2 stroke bike options?

[bakaneko]

I run an NGK Iridium spark plug at a 7 temperature level. It works like 100x better than the crap one they give you. Iridium spark plugs are suppose to last 100k miles and is 2-3x more expensive than a basic spark plug but well worth it.

 

[Island Racer]

Hi Steve & Bakaneko,

Changed the list of spares, so hope that will be ongoing .... shipping is 42 days due to Chinese new-year, I'm not in a hurry ... new hobby takes time to do it right ...

It looks like my plug is in the center of the cylinder-head, but agree with a cooler plug next to the inlet ...
Sanding the head is one of the many on the newbie list ... when the engine is here I'll start a new tread regarding the list-to-do and pictures
I read the stock plug & wired are apparently bad quality, but we have STILL shops here and hopefully can find plenty stuff for that, that's half of the fun trying to make it better ....

Andre

 

[sbest]

A centered vertical plug is generally thought of as the ideal in 2 stroke circles but the chamber shape is more crutial.
This is a typical China Girl cylinder head as delivered:

This is the comic book version of the ideal cylinder head chamber shape and an illustration of what it does to the mixture inside:

I think the Iridium spark plug is overkill over Platinum. I have used them in my KTMs and they come standard in my snowmobile at $22 apiece! There is definitely a performance advantage of the platinum over conventional due to electrode wear and size, but the main problem of 2 stroke plugs is carbon and oil fouling, not electrode wear. With the jetting right and oil at 40:1 in my KTM I get years of use out of a conventional plug. Modern synthetic oils and proper jetting make plug fouling a thing of the past. The stock spark plug is not as good as any NGK.

Andre, it s "cooler plug over the exhaust". When running the plug electrodes turn red hot at about 500f and actually helps self ignite the mixture and burn off deposits. If the plug base and treads are in a hotter section of the cylinder head (like over the exhaust) it only follows the electrodes will become hotter and can contribute to pre-ignition and detonation. I am using an NGK BPR7HS in the rear position, which I know is too hot in the forward position. Perhaps I should try a BPR8HS in the forward position?

You have a centered plug? No worries!

My as supplied wire and coil seem quite good, and I doubt you need to sand the head as supplied, the warp comes from overheating and detonation, but I would suggest sanding a larger squish surface in the head right out to the edge of the bore (look at above sketches) and setting the squish gap to 1mm+-0.5mm.
I apologize for poor photo quality but here is an example of stock and modified squish area. This was done using only sandpaper.

I'd also be inclined to clean any burrs in the ports with sandpaper and a small file set while you have the engine apart.
Clean it well before assembly. Not that complicated.

Steve

 

[bakaneko]

Yeh, platinum or iridium is fine over the stock spark plug. The better metal spark plugs help not only with wear but with ignition and voltage requirements. The iridium NGK I purchased was only $9 of off eBay and a normal off the shelve one costs $2-3 here in Wisconsin. It sounds like Island Racer has a lot of time before the kit actually arrive; I would use that time to really shore up the braking system on the bike. Don't underestimate the speed these engines can achieve and the braking power needed to quickly come to a safe stop.

 

[Island Racer]

NGK plugs are available here we have STIHL weed cutters here, so they have good plugs here ... well lets hope ... for my landcruiser i had to get them from the USA ha ha ha

I bought this brake for the front .... hope it will work ... if not I will change the front wheel that has a disk on it ... not that problematic i think ....
http://www.amazon.com/gp/product/B000NNQKGC?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00

I like your suggesting QUOTE, """sanding a larger squish surface in the head right out to the edge of the bore (look at above sketches) and setting the squish gap to 1mm+-0.5mm."""
I will measure all the angles & tolerances TOP piston to TOP cylinder, as if you take material away the compression chamber will become bigger, but if you "mill" a very small layer of the cylinder head then it could work and keep the compression chamber perfect like your second drawing, I like it ... tinkering around is fun .... measure 5x mill once .... that's why we order more spares .... glorious ...

QUOTE """ I'd also be inclined to clean any burrs in the ports with sandpaper and a small file set while you have the engine apart. """
yes that was on my list too clean the burrs

I'm starting to like this forum already ... working on these little engines is just soo much fun ... I'll never ride long distances, it is just for FUN, and nobody has ever seen one of these things here, maybe ill start a new HYPE ... who knows

 

[sbest]

Yeah Andre, you got the spirit!
I am guessing that you could go out and buy a ready made bike for 4x the price and not bat an eye, but that wouldn't be as fun, would it?

What a wonderful little motor. Get an idea into my head, pull the cylinder off in 5 minutes, whittle my idea into metal in the living room while enjoying a beer with the dog, and have it all back together again in less than an hour. Awesome fun. I do plan to do long rides on mine. In fact I go out for 2-3 hour rides most times I go out. Back roads, old woods roads, old rail tracks and woods trails. I plan on riding it 45kms to work every day when the weather gets better. Besides the experimenting, a bit of exercise and sightseeing are part of it. I see a lot more on my motorbike than in the car, and see more on the motored mountain bike than the motorcycle.

About setting the piston to head clearance (squish or quench gap). Most use 1.5mm (0.060") soft electronics solder. It is getting harder to find soft solder as they get rid of the lead. Rosin core does not hurt the measuring capability of solder. You can measure thru the spark plug hole too.

Yeah, the spares come in handy. Work on the next new idea while still keeping the bike running. And if you screw up and go too far... you have a fallback plan.

Steve

 

[HeadSmess]

the china engines have this horrible pocket, that i highlighted in red. good pic steve first realistic drawing ive seen.

you HAVE to get rid of this wedge shaped gap.

a few direct cut and pastes, from one of my favourite books found here http://www.amrca.com/tech/tuners.pdf (australias technical know-how has sadly degraded since it was released, i hate to say) on squish band theory...

Combustion chamber form should be established with an eye toward only a very few
special considerations, and these cannot account for even half the chamber shapes I have
seen. Listed - though not really in order of importance - these are: surface to volume
ratio; spark plug location; thermal loadings; and combustion control.

Surface to volume ratio is important because even in the part of the combustion
chamber fully exposed to the advancing flame front, there will be a mixture layer
adhering to the metal surfaces that does not burn. These layers, like that trapped within
the squish band, are cooled by their proximity with the cylinder head, or piston, and
simply never will reach ignition temperature. And, like the end-gases from the squish
band, they eventually find their way out the exhaust port, having taken no part in the
conversion of fuel and air into horsepower. Thus, the best combustion chamber shape -
taken strictly from the standpoint of surface/volume ratio - would be a simple spherical
segment sweeping in a continuous arc from one side of the cylinder bore to the opposite

if you want to use a true (measured from exhaust-closing) compression ratio
much over 6.5:1, on a high-output engine, combustion control beyond that afforded by a
non-squish cylinder head will be necessary


The clearance space between piston and cylinder head must be enough to avoid
contact at high engine speeds, yet close enough to keep the mixture held there cooled
during the combustion process. This vertical clearance between squish band and piston
should not be greater than 0.060-inch (1.5mm), and it is my opinion that the minimum
should be only barely enough to prevent contact - usually about 0.015-inch in small
engines

holding squish band clearance to a minimum means that there will be the
smallest volume of end-gases escaping the combustion process, and that can be more
important than you might think. For example, a 250cc cylinder with a full-stroke
compression ratio of 10:1 will pack its entire air/fuel charge into a volume of only 28cc
by the time its piston reaches top center. Assuming that it has a 3-inch bore, and a 50-
percent squish band with a piston/head clearance of .045-inch, then the volume of the
charge hiding in the squish area will be in the order of 2.6cc, or almost 10-percent of the
total. That can be reduced to 5-percent merely by closing the squish band's clearance to
0.020-inch - and you'll never find an easier 5-percent horsepower difference. True, the
difference measured at the crankshaft might prove to be more like 2-1/2-percent, but the
addition of those small percentages can make a very large final difference.

 

[HeadSmess]

detonation is also mainly caused by these "trapped pockets" of mixture having the pressure increased suddenly after ignition of the main charge, past the point of self ignition (dieseling) which then explode uncontrollably rather than burn smoothly.

not to be confused with pre-ignition that is caused by hot spots in the cylinder lighting the mixture up BEFORE the plug fires.

i once tried a champion "easy start" mower plug... it ended up being equivalent to an NGK with a heat of 2! super hot!

it ran fine when i had it rich, but when i swapped jets to something leaner, and rode back home...halfway up a hill at full throttle/load, i thought the engine had seized. it slowed right down,lugged, then died. a few minutes later i tried starting it again and wow, nothing wrong! until i got another few hundred meters and it repeated... i got home by keeping it below half throttle, then researched the plug i had used...rather than being what i had assumed it was...it was, as i said, an extremely HOT plug! the electrode must have been glowing so hot it was preigniting, explaining why the engine felt so "seized"... preignition was so advanced it was fighting the piston on the compression stroke...

if your plug is fouling or coming out black...drop a number or two and it will start self cleaning. if it starts coming out bone white, melts electrodes or acts like its seizing...go up a number or two.

(my first MB years ago, with a husky brushcutter...i remember looking down one dark night and seeing the plug insulator glowing blue! not an issue, just good mixture and a fairly stubby plug with semi-translucent porcelain)

never use champions cus they have some really strange, illogical number system that makes no sense whatsoever... other than the trusty CJ8.

NGK all the way!

 

[sbest]

Hmmm, I see you subscribe to the same Church of Brother Gordon.
When that book came out in the 1970s it was an amazing revelation. Some people knew this information in the racing circles but Gordon Jennings released it to the public in an easily digested form. In 40 years most of his book has still held valid and is still the best place for any 2 stroke fan to start. I used his formula to build a pipe and modify a 1974 CR250 and 1976 DT250. Amazing, almost magical increase in power in both. I sold the 74 CR250 to a friend when his new 1985 CR250 could not compare in power to my decade old air cooled bike. The work done was stuffing the case, raising the exhaust by cutting the piston, homemade pipe and squish set up. Oh yes, and a jump from Champion to NGK.
With the low output coils and heavy ashy oils, NGK's platinum plugs were like magic when they first came out.

Here is my sketch illustrating detonation as you accurately describe it in a stock China Girl engine:

Here is some of the work I did on Yamaha Blasters which make 17hp stock, compared to a very similar 1985 DT200 that makes 32hp.

The chamber volumes need some description. The stock Blaster is about 25cc with 3mm squish.
I set the squish to 1mm (by removing the base gasket) and had a modest power increase.
I milled the stock head down to 23cc, reinstalled a base gasket (to get the ports up) to set squish to 1mm and had huge detonation problems, which started the investigation into chamber shape and this drawing. The original 28cc head (as received from the machine shop) was a disappointment when measured for volume, but was a huge power increase over the stock head. About 5hp with no other changes in spite of less compression, that is 17hp to about 22hp with a lower compression head! I kept cutting the head and changing shape until I was down to 18cc with no detonation. There really was little power gain lower than 20cc, and compression pumps huge heat into the piston.

I also played with squish from 0.2mm to 2mm and conclude that 1mm is probably optimum for most engines, with power lost if you are smaller than 0.5mm or larger than 1.5mm. Toroidal (bathtub shaped) chambers and large squish areas work better at low speeds from off idle to midrange and Hemi shape and narrow squish is better for top end. Compression I vary with exhaust port timing. More radical timing, more compression, with piston temperatures deciding how much is too much. Our conservative "trail" Blaster engines are about 35hp with easy starts and idle, broad power and running on regular 87 octane fuel if they have to.

I guess I am WAY off topic, aren't I? It's your fault Headsmess, you started talking Religion...

Steve - fellow follower of Brother Gordon...

 

[Island Racer]

Hi Guys,
It's been a while sins i posted, i made my first test-ride and liked it very much .... the build took some time as i tried to follow most of the advice, and made a REAL GOOD sprocket holder .... it is still a work in progress, as after 30 minutes I broke my chain so i will be getting a new one / better quality

I got some nice pictures too ... I even found a better spark plug on the Island ....
Andre