250 Chinese bike knocking under acceleration
 

I bought a Chinese bike in Lima because I couldn't afford a Japanese bike and after 1600 km or so the piston fell to bits and the cylinder cracked. After a new piston and resleeved cyclinder (US $140) he engine sounds like its knocking when under acceleration. I am new to motorbikes but am learning about the mechanical side so any input is appreciated,

It could well be 'pinking' possibly you have a tankful of low quality fuel.

Are you at altitude?, that does not help!!


Engine knocking - Wikipedia, the free encyclopedia

Do you have a cellphone or something you can record the sound on?

Chad, I agree with Dave Ede it does sound like pinking, try richening the mixture a little ( easiest way is ride with the choke 1/4 or 1/3 or even 1/2 on to see if it makes a difference. if it does make a difference for the better then the mixture is too weak and need richening OR the timing need to be retarded by a degree or three.

try it and let us know what the results are!

If you change down a gear and then continue to accelerate at the same rate (but higher revs), and the knocking disappears, then yes, it's pinking.

So better petrol may be needed, richer mixture (but not if you're at high altitude) or retard the ignition a little if that's possible.

But often, pinking is your engine telling you to change down a gear.
Hope that helps.

Hi, Thanks for the replies. I don't think it is fuel, as I have ran a few tanks from different gas stations and it doesn't affect it. I have been using 90 octane fuel mostly and have tried 95 but it doesn't seem much different. Not at altitude anymore and the problem is still there. I have tried the choke method and it didn't affect it. How do I retard the timing? It seems to occur at higher throttle (over 60-70%) rather than being from a higher rpm. I don't think it's a need to change down I am new to motorbikes but have driven a lot of manual (stick shift) cars/trucks. I have no cellphone or anything to record it but thanks.

Hi ChadNZ,

"It seems to occur at higher throttle (over 60-70%) rather than being from a higher rpm."

That's a pretty precise description of pinking.
On a good engine with appropriate fuel it should never occur.
Modern cars (don't know about trucks, but diesel engines don't pink) have mechanical and electronic gizmos to prevent pinking. But you've got a Chinese bike there, resleeved and rebuilt. A bit of new cylinderhead gasket hanging inside the combustion chamber could cause it to pink along with dozens of other possibilities. (Is your spark plug of good quality?)

Most causes of pinking aren't particularly serious or damaging, but the pinking itself can be.

(Don't want to be contentious here, but there's a good chance, if you're under a certain age, you've never heard pinking. Once you have, you'll find it absolutely unmistakable next time)

When you hear it, change down a gear then open the throttle to the same 60-70%. If the sound has gone, then you've found the problem.

Retarding the ignition a tad will fix or reduce it. Maybe it wasn't set correctly after the rebuild.
As far as I know Chinese bikes are copies of older Japanese, so they probably have mechanical contact breakers.
Best thing is to go to a motorcycle shop and ask them to show you how to do it, or explain the problem and ask them to do it. Just 2 or 3 degrees, or they may already know the problem and know exactly how much adjustment is needed.

If, on the other hand, after changing down a gear the noise is the same, or worse, then you've got something else wrong, probably requiring as a first step a look inside the cylinder head and lifting the barrel. Maybe an after effect of the previous rebuild.

One other thing to consider, is there something loose and rattling? After the rebuild, has the exhaust been bolted back properly, and any heat guards that might be bolted onto the exhaust? The silencer?
All the engine bolts secure?
The horn bolted back on properly? All that sort of stuff, that can sound like the engine rattling but often isn't.

Good luck

Serows (XT225/250's) are notorious for this and you have to richen the mixture as they come very lean from the factory. My bike was exactly the same until I adjusted it. Of course there was very detailed info to do this on XT225.com which made it easy. Try to find a similar site for your bike and see if other owners are having the same problem, maybe they have a fix. If not search XT225.com for the "3 turns out mod" but remember that the carb may be different to the one on your bike.

The easiest way to tell if it's running lean is to check the colour of the spark plug. It should be a nice light brown colour but will be white if it's lean.

Thanks for all the help, the adjusting the mixture by a few turns has improved it a lot. The spark plug is looking a healthier brown now. Explaining things to a bike shop is difficult with my beginners spanish...

Somewhere on this site is a translator page for motorcycle parts and service: Spanish, Portuguese, Italian, etc. Can help with confidence as well as get you what you need.