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Some time last year I figured my kdx 220 was running to rich. The exhaust had an dirty 2stroke oily outlet and it was eating spark plugs.
I turned the air screw of the carburettor out another half a turn. (It might be more but I can't remember). Anyway - everything seemed to work and much less spooge!.
Continued like this for another two rides / 5 hours riding time.
At the end of the last ride I noticed that the coolant were being pushed out the overflow pipe. The bike was hot!
Went home and started with investigation. Initially I thought there was a leak with the cooling system. Fastened all the pipes, filled up with new coolant 50/50 and took her for a ride.
Got home and heard the bubble noise in the radiator.
Is the coolant boiling? This means that there is no pressure in the radiator. Faulty radiator cap ?
Next day started her up - immediate bubbling noise in the radiator - bike is cool!
Air must be forced into the radiator via the head gasket.
I have drained the coolant. There is definitely 2stroke oil / carbon in the coolant.
So guys .. I now have to order a new head gasket.
No loss of performance was experienced during/after the coolant loss.
The question is, did the fiddling with the air screw really result in the blown head gasket...

No highly doubt it.. The air-screw adj air for the pilot jet. The pilot is only for idle and low rpm. If you were too lean you would have had a bog down low just off idle. As long as your throttle response is crisp you got it right. If you need to turn it out a ton you need a smaller pilot. If you are under 1 turn you need a larger pilot.

BTW I would recommend a OEM gasket. Rockymountainatv has them. The after markets are fine but I noticed they are just embossed steel. The OEM ones are too but have a light silicone type coating on them.

Ran a fully built high octane race motor with no air/fuel screw with no harm done to my motor. Fell out, twice and didn't know it.

And I believe it's the other way around s10gto. If you're more than 2 turns out you need to go with a bigger pilot.

Thanks but No I am correct. The screw controls the air flow to the jet not the fuel. Turn screw out for more air to mix with the pilots fuel. The pilot only fuels what it can flow.

Specs are not for a KDX but same carb.

Yep, you're right. And i just went through this process too. Must be this cold weather.

Now if I can only get the wife to say that!

s10gto wrote:No highly doubt it.. The air-screw adj air for the pilot jet. The pilot is only for idle and low rpm. If you were too lean you would have had a bog down low just off idle. As long as your throttle response is crisp you got it right. If you need to turn it out a ton you need a smaller pilot. If you are under 1 turn you need a larger pilot.

I am no expert at this but I was under the impression running too lean can cause a head gasket failure.
What else could have been the culprit?
I bought a new OEM head gasket for the 220. Did not know there is a difference between the 200 and 220 - gasket thickness.

Lean can cause a head gasket failure ,but the air screw is only controling up to 1/8 throttle. After that the needle and main take over.

The difference of the 200 gasket and 220 will be the bore dia.

Was the top end done? If so was the topend re-torqued after some heat cycles? If not that can do it.

Also before you change that gasket check the head for warpage. Use a straightedge and a .001 thickness (feeler) gauge. If out of spec you can resurface it yourself with a thick piece of glass and 400-600 grit wet paper.

I can also resurface it at the machine shop for you. Just cover shipping. Glad to help

s10gto wrote:Lean can cause a head gasket failure ,but the air screw is only controling up to 1/8 throttle. After that the needle and main take over.

The difference of the 200 gasket and 220 will be the bore dia.

Was the top end done? If so was the topend re-torqued after some heat cycles? If not that can do it.

The difference between the 2 gaskets is only the gasket thickness. The are exactly the same size.
I bought the bike 3 years ago and have not done a lot of riding - max once every 2 months for three to four hours (funduro events).
Id say I have put max 60 hours on it since the previous owner.

didn't know that about the gasket. Interesting. Thanks

I think that is incorrect about the gasket. The 200 has a 66mm bore and the 220 a 69mm bore. I doubt that the bore size is the same between the two gaskets.

TheRadBaron wrote:I think that is incorrect about the gasket. The 200 has a 66mm bore and the 220 a 69mm bore. I doubt that the bore size is the same between the two gaskets.

Ok, maybe I am mistaken... will check again...

Most probably just a head gasket gone bad, allowing compression loss which caused it to run hotter, all the while the coolant system was over-pressurized by the added pressure from the leaking gasket , causing the coolant to overflow.

How can running lean cause a head gasket failure?

To answer your question jjavaman:
Running too lean can cause pinging or detonation. Doubt the Idle air screw had anything to do with it, more likely just a gasket failure as already mentioned.

Detonation (also called "spark knock") is an erratic form of combustion that can cause head gasket failure as well as other engine damage. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to auto-ignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.
Mild or occasional detonation can occur in almost any engine and usually causes no harm. But prolonged or heavy detonation can be very damaging. So if you hear knocking or pinging when accelerating or lugging your engine, you probably have a detonation problem.

s10gto wrote:Lean can cause a head gasket failure ,but the air screw is only controling up to 1/8 throttle. After that the needle and main take over.

The difference of the 200 gasket and 220 will be the bore dia.

Was the top end done? If so was the topend re-torqued after some heat cycles? If not that can do it.

That's what I was replying to. I'm trying to figure out how being lean can cause a head gasket failure?

^^ My 2c is it can't. MAYBE the head gasket can fail due to extreme heat, but doubtful. I'd venture to say a lean condition this bad would be obviously detonating, and hole a piston before this would happen. Circumstantial failure.