28, I may have to get one and so some buildup of the dome.

Well like your race gas, I like the methonal for the fact that we usually have at least a drum for the car, but if you are trying to use a smoke system, might not work, lot less heat. I really liked that 10 percent methonal that I mixed up, stays mixed, and I havent had any performance problems from the fuel, mixed almost 3 months ago, using the last of it come monday. I dont' know how it will work out with the gx31 that you have, but moving the timing I would bet on, and if you did that the meth mix would work.
That car sounds like a beast!

What do the intake and exhaust ports look like in that 4 cycle?
What kind of exhaust are you running on it? SS

It is great to just hit a switch and after about 30 seconds for the line to clear I have another 100Hp on tap over the 463 normal. The exhaust port is just fine, I might run a felt cone over it with some compound to smooth it out. The intake is choked off a bit at the top of the valve pocket and the port at the head is a bit small. The carb is small by all comments I have read, and lean running for emissions reasons. I do have some carbs from 36 and 49cc skateboard and scooter engines laying around that I might try but I read a promising post about a fellow who says he bored out the stock jet and got gains each pass, then removed it all together from the housing and solved the lean condition accros the RPM band. I want to give this a try. Once I get it mounted and running I will tach it with the various changes.

Wretched excess,,www.conleyprecsion.com
The roots blower is only $1200 easy cure for low compression

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.