Plug looks good but piston and exhaust port have excessive carbon build up?

[Kenneth S (222)]

Hi Guys,
I pulled my 79 CR 250 down this weekend and was surprised to find so much carbon build up on the crown of the piston and in the exhaust port. It is a worked engine I bought from the US which includes a nice port job and a piston port which I have been trying. I have been keeping an eye on the plug which appears to have a perfect colour of light brown. There is also sum black sticky oil leaking out of the exhaust manifold. Any comments from the jetting experts?
Thanks,
Ken

[Stewart Allen]

I think carbon is more likely to be from oil mix rather than rich fuel/air mix. What oil you using @ what ratio ?

[Kenneth S (222)]

Castrol 2T 30 to 1

[Stewart Allen]

Mineral or Synthetic ?

[oldyzman]

Any possibility its gearbox oil getting sucked into the crank?

[80-85 husky]

The plug is that color when? when you take it out? is it that color during a midrange and or a main jet chop? or are you getting an idle jet color.

Fuel is ?

does the bike run clean through the range?

what air filter oil do you use and is it saturated or just wet through?

That mix will develop some "drool" in your pipe

Try a full synthetic at 50 to 1 after a full clean out. polish the top of the piston and the cyl head a bit to assist carbon denial.

cheers

[JohnnyO]

Try a decent oil like Motul 800 at 40 to 1, you'll have no carbon deposits. I don't see the point in using those old school outdated oils.

[evo550]

Castrol 2T 30 to 1

..as in TTs or R30 ?
The Castrol castor oil leaves massive carbon build up, as J.O says Motul or similar and if it has to be a castor oil ( and why not I say) I'd go with Maxima 927.

[Kenneth S (222)]

I will check the crank seals

Oil is just standard Castrol 2T Mineral oil, not TTS or R30. I have bveen running it for 2 1/2 years and had none of these problems with the other engine using the same oil and mix.

I am using a Twin Air all in one air filter and boot which I used on the other engine as well. Oil is Belray air filter oil sparingly worked through the filter with no excess.

Bike is running really well, strong mid range but doesn't want to rev out like the other engine did.

Plug colour is from after a hard ride. I wasn't jetting so to speak, I was practicing.

Since I posted this question, I found a crack approx 15cm long in the belly of the exhaust and a mate said that could be contributing to it, excess fuel is not exhausting properly.

[80-85 husky]

plug will read exactly what it was doing when the spark stops so if it was idling for 5 secs after a hard run, it will reflect the idle mixture.

sounds like your rich on the main jet which is stifling the rev out and the mid range (needle jet) is ok as its pulling well through there. you need to do a  plug chop with the throttle wide open in 4th gear ( just jam the back brake on and lock her up when it hits about 3/4's through the rev range), don't back off when you hit the brake until the motor is dead...( massive brake slide for sure .) don't forget to pull the clutch after the motor has stopped and roll out. takes a good long dirt road to do this properly

Now pull the spark plug out and look (yep its f*kn hot!). it should be that nice choccy color but im betting it may be dark brown to black if your rich on the main jet.

Fix your pipe then try this

[Lozza]

Carbon build up is caused by a combination, mainly jetting and ignition. You oil brand/ratio has no real bearing on the carbon build up. A well tuned 2 stroke should get a bit of slight surface rust inside the pipe if it hasn't ran for a couple of weeks.
The crack will just be costing HP, still wondering what 'exhausting' is ?

[John Orchard]

Are you still running a 34 year old carburetor on the bike?  Trust me, if you are, the needle jet will be worn making it bulk rich at mid throttle.

The plug colur difference could be, as said earlier, running the bike at different fuel circuit before turning the bike off.  Plug colour is basically a sign of combustion chamber temperature, hotter temps tend to burn a plug clean.

I really think any two-stroke oil is cool to use, I feel mineral based oils make it easier to tune via plug colour.  In my RM250Z I use to run it on the cheapest Valvoline two-stroke oil I could find, after 12 months of riding it 3 days a week, there was still no signs of blow-by past the single piston ring! I might want a little extra protection for a 125 kart or roadracer though.

Do some plug-chops, you are miles too rich somewhere, there is a lot more horsepower and throttle response to be found.

[Simo63]

Try a decent oil like Motul 800 at 40 to 1, you'll have no carbon deposits. I don't see the point in using those old school outdated oils.

Interesting point of view John. 

My experience is that I've been using good old fashioned mineral based Castrol 2T in my VMX bikes at a very rich 20:1 since the 70's and never, ever had any problems with carbon build up.  A few months back now I pulled my 77 YZ250 and 77 YZ400 down and aside from a nice coating of oil over all the crank, piston etc and a very neat light brown burn patterns (a bit heavier in the lower revving 400), the internal workings were perfect. 

To be honest I couldn't be happier with this oil mix.  I know quite a few others that also run it without any problems.

[JohnnyO]

It does sound like you have other contributing issues, possibly too rich as others have mentioned or you may even have an ignition problem.
Has it always run like that or only just started doing it?

[JohnnyO]

Try a decent oil like Motul 800 at 40 to 1, you'll have no carbon deposits. I don't see the point in using those old school outdated oils.

Interesting point of view John. 

My experience is that I've been using good old fashioned mineral based Castrol 2T in my VMX bikes at a very rich 20:1 since the 70's and never, ever had any problems with carbon build up.  A few months back now I pulled my 77 YZ250 and 77 YZ400 down and aside from a nice coating of oil over all the crank, piston etc and a very neat light brown burn patterns (a bit heavier in the lower revving 400), the internal workings were perfect. 

To be honest I couldn't be happier with this oil mix.  I know quite a few others that also run it without any problems.

Simo I work on bikes every day and from what I've seen inside engines those using quality modern synthetic oils at 40 or 50 to 1 are cleaner with less carbon, less oily deposits running out the exhaust and run cleaner and crisper than the others. The wear factor is also very good, a lot of these bikes do way more hours than we do. I want the best in my bikes, I've used all those other oils 20 or 30 years ago and I really believe the newer stuff is better.
Modern technology is a good thing and oils have come a long way..