OzVMX Forum
Clubroom => Tech Talk => Topic started by: Kenneth S (222) on August 18, 2013, 08:01:48 pm
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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
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I think carbon is more likely to be from oil mix rather than rich fuel/air mix. What oil you using @ what ratio ?
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Castrol 2T 30 to 1
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Mineral or Synthetic ?
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Any possibility its gearbox oil getting sucked into the crank?
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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
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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 I don't see the point in using those old school outdated oils.
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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.
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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.
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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 :o ;D.) 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
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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 ?
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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.
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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.
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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?
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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..
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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.
Modern technology is a good thing and oils have come a long way..
Interesting John. I don't work on them every day but I just don't have a reason to change what has worked for me for 30 plus years. I've never had any problems to be honest.
When I was road racing my RS125's back in the 80s and 90s I used A747 as it was a mineral based and synthetic blend but the engines were revving so much harder. And I have friends in the go karting world that use it for the same reasons.
I don't know mate, something about old dogs and new tricks but do you honestly think I would notice the difference? I ask that because I am the sort of person that doesn't like to change oils at all. And by that I mean if an engine has been running on oil A then I just could not change to oil B.
Also my bikes are jetted pretty well on Castrol 2T at 20:1 mix and I don't have any issues (except my Evo but I'm blaming Carl for that ;D)
Anyway, I digress, this thread is about Kenneths carboning up problems not my resistance to any change ;D I'm always happy to listen and learn so maybe at Toowoomba we can discuss further?? That would be 8)
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Fair enough Simo we all have our own experiences & opinions. Happy to discuss it any time..
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Just an observation on the topic, at the recent conondale classic there was nearly always someone on the start line beside me who's bike was running so rich they were constantly revving the ring out of it to try and clear it out for the start..
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face it guys, not a real lot of people can figure jetting and that magic art of tuning all that well and things stay standard...
I took up racing when a mate conned me into it. he was on his second PE 175. I bought a PE 250 X and quickly figured it wasn't right... Mr McHanic jetted and timed and exhausted the muffler and she was off like a shot!
My mate (who I thought had half a clue) was staggered at the difference and rushed off to get his bike done..he had been riding them stock out of the crate with just the exhaust washer removed and the airbox cover tossed... he had no idea what was available in those engines..so it doesn't suprize me at all when people are running what it has in it.
they are either to stubborn to fiddle or too lazy or lack confidence in their ability to sort things out. (face it , trying to shift a needle clip in the scrub is one of lifes high tension points...)
its like 2- oil...look at the harrow video and some of those old smokers are near on fire there is so much oil in the mix......and that xl 250 s
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Just an observation on the topic, at the recent conondale classic there was nearly always someone on the start line beside me who's bike was running so rich they were constantly revving the ring out of it to try and clear it out for the start..
Lol, that might have been me on the Evo (YZ250H) .. but like I said, I'm blaming Carl for that. It's a new Mikuni but I was trying bigger jets to overcome my detonation problem but I went too far :-[
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Thanks guys for your comments,
John, to answer your question, I had a borrowed 38mm Mikuni in it as the guy who worked the engine had recommended the 38mm Mikuni to match his port job. I have now installed a brand new 38mm Mikuni so I'll have to start again. This thread has been good though, I always thought if the plug colour was right, then the jetting was the best it could be. There seems to be some good analysis I can work with. By the sounds of it the main is too big. Exciting really because it is going like a train now and if it has that much more horsepower and throttle response with the jetting sorted out, woo hoo!
Ken