Engine Trouble Pre ignition / detonation at WOT

[HeadSmess]

if the piston doesnt look like its been sandblasted.. it isnt detonation. and detonation happens at low rpm with WOT. not at high rpm.
with a 7hs in there you can almost rule out pre ignition though theres a chance the insulation is cracked and overheating. if its coming out white. change plug. the porcelains cracked. unusual though.

sounds more like the standard issue with these... a bit of everything EXCEPT what you have diagnosed it to be.

 

[Spare_Parts]

Try an extra base gasket. It will change your squish by the thickness of the gasket.

That didn't come out right.
The extra base gasket will drop compression but it will also change squish, possibly making the the squish no effective. Since you have the stock head squish isn't something to worry about.

 

[HeadSmess]

and will also increase transfer and exhaust openings whilst reducing induction time... way to kill the bottom end

 

[Vikingimike01]

Alright, back. Summer break is here for me, so I'll be working on the bike again.

I'm gonna have to weld the exhaust pipe for myself, with an old soviet stick welder...
Anyways. I'll go buy some welding stick thingies tomorrow, and report back in about a week or two.
If anyone has any tips for welding, please tell me.

After that, when I'm done, i'll come back to this thread's original problem. I'm going to try some 100 octane gas tho first. Nothing to lose. If the issue is still there, then I'll try to look for an extra base gasket, or head gasket, or whatever you guys suggest would be better.

 

[Vikingimike01]

Hey all. Back again.

I had a short ride a couple of weeks ago. I still have this thing with my new exhaust, wich is a proper expansion chamber. Meaning, it's more noticable (Mainly in the pull now, not the sound. its so quiet) it starts doing it at 40km/h and untill i hit 48-50km/h it like pulls for a second and the pulls slower then normal again then slower

I tried 100 octane premium gas. Still happens. Size 80 main jet. Needle in 3rd groove from the top, from the 4 grooves.
So far I don't know what else it could be.

Changes: I'm installing some mods, here's a list. I haven't tested the bike and can't for a while untill i either file or grind my jackshaft kit sprocket down so the chain fits, or install a bike sprocket, and the piston arrives.

Mods list:

• CNC Racing HP head (The circular one with a deeper combustion chamber)
• PZ20 20mm carburetor
• CDH G2 racing reed valve
• NGK B7HS
• Giannelli Next 31623x sport exhaust (Welded to fit the bike without changing up any header lenghts)

My question is. What else can I try other than try an extra head gasket? With the jackshaft kit maybe I don't have to rev that high, or even with the new carb it won't do it. But if it does, what else can I try?

 

[HeadSmess]

just keep throwing money at it, its sure to fix it! lol.

how about taking it back to STOCK then figuring out whats wrong? rather than chase your tail putting more and more problems on it?

ah well. again. it aint detonation, but that last post might be getting somewhere... stick a timing light on it. they arent always timed perfectly outta the box. could even be a simple thing like the magnet stuck on backwards.

but youre convinced its detonation.. so why listen to us?

 

[FurryOnTheInside]

it starts doing it at 40km/h and untill i hit 48-50km/h it like pulls for a second and the pulls slower then normal again then slower

I agree you should look at the ignition timing. If incorrect ignition timing is causing excessive vibration at higher RPM then that vibration could be shaking the carburettor and foaming the fuel, perhaps?

The base-timing-by-woodruff-key mod is a free and easy way to experiment with the ignition timing. A better CDI with a proper timing curve would be ideal; but filing a notch in the key and using the stock CDI is free. File a notch in the side that matters to try it with a little bit later base timing, then simply reverse the key if you don't prefer the new timing.

I used a thicker key to make an offset key but.. same difference.

Offset timing by magnet key

• FurryOnTheInside
• Sep 17, 2017

1.2mm offset key for a 5.4° postponed base timing (if I'm correct).

 

[Vikingimike01]

just keep throwing money at it, its sure to fix it! lol.

how about taking it back to STOCK then figuring out whats wrong? rather than chase your tail putting more and more problems on it?

ah well. again. it aint detonation, but that last post might be getting somewhere... stick a timing light on it. they arent always timed perfectly outta the box. could even be a simple thing like the magnet stuck on backwards.

but youre convinced its detonation.. so why listen to us?

I'm not throwing money at it so it gets fixed. This is my hobby, I'm not just buying things, so it gets fixed. I'm buying performance parts so the thing gets better. I never said I'm still convinced it's detonation. In my last post a while ago, I said I will try 100 octane gas as well, wich I only had time to try now, to see if it IS detonation.

Anyways, I'll look into this ignition retarding. I don't really know how to start cutting away or what I do.
Furry, I don't really understand your post fully, and the pic. Where do I file and how and how big? Lol.

 

[HeadSmess]

im anti-bling if i dont make it myself... its not there.
it just seems every post you list more parts but still no cure.

last post again starts mentioning interesting things...

fueling.

such as... excessive vibration causing foaming, or the floar level being too low, or another great one... the tank breather isnt breathing. restricted fuel flow... simple test for that one is to pop the fuel line. a cup should fill in about 30 seconds.

they all cause the surging you describe.

take all bling off. even take the exhaust off. (get some earplugs) eliminate EVERY POSSIBLE CAUSE before progressing.

even if it takes a new engine.

face it... some of them are lemons. and no amount of hairpulling will fix it.

lil chinaman only has to leave one shaving in the jig and oops, suddenly the cranks utterly useless... looks fine though. until you have a pair to compare... youll never know.

just reading one elsewhere of a kx125 crank from a reputable supplier having been machined 90 degrees out...

 

[HeadSmess]

Offset timing by magnet key

• FurryOnTheInside
• Sep 17, 2017

1.2mm offset key for a 5.4° postponed base timing (if I'm correct).

[/QUOTE
dude! you cant say that!

it has to be "ignition postponed at 5.4° beyond absolute top center"

cant say dead either...

even saying "top" has its suspicions. or "center" for that matter...