Zongshen 155z Running but wont rev properly and backfires

[grech_nathan]

Hi there i really need help with my pit bike I've done a rebuild on it (done the piston and rings, timing chain, valve clearances) also cleaned out the carby, it was running before i did this and i also did rebuild it one before.. It does start but it is a bit hard to get running and everytime i kick it it fires and kicks back on me but when i do get it running it kinda idles fine but when i rev it its really lumpy and i only can rev half throttle or it just kills itself but sometimes when i rev it, it does rev properly for like a second and then it back fires either through the exhaust or out the carby, I have also checked the hole loom and everything is fine i even tried a different CDI what it didn't change anything and the magneto looks fine.

thanks

 

[my67xr]

Welcome to Minirider's.
First up drop in here and Introduce Yourself.


Have you rechecked the timing mark's both line up ?
And what have you set the valve/rocker clearance's to ?

 

[mikkh]

Had the exact same problem. Using different electrics system cured it.

 

[grech_nathan]

Yeah the timing marks are perfect I even had a shop look at it cause he was doing the timing chain cause it was stretched and after he did that it still was running like **** I've rebuilt a lot of pit bikes before an I've never had this problem.. If it was the electrics would it be the magneto cause I've tried a different cdi


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[kosta]

Check the spring loaded pin under the 10mm bolt on top of the head isn't broken. Had this happen to me once and that was the problem. The pin holds the decomp in position once the engine is started. If the pin is broken, the decomp keeps spinning and the engine runs like crap, if at all.

 

[grech_nathan]

Check the spring loaded pin under the 10mm bolt on top of the head isn't broken. Had this happen to me once and that was the problem. The pin holds the decomp in position once the engine is started. If the pin is broken, the decomp keeps spinning and the engine runs like crap, if at all.

Yeah I did check that and in my motor it's a 8mm bolt but I'm not too sure what they do look like when they are broken it didn't look like it was


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[grech_nathan]

I'm just so confused why it doesn't run cause before the rebuild it was running perfect and now it doesn't... How it runs, it's hard to start and it kicks back every time I kick it then when it does start it idles with a bit of a lumpy ness to it but when I I rev it it becomes really lumpy and when I rev it too half way it usually wants to stall and randomly some times when Im revving it it suddenly revs normally for a second and then makes a huge back fire then goes back to being really lump... I can even ride it haha it doesn't even want to go forward cause it's bogging down so much


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[gottagetacheaperhobby]

Sounds similar to when I did my timing 180 out on the gpx, very similar symptoms. Are you 100 percent sure it is timed corretly?

 

[grech_nathan]

Sounds similar to when I did my timing 180 out on the gpx, very similar symptoms. Are you 100 percent sure it is timed corretly?

Yeah I'm pretty sure the timing is right the shop checked it and it was all good I even did and I also tried advancing and retarding the timing and it still ran the same but a bit worse... I will have another look at it today


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[gottagetacheaperhobby]

When you re assembled it, did you make sure the cam was set to open the exhaust valve next stroke? And when you lined up the timing marks did you make sure the pick up on the flywheel had just moved past the stator pick up?

 

[gottagetacheaperhobby]

I would also be running through the electricals, triple check all connectors, I actually did away with most of the connectors in my loom and soldered it all together. Try using a different coil, spark plug etc

 

[my67xr]

Could be a pilot circuit blockage in the carby too, do you have another pilot jet you can fit to test it ?
Off idle if you blip the throttle does it want to stall out then ?

 

[grech_nathan]

I just did the timing again and it's still running the same and I cleaned out the carby and it's spotless and put new carby gaskets on I kinda think it's something to do with electrics but I did try a different CDI and it didn't change anything


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[my67xr]

Off idle, if you blip the throttle does it want to stall out ?

 

[grech_nathan]

No it doesn't it still revs but it's really lumpy and it randomly does rev properly then back fires


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[my67xr]

Start the bike and let it idle,
try spraying all the joins on the carby to manifold, manifold to head etc with degreaser or wd40 etc, and see if it runs any different at all while you spraying.
If it does then there is a vacuum leak, that's cause similar problem's too


Have you checked you exhaust gasket is ok, and not crushed up ?
Try nipping the bolt's up a little to see if it was leaking.
If it is then that could cause the backfiring.

Have you tried a new/decent spark plug like an NGK C7HSA ?


Can you post up a video of it running, we might be able to help you better ?

 

[kosta]

Yeah I did check that and in my motor it's a 8mm bolt but I'm not too sure what they do look like when they are broken it didn't look like it was


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I really think it's the pin. Has anyone got a pic of what it should look like?
Or maybe post a pic of what yours looks like.

 

[kosta]

I really think it's the pin. Has anyone got a pic of what it should look like?
Or maybe post a pic of what yours looks like.

So this is what it should look like.

 

[grech_nathan]

Yeah that's what it looks like on my bike and the spring is fine but I will take a video of it in a few minutes


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[grech_nathan]

https://vimeo.com/136177867 that's a video of how it back fires when it starts and how it runs but some times when it is running like that it does rev properly and backfires and god back to lumpy..... Just put a new spark plug in it and it didn't change anything


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