CARBON BUILD UP.. Making valves stick open?


10 replies to this topic
  • OUCR250R

Posted April 22, 2007 - 05:51 PM

#1

Today when my brother went to start his 01 YZ426, it had absolutely no compression (he still has the original exhaust cam in it as well). The bike ran fine when he parked it. It uses no oil and starts first kick every time. The bike has been running VERY rich for the entire time he has owned it, approx 1 year. We could hear the pressure from the cylinder excaping through the carb, so we figured the valves seats might be worn out, but couldn't understand why it would be running so strong one day and then just go out without any warning or symptoms. We took the head and carb off and did the carb cleaner in the intake port test and found that 2 of them were leaking. What we also found was an ENORMOUS amount of carbon build up all over EVERYTHING. We cleaned up the carbon and opened each valve and cleaned the carbon build up off of the seats and tried the test again. This time no leaking. I could not believe what was happening here so we put the head back on with the old head gasket (just to test, I am waiting for my new one to get here for final assembly), and kicked it over, the compression was just like it had been the day before when it was running perfectly. we checked the valve clearances and the .005 gauge fit perfectly (the manual said the service limit was .0039). Anyone heard of anything like this before?

  • grayracer513

Posted April 22, 2007 - 08:17 PM

#2

It's not uncommon. One way to prevent it from sticking open is to get in the habit of rolling the bike up on compression to close the valves when you park it.

  • CrusRuss

Posted May 29, 2007 - 06:13 PM

#3

Today when my brother went to start his 01 YZ426, it had absolutely no compression (he still has the original exhaust cam in it as well). The bike ran fine when he parked it. It uses no oil and starts first kick every time. The bike has been running VERY rich for the entire time he has owned it, approx 1 year. We could hear the pressure from the cylinder excaping through the carb, so we figured the valves seats might be worn out, but couldn't understand why it would be running so strong one day and then just go out without any warning or symptoms. We took the head and carb off and did the carb cleaner in the intake port test and found that 2 of them were leaking. What we also found was an ENORMOUS amount of carbon build up all over EVERYTHING. We cleaned up the carbon and opened each valve and cleaned the carbon build up off of the seats and tried the test again. This time no leaking. I could not believe what was happening here so we put the head back on with the old head gasket (just to test, I am waiting for my new one to get here for final assembly), and kicked it over, the compression was just like it had been the day before when it was running perfectly. we checked the valve clearances and the .005 gauge fit perfectly (the manual said the service limit was .0039). Anyone heard of anything like this before?



Hi, I think I have the same problem on my RMZ 450.. it ran fine and then I parked it for a week or two and when I went to start it it was lacking compression and I couldnt find the "hard spot" to kick it - it was like an exhaust valve was stuck open.

What I am wondering is, can you take the head off and clean the valves and valve seats (as you say) without having to take the valves actually out, because I dont have a valve compressor tool to put them back in! I know I will need to get a new gasket if I do this anyway...

Thanks for any advice or tips!

  • grayracer513

Posted May 29, 2007 - 08:10 PM

#4

One thing you might try first is to make a point of pushing the engine up against compression when you put the bike away. Sometimes, what actually happens is the the valve seat, without the valve sitting on it, will rust enough to keep the valve from sealing after a couple of weeks.

The ony way to clean the carbon off the valves effectively is to disassemble them.

  • CrusRuss

Posted May 29, 2007 - 08:49 PM

#5

Well the valves are titanium and the head is all alloy... so I didnt think it could rust. A mechanic friend also told me I would need to do 100,000k's to get enough carbon build up to clog a valve... so maybe its something else :)

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  • Ga426owner

Posted May 30, 2007 - 04:34 AM

#6

The type of fuel and oil type used plays a big part on how much and how fast carbon will build up.

  • OUCR250R

Posted May 30, 2007 - 04:51 AM

#7

A mechanic friend also told me I would need to do 100,000k's to get enough carbon build up to clog a valve... so maybe its something else :)


I can assure you that your mechanic friend is wrong about the 100k's comment. All it takes is a bike that is running a little on the rich side, which makes it build up carbon quicker than usual. All we did to fix the problem was pull the head, open each valve individually by pushing on the top of each valve (after removing the shims and buckets) with something softer than the valve itself, i.e. wood block (this is a two man job) and spray it down with a healthy dose of carb cleaner and it cleared up the carbon deposits on the valve and seat. Then we put the head back on and the motor was back to running better or at least as good as it was before. It's a cheap option when compared to unnecessarily rebuilding your head. At least check it out. I assume you have already checked to make sure that your decompressor is not causeing the problem?

  • grayracer513

Posted May 30, 2007 - 06:00 AM

#8

Well the valves are titanium and the head is all alloy...

And the valve seats are iron.

  • CrusRuss

Posted May 30, 2007 - 08:25 PM

#9

I can assure you that your mechanic friend is wrong about the 100k's comment. All it takes is a bike that is running a little on the rich side, which makes it build up carbon quicker than usual. All we did to fix the problem was pull the head, open each valve individually by pushing on the top of each valve (after removing the shims and buckets) with something softer than the valve itself, i.e. wood block (this is a two man job) and spray it down with a healthy dose of carb cleaner and it cleared up the carbon deposits on the valve and seat. Then we put the head back on and the motor was back to running better or at least as good as it was before. It's a cheap option when compared to unnecessarily rebuilding your head. At least check it out. I assume you have already checked to make sure that your decompressor is not causeing the problem?


I have the head off at the moment and it looks ok, its a bit dirty but thats to be expected for 100 hours. The valves move ok but there is a bit of crud around the valve seats, so I am going to do as you suggest and also do some rings because there is a small amount of wash on the piston.

I would love to check my auto decompression but I dont know how to. I dont know what to look for. I dont even really know what it is, I know its somethihng on the exhaust cam that operates on centrifugal force but the only thing I can see on there is a little spring thing and it looks ok... if someone can point me in the right direction or tell me what I need to look for I will be happy to inspect it.

Personally I think its my rings because the engine compression feels a bit mushy when the engine is code, but when it heats up (from repeated kicking or when i have had the bike started in the days previous to pulling the head off) it would feel ok. Also with the head off I can almost see down the side of the piston to the oil ring at some points!

  • grayracer513

Posted May 30, 2007 - 08:58 PM

#10

You'd get more accurate specific info on the RMZ forum, but in general, the AD mechanisms all work on the same principal. At speeds lower than idle (about 700 rpm and less on a YZF) the device causes one exhaust valve to lift off its seat as the compression stroke begins, and allows it to reseat somewhere around 80 degrees BTDC. The effect is that the compression stroke is shortened, and the engine develops about 100 pounds of compression, instead of 200. Once it starts, the system disengages centrifugally, as has no effect on any aspect of the engine's operation.

  • rexbond007

Posted May 30, 2007 - 10:53 PM

#11

Are you using low octane fuel? when i had my RMZ 250 i checked valves every 10 hrs. (i did not trust Suzuki!)
one time i checked and i had loose valves, i had started using cheap low octane gas.
my Dealer told me to use high octane and blow the carbon out of my valves.
one tank later on high octane. the Valves were back in Spec.





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