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2006 Virago 250
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I have a 2006 Yamaha Virago 250 with 18,000 miles on it.

I think I might be having issues with preignition, although I'm not 100% sure.

It seems to be temperature dependent. On warm days under a moderate/high load, there's be an even tapping sound from the engine. There were specks on the porcelain. Although I switched to a colder plug and there's no specks so far, though the noise continues.

I've checked for intake leaks by spraying around with fluids, propane, and checking for incense smoke being sucked up. I haven't found anything.

The foam air filter has been maintained.

This occurred with two different carburetors; first a Chinese aftermarket carburetor, then an OEM carburetor. I cleaned it before installing it (although it was very clean in the first place), and make sure the slide was acting normal before installing it. The fuel level in the bowl seems fine. I've also run a few tanks with seafoam.

Just tried increasing the main jet above spec from 110 to 115, no difference.

The timing isn't adjustable on this bike.
 

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I think the valves need to be adjusted.

Get a can of Lucas, Seafoam or my favorite, Chevron Techron and run half a can in the gas to clean the combustion chambers, spark plugs, valves and fuel system. Use 87 octane fuel only.

Good luck.

Sam:grin:
 

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If switching to a higher octane fuel reduces the noise, it is almost certainly preignition, and the next step is to find out why. At 18k miles, it shouldn't be carbon built up in the chamber; shining a light through the spark plug hole and slowly turning the engine should allow you to see if the valve heads or pistons have carbon on them. It could also be a tiny leak at the headers, which sounds similar to ping; you can use a length of fuel line, one end in an ear, and probe around the gasket area.

I had a similar tapping noise in my old 450, and octane didn't change it; it sometimes could be heard at idle when the engine was really hot. It turned out it was the spark plugs. I had installed plugs with a P in the number, which means the tip protrudes farther, and the intake valve was just lightly tapping on it; not enough to cause real damage, thankfully. New plugs, and no tapping sounds.
 
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