WOT 5th gear detonation

Hey all I've got a '97 Yamaha wr250 I just bought this year. As I progress and get better I have noticed it sounds like a handful of hardware was thrown in my engine. It only happens in 5th gear wide open throttle at higher rpm. If I back off the throttle it goes away. I can only assume this is detonation. I've done everything I can think of (and searched the forums) with no avail. I ride mostly tight trails with some wide open sections. Any thoughts or ideas would be greatly appreciated.


93 octane (with 10% ethanol) can't get it without

BR8G plug (stock)

Stock bore with weisco piston (freshly done less than 30 hrs ago)

new crank seals

11oz steahly FWW

timing 2 degrees retarded

mikuni carb jetted cleanly (no spooge, plug looks perfect)

maxima super M at 32:1

new reeds (boyseen power reeds on stock cage)

gnarly pipe with stock silencer

new chain and sprockets

new clutch


Thinking about jumping up a few sizes on the main jet to see if that helps, maybe due to temperature change?

try some 110 race gas if you can get it

I don't know if I can race gas or not. I'll have to shop around locally. Thanks for the idea.

You could add another base gasket or go to Athena and have them make you a thicker one. Also try retarding the timing abit more and see if it helps.

Will multiple base gaskets cause a leak? The one I installed with the fresh top end did seem thinner than the old one. It is a single layer steel, and the old one was regular thick fiber gasket. How far can I safely retard the timing?

Anybody else have similar symptoms or fixes?

That sounds a lot like detonation. Check your squish clearance first. It effects just about everything. You want the right squish, which for a 250 is I believe around 1.3mm. Maybe your WR250 is way out. Some YZ250s are apparently as wide as 1.9mm when new.  Once corrected, if it still pings, then get the head volume increased or use higher octane fuel.


No need to take the head off to check squish gap.  Get some 3mm soft core solder, pull the spark plug out, get yourself some right hand dexterity and put a piece of solder on both sides of the cylinder/crown as you gently turn the motor over using your left hand on the flywheel. Pull the solder pieces out and you'll see exactly how they were compressed, and you can either measure them up, or trim the ends and have another go at it. Experts might say it's more accurate to pull the head off, but I'm not sure why.

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