Lexmoto STR125 YB; help anyone

[Sbunny]

Hello everyone. Im new to this site and was wondering if i could get some expert advice on something. I purchased a Lexmoto str125 yb as a none runner. Put fresh fuel in cleaned the spark plug and it fired up. However when i give it revs it dies out. The only way to get it to rev is to flash my light and that is also the only time the main light comes on. It also has what i belive is an engine from a ybr what has been put into it and looks like someone has attempted to wire it in. Probably why electrics look like the spaghetti monster. What can i do to get this runnig. Thanks.
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SteBunny

[NJD]

Flog it to some unsuspecting yout on Facebook marketplace as a project with about as much detail as the buyer requests.

Not helpful, sure, but the wiring on Lexmoto's when new make them look like they were assembled by monkeys at the factory; and that's failing to mention the often poor quality, or at least one electrical failure in its life.

Here's a link to what the engine should look like from standard.

If someone has changed the engine and clearly done a poor job in the process you really do have a Friday special and have to question if its really worth attempting to fix. I mean you may get it running, but weigh up the time and cost versus reward before starting.

Other than that keep in mind not only are you attempting to fix a bike with issues, but attempting to understand what's been changed -- which is important because it could be something that's been swapped over causing an issue.

More helpful suggestions may follow, but I can't help being the boring sensible one.
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The do it all, T̶h̶e̶ ̶b̶r̶o̶k̶e̶n̶ ̶o̶n̶e̶,̶ ̶T̶h̶e̶ ̶b̶i̶g̶ ̶l̶u̶m̶p̶,̶ ̶C̶h̶o̶n̶g̶ ̶N̶o̶o̶d̶l̶e̶

[Sbunny]

[Easy-X]

I'm curious: this sounds like a project where someone bit off more than they could chew, why take it on? The only reason I could see is a) much cheapness and b) you want to learn a lot about bikes in a very short time

If you want to look at this as a learning experience I'd get hold of the "Chinese Bikes Haynes Manual" which will cover most things. Most of these bikes have Chinese copies of classic Jap engines so a "real" ybr engine won't be dramatically different to what it would have had.

How much time/money/patience have you got?
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Sbunny]

[Easy-X]

The fact you can get it to turn over and catch is about 9/10ths of the battle. After that you need the patience part

Beyond idling you'll need to look at:

Am I getting enough / too much air - look at the intake and the exhaust for blockages or leaks.

Fuel too rich or too lean - are you, for example, using a YBR engine with the original Chinese carb? Then you're looking to compare what jetting a YBR might expect versus what you have. i.e. a lot of guesswork.

I'm assuming you have a nice bright spark but is it the original CDI or the one from the YBR engine?

Then there's the issue of timing. So a random engine has been lobbed in, has someone got the cam chain lined up right...

Having fun yet?
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Robby]

Sort out the earths. For a test, run a fairly thick wire from one of the bolts on a crankcase cover to the negative terminal of the battery. See if that solves the problem.

[Sbunny]

[Sbunny]

[Easy-X]

AFAIK there isn't a central repository for engine numbers to model beyond what the DVLA have, good luck chatting with them during lockdown Yamaha would know something if it is indeed one of theirs but again, this is not the best time to be hassling ppl for what they would see as trivial information. However, there are fanboi forums out there for all sorts of bikes so I expect there's a list someone's compiled somewhere. A nice cup of tea an 10 hrs lost on Google Search

BTW, inserting "mate" into every sentence is a bit unnecessary, we're all friends here
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Sbunny]