When I pulled the cylinder off of my 2001 yz 250 today, I was shocked to see that the intake ports were completely black. also, the bottom of the piston was completely black. The only thing I can think of is that the rings were so worn that the exhaust was blowing by into the crankcase, and being drawn through the ports back into the cylinder. This is the frist time I have had this bike apart, so maybe I just waited too long? I normally put 2 to 3 pistons in my 125 per year, but I thought I could get away with only one rebuild per year since this is a 250. Anyway, if this is what happened, should the cases be split, and the main bearings be inspected for damage? The last thing I want is for the crank to sieze on the face of a triple. Or was it something completely unrelated to worn rings? Thanks.
2001 yz 250 intake ports full of carbon
- Thread starter yz_387
- Start date
aharoncross
Member
- Oct 22, 2001
- 51
- 0
Look at Eric article
Eric poste an articale "PISTON DIAGNOSTIC GUIDE BY ERIC GORR" and he describe a similar situation like you have :
"BLACK SPOT HOT
The underside of this piston has a black spot. The black spot is a carbon deposit that resulted from pre-mix oil burning on to the piston because the piston's crown was too hot. The main reasons for this problem are overheating due to too lean carb jetting or coolant system failure.
"
you can see the all article at Eric Gorr articles - Piston Diagnostic
Eric poste an articale "PISTON DIAGNOSTIC GUIDE BY ERIC GORR" and he describe a similar situation like you have :
"BLACK SPOT HOT
The underside of this piston has a black spot. The black spot is a carbon deposit that resulted from pre-mix oil burning on to the piston because the piston's crown was too hot. The main reasons for this problem are overheating due to too lean carb jetting or coolant system failure.
"
you can see the all article at Eric Gorr articles - Piston Diagnostic
- Thread starter
- #3
Thanks aharoncross. I did check out Eric Gorr's sight, and like you said that does seem to describe my piston. However, I find it hard to believe that the intake ports on the cylinder would have been hot enough to cause the same result there. If you are right, then I have a whole new set of problems to worry about, because I have no idea as to why my bike would have overheated so badly, or for that matter when (I have never even had to add any coolant to it, so it has never boiled over). Do you have any experience with this type of overheating problem, should I worry about crank/main bearing damage? Thanks for your help.
aharoncross
Member
- Oct 22, 2001
- 51
- 0
I used to have the "black spot hot" problem at my CR125 1993, But I thought it was normal (silly me..:silly: )
My intake ports ware not black at all.
In case your engine suffer the "Black Hot spot”, I guess the reason to black intake port is having this problem for long time and the intake ports are getting black just after the piston get black, But I am far from being an expert so don't take my words for granted.
May be the intake ports got black some time in the past (before you bought it), and the guy that sold it to you already fix it so you can't find any reason for it now.
Thanks
My intake ports ware not black at all.
In case your engine suffer the "Black Hot spot”, I guess the reason to black intake port is having this problem for long time and the intake ports are getting black just after the piston get black, But I am far from being an expert so don't take my words for granted.
May be the intake ports got black some time in the past (before you bought it), and the guy that sold it to you already fix it so you can't find any reason for it now.
Thanks
- Thread starter
- #5
Thanks again aharoncross. However, I bought the bike new, in fact, I even brought it home in the crate, so I know any problem must have occured when I was riding it. When you experienced the black hot spot problem did you lose any coolant, or have any other signs that you might be overheating the engine?
Anyone have any opinions about the main bearings?
Anyone have any opinions about the main bearings?
- Nov 13, 2001
- 1,178
- 0
I had this problem with my '92 CR125 when I tore down the topend this winter cause my bike had gone low on compression I found my piston had a black hot spot on the piston and some black going down the ring area. My intake port wasn't black at all tho but maybe because me and aharoncross have a case reed design and you have a cylinder reed desgn?:think
To answer your questions I did not lose any coolant, There was no signs of my bike overheatngd and if anything my bike was running way to rich cause I kept fouling plugs after a few hours of riding. To check your main bearings all you got to do is remove your magneto and lift the end of the crankshaft up and down to see if there is any play. I doubt your main bearings are bad tho.
To answer your questions I did not lose any coolant, There was no signs of my bike overheatngd and if anything my bike was running way to rich cause I kept fouling plugs after a few hours of riding. To check your main bearings all you got to do is remove your magneto and lift the end of the crankshaft up and down to see if there is any play. I doubt your main bearings are bad tho.
Pete Payne
MX-Tech Suspension Agent
- Nov 3, 2000
- 933
- 38
Carbon build up in the intake ports normaly comes from too short blow down timing. The burning previous air/fuel charge did not have enough time to leave the cylinder before the new fresh air/fuel charge enterd the cylinder. This cuased the fresh charge to burn down into the intake ports.
Or from excessive blowby from the rings.
Or from excessive blowby from the rings.
aharoncross
Member
- Oct 22, 2001
- 51
- 0
Losing coolant was very common on my CR125 since I smutch my radiator, so I can't tell if I was loosing coolant because of over heating or because my radiator was leaking.
I don't remember my engine got any excessive over heating.
Thanks
I don't remember my engine got any excessive over heating.
Thanks
- Thread starter
- #9
Pete,
I was beginning to come to the conclusion that the carbon was caused from burning back into the ports, so what you said makes sense. But what actually causes this? Retarded spark timing? Engine geometry? Wave dynamics of the pipe? Thanks to everyone for their help.
I was beginning to come to the conclusion that the carbon was caused from burning back into the ports, so what you said makes sense. But what actually causes this? Retarded spark timing? Engine geometry? Wave dynamics of the pipe? Thanks to everyone for their help.
Pete Payne
MX-Tech Suspension Agent
- Nov 3, 2000
- 933
- 38
YZ_387
It is in the geometry of the engine.
Also if the powervalves were stuck in the closed position there would not be enough exhaust area for the exhaust gases to leave the cylinder . The powervalve opens up at higher RPM's to make the axh port bigger . More area is needed for higher RPM's sincew the window of the exh port stays open for a shorter time.
It is in the geometry of the engine.
Also if the powervalves were stuck in the closed position there would not be enough exhaust area for the exhaust gases to leave the cylinder . The powervalve opens up at higher RPM's to make the axh port bigger . More area is needed for higher RPM's sincew the window of the exh port stays open for a shorter time.
Pete Payne
MX-Tech Suspension Agent
- Nov 3, 2000
- 933
- 38
It does not account to the black under the piston . And I have not seen it hurt the crank or bearings . Although I have seen a few cooked reeds.
- Thread starter
- #14
yam 3, I guess nothing really came of it. I have a friend who also has a 2001 yz250 and it did sort of the same thing, only to a lesser extent. I never really did get one answer that would account for both the black piston and the black intake ports, only one or the other. I just cleaned everything up and put it back to gether. I haven't run it yet though, its still winter here in MI.
Pete Payne
MX-Tech Suspension Agent
- Nov 3, 2000
- 933
- 38
The way to solve this problem is to have your cylinder ported, to correct the blown down problem . Have the cylinder head moded for the type of fuel that you are allways going yo run .And have the cylinder honed for proper cross-hatch to aid in seating the rings. Then do a proper break-in and enjoy!!!!
- Thread starter
- #16
Pete,
Thanks for all the advice. I was discussing the idea that fuel may have played a role in the carbon buildup with a friend who has the same bike so I find it interesting that you mentioned it in your last post. We both run a 50/50 mix of cam2/premium unleaded (110/93 octane) and were wondering if this might be partially to blame. We both had 98 yz125's that ran excellent on that combo, so we just continued using it when we bought 250's. Should we have uesd a different fuel? I also wondered if I could get away with advancing the spark slightly to try to get all of the fuel burned before the transfer ports open. Thanks again.
Thanks for all the advice. I was discussing the idea that fuel may have played a role in the carbon buildup with a friend who has the same bike so I find it interesting that you mentioned it in your last post. We both run a 50/50 mix of cam2/premium unleaded (110/93 octane) and were wondering if this might be partially to blame. We both had 98 yz125's that ran excellent on that combo, so we just continued using it when we bought 250's. Should we have uesd a different fuel? I also wondered if I could get away with advancing the spark slightly to try to get all of the fuel burned before the transfer ports open. Thanks again.
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