Yamaha ef2800 backfires and will not start

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Check out the link. It it backfires through the exhaust the timing is retarded and firing too late.

https://www.google.com/search?q=yamaha+yz80+set+ignition+timing

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Ya got a sticky intake valve

if it is backfirering sound like timing is out of spects

take out the spark plug and clean it or replace then check the gap

sounds like you have spun the timing. check your electronic ignition seems that it has gone bad or the relay went bad and it is not advancing the timing

is it a straight thru exhaust, or is it just a head pipe with nothing attached? If it's just a head pipe, you need to get some sort of muffler on it or you'll burn your valves out from a lean condition. The backfire is due to the vaccum fed carbs that yamaha runs. They work wonderfully, but under heavy deceleration, the vaccum still feeds the bowl, and it builds up excess fuel, which eventually burns off as a backfire. If it were fuel injected, you could have the fuel map adjusted to lean it out under deceleration.

ya u got a exhust leak