Engine Cannot stay running, easy to turn engine with clutch in

I don't think that would be a good idea, extra head gaskets will give head clearance but won't help on the port timing. The top of the piston needs to travel to the bottom of the stroke to run corectly.
 
Well, finally got the replacement piston and necessary parts to rebuild the top end. Made a mistake of overtorquing the head bolts and eventually lead to stripped studs after the cylinder head exploded, launching acorn nuts and washer alike into the air. Upon closer inspection, I notice both the thread in the stainless steel acorn nuts that I bought to replace the original ones and the threads on the studs are destroyed. I gotta say, it's been frustrating but at the same time a great learning experience. After searching around the forums, I learned the proper torque specs for the head and to torque them only when the engine is cold.

I was able to get a new china girl engine kit and installed the new engine on my bike. I decided to use the old engine as a specimen for dissection.

With the new engine, I am experiencing greatly reduced performance, even though everything is fine. No leaks, all bolts properly tightened. I even used the carb I previously tuned for the older engine and a brand new spark plug of the same type as my old one. But it just isn't as powerful as the old one. On the old engine, I can get up to 34Mph on flats without a problem. This new engine barely reaches 26mph on flats, and struggles greatly going up hills that the previous engine had no problems with. Seeing how it's the same "80cc" Raw engine I had before, what could be sapping the power from the new engine? The clutch is adjusted correctly with no slippage. There are no signs of airleaks.
 
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I feel your pain. That exact same thing happened to me. I'm homeless and I do not have the money for repairs at the moments. I used the bike to get back and forth from my job at taco bell. Sucks but it's apart of life.

I use the internet at work. Heh.
 
I just about burned through 2 gals of fuel over the past couple of weekends and the engine is still as low-power as when I got it. I assume the power increase is gradual? I have not noticed any improvements in its power, as it pulls up the same hill everytime at 14mph max (old engine did 18mph up the same hill). Did another close-up exam on the engine and found no leaks near the exhaust, upper and lower cylinder gaskets, spark plug, or intake manifold. I tried three different carbs and they all provided roughly the same power. One of the carbs even had a different jet (.72) than the stock carb and it still performed low power. I top out at 25mph on flats (old engine topped out at 34mph flats). I somehow doubt adjusting the clutch will improve the power, as the clutch works perfectly fine, with no slipping when engaged. Can a weak magneto or problematic CDI cause weak power? Then again, the bike idles perfectly fine. At this point, I am not sure why it is still weak, given everything else on the bike is exactly the same as when I had the previous "80cc" engine.
 
Well.... as the quality control is basically non existent in these engines, it could have lower compression than your older engine, it could have poorly finished cylinder ports, it even could be the muffler.
The power of the ignition spark will not really affect the power of these engine.
Now if you are into drag racing, then it does matter a little bit.
 
If the intake manifold gasket opening is smaller than the intake port, will that cause reduced power? I been reading up on the port and polish threads and learned that port matching and port polishing mainly makes the engine run smoother and not necessarily make it noticeably faster. In my case, the engine runs fine, just in a weaker state. What can be causing lower compression, if it is not a leak? My head bolts are only torqued to 55 in-lb, which is lower than the 100-120 in-lb of torque recommended here. But the top and bottom cylinder gaskets don't leak at all after long rides, so I rather err on the side of not stripping the bolts or studs, like I have done with my previous engine.
 
Any gasket or poorly manufactured engine part which protrudes into the flow of gasses inside the engine is a restriction of flow and will definitely reduce the overall power output.
Port matching and polishing when done correctly will certainly make the engine more powerful as it increases the efficiency of the engine. Less internal resistance to flow equals more flow.
A very crude explanation:
A gasoline (or diesel) engine is basically an air pump only the "air" is highly flammable.
When the "air" gets introduced to a source of ignition at the correct time and conditions it burns producing heat which in turn is used to push the piston down producing power.
If you want to learn more, wiki it.
If you make the engine's intake, transfer, and exhaust internals flow better it makes more power. Smoother operation is a side effect that can result too.
I am not suggesting that you try to modify the ports with a port job unless you know what you are doing. Porting done wrong is counterproductive.

Basically the compression ratio is determined by the swept volume of the piston and the volume of the combustion chamber in the head.
For example: If the swept volume is ten times larger than the combustion chamber you will have a 10:1 compression ratio.

Other things can affect it such as piston shape and construction, cylinder head gasket thickness, base gasket thickness, ect.

Ignition timing will also have an effect on overall power output.

There are sooo many variables in how these engines are made, and quality control is not very good to say the best.
It is no surprise that 10 engines made in the same factory on the same day one right after the other, will run totally differently from each other.
 
After reading through what you have posted, have you cleaned up the intake? I like to remove all of the welding slag and smooth it as much as I can, I also cut the gasket out to the size of the flange after deburing it.
 
Greg58 said:
After reading through what you have posted, have you cleaned up the intake? I like to remove all of the welding slag and smooth it as much as I can, I also cut the gasket out to the size of the flange after deburing it.
Click to expand...


Just did that today. I removed as much as I can without busting through the flange. It is larger than what it was before and the gasket is also matched as much as I can to the cylinder intake port without making it too thin in certain areas. No appreciable increase in power or speed. It was wishful thinking that I would have, out of luck, a faster, good engine out of the box. I'll keep researching on this forum to learn more.

P.S. Why is the long exhaust called a "poo-poo" pipe?
 
Because the exhaust note sounds like "poo-poo" instead of "ding-ding"? :D

I have never heard of an explanation for that name other than that's just what they decided to call it.
 
Can a CDI contribute to a weaker engine? Or is it all or nothing type of thing? Like if it works, then it works at full power, or if it's broken it doesn't work at all? Or can there be variable power. Would he magneto be all or nothing as well? Or can it have different power output, leading to a weaker spark? Is there a be a big difference between using a slant or straight cylinder head? The engine originally came with a slant head, but I swapped it out for a new straight head after reading about issues with uneven explosions with the slant heads. Now it seems my engine will go faster, up to 32 mph if there is a slight decline, but any incline, then it tops around 23mph or slower, depending on how steep the incline is.
 
No 2 CDI's are the same even if they are made for the same engine. Every component inside them has acceptable tolerances to qualify for use. All the parts start out in bins at the assembler's soldering station. Sometimes the randomly chosen parts work in perfect harmony and you get a very, very good CDI. Other times it is the opposite is true. The same goes for the Mag. They also can have internal failures which can hamper their output yet still operate the spark plug.

The straight plug heads tend to have a smaller combustion chamber which raises the compression ratio, compared to a slant head.
Higher compression = more power.
Any uneven explosions that the slant heads may have you would not really notice unless you are drag racing the bike against the clock.
 
I was riding today and all of a sudden I hear a slight pop noise and the bike suddenly got louder, as if a gasket blew. The engine immediately loss power and could not idle anymore. I looked at all the gaskets and made sure none of them were leaking. The spark plug is still tightly sealed and there are no leaks at any of the gaskets. I tried a new exhaust, thinking the sudden increase in engine noise is due to a faulty exhaust. Turns out nothing changed even with the new exhaust. I can go up a specific hill at 17mph prior this incident, but afterwards, I can only go up the same hill at 12mph. Something happened that resulted in a loss of power, even though I can still reach max speeds around 32mph on flats. The engine also plateaus around 26mph for ~5 sec before continuing to speed up to 32mph.

I also notice sometimes after a higher speed ride of 85-100% throttle for a few minutes results in the engine idling really low RPMs, to a point that the engine dies. But the bike starts back up immediately without problems and I can continue riding.

I am not sure what is causing these issues and if they are related to each other.
 
Possibly, but not necessarily. If you don't have a compression tester, you can spray carb cleaner around the head and listen for it to start revving. Also - consider blown seals.
 
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