Chug-a-Lug

[KenM]

Yes I am with you! I was thinking about asking how far other people got on a tank of gas. I get about 40 miles. I still can not get it to lean out right. The top notch on the needle is to lean the second is to rich, I have it set there, I have the float set at 24mm. Have changed the air filter with a better breathing filter. Still to rich .I have about 220 miles on it so far, all is good. Keep looking up! Ken.

 

[M88]

I had just emptied 1st tank and then it snowed! I got the 150 from somewhere on the web, just trying to be hopeful I guess.
As for keeping looking up Ken - I did that once and stepped in a pile of doggy-doo!

M8

 

[sbest]

40 miles is about 60kms, I am probably doing about that, maybe better on long flat trips, less on my full speed hill testing. As the weather warms up, I plan to ride to work, 45kms total trip and lots of hills. I better have the figures right for the first trip!

This is one of the things I plan to work on. I want the most out of my engine, including fuel mileage.
My KTM 125 Ran well in the dirt, but when I put it on the road, I only got 35mpg!
It was jetted "rich for safety" in all of the carbs ranges.
Leaning it out gave much better performance and mileage in the 55mpg range. Not great for a 125, but for the performance, not bad.

If I really wanted better fuel mileage out of it, I'd make it lower and leaner.

Same with the bikes I guess. Ergonomics take precedence over aerodynamics, otherwise we'd be running 20" tires and crouching or recumbent.

I think there is fertile ground for fuel mileage improvements.

As far as the original poster's chug-a-lug post, here is what worked for me.
A better squish area on the head chamber.
This is what the average China Girl head looks like:

Below is what it should look like.
Note the "Pockets" in the corners that can trap and compress gasses.
Note the small squish area that has little effect.
The drawing below is what an ideal combustion chamber should look like and shows the tornado like swirl that will sweep the flame front to complete combustion.

So what happens in the stock China Girl head at the moment of combustion? DETONATION.
Stagnant combustible mixture is trapped in the corners and feels a rapidly rising pressure wave that causes a spontaneous detonation of incomplete combustion. This hits the piston and head like a hammer, causing the Hurky-Jerky you feel at low speeds.

Below is the flame front you need for smooth combustion:
The large squish area "squirts" the mixture out to the combustion chamber in a swirling donut shaped tornado.
There is no room for pre-combustion or detonation in the cool narrow metal squish area (often also called quench area).

Smooth combustion is the secret to getting rid of the Chug-a-lug...
Steve

 

[M88]

Yknow, I hadn't thought of that on this itty-bitty China Girl (as you call her)! Several yrs ago I wanted to calm down my Cagiva 250 (200 actually) as it was a wee bit too jumpy in the woods. So I reduced the comp by about .63 by machining the comb-chamber but I left the squish alone, and I added about 3lbs to the flywheel. Result was a more pleasant ride between the tree trunks but still outran my buds on their Ymaha's and Hondas on the top. No idea of mileage but I didn't really care anyway. When I get done with my other projects (rebuild antique u/light; getting '89 MBZ560SL ready for spring; finishing top to bot re-do of '92 Geo Metro convertible plus all the honey-do's my sweety insist I do if I want to eat!
Cheers all,
M8

 

[sbest]

It only took a few minutes to sand the squish area into the head. I was forced into it.
The detonation and heat warped my head. I had to sand 0.020 or more off it to clean up the warp.
This put the little squish there was too close to the piston.

So I took an old piston, wrapped some coarse (150?) sandpaper over its dome and ground it into the cylinder-head like a mortar and pestle. It could also be done with a wood block and sandpaper in the drillpress, or a flycutter in a mill. 10min of sanding gave me what I wanted:

The result was no more detonation. GONE!

This is not my first time at the rodeo.
My son and I found huge gains in the Yamaha Blaster cylinder head. To the right is the stock head;

The one in the back is a hemispherical cut, the head on the left is a KTM torroidal head.

Here are another couple of our experimental cuts:

We must of done a half dozen cuts and then recut for squish and compression, plus bought every odd shaped head to try.

What we learned is that the chamber shape and squish is critical. The stock head would detonate at 23cc, and yet we had heads we cut to 18cc and ran on regular low octane fuel. That is what I did with my Happy Time (I love these fun names) motor. To get rid of detonation I cut the deck surface down, RAISING my compression but shaped the squish to get rid of the detonation.

Something else we learned on those Blasters (and I from previous experience with 4 strokes) is that improved combustion needs less timing advance. Like blowing on a fire, it burns complete and fast with the right turbulence. You have to back off the ignition advance if you are doing it right. I have a Mustang and the guys like to brag about their total advance, as if that measures or makes power. Mine has like 10 degrees less total timing than these guys brag about, and I go faster.

I gotta give some credit to a guy named Larry Widmer here. Many years ago Larry gave me the clues to what went on in a combustion chamber and how to take advantage of it. He showed me how to run 10.5:1 compression on regular gas when they stopped selling premium. Larry "The Old One" was a voice of wisdom, against the crap of the Hot Rod magazines. And Ak Miller, and a few other brilliant renegades. I digress.

Take 10 minutes, pull your head off and either use an old piston or take another 5mins and pull your piston too. Hold a strip of sandpaper across the dome and grind the squish area out to the edge of the gasket (another 10 mins). Wash it all off in Mom's sink with her dishsoap, and trial fit it. Use some 0.060" (1.5mm) electronic (rosin core is fine) to check the distance between piston and head. Anything from 0.020" to 0.060" (.5mm to 1.5mm) is fine. Adjust with paper hand cut basegaskets or more sanding (deck or squish).

Can be done in an hour.
The result won't change the world, but it will make your China Girl more of a Happy Time.

Steve

 

[Purple Haze]

Great pics, sbest! These china girls have very rough castings, amazing what you can do with a little sandpaper and time. One simple thing to check is the needle height. Most of the stock NT carbs seem to run best with the needle lowered (1st groove top) They also seem to be jetted too rich from the factory (#70) I usually run a #62 - 64 with good results.

 

[sbest]

Great pics, sbest! These china girls have very rough castings, amazing what you can do with a little sandpaper and time. One simple thing to check is the needle height. Most of the stock NT carbs seem to run best with the needle lowered (1st groove top) They also seem to be jetted too rich from the factory (#70) I usually run a #62 - 64 with good results.

True story, and not an accident I think. Better to start out a little rich for break in.

Steve

 

[Purple Haze]

I wonder about the difference plug placement makes. I know from experience that the old angle-plug sbc heads were less efficient because of flame travel, they were a crutch for running a high dome piston. You certainly seem to have done your homework on this, sbest. As far as fuel mileage, and being an old Mopar guy, I found that the old 318 poly got better mileage than the newer 318 wedge. Both engines were held back on power by poor basic design; (varying deck-heights comes to mind, among other things) On these little 2-strokers, I think exhaust length has much to do with part-throttle mileage. I've been able to get about 70 miles from a tank by optimizing carburetion and exhaust, plus a little juditious porting of the transfers. It's always a juggling act between power, economy, and reliability; I do think engine effiencency is the key to all these things. Thanks to everyone for the feedback.

 

[sbest]

While these engines are simple and casting flash is bad, the three engines I have all had squish (piston to head gap) within the ideal 0.5mm to 1.5mm. Most Japanese bikes I have measured have been in the 1.5mm to 3.5mm range. Exhaust tuning is much more than just length. Even with the stock muffler I think a gain be had by finding the right length and then the right stinger restriction to keep gas/air charge in the cylinder and out of the pipe at max rpm. This would improve power and economy.

One of my earlier posts in response to a question about plug direction, before I ground more squish area into the head, I tried the head plug forward and plug reversed. I had noticably better performance with the plug to the rear. I now hypothesize that it is because the exhaust side gets hotter and the plug threads are kept hotter and the plug runs hot. If this is the case, I could run a colder plug to the front and get the same results.... Hmmmm. Another test?

 

[thefastestone]

like the 1st repley said they dont perform well at low lugging speed
if its a two stroke they run off rpms
it also matters about that pipe
i bilive if you have the expantion chamber exhause pipe it will give you more bottom end power
but they certainly like running hard
and that chugging sound like a turkey i think
the combo of the cluch and exhaust of those crappy pipes they send you in the kit
sure it works but dose it performe over time

20130803_142432

• thefastestone
• Jan 28, 2016