PE400 pinging at top of rev range

[rohan]

I have a recently rebuilt PE400 engine that pings (quite badly) right at the top of the rev range. In an attempt to correct this I have checked the carb specs, the ignition timing and rode it without the muffler as I was concerned the head may have been getting to hot as the gasses couldn’t get out properly (exhaust flange was leaking oil slightly).

I am now thinking to up the main jet by a size or two to make sure the bike is getting enough fuel up top. Does anyone have any ideas on why I am getting this pinging or how to fix it? Or could I simply be over revving the engine?

[Freakshow]

HAve the same problem on a few YZ, still not sorted it out. Tried jets and timing no real change just shifted the problem and scope.
 I have had better luck with changing fuel supply, I suspect its not dence enough.   Try some avgas and see if it still happens

thats my next step

[oldfart]

Rabbit, use a dial indicator to check timing and check this against factory settings.  I tend to short shift mine and not rev it out as they tend to develop most of thier power about mid range.
 

 

[frostype400]

Yeah I agree with Stew on the shortshifting the 400 you don't need to rev it out like a 125 I find mine dies off up top too on stock jetting.

I found it performed better with a clean new muffler also the flange they always leak I have had good results though with some high temp red silastic no leak anymore.

Also I run Premium I have heard there is no difference in some people's opinion but I think you get alot more bang also there is further octane additave you can buy and try.

[evo550]

You could also check the thickness of the head and base gaskets you rebuilt with, If they are thinner, you may have  raised the compression. This can sometimes bring on the death rattle, with dodgee feul.

[flattracker]

I had the same problem with my KX 420. they are a very peeky motor and make all there power at the top so need to be reved. I have had to retard the ignition and it will run fine on BPpremuim (98). I have secured a suply of avgas that i run my Tracker on and the KX likes a 50:50 mix of this with premium.
None of the fuel today is as good as super was with lead!

[Lozza]

It's a universaly shit ignition.

[cheapracer]

Try a colder grade plug and experiment with different oils. I favour plain Mobil 2 stroke and Nippendenso plugs - certainly fixed the problem with my 465.

If that fails try adding an inch or 2 to your header pipe length, that will add some more exhaust radicals to the mix at top end cooling and slowing the combustion burn a little.

[Mike52]

Guy on the PE forum (elsewhere) simply machined his head to drop the compression ratio.
Ugly fuel .

[Lozza]

If that fails try adding an inch or 2 to your header pipe length, that will add some more exhaust radicals to the mix at top end cooling and slowing the combustion burn a little.

How would hot exhaust gas cool anything?

[bazza]

Do a plug chop at wound out,for Main jet.  If ok then Av gas

[gettinold]

Ahhh a big bore 2 stroke pinging. Bring back memories.YZ 490's did it all the time.Take the advise from above.Pinging usually  means lean ( and power). Does it have the std air box etc still on it? If not that could also be not helping the problem.

[frostype400]

What are you running I run 8's or 9's

[frostype400]

8 is hotter though try an iridium 9 or 10 I know they cost like $20 but then the bike starts easier and I have had a good run lately with iridiums.

[LWC82PE]

Look on ebay for iridiums. They are pretty cheap in boxes of 4, well under $20 each

Otherwise the head mod on the PE forum seems to have worked.

What fuel you running by the way?