RD 350 31K hits wall at top power - melt down

[RDrocket]

And if the ambient air temperature is 100*F it will just compound your problems. At very high ambient temperatures and WOT the engine can't dissipate heat fast enough and you'll have problems.

I have riden the bike on 38°C no prob, whet it melted it was ~25°

[RDrocket]

The piston is toasted, the cylinder will be like new after muratic acid and scotch brite (no scars under fingernail). The crankshaft is oiled but the oil is dark gray. Any way to wash it without teaking it apart? Like 100 rinsing with gasoline?

[silverstrom]

That dark gray in the oil is aluminum.

You now have 2 options.

Option #1 is you just fix the affected cylinder and then hope for the best. How lucky do you feel?

Option #2 is you open it up and do a proper inspection and repair as necessary.

If you don't open the bottom end you'll never know if the melted piston is now in the rod big end bearings or crank bearings (and elsewhere). Imagine how you'll feel if you repair the top end and then have an even bigger problem with a connecting rod big end failure. That event may grenade the cases.

Not a hard decision to make really.

You need to sort out why this happened. Was it an air leak, or something else. Left side meltdowns are typically due to an air leak. Pressure/vacuum test before opening the engine.

[RDrocket]

https://mcbperformance.com/p-25418-pist ... auses.html
# 5 any thoughts? Because my piston head is melted in the middle and towards the exhaust but also the crown ower the top ring is missing on the exhaust side.

[MK]

I wonder why Yamaha recommended BR9 plugs when your bike ran so fantastic on BR8.
Oh, wait, it's melted, so maybe using hotter plugs together with increased compression wasn't that smart.

Seriously: If you know what you're doing it's no problem to run on #8 heat range, but then for "city usage" or in winter with no excessive WOT and maybe even under EGT surveillance.
A piston melted exactly in the middle is a hard prove for a too hot plug.
People even run colder than 9 in race applications. 9.5 or 10 would be appropriate here, but the latter ones are extremely prone to fouling.

[RDrocket]

I wonder why Yamaha recommended BR9 plugs when your bike ran so fantastic on BR8.
Oh, wait, it's melted, so maybe using hotter plugs together with increased compression wasn't that smart.

Seriously: If you know what you're doing it's no problem to run on #8 heat range, but then for "city usage" or in winter with no excessive WOT and maybe even under EGT surveillance.
A piston melted exactly in the middle is a hard prove for a too hot plug.
People even run colder than 9 in race applications. 9.5 or 10 would be appropriate here, but the latter ones are extremely prone to fouling.

I was riding 3 years without any marks of overheating, melting.. And the manual says BR8ES..

[85RZwade]

Speaking of manuals, I recommend Martin's. Lots of great information about the very engine you're dealing with. Maybe John has written one as well that I haven't found yet, but he clearly knows his way around a two stroke as well. I'd bookmark their advice.

[RDrocket]

RH Piston - https://hosting.photobucket.com/images/ ... &crop=fill
LH Piston Crown - https://hosting.photobucket.com/images/ ... &crop=fill
LH Piston - https://hosting.photobucket.com/images/ ... &crop=fill
LH Piston Exhaust side - https://hosting.photobucket.com/images/ ... &crop=fill

It isn't the spark plug because the right side piston will have at least marks of heating if it was..
I think that it was an base gasket leak because when I pulled the cylinder the gasket's center was dry and clean all the way but on the 4mm wide part towards the right cylinder it was wet and it wasn't stick to the base (I use sealant on all gaskets)
The day before it melted I had the pipes of and checked the cylinders and pistons, THEY WARE THE SAME in perfect condition. Next day, 80km later it sized. I wasn't riding so hard, 4000 - 7000RPM with lets say 5 times WOT 4. 5. and 6. gear.
A few km before the disaster, while breaking I pulled the clutch in and after 5sec on idle the engine died like the idle is to low. Quick blip with clutch and it restarted like nothing happened.

[MK]

Yamaha recommends a BR8ES for the 48H, that's right.
But that is intended for whimps obeying the local 88kmh speed limits.
They mentioned that for high speed riding you need a BR9ES.
http://www.rd350lc.net/specRZUSchass1.jpg
(I'd add that high speed starts earlier than you may think. Stock bikes easyly get above 600 deg C EGT going a constant 130 kmh for a while!)

They learned from failure and updatet that to only BR9ES (except the 27HP restricted 1WX).
http://www.rd350lc.net/spec-86chass1.JPG

Your piston pics exactly show what I mentioned. The heat started from the plug region.
The BR8 plug narrowed down your margin enough that any slight issue brings you above the acceptable level.

[tacky1]

That ran way to hot, lean and possible air leak, I would tear it down, get the crank checked, replace the seals, hone the cylinders and fit new pistons, Wash it all out, B9 plugs, and definitely up your main jets or fit some real PWK carbs and be done with it.
All that Gray shit is melted piston that is now washed in around your bearings, Thats basically grinding paste, any you miss when you wash out with gas without stripping the bottom end will eat your new pistons.

[RDrocket]

The wery tip of the ground electrode is melted. So lerned that on the hard way. Will keep that in mind in the future. After muratic acid the cylinder is like new. Will keep you updated how the rebuild goes.

[silverstrom]

Just so you know, a BR8 plug is 100*C hotter than a BR9 plug.

Make sure the small holes in the top half of the cases, just under the cylinder, are not plugged with melted aluminum. There should be 2 open holes under each cylinder. Those holes direct oil to the crank bearings.

[RDrocket]

Just so you know, a BR8 plug is 100*C hotter than a BR9 plug.

Make sure the small holes in the top half of the cases, just under the cylinder, are not plugged with melted aluminum. There should be 2 open holes under each cylinder. Those holes direct oil to the crank bearings.

100° didn't know that..
They aren't plugged, like I said the oil in the crankcase is a bit grey-is. No black or solid grey, clear oil with grey tone.

[RDrocket]

So I went for the scary solution without pulling the crankshaft out. The oil on it was clear but with gray tone. I rinsed it and the main bearings thru oiling holes with gas and an syringe. Two syringes gas in and then all out with constantly rotating the crank. After 30min and like 1/2L gas the gas I pulled out was the same color like the one i put in. Then I cut a small white rag and rotated the crankshaft with it in the crankcase. When I pulled it out it was clean. So I'm 99.9% sure that I cleaned it good. Made all new gaskets and new prox piston.
Now, i cleaned all jet seats with an end mill on the PWK 28 nockoffs, reassembled them and started the bike. runs 1000 times different (better) than before the melt down.
I will let the new piston break in before I'm going to rejet the carbs (35pilot, air crew 1turn out, 150main, JJMneedle second from bottom, 30power jet) runs rich but strong.
Yes BR9ES spark plugs are in there

[RDrocket]

So I went for a small ride and the bike is so rich it won't rev past 4500-5000 rpm while riding. If I rev it with clutch in and accelerate like this so I get past 5000rpm it is rich but revs fine. As soon as the revs drop a bit it starts to act rich. So I went on the jet needle from second from bottom to second from top clip position and main jet from 150 to 145. The bike runs cleaner but as soon I start riding it acts lean like the floats are empty. It idles forever without a change. So I'm on the beginning where I was a month ago... I came across a topic with similiar symptoms and the fault was in the failing stator?
https://rdlccrazy.proboards.com/thread/ ... starvation