I thought I’ start a topic about modifing the small frame (about 200cc) Honda clone engins. Although most of this could apply to other engines.
SPARK PLUGS:
I tried an Autolite AR3910X (magic spark plugs; Wallace) about a month ago and noticed that it’s not easy to adjust the gap. I found that my engine, with a stock plug, starts better with a wider gap.
The idea for this plug came from cutting standered plugs in order to raise the compression a bit. Some of the shoulder material is removed in order to place the plug further into the combustion chamber with the home made cut plugs. Also people remove the washer. Just don’t run the plug into the piston. The 3910 design has more material that takes up more room. Also the 3910 claims that indexing is not needed.
So did I see any improvements? The first test didn’t show any improvements if anything it may have run a bit off.
This weekend I opened the gap up a bit with a cut off wheel and Dremel. Still needed a harder pull to start the engine. I pulled the brush hog around for over an hour and thought it lacked a bit in power. So I shut down and reinstalled the stock Chinese plug. It started better and the air/fuel ratio was much wider. Power seemed to be restored.
Maybe later I’ll open up the gap more but right now I would say stick with a quality standard plug that is gaped and indexed for your engine.
The other issue with this plug is the hex size is 5/8" and I cannot use the plug wrench that comes with the engine… 
Jeff
I found several natural gas and propane conversion discussions recommending closing the plug gap by about 20%. This seems to have improved the performance on my old Onan running on CO. I wonder why you are getting better results with a wider gap? I’ve used offset keys to advance timing, but have lately been thinking about advancing timing by moving the CDI pickup. What do you think?
Bruce
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My boss at work is into go cart racing using that 200 clone. There are racing web sites with all kinds of performance parts.
http://www.bmikarts.com/65-HP-Honda-Clone-Engines-Parts_c_14.html
That would be because the spark is shrouded where ever you put it I think. Have you tried a hotter plug?
Bruce, I should try going narrower than stock and see how that works. Maybe CDI has a hitter spark than points? Maybe a narrower gap advances your timing, i don’t know… I looked at moving the pickup but not enough room under the tin housing. I plan on tying Wallace’s method of lapping the flywheel to the shaft. Then toss the key and set the flywheel anywhere on the shaft next torque to spec.
Marvin, I sure would like to try other heat ranges… 
Carl, yup lots of goodies out there. A lot of the hot rod stuff don’t apply to us because of our low rpm but lots to learn from the race community… 
Jeff
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I was just looking around at some articles about small engine timing and came across one about tuning by exhaust temp. This seems so much simpler and less expensive than an O2 sensor. The idea is to mount a thermocouple in the exhaust flow close to the engine port and watch for the highest temperature. Both too lean and too rich will give a drop in temperature as will timing that is too advanced which causes ping. There may be problems with this approach on gasoline because we can overheat the head with the optimum smaller jet size or if airflow through fins is blocked. I think with charcoal gas, which is cooler, we can safely test for optimum mixture and for best timing by simply watching the temp. We might also be able to check the effect of water and oil drip. Keep in mind that use of these drips will also affect optimum timing.
Bruce
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Bruce, interesting stuff. Gas still burning out the exhaust might confuse the interpetation.
Jeff
Hi All
I do have limited woodgas fueled (not charcoal gas) experiences using the brand of E3 spark-plugs in different four-stroke small engines. Helps a lot on the misfiring and power smoothness.
These have a square holed positive electrode. Unlike side-gapped spark plugs like the Bosch Quad’s it is still possible/easy to open and close up the gap on these. All my bought 3E’s were just a single legged positive electrode.
Actually used these with noticeably better results for 7 years on all of my gasoline fueled four stroke outdoor equipments: point type/magneto’s; points-battery; and CDI’s.
www.e3sparkplugs.com/lawn-mower-spark-plugs
Ha! My expensive Stihl’s 2-cycles still get their designed for standard Bosch plugs.
The Wife’s Hyundai and the Plymouth mini-van get Nippon Denso and Champion/NGK platinum’s. The Ford pick-up semi-platinum Autolites.
Processor “smart” ignition quality sensing/adjusting systems; OBDII’s!! are spark plug brand and style picky in my experiences. Careful. Problems? Go back to the OE manufacturers engineered, spec’ed plugs.
Regards
Steve Unruh
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Hey Everyone,
Just thought I’d throw this out for general info.
The phrase exhaust gas temperature brought me back to my Cessna 172 days. Here’s a link to EGT and the parameters that effect mixture control, etc. What happens to my normally aspirated engine when I climb Pike’s Peak? A good read in general and a must for automation. Temp, altitude or elevation, air pressure, etc.
http://www.gaceflyingclub.com/Member%20Download/LeaningArticle.pdf
Pepe
I removed a statement here from another article, which I can’t seem to find right off.
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I have seen many rants against E3, but they have worked for me. My bottom line: a clean but poorly designed plug works better than a well designed plug that is fouled. Some suggest reading the condition of used plugs and adjust brand based on temp. Colortune plugs are made from clear glass so you can see the color of combustion thru the plug body.
Bruce
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I looked around a bit more about EGT (exhaust gas temperature) and found that tuning is not quite so simple as shooting for the highest temp. Several experienced mechanics agreed that O2 and EGT sensors are better as diagnostic tools than tuning tools. Bottom line: “When the engine sounds crisp and makes great power, you’re there.”
Retarded timing can raise the exhaust temp a lot while giving lower power because more unburned fuel is passing into the exhaust stream.
It seems that a combination of head temperature and EGT works best to read burn conditions.
EGT and head temp should both be high and rise together. Remember head temp changes slowly because of the mass of metal and is also affected by ambient conditions.
EGT will rise going from rich to lean then drop after passing stoichiometric AFR (air fuel ratio). Note that neither maximum EGT nor ideal AFR necessarily give best performance/economy because of many engine and load variables.
Advancing the ignition will lower EGT. A rising head temp with dropping EGT under load is a sign of detonation (often audible as knock or ping under load with acceleration) which is caused by too much spark advance.
When AFR is going too rich the head and exhaust will both cool.
The preferred approach to finding the ideal EGT, which is not necessarily peak temp, is to start rich and under load then go lean until power/torque drops. Note the EGT just before power loss.
A little bit (50 degrees?) cooler EGT on the rich side is generally best for overall performance.
This optimum EGT point can be used thereafter to approximate best AFR and read combustion conditions.
Bruce
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Hi Jeff
Its my favorite and most used engine 
So, i hope to learn some tips and tricks here
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Well, ran out of time this weekend before I could do some head work.
Got some stuff rounded up. Gasket, sealer, valve lapping compound and valve tool. Not to mention sand paper (plus the tile) to mill down the head. I also have a belt sander and a small mill but will give the sand paper and granite tile a try. Might also work as a surface plate. Not much of a hight gage there but its all I got. Maybe I should have got two tile to level each other out a bit more with some lapping compound or just test them out with some blueing.
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I am working on this ignition for the Honda clone, a kind of programmable digital ignition, going to use a hall sensor instead of a pickup coil.
http://www.sportdevices.com/ignition/ignition.htm
And this tutorial i did find very useful to modify my GX35, probably going to do the works on the GX200 and the GX160 as well
And the cherry on the pie…
A wet dream for the tuners…
http://www.ecotrons.com/products/gaseous_small_engine_fuel_injection_kit/
and many more useful stuff
very cool fuel injection set up… didn’t see a price but that’s all good.
and many more good stuf, programmable ignition included, and throttlebody’s, and a small turbo set,… and
The briggs junio 206 kit is 699…
Here is the small engine ECU. It is the same chips, not the same board… It looks like the ecotrons used the rs232 serial port instead of the usb port…
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=KIT33812ECUEVME