I actually don't know what the spec is. Its not in the manual. 38 is something I aimed for because the stock spring is black and stock needle is 1.5mm. So I should be aiming for 38... But besides that I've been using the specs for an XP.
Two things just crossed my mind.
1 since the compression is higher, should I up to octane from 87 to 89 or 91 ? as in: could it be pre-ignition? The sound is kind of tingy
2. would the higher compression create a strongre negative pressure requiring smaller pilot jets ?
Detonation occurs when the fuel charge stops burning prematurely. Lower octane fuels burn faster thus one way of avoiding detonation is by increasing octane. Another way of avoiding detonation is by increasing the fuel/air ratio to the point all the fuel cannot burn. Peak pressure should occur after TDC. Cold air can increase the chances of detonation b/c air density changes air/fuel ratio. A thermally hot chamber can also cause the fuel to burn faster, adjusting compression ratio and combustion chamber shape can also be manipulated to avoid detonation.
If the fuel charge burns too quickly, it may be necessary to ignite the charge later in the compression stroke.
But I doubt detonation is occurring at low speeds, especially if you are experiencing rich 4-stroking. I would be more concerned with detonation at WOT. A 2-stroke nearing detonation will begin to run like a bat out of hell just before it seizes, so if there's any question about top end detonation you can add more fuel using the HS mix screws. If you were to play with HS mix and begin bumping into the rev limit then I'd say it's probably too lean and you should add more fuel. If you weren't able to hit rev limit it's still possible it's too lean so in that case find max RPM and then add enough fuel to drop WOT RPM by 50RPM then you should be safe from detonation (at least you know it's not as lean as it would've been).
Basically, add fuel if you suspect detonation is occurring and try to achieve that cardboard insulator color. This color comes from the oil film, not carbon from the fuel. Carbon from the fuel will be black.
Keep in mind, if there's way too much fuel somewhere in the throttle, the brown oil film can actually be washed away from the insulators (similar to washing the plugs with brake or carb cleaner solvent but tons of fuel can be even more effective).
So if you're pig rich and heavily 4-stroking, then it's possible your plug insulators are being washed clean by the excess fuel. Also, oiling will be negatively affected by excess fuel.
I think it's okay to adjust low speed mixture (via jet orifice) till you reach a point where a lean hesitation occurs then richen it up from there. The reason I think this approach is okay is b/c the engine isn't making a lot of heat and the oil film is unlikely to fail under these conditions.
I like to hear a hint of 4-stroking during mid speed, this tells me there's plenty of fuel there for when I come off WOT with a hot piston a lean seize is less likely. I don't want constant 4-stroking in mid range though, too much fuel can wash the oil film off the cylinder walls.
So it's hard to know if you're experiencing detonation but if you are it's most likely to occur at WOT. Your piston gains a lot of heat at WOT, so way too much fuel in mid range could wash the oil film off the cylinder and cause a momentary seizure when coming off WOT. One warning sign of too lean HS is the WOT RPM tends to decay but in that case there's no doubt the piston has overheated. If there's any question then adding fuel (enough to drop WOT by 50RPM) with the HS adjuster can help avoid WOT detonation.
So what WOT RPM is enough? The higher it is, the more chance of reaching the danger zone. If you can gain WOT RPM by adding HS fuel then it's definitely too lean, adding HS fuel should always drop WOT RPM.
Preignition is combustion that occurs before the ignition spark occurs, such as immediately after the exhaust port closes, and this can happen if the plug electrodes are overheating. Preignition occurs while the piston is still on the compression stroke but now it's compressing an expanding charge of gases. There is no rapid pressure spike to resonate the block as in detonation, the pressure builds to an abnormally high level as the piston is compressing the expanding gas charge and the piston breaks due to intense pressure.
So have a look at your plugs to see if the electrodes are melting, this tells you you are nearing electrode temps that may cause preignition to occur.
It's on my list next time the carbs come off though, I'm curious to know if I can notice a difference.
