Engine Trouble Running great until...

[Mr. Beaver]

yeah got lots of fuel... thats definetely not the problem... might be way to rich or lean, again im completely new to this but i had it running again today... had to replace the rear tire inner tube.... took it out for about an hour, maybe 2 and as i pulled into a gas station... just dead.... ran with a bit of a sputter for those 2 hours but reliable and turning.... all of a sudden just nothing.... right back to where i was pushing the engine with my pedals... no power all the way home... tried playing with the choke... nothing... cut the fuel thinking it was flooded, nothing... i really dont know what to do with her.... pls help.... im at the point im looking at buying a brand new motor, and a nice used bike... invest some money and use this one for spare parts.... i just dont get it... she ran like the breeze for 3 days then all of sudden... issue after issue... i dont want to drop another 500-1000 dollars to get to work and back... but truth be told after the experience i've had im kinda willing (again wife might kill me ) let me know what you think.... also if you guys have ideas... like i said ive changed the carb... changed the spark plug, and magneto... maybe i need new parts? :'(
- new and clueless

 

[Tyler6357]

Sounds like an intermittent problem, those are always the hardest to solve. Remember that these little engines are very simple, to get them to run you need mostly just 3 things, fuel, spark, and compression.

Fuel: The first thing when looking at fuel is the mix you are using. There are lots of opinions on what kind of oil and oil/gas ratio is best but I think that most everyone can agree that the 16x1 mix ratio often times recommended from China is too rich and will cause oil to gunk up your exhaust and cause problems. I would say a mix between 24x1 - 32x1 would be acceptable. I recommend using a good 2 stroke semi-synthetic oil. Check to make sure your tank, petcock and fuel line are clean of oil build up, use a good clean oil filter in the fuel line to the carb. Check for excess oil build up that may be clogging your exhaust pipe. Clean the carb and make sure the float is as level as possible, it's okay if it's not perfect. Take off the air filter and test for any change, sometimes they can get clogged and restrict air flow, the air/fuel mix is critical to getting the tuning correct.

Spark: Since you have replaced the spark plug, I suspect that the spark is not the problem unless perhaps you are using one of those horrible plastic spark plug boots that require you to remove the cap off the spark plug to get them to work. If you have one of those you should go get an automotive quality spark plug boot from the auto store (about $5) and replace the one on your CDI then use a new spark plug with the cap on.

Compression: This is most likely your problem, check for air leaks. Start it up if possible and feel with your hand around the head to see if you can feel any air shooting out from around the head gasket. Check for leaks at the carb connection and the intake connection. You have 2 crank seals one on the mag side and one on the clutch side, take off the mag and clutch covers and see if there is any sign of oil inside. Check for oil leaks at the cylinder base gasket, and along the bottom where the case fits together, Tighten all the case bolts as tight as possible without stripping them!! Head bolt nuts should be torqued to about 15 foot pounds.

 

[Stoneman]

It’s not the kill switch playing up?

 

[Mr. Beaver]

OK, so i have spent maybe 12 hours in the past two days ripping her apart... losing my mind... i took off the head gasket, cleaned it up real nice and re-installed it... same with the carb and the exhaust.... so i can confirm i have absolutely no leaks... (no oil leaking anywhere). i took off the magneto and gave it a little buff with some sandpaper to give it a decent connection, i also checked for spark on the plug (while not looking powerful at all, it does appear to be firing) i took out the plug and ran it for idk like ten minutes with my drill turning the crankshaft (or whatever its called) to ensure it wasnt flooded, replaced the plug and still ive got nothing... cleaned the air filter... nothing.... i noticed that the spark plug appears to be a little short, and is not sparking like it did when i first replaced it a few days ago... still sparking but not as strong... considering replacing CDI and magneto assembly... also considering buying a whole new motor and using this one for parts...
Please advise

 

[Mr. Beaver]

OH and i almost forgot its not the kill switch (disconnected and still had same problem) and there is also a white wire coming from the magneto that goes nowhere.... i assume that is perhaps for an optional light but its possible that maybe is the issue? i noticed most magnetos have a little wire going to one of the mounting screws however mine doesn't

 

[Tyler6357]

The white wire is not used and should be capped. It has been used for a light but I don't recommend doing that, at least without a generator. Most of the new engines don't even have a white wire anymore. Have you checked your electrical stuff with a meter? And just so I'm clear, at this point it won't start at all now, correct? Try reading this thread at motorbicycling.com , it's an old thread but it might give you an idea of what could be wrong.----> https://motorbicycling.com/threads/so-your-ht-bicycle-engine-wont-start-well-lets-take-a-look.35090/

 

[Tyler6357]

Also, if you decide to just buy a new engine you should know that these engines may all look alike but they are not all the same. Some parts are interchangeable but other parts may not be depending on which engine you get. I recommend one with a balanced crank, 47mm piston, 40 stroke.

 

[FurryOnTheInside]

The white wire should be capped off and tucked away where it can't possibly short anything out.

It wouldn't hurt to solder an extra wire from the black wire to a connector you can tuck under the mounting bolt, just in case.

It's never a bad idea to have a spare magneto and CDI anyway, so I would advise you to get these and try them one at a time, just in case.

The plug wire and boot from the kit are super low quality and so cheap to upgrade that it isn't worth not upgrading them.

You'll be able to use them on your next kit anyway if you go that route, so I believe these things are an investment in your MB'ing future, rather than just more expense.

 

[gary55]

Replace the CDI.

 

[crassius]

if you have spark at cranking speed, then I'd check if plug is wet after trying to start it - if wet, then I'd check compression - if good, I'd check that timing is right - if right, I'd loosen muffler a bit