Hey guys, there are a lot of posts regarding this topic that are spread over a number of threads so I thought I would consolidate them into one post to help someone else who may have the same problem I recently had.
The problem I had on my brand new Atomik 250 (Loncin engine, mukuni carb) was that the engine wouldn't start without heaps of priming and when it did eventually kick over, it would run rough, coughing and backfiring until it cut out. The following is what I did to remedy the situation:
1. Changed the oil - replaced the Chinese glue it came with to a premium 10w-40 synthetic oil.
2. Changed the sparkplug - although my Atomik came with an NGK sparkplug, I have doubts whether it was a genuine product. When I bought a replacement NGK, I compared the two and the difference in quality was pretty apparent - the replacement also had the words "NGK Japan" engraved in it whereas the original only had "NGK".
3. Cleaned the Carby - took the air filter off, cleaned out the carbie, and double checked to see which way the choke lever worked. On mine (and it seems on most Chinese bikes) the choke is applied by lifting up on the lever, which is the opposite that you would think.
After doing the changes above, and leaving the air filter off, I sprayed some "start ya bastard" into the air intake and the motor started up no dramas, but I was getting a backfire through the carby every 10 seconds or so. This was enough to stall the motor at idle but the engine could recover if the revs were high when the backfire occurred.
After doing a bit of research I figured it was either an issue involving the intake valve clearance or a air/fuel mixture problem. I figured I should start with the air/fuel mixture in order to rule it out before stuffing around with the valves.
What I noticed was that the engine would backfire most often with the choke off. With the choke half applied, the backfiring would occur less often, and with the choke fully applied the backfiring never occurred. This indicated to me that it was an issue with the air/fuel mixture. If the engine did not backfire when the choke was applied, this obviously meant that the engine was getting too much air and not enough fuel when the choke was off.
There was two possible causes of this problem - either the air screw required adjustment to correct the mixture, or there was an air leak somewhere in the carby. I grabbed my can of 'start ya bastard' and sprayed around the seams and joins of the carby while the engine was running. I found that the revs increased when I sprayed around the manifold joint, which must have been leaning out the mixture and causing the backfire.
I applied some liquid gasket sealer between the parts (which sealed the joint) and the problem appears to have been resolved. Total cost of the repair was around $20 (sparkplug, gasket, carby spray and oil).
Hopefully this will help someone out in the future.
The problem I had on my brand new Atomik 250 (Loncin engine, mukuni carb) was that the engine wouldn't start without heaps of priming and when it did eventually kick over, it would run rough, coughing and backfiring until it cut out. The following is what I did to remedy the situation:
1. Changed the oil - replaced the Chinese glue it came with to a premium 10w-40 synthetic oil.
2. Changed the sparkplug - although my Atomik came with an NGK sparkplug, I have doubts whether it was a genuine product. When I bought a replacement NGK, I compared the two and the difference in quality was pretty apparent - the replacement also had the words "NGK Japan" engraved in it whereas the original only had "NGK".
3. Cleaned the Carby - took the air filter off, cleaned out the carbie, and double checked to see which way the choke lever worked. On mine (and it seems on most Chinese bikes) the choke is applied by lifting up on the lever, which is the opposite that you would think.
After doing the changes above, and leaving the air filter off, I sprayed some "start ya bastard" into the air intake and the motor started up no dramas, but I was getting a backfire through the carby every 10 seconds or so. This was enough to stall the motor at idle but the engine could recover if the revs were high when the backfire occurred.
After doing a bit of research I figured it was either an issue involving the intake valve clearance or a air/fuel mixture problem. I figured I should start with the air/fuel mixture in order to rule it out before stuffing around with the valves.
What I noticed was that the engine would backfire most often with the choke off. With the choke half applied, the backfiring would occur less often, and with the choke fully applied the backfiring never occurred. This indicated to me that it was an issue with the air/fuel mixture. If the engine did not backfire when the choke was applied, this obviously meant that the engine was getting too much air and not enough fuel when the choke was off.
There was two possible causes of this problem - either the air screw required adjustment to correct the mixture, or there was an air leak somewhere in the carby. I grabbed my can of 'start ya bastard' and sprayed around the seams and joins of the carby while the engine was running. I found that the revs increased when I sprayed around the manifold joint, which must have been leaning out the mixture and causing the backfire.
I applied some liquid gasket sealer between the parts (which sealed the joint) and the problem appears to have been resolved. Total cost of the repair was around $20 (sparkplug, gasket, carby spray and oil).
Hopefully this will help someone out in the future.
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