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Re: flat spot just off idle
It has always been my experience, no matter if it's a car, bike, lawn mower, etc, if it backfires, it's the timing. If it's a constant poping thru the induction system, it's something to do with the valve train. Flat cam. lifter out of adjustment, bent push rod, bad lifter, etc. I think you get the picture. Mentiioning that you have a "Flat Spot also points to timing. Since it's been like this since new, I would look at the possiblilty that the timing marks on the cam and crank were not properly aligned when the unit was built. It's a long shot but work looking into.
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You seem to be describing a flat spot - a hesitation during initial acceleration that is commonly caused by either insufficient spark power or the fuel system being slow to adjust to the new requirements.
A backfire is an explosion in the exhaust system and I wonder if you really meant a spit-back (through the intake), which is common with a fuelling caused flat spot?
Replacing the spark plugs is merely half the job and testing the spark power is the other half...
I suggest you check the fuel pressures and pressure regulator function and the injector spray patterns. Only if all tests good it could be worth testing the oxygen sensor...
These will backfire when you let off the throttle if the idle is set low. When set low enough to sound "right" you will get the backfire. Set the idle up a bit and you will lose the backfire but it will sound like it is idling a bit fast.
Your posted symptoms is as you described, running lean which can be verified by the colors of the plugs, brown, light brown to almost gray / white. While the jets and the exhaust could be worked on, the simplest try of a fix is to restrict the air intake; the lower the RPM of the flat spot, the more air restrictions should be added. Experiment by temporarily placing upholstery foam on the outside portion of the air filter. You may have to try different thickness / porosity.
I am not sure what you mean on the "running fast" statement. If idle speed then turn the idle screw outward to slow down the idle.
The backfire is caused by turning off the ignition key when the bike is at high revs. let it idle down THEN turn it off. The backfire is there because the carb idle circuit is active anytime the piston is in motion. Gas goes into the cylinder as the bike revs down then a hot spot in the exhaust pipe finally ignites the built up gas. There is your backfire.
Is the running fast question answered? Give me more information on the problem if it isn't.
Initially, pls confirm that it uses a two-stroke 50CC engine. Additionally, your description "It refuses to rev beyond idle speeds." could indicate several conditions:
throttle up above idle speed, engine stalls/dies - fuel related issue, fuel starvation, clogged jets, nozzles, too much 2T oil, contaminated fuel (water). Basically a carb rebuilt;
throttle up above idle speed, no change in engine RPM - mechanical problem, possible broken accelerator/throttle cable, stuck plunger in carb, detached needle valve;
throttle up above idle speed, engine backfires/dies - electrical issue, check CDI, stators, trigger/pulse, their physical state/mounting, electrical connections;
throttle up above idle speed, engine hesitates/flat spot - air/vacuum issue, check rubber coupling from carb to intake manifold, blocked or absent air filter.
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Yes the backfire is an indication of being lean. Have you checked the valves lately? May be a sign of tight valves although that often cause a rough idle as well. Have you modified the filter system or added a tuned exhaust? Any changes like that can and will cause a change in carburation in which case you may need to fatten up the mid range jetting.