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post #1 of 4 (permalink) Old 05-14-2019, 12:05 AM Thread Starter
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50dx help won't start

Ok so our sons 50sx stopped dead in its tracks the other week. It's got 8 hours on it since a full rebuild!

It has spark
It has fuel
It has compression

I've done the following and it's still won't start......

New plug
New stator
Rebuild carby
New genuine cranks seals and o ring

For the life of me I can not figure this thing out! There should be no reason why it won't start.

I'm at the point I have no more ideas and nothing else to try!

And to top it off I've got the young fella beside himself cause he hasn't been on him bike for nearly a month.

If any one has any ideas or things to try it appreciate the help so much

Thanks ayrton
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post #2 of 4 (permalink) Old 05-14-2019, 06:04 AM
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Feeling your pain

Our friends @Axzon and @cors187 are our resident mini weedwacker experts, but I would offer some generic tips for this no start mess. Your troubleshooting is process of elimination and begins at most likely trouble until trouble is resolved. Rule #1 never trust a spark plug! Same with plug resistor cap! They can fire outside the motor and wonít fire under compression inside motor. Same for other ignition components. Donít feel bad about the cost of substituting ignition parts for testing. Parts cost is cheaper than repair shop labor and you may need them later or lend them to another poor soul trying to fix his own no start troubleCheck for mis-timed rotor from damaged key/keyway. Drain fuel system completely and quarantine fuel until problem is resolved. Test starting with spray ether when cranking. Verify compression and leakdown by proper test and not feel. Some guys think resistance to kicker indicates proper compression. Test properly and verify. Donít be afraid to try a module. It is a fairly common failure. Good luck. Check back as needed

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post #3 of 4 (permalink) Old 05-14-2019, 06:37 AM
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Turning fuel off and spraying easy start can make you find your steps again in the process of elimination.

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post #4 of 4 (permalink) Old 05-14-2019, 08:00 PM
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Post I feel your pain!

The kids trust us to 100%. We are the ones who can fix everything... and then when it does not go to plan that trust is challenged. Makes us want to do whatever it takes for the little one to get back out and for us to prove that their trust was not misplaced!

So to your bike. You already have some sound advice above. And I will parrot some of it here while trying to find the issue.

First a few questions:

What year is the 50sx?

When you say "rebuilt carby" what does that include? Was it a "clean of all bits" or have you replaced bits? and if so what ones. And if you have replaced bits, have you replaced them with exactley the same bits?

Is the new stator brand new OEM or a s/h one that "did run well" or a after market one?

Did you replace crank seals AFTER the issue happened or at a resent rebuild before the issue?

Have you had the cylinder/head off recsently? If so, did you fit new O rings (head gasket O - rings)?


As the bike was running well and suddenly stopped and will not run again I rule out "faulty" fuel. (But I do think Augie has a point re the fuel. If you provide new fuel and store the old stuff until later, you will know that it is fresh and OK!)

These small ones are very sensitive to leakage in cylinder: head gasket leaking, a scratch in cylinder, damaged piston or ring. Have you checked all these?

How are your reed valves looking? No rips or fraying on the edges? And they are closing well??

When you turn on the fuel tap with the hose to the carbi off, is the fuel happily flowing? (let it flow for a short while in to a container to confirm).

If all the things above are well we need to look at the ignition.

Disconnect the kill switch. (They can give issues)

Fit a brand new spark plug and test that you have nice blue spark.
I assume that you have fitted the new stator exactly in same position as the old one?

If the fuel and carbi + reeds are all OK, if the cylinder has no marks, the piston is fine and so is the ring, the head gasket is in its groove and head is torqued down properly to specs, if the stator is in right spot and plug has a nice blue spark the bike should run... as long as it gets fuel/air and can blow the fumes out.

Have you tried to take plug out and spray some "Start-Ya-Bastard" in to the cylinder and then refitting the plug and tried to start the bike?

Crank seals can give some strange problems. But as long as they are there, even if in a bad state, the motor should be able to get the fuel/air mix up on to the piston where it can get ignited, so even if poor crank seals are a slight possibility, it is down the list.

The option is to find a friendly dad to another small racer with same bike and see if you can test your carbi and stator + coil and plug lead on his bike. Might cost you a doz of beer, but worth it for your peace of mind.

Once you have gone through all this and if the bike still refuses to run, come back and we dig deeper.

EDIT: Augie has a good point re the key-way on the shaft for the rotor. Make sure the key is in place and not damaged and rotor properly fitted!

.................................................LET THE BRIDGES I BURN LIGHT THE WAY!

Last edited by Axzon; 05-15-2019 at 07:17 PM.
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