Spark plugs timing and piston stops

I have here a couple of plugs to show you, to put a fine point on things.
Typical of flat heads here is a J8C a very conventional spark plug. Low compression engines with rich mixtures.

Next this is a plug out of my car a direct injection gas engine with what I suspect is a lean burn or stratified burn system. Its got a very fine point ( boy that’s lean and clean burning too for 120k. )

Next to that is a piston stop I discussed in another thread a way to do this head off and this is for head on timing.

Last I have my famous BGE head with the factory spark plug and it has a pretty interesting open design and fine point on it too!!! Onan was trying very very hard to make the BGE lean clean and efficient. Notice I have this indexed not that it makes a huge difference but its angled in way to ensure the spark has a clean open window on the combustion chamber.


More to follow:
I am going to talk about how and when to use that piston stop and when and where its not going to work for you.


So here is a pretty good video of a guy with a piston stop in a cut away engine.
Once installed in a properly fit engine we can rock the flywheel back-and-forth. We mark one side then the other on the flywheel at a fixed point we can hit with a timing light. The centre of these two marks will be top dead centre.

Lets assume for argument sake we have a flywheel with a 4.5 inch diameter.
C = 2πR Radius is 2.25 inches so lets plug in the numbers.

`C = 2 * π * R = 2 * π * 2.25 = 14.137 inches

To set the graduations on the flywheel

( 14.137 inches / 360 deg ) * 20 = .785 inches

Lets make a mark there, that is base timing for a clone engine like the GX200. Lets run the engine with the timing light and shine it on our witness mark and see if we are actually firing at 20deg before top dead centre.

Assume we are where we want to be stock but we want another 8 deg of advance.

Lets say we want to run a test and run about 28 deg advance.
That’s a new mark at 1.1 inches ( rounded for simplicity )

Here is a great video of how to lap a flywheel, we need to do that next.
We are not going to use a flywheel anymore…
I am not such a big fan of sanding I would suggest Naval Jelly or a paste made from lemon drink crystals to clean any rust.

We are going to lock the rotating assmebly in the advance possition with the piston stop at 20 deg BTC as checked buy our witness mark, but we are going to turn the flywheel to the 28 mark we measured at 1.1 inches. Install nut tighten flywheel and do a timing chek by the light.

Run the engine test and see if your happy. Advance, retard test find the best place for maximum power and economy.

Good luck

Last thought maybe you have a flat head like BF CK BGE LK AK ect Onan a Wisconsin or other engine that does not allow a screw in piston stop. In that case your going to pull the head off and make something that looks like this and your going to have the extra step of head removal, but everything else is the same…



Lets talk about small block clone heads they range in size as small as the Chinese and early Honda GX160 head at 14cc to as large as the 24cc head on some of the 212 and 208 chinese clones and there are variation on shape and design too. Lets ignor all the canted valve heads and the trickery they include

14cc head

18cc head next to a 14cc look at the difference.

The 22cc head, see how the chamber is opening up???

The other heads for the 208 and larger small blocks work out to 23 or 24cc in most cases.
Here is a Semi-hemi as I call them that clock out at 23.
Thats a big wide open holes…

Yes it has bigger valves but it offers no advantages to woodgas. In fact we are better off with the restrict flow on the 14cc head. That head on a 208 with a flat top or a 636 predator will give us 1bout 10.5 to 11 : 1 compression without much in the way of modifications. The smaller head also presents a smaller combustion chamber at ignition so we might find we need less timing than if we milled a bit open 22cc head down to match the 14.
My advice buy a 14cc head and use a GX200 head gasket for all engines piston combinations on a Gx200 or larger engine. It will work and you can tune around it.

Chart Courtesy of Marty Stout, all around great guy that helped me solve lots of weird tuning troubles. Deck height is how far down in the bore of the engine is the piston from being fully flush. Ideal you want to be in the hole .020 with a gasket that is .010. that figure is the absolute minimum for safety on a clone. You don’t want to be more than .060. You can buy connecting rods that are slightly longer to make up too much depth in the bore.

Genuine Honda 13101-Z4M-000 STD OEM piston fits all GX160 and 200 engines. I was the first in Canada to sell these when they came out and man was that a hot race part back in the day.

And here is the lower compression dish you find in many stock engines.


Here’s a brand new and cheap Chinese rod that measures .010 longer than stock. Almost all Chinese engines are built a little deeper in the hole than I like and almost universal this slightly longer stock rod will help close up that gap and make a tighter quench.

My reasoning:
A flat top piston with a tight quench and a closed chamber head like the 14 will squeeze the gas mixture out of the edges when the piston comes up and force it a high speed towards the centre just as ignition is taking place. This makes for better mixing and faster burning. Woodgas is slow burning, anything we can do to help that speed up so combustion is complete at the correct time ( timing involved here too remember ) will give us our best performance and economy.

THIS ROD ONLY WORKS in the GX200 and the GX160. Other stroke length engines will require a difference rod


Nice! Ironically I just ordered a set of hotter plugs for a 79cc clone. The Torch plug I suspect is bad as it keeps fouling out and the engine does not run good on gasoline let alone chargas. So throwing a hotter plug at it to see what happens.

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Hi MattR,
M-a-y-b-e there is a good small engine Chinese brand name spark plug out there . . . ?
Just switch to made-in-Japan or made-in-USA and be happy.
Wallace is correct half across clipping back the ground electrode works. You see this on factory 2-stroke Bosch plugs across the board.
I tried the small engine single support electrode 3E brand of “Diamond Hole” starting about 5 years ago. Now I have evolved to these in all of my working 4 stroke and 2 stroke engines. Even the expensive Stihls. The engines start much better, have much less plug fouling on the old oil burning ones.
Seem to have better power. That is subjective of course.
I would NOT use the three leg supported 3E’s. Seem dumb. Single legged can be gap changed.
I did try a set in an EFI V-8. Mistake. These later feed-back auto/light truck 2000 and on from GM and Toyota; especially most all using the individual coil sensed spark quality to adjust that cylinders individual fuel and spark timing. I got weird codes and forced into Limp-ins.
Stick with the factory Iridium spark plugs for gosh sake!

And Wallace many of the small engine manufactures once into larger engine sizes use minimum three timing sets sloped coil pacs.
You can look up online specs for this. See it with an adjustable advance timing light.
I never done this (yet) but could be a fellow could stock position fir one of these to get the loaded engine spark advanced timing.
Steve unruh

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I generally only use NGK or Bosch plugs on anything. So I ordered the NGK equivalent but just a step a hotter. Ive had issues with the Torch brand before, most of the time they are good plugs but as soon as I have issues with an engine that have them stock, they are the first thing to go and that usually solves the issue.


Yeah modern cylinder heads that have the smaller chamber opening but cut deeper are designed to atomize the air fuel mixer. The flat portion of the head extending over the area of the cylinder bore is to create as much turbulence and contact of the head surface to AF mix to optimize atomization. how that effects a gaseous fuel I don’t know, but this is what was relayed to me by a professional head builder for race engines.

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Ya Matt Steve is right the Torch plug is very very hit and miss, mostly miss after a short time.
Trying to think now the heat on an NGK for that BPR6ES sounds right but thats off the top of my head. Hotter plugs are good for burning carbon off and keeping the electrodes clean the lower the number the hotter the plug. Woodgas does not foul up plugs as much as gasoline does. But it might do something else long run like effect the glaze on the insulator. Higher mileage DOW guys here would have a better suggestion on that than I can offer as far as heat and glaze.
A smaller gap is probably better, removing the resistor button from the plug boot might let through a little more spark energy ( at the expense of RF interference ). Making sure the gap between the flywheel and the coil is about .025 and not to wide or too close is important for safety and best spark too.

Something I forgot to mention.
The newer Honda Ignitions went to a CDI system that retards the spark for easier pull starts, but these are only for use on the big blocks as far as I know. I will look up some part numbers. They also have a rev limit built in and that’s a real handy thing for those who are not using governors.

Its called quench and it does speed up mix up and generally enhance combustion. Another is called swirl and it helps too again for mixing.

Swirl ports mix well but do not flow well. For a general purpose engine a swirl port make a cleaner burn and better economy. If you look at the photo you can see its not hard to port a Small engine head and put some swirl in it. it will not hurt flow on a Honda head. Its very hard to make a Honda head flow worse than it does, a lot harder to make it flow better.

I have no photos of this, but Honda and Fuji ( Robin engines ) used to make a swirl in the intake of the flat heads like the G200 ( 5 hp ). I used to work at taking the swirl and smoothing it improve flow. I am not sure if it was worth the effort. I would not waste my time doing anything more to a small engine head than just smooth off any sharp edges like the flat cut around the guide or the sharp edge on the short radius. Those will not make a huge difference anyways but you might be able to sculpt a little swirl in to make more turbulence and get better gas mixing


Im not a fan of the Torch plugs; not at all. I would never intentionally buy one, they are just what comes on the engines when I buy them new. Most of the time I get good use of the stock original “Torch” plugs but when I have issues those plugs are the first thing rip out of the engine and throw accro… ur I mean in the garbage. lol


Break out the centre of one and make a piston stop like the one I posted a photo of.
Good use for a Chinese plug…
Now and then there are counterfeit NGK plugs sold and you get a box of duds on sale someplace. No such thing as cheap and good.

Here is a real good photo of someone showing where they removed metal on the bowel of a 14cc clone head to try and make it swirl. If you have a dremel and time this is the spot to knock the edge from the plunge cut when they machined the head and I would leave the other side alone.


Here is the CDI ignition for the GX340 and 390. It will probably work on the 240 through 301 clone and 420 clones too.
To make this work you need a switch that is wired normally closed to enable the ignition. This is opposite to a dumb inductive ignition that uses a Normally open contact to short out the primary of the coil.

YOU MUST use a piston stop and re-time the CDI it will not work with the stock key in the flywheel. This ignition works by retarding the timing at low speed and starting before it moves the timing up. So in order to use this you will need to turn the flywheel even further than you would normally expect. Possibly as much as 2 inches of radius rather than 1 ( maybe 40 or so deg of advance to get say 28 at high speed and 10 or so for easy starting and low speed operation )



What kind of measuring devise are you using that will allow you to mark the .785 on the flywheel? Digital caliper and somehow compensating for the arc of the wheel?


Not 100% following so I could have this backwards. Could mark out 0.785 on some thing flexible like paper or roof flashing. Cut out then conform to flywheel.

Best way to do it is with a piece of masking tape and Caliper. Transfer the marks once you accurately stick the tap on the flywheel.

Lest look back at this statement

“but we are going to turn the flywheel to the 28 mark we measured at 1.1 inches”

That’s a rounded number and I skipped over the decimal places.
These are also numbers made up for a fictitious 4.5 inch flywheel and that’s pretty small so do not assume 1.1 inches is 28 deg on a clone flywheel you need to measure what you and right now I don;t have one in front of me to use as an example.

This is an offset timing key. But it starts at the assumption that you know what your timing is. I think filing the trailing edge .009 give you 2 deg advance. Its very hard to file to that accuracy. I prefer to mark the flywheel and using a timing light to know what I started with and then add to that what I want to try. But some people prefer the key and that’s OK too!

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My 74 year old brain is partially calcified. Simple things are starting to elude me.

Maybe I make a video.

it will be a later project but I have a clone on the healing bench that’s been waiting for final assembly a long time and I should finish it for you and do the timing set up just to see it.

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I did not make a video its kind of a pain right now.
The Google corporation has been diligently monitoring my browser and recommended this video. AS it turns out there is a pretty good video here about timing a clone for EFI.
Ignore most of it and just watch the piston stop and degree wheel. Its yet another take on how things can be done to set timing accurately.

No I am not not paranoid about Google monitoring my browsing habits. I wear a tinfoil hat strictly for fashion reasons


At the age of 40 the average man’s skull plates have begun to calcify and fuse. This is completely normal and proof that you have reached the age of reason… Sometimes this classification penetrates a little deep and hardens our position on things. it just means we recognize that we are right almost all the time and everyone else has stupid ideas…

So in the words of my father…
“if you want to be a genius like me, just say everything new stinks and your never wrong…”



Amen. I know that’s right. Just can’t seem to convince everyone else.