Help please motor won't start

Joined
Nov 1, 2017
Messages
32
I was on my way to work on my motorized bicycle and the engine just died out of nowhere. I changed the spark plug and put a brand new cdi on and still won't start. Took head off and yeah there's a little carbon buildup but no scoring. Cleaned air filter and checked to make sure there was fuel and there was. It definitely has compression because it is very difficult to pedal and i can hear it. I am not understanding what is going on. I do notice there's a metal thunk when im riding it while the clutch is both engaged and disengaged. But other than that I don't see a reason for it not to start back up. The sound seems like it's coming from the crankcase so I will try to take the motor completely apart tomorrow and the magneto and wires look good as well so I don't believe it's that but I would really appreciate some feedback as to what could be wrong. I haved looked all over forums for about 3 hours now and it seems like I can't find any answers so please help me out. This is my only way to get to work
 


Joined
Nov 1, 2017
Messages
32
@crassius I will see if the plug gets wet when I get home from work. I did check spark with the plug held on top of motor head and there was spark. Thank you very much for your time. I will be able to update on this in about 4 hours
 
Joined
Nov 1, 2017
Messages
32
So the bike starts now and stays running but only if the rpm is high. Will not start up without extra strength starting fluid and the piston is knocking which it wasn't doing before
 

6blueoval9

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Aug 28, 2017
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1,120
Do you notice any oil around the cylinder or the head?

Can you tell us what oil/fuel mixture ratio you are using?...did you use the proper ratio for break in?

A knocking sound unfortunately is not a good thing to hear.

How old is the motor?..what motor is it?
 
Joined
Nov 1, 2017
Messages
32
Yes and i ordered a new head gasket but I'm not sure if that's the only problem. I'm using a 20:1 mix right now and I used 16:1 during break in. The motor is only a couple months old and i got it from californiamotorbikes.com
 

6blueoval9

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Aug 28, 2017
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1,120
A blown gasket itself can make a motor difficult to start as it then has very little compression so that could be your running issue.

As far as a knocking piston as you believe it may be that needs to be looked at and chances are repaired before going any further.

You would need to check for any excessive rocking side to side with the piston and even up and down on it's pin.

It's not uncommon to have upper bearing failure so this may be what has happened.

Wish I could do more but you'll need to check that out first...Maybe even check the rod on the crank for excess movement in any particular direction.
 

6blueoval9

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Aug 28, 2017
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PS....also be sure you aren't running the motor to lean with improper carb tuning.

Running lean can cause alot of possible failures such as with yours now

Lean creates excess heat which can warp a head and leave open a chance to blow a gasket and also not provide enough lubricant causing bearing failure.

A top end rebuild may be needed unfortunately.
 

6blueoval9

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Aug 28, 2017
Messages
1,120
By looking at your plug...Hearing how the motor sounds (a little advanced method)

If your plug is a light grey or ashy looking it's too lean.

If it's black and covered in oil then you were rich and maybe not the cause of failure.

You're looking for a light brown in color.
 
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