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

[Questo vecchio rz]

This is where I lose all interest in performance 2 strokes...
There are SO MANY modifications and changes...that I wouldn't know where to start.
Generally..I would say that any bike that has been jetted just will not stop in the middle of the ride one random day.
But on a heavily modified 2-stroke jetted on the ragged edge for maximum performance it very well could have something to do with your temperature that day or seasonal changes.....? But the fact that it runs cold and then runs bad or not at all when hot ...is a question mark especially when you said you fix the venting.


You mentioned it once in your previous posts once you get to a point where it's rideable I would invest in some dyno sessions,

[RDrocket]

BR8 seems a hot plug for a tuned engine, i'de use at least 9's and probably even 10's.

If my readings are correct, the BR8ES are spot on because the line on the ground electrode is right on the middle. But I plan to go to iridium spark plugs and then maybe BR9EIX, when I sort jetting..

This is where I lose all interest in performance 2 strokes...
There are SO MANY modifications and changes...that I wouldn't know where to start.
Generally..I would say that any bike that has been jetted just will not stop in the middle of the ride one random day.
But on a heavily modified 2-stroke jetted on the ragged edge for maximum performance it very well could have something to do with your temperature that day or seasonal changes.....? But the fact that it runs cold and then runs bad or not at all when hot ...is a question mark especially when you said you fix the venting.


You mentioned it once in your previous posts once you get to a point where it's rideable I would invest in some dyno sessions,

I always jet my bike on the safe side, and I mean fat on the safe side

So I maybe am on the right way, the PWK28 nockoff..
I put back the stock carburators pilot 22.5, air screw 1/2turn out, jet needle 4'th clip position, main jet 270 drilled out to something like 320 (this is just for test because I don't have main jet set for the mikuni, will order it)
All in all couldn't make it to make the same bog like with the PWK
So I need to know why. After a few hours I think I found the problem.. the carb is full of leaks where jet seats, power jet cover... Maybe that's way I had a hard time jetting them and when summer came, came the problems with less humidity and higher temps. Tomorrow I will by more gas and test it again an let you know of the results but like it is now I should have by the OEM keihin..

[tacky1]

Bin the carbs and do yourself a favour and get some decent carbs, You spend all this money on your engine and pipes and then cheap out with the carbs!!!!

[waltmil]

We're on the second page of the post and the O.P. finally admits his PWKs are knockoffs!

[RDrocket]

Yes I did a big mistake. I was thinking "how bad can they be?" I found the best ones an buy them, next I disasambled them and checked ewerithing (minor casting flaws) then put in polini jets. The last thing I had in mind was the jet seat. So now i will rebuild the oem carbs fully and then over winter buy kx 85 pwk 28

[brrrappp]

We're on the second page of the post and the O.P. finally admits his PWKs are knockoffs!

Now, now......
To be fair, the O.P. mentioned the carbs were "knockoffs" about 3/4 the way down in his very first post.
he wrote: "The carbs are knokoff but first thing i did was disassemble them and reassemble and they are very good."

The problem that even the best techs sometimes overlook is having a new part that is defective right out of the box. Professional techs often insist on using parts that they source from known-good vendors. This experience is exactly why 95% of dealers/repair shops will always insist on supplying the parts for engine builds.

[silverstrom]

Others are using the Chinese reproduction carbs with no issues, so you can't automatically assume the carbs are defective.

[MK]

I just read and smirk.
That always applies when people post overly complicated questions, already implying to know what to do (mostly highly theoretical stuff nobody cares for) and then giving a s... on sensible answers.

[RDrocket]

Jetted almost perfect. A bit rich on WOT to bee safe.
Huge difference, smoother running (not even close crisp throttle as with pwk-s)
The bike isn't hitting a wall, but revs clean to 10 000RPM
the low rpm wot is a bit stronger but with a little power drop at 5000RPM

[RDrocket]

Yeah, it wasn't the chinase carbs... The LH cylinder sized

[tacky1]

Did it seize from running lean. What caused it to seize

[RDrocket]

Just to make clear, Last year I was riding the hole season with the knockoffs without any problem. Not once I had marks of detonation on the piston. Now I'm missing the top of the piston on the exhaust side so I can see the top ring and the top is melted.. maybe I will have luck to go only with new piston and a good clean of all parts (if there are no pieces in the crankshaft). Then I need to find what is really wrong with the bike.. I just MOT-ed the bike yesterday, put new tires and full gas tank, made only 80km before the disaster. Yes the bike sized on a strait 5. gear WOT. pulled the clutch and coasted 10 sec so it cooled down. It restarted and I ride it 10km on one cylinder to home. The right cylinder is in perfect condition, the spark plug tells me that it is a bit lean, but nothing that will make a disaster like on the left side.

[silverstrom]

Those are the classic symptoms of a severely overheated piston. The damage will appear on the exhaust side.

Knowing it was overheated there are simple questions to ask when dealing with your piston failure.

Is it detonation?

Is the squish correct?

Is compression too high?

Proper octane fuel?

Correct heat range spark plug?

Is the left crank seal is OK?

Do you have an air leak somewhere else? Reed gasket, base gasket, carb top, etc?

Is your ignition timing is correct?

Hopefully none of the melted aluminum is in the bottom end and bearings. The only way to know is to pull it apart and look.

[silverstrom]

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.

[RDrocket]

Those are the classic symptoms of a severely overheated piston. The damage will appear on the exhaust side.

Knowing it was overheated there are simple questions to ask when dealing with your piston failure.

Is it detonation? Yes, the spark plug is also full of molten aluminum

Is the squish correct? Didn't changed since last year, 1mm

Is compression too high? Also the same

Proper octane fuel? The same as always, 100octane

Correct heat range spark plug? The only ever was in the bike BR8ES

Is the left crank seal is OK? No ozing behin the stator plate, replaced 3 years ago an has 5000km on it

Do you have an air leak somewhere else? Reed gasket, base gasket, carb top, etc? Checked all to many times

Is your ignition timing is correct? Didn't changed, stock

Hopefully none of the melted aluminum is in the bottom end and bearings. The only way to know is to pull it apart and look.