Tuning the Chinese Motor, pictures to come

[sbest]

I bought a Grubee Skyhawk, followed the instructions, but it together as it came and drove it moderately while breaking it in. I was quite pleased with it, am improvement over only pedaling. According to the website calculator at:
http://www.mne.psu.edu/simpson/courses/me240/bikecalc1.htm
it makes about 1.5hp to 2hp and has a 44t sprocket on back.

I put 3 tanks of gas thru it, swapped the plug to a NGK BPR6HS because the original 3 pronged plug fouled, tried the head plug forward and plug to the rear (worked better to the rear) and leaned the needle one notch. A fine wire in the main jet leaned it too much so I left it alone for now.

I did play with exhaust pipes too. I have the stock black catalytic pipe and a shorter chromed pipe. The shorter chromed pipe seems to be slightly stronger and quieter, but it is a close call, so I am using it. Uncapping the pipe is out of the question because I want to keep this sport unregulated, so I don't want to pi$$ off the neighbors with too much noise.

I have a couple hills to try for power, with the one at 45.021146, -64.630514 being about 10% grade. A great test for this little motor. Stock, it will not make it up here without pedaling, so let's see if I can improve that. Top speed on the flat is 59.9 kph.

So the first principle of trouble shooting is to return everything to as designed configuration. That is pretty much where I am. The next principle is to make minimal modifications (one at a time) so you can see what does what. Also best to start from easiest to hardest items to modify as well.

I figured the terrible gasket and port mismatch is probably the first thing to correct. So I pulled the cylinder and got some pictures. More coming of the corrections as soon as I can load the pictures...

 

[dbledsoe]

sbest,

I think you are off to a good start. 59.9 kph is a very respectable speed on level ground for a stock engine kit installation. I also think your starting point logic is sound. Start from stock and make one change at a time. Your first change is a good one i.e., port match the gaskets. There are a lot of different suppliers/sources of gasket for the Chinese engine and like so many of the other various engine parts, quality and configuration vary greatly. I too often see gaskets as choking points because they have smaller or different shape cut openings than the two ports they are intended to mate.

Please keep us informed of your progress.

Don

 

[sbest]

I apologize for the poor picture quality, but a camera is a low priority so I am using my cell phone. Here is my bike, including a bit of mud:

On the last hard run up a steep hill, I hear an exhaust leak. Head gasket blew and was loose when I got back home. It and the cylinder showed up as warped when sanded on a glass plate.

Gasket mismatches are legend on these motors. This is the intake:

And the exhaust:

The start down the road to improvement, the cylinder head half sanded to show traces of where it was warped. Further sanding removed even this.

 

[sbest]

Engine is in the way back together. Cleaning up the warp in the head and cylinder took too much off. The squish distance between piston and head is down to 0.015" and is terribly shaped. It is actually thicker at the cylinder wall and tightest near the crown. It really cannot be assembled with this distance less than about 0.020" so it is time for more kitchen machining.

This is tested by using a strip of 0.060" lead solder. 0.060" is about the maximum needed for any engine and rosin core solder is fine, it crushes easily. Lead free solder is very hard and is not a good idea. I measure this distance on every engine I put together. You are looking for about 0.030" to 0.060" in any engine, closer for smaller or low reving engines, more for larger and higher reving engines. I've included this sketch to show what the squish gap does to keep the mixture swirling in the chamber:

Actually it is amazing the close tolerance they have on the squish distance.
Much better than most Japanese and American motorcycle and car engines.

Another important thing is that the gap tapers from tightest near the cylinder wall, most open toward the piston crown. This gap sets up important turbulence in the combustion chamber that will get rid of detonation and speed burn and power.

Steve

 

[Bikeguy Joe]

What fuel mix ratio are you running?

 

[cannonball2]

I revisited the CG engine a bit last year and they have improved since my first experience with them about 10yrs ago when they first appeared.

I found the cylinder head you are running is the least desirable. It has less chamber volume making excessive compression for a stock(or nearly so) engine and ran the hottest. It also has the most potential to blow gaskets. Decking the cylinder and head as you are doing is a must.

The other common slant head with the much higher fins is better. I found the stock center fire head to be the best one for a stock engine and a good starting point for mild modding.

Of the common engines out there the GT5 style is the best out of the box runner in my experience. I still have two CG bikes but rarely ride them anymore.

There is a wealth of info here if you explore the entire site.

http://www.grubeeinc.com/index.html

 

[sbest]

BikeGuyJoe, at the moment I am finishing up my third tank of 20:1. Much too rich. Oil everywhere. Surprisingly it doesn't smoke much but I notice a thick coating of soft carbon sludge on the exhaust port and pipe bore. I may refuel with 32:1 shortly after retesting. I am trying to change only one thing at a time but got in a mess with the head.

About the head, I was forced to sand it flat due to the warp, but this brought the quench gap (squish) too close at 0.015". Properly, to change and test only one thing at a time, I should have shimmed the headgasket to make the gap right. To heck with that. The gap and squish area and angle was a huge mess anyway. I decided to do it right. It will show more effect than just the port matching I originally set out to do because I have raised compression very slightly and increased the quench area and squish effect.

My squish band went from 0.100" to 0.300" and from a hugely wrong angle to a consistent 0.038". It should really taper a bit but I had trouble setting up my machine tool to do that.

This is what the intake manifold originally looked like:

This is the best I could grind it out to provide a smoother transition:

There is some thought that the sharp transition is a good thing on a piston port engine to help stop intake reversion. I will be able to test it with a spare intake I have, but I thought I'd do the grinding while I had it apart.

The base gasket was trimmed to fit the cylinder but it turns out the case transfer ports are much smaller than the cylinder transfer ports anyway. This leaves 2 options, grind the case or fill the cylinder ports with epoxy. Not going to touch it this time.

Anyone notice the intake/exhaust size ratio is backwards? The intake should be much larger than the exhaust. The exhaust looks too large for the size of the engine to me. Thoughts?

Steve

 

[sbest]

I went bike riding. It is about 10c and foggy here. Some very interesting results from the changes. I lost some top end power, quite a bit actually. I was able to run 53-58kph with the stock motor, I now run 50-55kph with all these mods. Wow. Disappointing.

I did gain some midrange, the hillclimbs were better, with the engine running cooler on the hill and no "popcorn popping" sound which I am sure was detonation. Below 30kph the torque fell off.

I refueled, tank was half full, so I put clear gas in with no oil. This should bring me from 20:1 to 40:1. Took 1.3 L to fill to the brim. I ran it for a few miles to clean out the carb and tried it out with the thinner oil mixture. Sounds crisper on the accelerator with no load. Fuel line is still blue but not quite so dark. No noticeable difference in acceleration. There was no smoke with the high quality oil before so none noticed now and no smell at idle. This is Motul brand synthetic oil I believe. I have it in a mixing bottle so I don't remember the exact type. It starts and idles better on the 40:1 mixture.

I was very underwhelmed with the gains. Loss of top speed and a moderate gain on mid-range. It even felt like it lost some low end torque. Hmmm. Time for a change. There is a theory that piston port engines need anti-reversion help. That is often accomplished with long intake tracts or mismatched gasket faces. Maybe those Chinese engineers knew what they were doing with that terrible intake flange:

Just to try it, I put my spare stock intake flange back on. Wow. Big improvement.
Power increased in all ranges, stronger than stock but still lost top end speed. Still only 50-53 kph but hold it better. Engine just doesn't want to rev any more than that. There is a very strong torque peak at 35 kph now. It will power up hills it couldn't stock at that speed. Torque drops off quickly under 30 kph but is still better than stock. Still no sign of detonation even with the increased compression.

Top speed was 55.9 kph and hard to get that last 2 kph.
That is down from the 59.9 achieved stock, but power is noticeably up everywhere but at maximum revs. Seems smoother too, less vibration. but that is subjective. Very enjoyable to ride with the increased torque and no popcorn rattle.

Why the loss of top speed?
Exhaust flange improvement allowing mixture to escape out the pipe?
More squish area limiting rpm somehow?
Both can be tested by swapping a stock head on or a stock exhaust gasket on, some other day.

Steve

 

[2door]

How many total miles on your engine?

Tom

 

[sbest]

How many total miles on your engine?

Tom

I cannot begin to guess Tom. I have put about 9 litres of gas through it so far.

Steve

 

[sbest]

Out for another ride today. Below freezing weather, some icy patches. Saw a max speed of 58kph and hit 67 in the opposite direction. Did my moderate hill without pedaling, could not do the steep hill, it has 4" of snow on it. Big mod this time was merely lowering the float level again, about 1mm. This leaned the engine enough to run crisper. The cold weather tends to lean it as well. Just finishing up 10 litres of gas thru this engine. The engine is strong in the mid-range (30-40 kph), not so much at top end, making for a variable top speed.

Steve