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GY 200 engne driving me nuts!

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So here’s the story…

I bought a gy 125 a couple of years ago off a friend’s son. It was only 3 years old but he had trashed it so I got it for only £100. I sorted it out and put it back on the road to use for a bit of green laning.

I fitted the gy 125 (OHC) with a foam performance air filter and noisy pit bike muffler. It ran well when I sorted the jetting but I wanted more power so I fitted a 144cc barrel of a Honda cb150. The Honda engine has the cam chain tensioner at the bottom end so I had to make my own and fit it to the side of the barrel. Once I got this engine sorted it ran very well, it would stand-up wheelie in 1st gear and top out at 70 mph. (I have kept this engine!)

I then came across a good value new gy 200 OHV engine off a reputable dealer. I fitted this engine hoping it would give a bit more torque for off road riding but still give a 70 mph top end. I originally ran it off the pz26 carb which I ran the gy 125 / 144 from. I had to lean it right down to get it to run right due to the increased vacuum. It pulled like a train in the mid-range but would not pull at all at the top end. I decided to fit a bigger carb, the pz30, and here’s where my problems started!!

I cannot get it to pull from ¾ throttle. At 3/4+ throttle In low gear it miss fires and is generally rough. In high gear it’s lifeless. The pz30 carb came with a #104 main jet and I’ve tried a #105, #115, and #95. Nothing seems to improve the top end. It’s totally lifeless with the #95 and miss fires even more on the #115.  There was no improvement on top end going up or down. Still lucky to get 50mph.

For information…

  • The bike idles beautifully.
  • The plug is light tan.
  • I changed the oil at 150 miles and it was clean.
  • I re-set the valves at 150 miles, they were tight. This had no effect on performance.
  • The compression test gives 140 psi.
  • The spark is a weak blue when I crank it over and I’m going to fit an iridium plug when it arrives. I have discounted this as a problem as it idles so well and run well at mid-range.

Does anyone know then standard card size for a GY200?

The standard jetting for the GY200?

Any other ideas…..?????


I know I could refit the old engine but I really want to get this one sorted!!

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Thanks for the info humanbeing, I am coming to the conclusion that it is very likely the timing too - I put in an iridium plug and it did improve things a little (I think the more efficient sparking slightly advances the spark)


I can understand the curves, I just wish I could read the text.


Thanks once again.

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Engine Sorted!


Here's the story.......


After trying everything with fueling and ignition I decided to delve a bit deeper into the engine, this was mostly because I suspected that the compression was too low. The quoted compression for the engine is 9:1, when I had got the engine to bits and properly measured it I found it to be closer to 8.5:1. This was sorted by replacing the 63.5mm flat top piston with a 63.5mm domed piston from an xr185. This cranks up the CR to just over 10:1. Perhaps a little ambitious for an air cooled bike! 


When I got the engine to bits I also found the cam lobe to be well worn on the nose, over 1mm of wear. This was replaced. I also found that the cam followers were binding because of a dodgy thrust washer, again this was fixed.


With the head off I decided it also needed a bit of "optimising" this was done by porting the Inlet and exhaust ports as well as back-cutting the inlet valve (the stem had a terrible over-hang).  For good measure I also machined off the massive valve bosses that stick into the inlet and exhaust ports.


Finally I lightened the flywheel by taking off the starter clutch (loads of weight)


The result...... well the thing certainly goes now - it does 0-70mph in no time at all then revs out. I suspect it will do about 80mph. It will also easily power wheelie!


The downside is that the engine now gets stupidly hot - the next project might be an oil cooler.


In conclusion, these Chinese engines are fine for pottering about and do have potential. You just have to be lucky to get a good one straight away or put some effort in to get them set up.

Edited by YellowBellyFenRider

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