Piston wash

I will be pulling my bike apart on monday for it's second rebuild at 20hrs. I have heard a little about piston wash put don't have a really good idea on what to look for. I'm a jetting freak, i love having my bikes jetted right on the money, i have cut plugs, gone off feel, but i would love to add another method to my list. So any information and discriptions would be great.

I'm a jetting freak, i love having my bikes jetted right on the money

Then you should just go and buy a wideband. :thumbsup:

So a stock YZ piston is classified as a flat top?

i believe they are domed. and i didnt think a wide band could be used with leaded fuels or 2stroke oil.

A wide band can be used on 2 strokes with leaded fuel. You really do not want to leave the wide band on permanently though, just use it for tuning. 02 sensors will not last as long on a 2 stroke but it will work. There are a lot of guys running wide band data logging systems on turbo charged 2 stroke snowmobile engines making in excess of 300+hp with leaded fuels. I have read about an 02 sensor lasting all year if placed in the correct spot.

Nitrous, I know wide band is being used on 2stroke, but I dont understand how it can provide consistant results concidering the charge short circuit throwing off the O2 sniffer???

It takes a good amount of time for the 2 stroke oil to contaminate the 02 sensor before it starts to see inconsistent results. It's also a bit more critical on where you place the 02 sensor.

Nitrous, I know wide band is being used on 2stroke, but I don't understand how it can provide consistent results considering the charge short circuit throwing off the O2 sniffer???


Quite a while back I had the same question and I received this answer from a very knowledgeable and trustworthy source:

The pressure waves used for an advantage in a two-stroke expansion chamber are finite amplitude waves, and they work pretty much independently of the actual gas flow in the exhaust. In other words, the gas particles flow out while the finite wave pass back and forth. Once you get far enough away from the exhaust port (say the dwell section of the chamber) an O2 sensor will respond correctly and send accurate information.

It makes sense when you sit back and think about it. :thumbsup:

leaving out the possibility of oil fouling the sensor, what I am questioning is the fair amount of charge short circuit that gets out the exhaust port well ahead of the main charge that is pulled out by the pipes returned negative wave. The short circuited charge never has the chance to be stuffed in by the pipe later.

For those that dont know what charge short circuit is, it is the charge that is lost directly out the exhaust port between the primary (the transfer port closest to the exhaust port) and the exhaust port without making the intended loop around the cylinder. In a perfect scenario all the charge is pointed toward the rear of the cylinder and the boost port circulates it around the head then down and out the exhaust port, that is what has been described as loop charged.

So my thinking is that the lost fresh air would show incorrect readings from a O2 sensor, and that compensating for it would be difficult as the lost charge is varies according to load and rpm. I dont have anything to back it up, just thinking........

on edit: and oh, keeping it with the piston was subject, you can see the short circuit "trail" of wash from the primary to the exhaust port.

Yes, extended use with either leaded fuel or 2-stroke oil will more than likely kill the o2 sensor (leaded fuel definitely will!). I use my Innovate LM-1 with a tailpipe sniffer that I normally use on customer's turbocharged Subarus (I tune those as a hobby in my spare time). Stick the sniffer in the end of the muffler and grab some pulls at a constant throttle position that correlates with whatever jetting circuit you are working on. Readings have been very consistant for me and have verified problems that I initially suspected exsisted with both of my bikes.

Now back to the original topic! Sorry for the unintentional side tracking, hehehe.

no please keep talking about the wideband i have been trying to find research on these for my yz and my crf. but never got a clear understanding

how far back should one of these be place. like rite be for silencer and pipe conjunction?

I have the optional tailpipe sniffer thingy for my LM-1 wideband. What it is designed for is portable usage on things like dynos -- the wideband sensor screws into the sniffer and then you clamp the sniffer into your car or bike's muffler. Thus, no o2 sensor bung is needed.

Innovate LM-1 wideband kit:


tailpipe sniffer:


Works well for me. :thumbsup:

Is the tail pipe sniffer as acuarte as the a welded in bung?

As long as you don't have a humongous exhaust leak, yes it will be. The thing I've noticed the most was the 'lag' in readings vs. engine speed -- but that is in the area of milliseconds on cars, something that wouldn't be an issue on the rudimentary fueling & ignition systems on our bikes.

If anything, readings might be skewed by a tenth or two of an afr point when using the sniffer on a car.

Trying to play CSI forensics by reading plugs, piston tops or whatever is going to cover a MUCH MUCH MUCH wider AFR range (as in several AFR points).

The piston wash concept is new to me, but it makes sense. What doesn't make sense it what I am seeing on my kids 05 YZ125.

I did a top end a few months back and the top of the piston was nearly perfectly clean, which was the first time I ever saw this on any of our bikes. If the bike were running rich I would expect to have seen excessive smoke and been fouling plugs, but this never happened. In fact the bike smoked less then most 2 strokes and if anything the plugs were a little lean and others have agreed.

The bike is stock with the exception of a V Force reed & Procircuit pipe. I forget the numbers but the main jet is one size larger then stock, the pilot is stock and the needle clip is in the center.

We are running 40:1, 100 octane gas, yamalube oil.

Any suggestions?

my bike was the same way it didnt seemed like i was a few jets rich, but i was. i was running a 175 but now im all the way down to a 170. when i did run the 175 i never felt any signs of richness. no smoke no plug fouling. and my piston was clean. so maybe try and lean the bike down main by main till you find it perfect.

I increased the main jet 1 size from stocl a few rides after I added the PC pipe because the pipe was turning colors from running so hot. I went up one jet and the pipe ran cooler.

I am thinking of trying a Phathead and flat top piston in a few weeks so I will see what the current piston looks like when I take the head off.

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