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Discussion Starter · #1 ·
i have a 4 stroke 50cc fosti f50
its less then 2 years old and i am the first rider on it, i got fed up with the power and found a big bore that fitted it even tho there is nothing on the web about this bike.

i took of the old bore and replaced with new one doing all the correct steps that needed too be taken like changing the timing belt ect
after it was all togeter i lightly kicked it over a couple times too get oil in the head, i started the bike and it started backfireing though the air intake, the timing is spot on and the valves are seated on 0.05 0.06.

what do you think i should do
 

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Discussion Starter · #3 ·
backfiring through the intake means the valves are at least partially open during the combustion stroke. double check the timing.
ive checked the valves and they are set correct the bike no longer back fires but the bike is stuck below 3000 rpm and is running really lean
 

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was the carb upgraded with the big bore kit? if not, youre going to be lean as the carb will have twice the displacement (plus change even) to feed. depending on the carb it might as simple a fix as a new set of jets.
 

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Discussion Starter · #5 ·
was the carb upgraded with the big bore kit? if not, youre going to be lean as the carb will have twice the displacement (plus change even) to feed. depending on the carb it might as simple a fix as a new set of jets.
the bike is efi, im going too buy a carb leave all the wires plugged in like fps and stuff zip tie them too the wireing loom and put in the car so the bike will start as the ecu will read that its all "correct"
 

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Backfiring thru the intake is a symptom of lean mixture. You need talk to a dyno shop and see if they can set up what you have . maybe throttle body and injector from another bike would be a good start. Or go talk to a dirt bike shop about swapping to a carb and cdi.
I don't know the answer but that's what I would do
 

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EFI eh? then GT alex is right, a trip to a tuner will be in order. the stock fueling system can only adjust for so much variance and I doubt itll correct for double the displacement.
 
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