Spark plugs?

[Robert Taylor]

Leaner oil ratios = richer air fuel ratio because the specific gravity of the fuel / oil mix is lighter and therefore draws through the jets more readily.
Richer oil ratios= leaner air fuel ratio because the specific gravity of the mix is heavier.
I totally disagree that more oil always = more power and can back that up with about 10 national gearbox kart titles and much more national mx titles when I was tuning / porting 2 stroke engines through the late 80s to early part of this century. During this period we had a dyno and spent many hours on it.
Our favoured oil in latter years was Motul 8002t, we ran at 40:1 ratio in 125 and 250cc mx and 30:1 in gearbox karts. Only at 30:1 in karts to preclude engine seizure when dethrottling at the end of long straights. When you dethrottle after sustained high rpm the throttle slide is shut and its predominantly only receiving lube off the pilot jet and residual oil in the system, and as the revs slowly decrease the ignition timing actually advances.
There are many causes of detonation that I will not go into here, but suffice to say retarding the ignition is sometimes a cure, as Danger said.
What is often overlooked is how our fuels are rated at the pumps, its a RON rating ( research octane number) which is a higher figure than the more important rating of MON ( motor octane number ) To confuse things further the Yanks rate as an average i.e RON plus MON divided by 2. Reading Yankee MX comics is often very misleading and their jetting specs very rarely work here because the specific gravity of their fuels and burn speeds are different.
Many 2 strokes are very marginal on pump gas. For MX we always favoured 60% avgas ( to attain a decent anti knock property, 35% 96 and 5% acteone to compensate for the lazy burn speed of avgas.

[B0000M]

[QUOTE=Reckless;1129284746
I'd still want to know where factory was before changing anything if it was me [/QUOTE]

theres also markers to line up to see where factory is. so knowing where factory is isnt a problem.


lol at removing the little plate, i went to the trouble of slotting it to get the adjustment as per the service manual... but removing it wouldve been a lot easier lol. nevermind its done now, ive used my eyecrometer to retard it a bit and will hopefully get my shifter seal and another plug before the weekend and be able to test it sat, before racing sun.

[CookMySock]

Leaner oil ratios = richer air fuel ratio because the specific gravity of the fuel / oil mix is lighter and therefore draws through the jets more readily.
Richer oil ratios= leaner air fuel ratio because the specific gravity of the mix is heavier.

Yep, thats exactly what we are getting.. and thats why adding more oil leans your engine out and makes it heat and eventually detonate. More oil = leaner and hotter burn = bad!

Take your mixture up to 50:1... at least.. Your engine is probably jetted for 50:1.

Two strokes are funny with ignition timing. It's not like a 4T engine.

Steve

[Robert Taylor]

Yep, thats exactly what we are getting.. and thats why adding more oil leans your engine out and makes it heat and eventually detonate. More oil = leaner and hotter burn = bad!

Take your mixture up to 50:1... at least.. Your engine is probably jetted for 50:1.

Two strokes are funny with ignition timing. It's not like a 4T engine.

Steve

No, 100% do not agree that tuning air fuel ratio by varying fuel / oil ratio is a prudent thing to do. Most of the modern 2 stroke synthetic racing oils ( that are any good ) I wouldnt run any leaner than 40:1 in a 125 / 250cc MX bike. 30:1 in a 85. But you can run 50:1 if you are trail riding. Run 50:1 in RM125s that have notoriously weak cranks and you will foreshorten their life dramatically. Spend a little more on a modern high quality oil that burns clean but still delivers excellent lubricity e.g Motul 800, Elf and to a lesser extent the top line Castrol TTS. If you really get a kick out of constant decoking of exhaust valves then buy a cheap dirty burning oil. Cheapness is costly!!!!
If the air fuel ratio is not correct change the jets, change the needle coding! Many oem 2 stroke needles are a coding or two too rich on their taper dimensions, starting from parallell section all the way through.
Many 2 strokes out of the crate are seemingly jetted for ice racing in Canada!!!
If you are lazy and dont like changing jetting to account for varying tracks, drag created by soil conditions, humidity, altitude, oxygen concentration etc then put up with it being rich and flat. There are no free lunches if you want crisp and maximum performance.

[Danger]

Ride it till the red and Booom, I don't recall removing anything when retarding the CR250 I did, but it was a few years ago so I'm having trouble picturing in my mind what was involved, as I recall only loosening the screws, rotating slightly in the direction of the rotor spin and retighten. You don't need to move the plate much. As you can see its a very easy mod to return back to where you started. And no problems doing it to your RM250, I did just that on one slippery slimy Riverhead ride and actually went a little further than an 1/8" and liked the results, but for all other riding an 1/8" is just the right amount to smooth out the power and cure any rattle I had that wasn't fixable with jetting.

Good to see you endorse my use in the 250 of Motul 800 at 50:1 for trailriding Robert. What I feel happens is that at higher oil concentrates for trailriding the exhaust just does not burn hot enough and the muffler packing becomes soaked very quickly, but like you for MX I would run 40:1.

[Danger]

If the air fuel ratio is not correct change the jets, change the needle coding! Many oem 2 stroke needles are a coding or two too rich on their taper dimensions, starting from parallell section all the way through.

This is a very good point and as I mentioned in another thread regarding the CR250 is exactly what I suspected was part of the problem with this bike. I recall recommending to my mate that we try a different needle after never being 100% happy with the results but, well it was his bike and he sold it for a 200EXC in the end and I don't recall the specifics of which needle we were going to try.

I also did the very same thing on my 200EXC, used a completely different needle and I swap between needles on my RM250 to suit the season after never being happy with the JD needles or the stock needle for my bike.

[Ktmboy]

Yes I total agree with the ratios from Robert. Learnt the hard way with the boy on his KX85. It was laziness really that keep me using 40:1 the same as my bike(one container you see) and I didn't realise he was on the pipe so much. Lack of lubrication killed it, not how lean it was.

Hes on about 30:1 with Silkolene and I'm using 40:1 with the same even with trail riding and our pipes are very clean.

I'm 100% sure it has a lot to do with the quality of the oil. Strange though we use 91 oct and have never fouled a plug.

[CookMySock]

No, 100% do not agree that tuning air fuel ratio by varying fuel / oil ratio is a prudent thing to do.

Yeah, I didn't mean to suggest that fuel/oil ratio was a useful tuning tool. I meant to observe how they were inter-related, basically agreeing with your statement.

We adjust the jetting on a race-to-race basis, but it's easy as the main jet hangs out the bottom of the carb. (pocket bike)

Steve

[Robert Taylor]

Ride it till the red and Booom, I don't recall removing anything when retarding the CR250 I did, but it was a few years ago so I'm having trouble picturing in my mind what was involved, as I recall only loosening the screws, rotating slightly in the direction of the rotor spin and retighten. You don't need to move the plate much. As you can see its a very easy mod to return back to where you started. And no problems doing it to your RM250, I did just that on one slippery slimy Riverhead ride and actually went a little further than an 1/8" and liked the results, but for all other riding an 1/8" is just the right amount to smooth out the power and cure any rattle I had that wasn't fixable with jetting.

Good to see you endorse my use in the 250 of Motul 800 at 50:1 for trailriding Robert. What I feel happens is that at higher oil concentrates for trailriding the exhaust just does not burn hot enough and the muffler packing becomes soaked very quickly, but like you for MX I would run 40:1.

1mm of stator rotation at the marks approximates to 1 degree of timing advance or retardation. You will take out midrange hit and maybe a touch of peak but also extend over-rev, the charge burns later, elevates the pipe temp and changes the speed of sound of the wave energy happening in the pipe.

For trail-riding its also about the bike not as constantly being ''on the pipe'' and therefore scavenging efficiently to give a complete burn.

Motul 800 is a very hard oil to beat, worth the extra money alone in extended crankshaft hours.

If you dyno an RM250 you will often find that the powervalve opening curve is not smooth and in total sunchronisation, there is often a noticable step in the powerband. That may have got a little better on later models. Late model YZ250s are very smooth in that respect.

[Danger]

Good info thanks.
Run a Bills Pipe with a few extra pipe gasket washers, a reed spacer (just like Ricky) standard muffler (shorty didn't help over rev) and yes I noticed when I got the bike the lack of over rev, its much better now and very smooth through out the range, very YZ like and great for offroad use, perhaps a dyno would show different.

[cheese]

Hmm I'm about to run out of TTS, and I used to use Motul 800, but went for TTS because its more easily available. Currently running 40:1 with a 8exv (I think it is) plug. My jetting is standard and I want to have a play with it, but to be honnest its running pretty good, but I'm sure it could be fine tuned.

[Reckless]

Yeh Robert generally I agree with you and must qualify my "More oil = more Hp" comment. It was with reference to the 100:1 mixes being put forward as a ratio for fuel mix. I was implying that in my opinion you'd get more HP at 25, 30 or 40:1 than 100:1. But any sane person would realise there is a point where you get to much.
Interesting your where running 30:1 in the gearbox kart, we where at 25:1 so not to far off your premium mix. But our compression and timing could have been different as well. We always run motul and I still do. Some guys where 60-80:1 though. LOL you must have read my plug a million times when I was travelling everywhere with Geoff Downs but kept your ratios pretty close to your chests LOL!! Mind you not many people could beat the great Ian Bias and Rimmer in those days he was certainly king of the gearbox class for many years!

Interesting you say 50:1 for trail riding my manual specifies 60:1 in the Ktm200exc. But I'm running Motul at 40:1 and its not perfectly tuned for max performance. But I'm happy with the amount and way it delivers its power. Doesn't drip oil outa the exhaust or foul plugs and delivers as much smooth power as I can handle so at my level of riding thats good enough! The 200 probably revs out more like a 125 than a 300 anyway so i definatly agree as I have previously said, tuning is way better than varying the fuel mix to cure any issues so I'll stick with my 40:1 I reckon, seems a good happy medium for the odd occasion when I go to Harrisville or Ardmore.

Interesting thread! Who'd have though a simple Spark plug question would lead all the way here LOL!!!

[Danger]

Reckless we were talking 250's with relation to the 50:1 ratio for trail riding using Motul 800.

Personally I also use 40:1 on the 200 so I think your all good.

[camchain]

Feeling more & more comfortable about moving to Motul 800. Reckless using it plus pics of stripped down engines (on KTMtalk.com) looking super clean and good nick prompted me to trying it. Reckless, I knew where you were coming from re oil mix!

[vifferman]

I think us karters and possibly the road race boys have discovered what all the literature I read said that: more oil = more ring seal = more compression = more horse power and as long as your not putting so much oil in that it inhibits the combustion you get more horsepower.

There's another aspect to this (something I learned from a mate who used to race an RD350). Using more oil allows you to run slightly more advanced ignition and/or more compression, as the oil effectively increases the octane rating.