Motorized bike starts but then dies. when trying to start engine, makes popping noises which is followed by short burst of acceleration

motorizedAUS

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I've owned an 80cc motorized bike for 2 months now. Out of the blue i get on my bike like usual after a day of not riding it and i drop the clutch, starts like normal, i go to twist the throttle and all i can hear is a hum sound of the engine turning over (the engine turns over just fine) as i twist the throttle however no power or acceleration. (when i twist the throttle, the hum sound increases for a second but it doesn't accelerate) I did this a few times and i was getting to the point where the engine would have these quick bursts of acceleration (a loud popping noise would go followed by the quick acceleration burst) but it would never actually take off.

it sounded like it really wanted to start and was on the verge of but just couldn't get there. i have spark as i tested it, my cables are ordered correctly and my clutch is fine. everything seems fine, im getting fuel, carb isn't flooded as ive taken it apart, cleaned it etc. ive cleaned out the chamber where the piston is. im very confused as everything seems fine. my mate came over and i put his spark plug on my bike and it worked perfectly. my bike ran like normal, so i thought it was spark related however he put my spark plug on his bike and his bike took off perfectly as well confusing both of us. everything was all good until i left the bike for a few hours and went to go ride it again later but same problem.

i don't know if its the engine miss firing but now when i try with the different spark plug in im getting more of those pop pop pop noises (its sounds like the noise it would normally make if it was running fine) and as these popping noises go off i feel a little bit of acceleration everytime it pops, but when its not making the popping noise it doesn't accelerate. im very confused at the moment and am just wanting to ride again. i keep my bike outside, under shelter obviously so no rain gets on it, and currently its cold where i am so idk if that has anything to do with it.

i have tried playing around with the carb, specifically the choke, trying to cold start it using choke and what not but nothing seems to work. what confuses me a lot is that it happened out of no where, it has been running flawlessly for months and randomly it does this, i didn't crash it or anything like that. keep in mind ive well gone past the break in period and have been running a 50:1 fuel ratio constantly. i dont know what running rich fuel and whatnot means so ive kinda just stuck to that ratio. i fill up a jerry can of 5 Litres of fuel and pour in 100 ML of 2 stroke engine oil into the 5 L making one big mix. could it be fuel ratio related.

ive gone through all kinds of forums and tried everything. please help!
 

motorizedAUS

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hey Gary,
not 100% sure what this means or how to do a compression test. what does warped head and blown gasket mean.
 

extremeodd

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This is exactly what I had happen when my CDI went out, including it running on a different plug once then not running after trying again an hour later. Getting it to spark when the plug is out of the head doesn't mean that it's any good, if the spark is weak it cannot spark in the compressed fuel air mix. I tried literally every single thing before replacing the cdi, the moment I plugged the new one in, it ran like a brand new bike.

I've noticed the things that can cause them to fail include but aren't limited to (and aren't guaranteed to cause damage, this is my first-hand experience): Pulling the plug wire to kill a running motor, tuning over the engine fast enough to trigger a spark with the plug wire disconnected or otherwise not grounded to the motor (like the plug is out of the motor to test for spark and its not being toughed to the head the entire time and it fires once or more without being grounded), or anything that will cause the CDI to fire without the circuit being 100% complete. I've had 2 CDIs fail due to these reasons, they would appear to work when I would check the plug for spark, sometimes the bike would fire up if I was pedaling the ever loving s**t out of it, but otherwise were completely dead for all intents and purposes. Both times when swapping the CDI for a new one, the bike would fire up on the very first try and run perfectly.

They are $7 and can randomly flake out despite typically being quite reliable. I'd order a couple to keep on hand.

The feeling of acceleration when it goes "pop pop" is when it manages to generate a spark strong enough to ignite the mixture, otherwise the engine is just cycling without ignition. If you open and close the throttle during this you will feel the resistance of the engine increase and decrease.

For s**ts and giggles, borrow your buddies CDI and see if that makes it run.

Otherwise, this could be your head gasket. Take a flash light and look at the underside of your head where it meets the engine. Look for any evidence of oil getting pushed out. It'll kinda look like this:
90307

90308
 
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motorizedAUS

New Member
Local time
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Sep 7, 2019
Messages
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Thanks to both Gary and Extrememod,
i will try my friends CDI but what im hearing is this is mostly spark related? should i just buy a brand new spark plug? I have checked my gasket head and no leaks whatsoever, i used the ruler to see if it was bent also and its perfectly fine. I also tightened the 4 bolts on top of the gasket head to make sure nothing was loose. I have another new cdi in my garage so i will give that a shot at some point and see if it works. i will let you guys know of the result with trying new cdi with current spark plug, and then trying new cdi with new spark plug.
 

motorizedAUS

New Member
Local time
3:46 PM
Joined
Sep 7, 2019
Messages
7
This is exactly what I had happen when my CDI went out, including it running on a different plug once then not running after trying again an hour later. Getting it to spark when the plug is out of the head doesn't mean that it's any good, if the spark is weak it cannot spark in the compressed fuel air mix. I tried literally every single thing before replacing the cdi, the moment I plugged the new one in, it ran like a brand new bike.

I've noticed the things that can cause them to fail include but aren't limited to (and aren't guaranteed to cause damage, this is my first-hand experience): Pulling the plug wire to kill a running motor, tuning over the engine fast enough to trigger a spark with the plug wire disconnected or otherwise not grounded to the motor (like the plug is out of the motor to test for spark and its not being toughed to the head the entire time and it fires once or more without being grounded), or anything that will cause the CDI to fire without the circuit being 100% complete. I've had 2 CDIs fail due to these reasons, they would appear to work when I would check the plug for spark, sometimes the bike would fire up if I was pedaling the ever loving s**t out of it, but otherwise were completely dead for all intents and purposes. Both times when swapping the CDI for a new one, the bike would fire up on the very first try and run perfectly.

They are $7 and can randomly flake out despite typically being quite reliable. I'd order a couple to keep on hand.

The feeling of acceleration when it goes "pop pop" is when it manages to generate a spark strong enough to ignite the mixture, otherwise the engine is just cycling without ignition. If you open and close the throttle during this you will feel the resistance of the engine increase and decrease.

For sh*ts and giggles, borrow your buddies CDI and see if that makes it run.

Otherwise, this could be your head gasket. Take a flash light and look at the underside of your head where it meets the engine. Look for any evidence of oil getting pushed out. It'll kinda look like this:
View attachment 90307
View attachment 90308
Also extrememod i love the part where you say you need to pedal the ever loving sh*t outta it, because thats exactly what i have to do to get anything out of it. i be pedalling so fast at the end i feel sick to the stomach.
 
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