Today's Whodunnit ...

A short video of a run last weekend. There is a tale to this, so watch until the end.

I think I know the reason why too. See if you can figure it out.

Love that 390 it's so freaking gnarly.

Whistle?? When hitting the second left trail. Air leak I knew it. Sucked in the base gasket?

I wrote about the whistle while the video was running I know that whistle.

Retorque often, replace the base gasket if you just purchased the bike. When these bikes are flooded and they sit it rots the base gaskets. My '78 husqvarna 390cr did the same thing. It was so flooded I turned the bike upside down to drain when I purchased it. I was new to the flooding situation. But learned quickly when it seezed in the trail. New piston, bearings, seals fixed it.

I had four 390's '78, '79, '80 and a '82 Frankenstein 390. My backup had backup.

Good try Bill, but no.
I wonder if anyone will get this one . I could give a clue .... maybe.

Looks like damage around the wrist pin boss. Did the keeper come out?

You took a clean bike out in england first mistake right there, looks like the piston got hot, an seized you didnt stop so im gunna say too tight a clearance and not enough oil.

I'm thinking something got into the cylinder.... a piece of reed pedal, or chunk of carbon?

Whistle, squeal, metal on metal. The engine did sound tight on the revs. Wrist pin retainer clip missing?

Loose spark plug?

You forgot premix, when you looked back after the restart I didn't see any smoke?

She was definitely not happier towards end, did not want to rev at all, any broken springs in your oil? I know what Bill means about the noise, but Autos make so much mechanical noise anyway I find it hard to pick up something going wrong

Oh - Ian - NO WAY!!!

Looks like it might be a bit lean (did I hear it pinking) and bored a bit tight....

Will the bore clean up?

Andy

The damage (or rather scratches) are where the wrist pin clip has been in and out a few times. If you remember from the build thread, we had to grind a few thou. off the angle on the end to get the clip to fit. Not that.

Juicypips is close. The piston over heated ..... but why ?

Nothing got into the cylinder, spark plug tight. Not that.

Nice new batch of fresh premix at 40:1 just before leaving. Not that.

Steve, the auto box is all good.

Clue 1 .... the temperature strips on the head tell me this was running at around 178 deg C. The strips on the left casing for the 1st gear clutch did not register on their strips. The barrel was not too hot. Not as hot as the head anyway.

The bore cleaned up good Andy. The slight pinking before it finally stopped was a heat related issue. The mixture has been perfect thus far.

Not bored too tight. We went to .1mm as I know they run hot.

Clue 2 ... my fault .... I should have measured once, twice, three times ! but what was it ??

Oh shucks! Hate when you do such good job and then something goes wrong.

Place of decompressor ?

Squish too small?
Or the head had a loose bolt and sucked in too much air into combustion chamber, or timing too far advance but doesnt normally show like that.

Do tell is am on the edge of my seat.

I may have to recoil a bit here. I have finally had time to double check the measuring I did the other evening. It may not be what I initially thought.
I need to repack the silencer on the other 390 this afternoon, so I'll have another ponder alongside a cup of tea.

Don't feel bad! I had a new piston and was putting rings on it. Of course my hands had two stroke oil all over them and piston slipped out of my hands! Hit concrete floor and mashed top of piston between top and top ring groove.
Instant trash!

Have you got a pic of the spark plug Ian and what plug was it?

Andy