Piston Deposits

[scooble]

this is the condition of both my pistons that have done one and a half track days and one race day.
I've seen pistons develop a thin even coating before, but not the kind of grainy appearance you see here.
What does everyone else think?

[falconman]

What kind of oil are you running? Also what ratio? What is your compression and plug heat range/type? I used to see alot of that back in the '70's and '80's from mineral and castor based oils when run oil rich.

[scooble]

the last meeting, I was running Motul 800 at 30:1 and prior to that i was running Castrol XR77 also at 30:1.
Comp ratio = 7:1
the plugs I was using was B10 Iridium plugs and have only been used for 1 trackday and one meeting.
The last meeting, I was running 60% Tesco 99 octance and 40% Avgas, prior to that, I was using Tesco 99 and octane booster.

[dave32]

Was the heads clean or did they have an area where there were deposits?
It could be the fact your running AVGAS mixed,it will effect the piston deposits/pattern due to its slower burn rate than super unleaded.
I wouldnt expect that amount of deposits in that short space of time and the way its not even from side to side points to something being amiss.
If your gonna use AV mixed then you will probably have to change the jetting,generally you need to lean it off a bit (if it was running good before) but dont take that as set in stone,thats just my experience with the 2 fuels on the same engine.
Your using a good oil,but double check you have the road race 800 not the off road 800 which isnt designed for the constant high revs of a road race engine.
if you have trouble getting this,then use Castrol A747 as it contains castor oil and so is less likely to nip up if your out with your jetting a size or 2.
Was the other piston the same?
Also do you know what the squish clearence is?
ATB

[falconman]

I'm not familiar with those products/fuel but best guess is the 2 oils don't mix well causing the build up. Especially if one is synthetic and the other mineral based. One thing I learned long ago about 2 strokes is pick an oil and stick with it. Not sure about roundy-round racing but in lsr you don't want thin wire electrode plugs. They act like glow plugs on the top-end. Since you're not WFO for the entire race you might be ok.

[scooble]

I was told the squish was about 0.7 but I havent measured it myself. I would have stuck with XR77, but Castrol only made it for a short while and now it has been discontinued, so I switched to Motul, as I know its also a good brand. The two types of oil were not mixed in the tank though and it was definately the road race stuff (although Oipe did send me the mx oil first and tried to convince me that the only difference was the label...so why does the label cost more?). Jetting was changed according to the dyno results. I have heard some people say dont use irridium plugs, so I might go back to the old şort. Both pistons are more or less the same. If I get a chance, I will try and build another spare engine to do a back to back

[dave32]

Iridium plugs providing there the right heat range wont cause any problems,most racing plugs for 2 strokes are fine wire (NGK 105's,BREIX,EV,EGV etc).
Your not running excessive compression or squish (providing it is what you think) so there is no reason to mix AV with Super,if you want to raise the octane of the super you can use silkolene octane booster,but there really isnt any necessity.
if your worried about detonation or such it would make mores sense to jet the engine to the conditions/track type on the day,you cant jet for the circuit by a dyno run,all you can do is get a rough base line as to what gives good power.
Invest in a weather station,make notes and inspect the plug/piston to confirm the mixture is right not too make a decision based on what the plug says.
There's too many variables to guess what jetting to use,usually when it feels right it is right,the you confirm with a plug check,note the RAD/temp etc and then you can work from there.
Also bear in mind,its possible to get a high HP figure by leaning the jetting on a dyno run,this maybe ok on a twisty slow circuit but 10mins round snetterton and it will lose power and eventually grenade itself.
Reason being,fuel is used to cool the piston,so a fast rider/fast track needs more fuel than a novice/twisty track.
A richer than ideal mixture will give a slightly lower initial reading but it wont suffer from power fade after a few laps like a "on the edge" setting.
The slight swirling on the crown is something to do with the ignition on the KR (which is designed for unleaded which is fast burning) being not suited to the slow burn rate of the leaded fuel.
GP 125's and 250's (TZ and RS) run a night and day difference in settings between leaded and unleaded,
Ignition advance curve,
Squish and combustion chamber design,compression ratio.
Even the exhausts are different.
Jetting is changed etc etc.
Take that into account and the only reason for running leaded in an unleaded set up is in the belief that its safer (stops det),it may in theory but a too lean/advanced setup will still go bang whatever fuel you use.
Your right about the motul being different,it is,the off road stuff has anti fouling additives and a lower SAE,it still works well on a roadbike just not suited to constant high loading.
While there's nothing wrong with 800 road race,a castor/synthetic mix will give added protection against seizure and parts last longer.
Elf HTX and 747 are what most GP 125/250 guys use.
HTH a bit
ATB