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Old 01-10-2013, 12:09 PM   #46
Nihon Newbie OP
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So I am having factory clone head gaskets made for the scoot. The oil leakage is coming from random places because it seems like the rings aren't seating right, or the end gaps got aligned somehow. The entire crankcase is way overpressurized, and the bike is lacking some power.

If I remove the timing cap on the stator cover while it's running, I can feel almost as much exhaust coming out as I can from the tailpipe, lol. What does it feel like on your scooter if you remove the timing cap while it runs? I know there is always blow-by, but this is way too much. So, as soon as I get my head gaskets, I'll go ahead and tear it down, hopefully for the last time, and figure out why there is so much piston blow-by...

Speaking of head gaskets...What do you guys do about replacing the OEM one when you pull the head??
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Old 01-10-2013, 01:51 PM   #47
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Exhaust plugged up?
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Old 01-10-2013, 05:01 PM   #48
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Have you explored the crankcase ventilation system? There's a good bit of information on www.ch250.net in the tech tips. If the ventilation system is plugged you will get a lot of pressure just from the single piston going up and down, and that pressure has to go somewhere. For some reason people seem to think that ripping off the ventilation system will released massive amounts of undiscovered horsepower, and so they do get vandalized from time to time. And, the engineering Honda used in the design was not great either.
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Old 01-10-2013, 09:42 PM   #49
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Originally Posted by Wentwest View Post
Have you explored the crankcase ventilation system? There's a good bit of information on www.ch250.net in the tech tips. If the ventilation system is plugged you will get a lot of pressure just from the single piston going up and down, and that pressure has to go somewhere. For some reason people seem to think that ripping off the ventilation system will released massive amounts of undiscovered horsepower, and so they do get vandalized from time to time. And, the engineering Honda used in the design was not great either.
Short answer: Yes, and that is not the problem, but thank you.

I know the captive-ball hiss vent in the stator cover is working properly. The CA model, my motor, has a hose that goes from that nipple to a cylindrical plastic condenser chamber, then over the head and into a filter in the airbox, where the liquids drain into a tube. I know none of these are obstructed, and though the motor runs a little easier when I hold that captive ball in place, with no tube connected, there is still an inordinate amount of exhaust escaping past the cylinder, down into the crankcase. The thing I really don't get is how exhaust gasses could be getting out from the housing around the crank itself, directly below the piston and con-rod. I don't want to pull the flywheel off, as I don't have the right tools to do it correctly...

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Exhaust plugged up?
Nope. Tested that before I put the whole motor back together. Even filed down the factory welds on the inside of the pipe.
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Old 01-12-2013, 04:39 PM   #50
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I've never really had much luck with ball hones as they only put crosshatch on the cyl walls but don't take off any high spots, Try a good hone on it.even if it means maybe a little looser fit, its not a high revving motor .
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Old 01-13-2013, 01:02 PM   #51
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I've never really had much luck with ball hones as they only put crosshatch on the cyl walls but don't take off any high spots, Try a good hone on it.even if it means maybe a little looser fit, its not a high revving motor .
That makes sense, but I still just don't understand how new rings in a freshly honed bore could lead to this much blow-by, regardless of a less-than-perfect hone...
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Old 01-13-2013, 03:03 PM   #52
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If the old rings had worn to a slightly out of spec cyl.they would probably seal better than new perfectly round rings.
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Old 01-15-2013, 06:12 AM   #53
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"The fuel line from the tank to the carb had sprung a leak. Clipped the hose and that was solved. The other leakage all appears to be coming from the valve cover...This has really got me stumped, because as far as I can tell, there shouldn't be anything more than crankcase pressure in there. The only thing I can think of is that, somehow, exhaust gasses are making it past the valve guides and into the cover? The oil looks fine, still clear, not burnt, not aerated or milky, but the stuff coming out of the valve cover, right where it is supposed to seat on top of the cylinder head, is almost foamy. When I reinstalled the o-ring into the valve cover, I used Hondabond as well, which didn't seem to help, as it leaked a little when I rode it today..."

I noticed in your photos that the rockers pivot in the valve cover. My Honda FT500 Ascot of many years ago had the same arrangement. I am not a mechanic and have no idea how common this rocker arrangement is with Honda or other engine makers, but I felt like it was problematic in my Ascot. He'll, my Ascot didn't even have your o-ring, just metal to metal. I am pretty sure all Ascots leaked from the valve cover. It is my understanding that no real valve cover gasket was used by the factory because the squish in the gasket would make tappet adjustments difficult if not impossible. This was made worse by owners, seeing the leaking valve cover, would try to tighten the cover bolts, stripping them out, so then you had fewer bolts holding the cover down. I read that Yamabond was the best for solving the valve cover leaking. Worked great for me.
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Old 01-15-2013, 04:56 PM   #54
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Custom head gaskets should be here soon. Hopefully my hypothesis about the piston rings will prove to be accurate, so I can hone the bore, or maybe even switch back to the old rings, and be done with it! If I can get the motor running well again, I'm going to Plastidip the wheels and bodywork and sell it ASAP.

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I noticed in your photos that the rockers pivot in the valve cover. My Honda FT500 Ascot of many years ago had the same arrangement. I am not a mechanic and have no idea how common this rocker arrangement is with Honda or other engine makers, but I felt like it was problematic in my Ascot. He'll, my Ascot didn't even have your o-ring, just metal to metal. I am pretty sure all Ascots leaked from the valve cover. It is my understanding that no real valve cover gasket was used by the factory because the squish in the gasket would make tappet adjustments difficult if not impossible. This was made worse by owners, seeing the leaking valve cover, would try to tighten the cover bolts, stripping them out, so then you had fewer bolts holding the cover down. I read that Yamabond was the best for solving the valve cover leaking. Worked great for me.
Thanks for the input. I think I mentioned a few posts back that it's actually the entire crankscase that is weeping oil, because exhaust gasses are making it past the piston rings and pressurizing the cases, forcing oil out from many places. The place it was leaking the most was the valve cover, but only because it was sealed in the least effective way, with just a rubber o-ring. Again though, thanks for the info.
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Old 01-15-2013, 06:34 PM   #55
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Originally Posted by Nihon Newbie View Post
Custom head gaskets should be here soon. Hopefully my hypothesis about the piston rings will prove to be accurate, so I can hone the bore, or maybe even switch back to the old rings, and be done with it! If I can get the motor running well again, I'm going to Plastidip the wheels and bodywork and sell it ASAP.



Thanks for the input. I think I mentioned a few posts back that it's actually the entire crankscase that is weeping oil, because exhaust gasses are making it past the piston rings and pressurizing the cases, forcing oil out from many places. The place it was leaking the most was the valve cover, but only because it was sealed in the least effective way, with just a rubber o-ring. Again though, thanks for the info.
I am not a mechanic, so expanding gas gets past the rings into the case, how does the expanding gas then make it into the valve cover? Maybe you are saying the gas is making it's way past the valve guides, but that has nothing to do with piston rings?
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Old 01-15-2013, 09:57 PM   #56
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I am not a mechanic, so expanding gas gets past the rings into the case, how does the expanding gas then make it into the valve cover? Maybe you are saying the gas is making it's way past the valve guides, but that has nothing to do with piston rings?
Well, I know there is a kind of enclosure around the cranks inside the main engine case, but I don't know if that is sealed. I'm guessing not, because as I mentioned, if I take off the timing hole cap on the side cover, that lets you look at the markings on the flywheel, exhaust gas is coming out. That whole cavity in there where the flywheel is, is connected to the area up top by the valve cover by way of the tunnel where the timing chain lives. The chain goes down from the camshaft sprocket and wraps around the gear teeth directly on the crankshaft. This is what lead me to believe that the rings are the issue at hand.

I wish I had my GoPro so I could just post a video! Haha. Might have to break out my DSLR and get some video the hard way...
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Old 01-16-2013, 04:53 AM   #57
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That makes sense, and I was thinking that oil must somehow make it's way to the rockers from the crankcase and exhaust gases could be coming up with the oil. I wonder if Honda learned from the FT 500 and that is why your scoot has the
o-ring.
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Old 01-16-2013, 12:47 PM   #58
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Originally Posted by andoulli View Post
That makes sense, and I was thinking that oil must somehow make it's way to the rockers from the crankcase and exhaust gases could be coming up with the oil. I wonder if Honda learned from the FT 500 and that is why your scoot has the
o-ring.
That job is handled by the oil line, that carries oil up from the sump and into a drip hole in the top of the valve cover to lubricate the entire valvetrain. In theory there is always oil flowing into that pipe, and the excess pressure in the cases would only increase oil pressure, not flood that line with gas. In theory...Ha.

My friend had an FT500 a few years ago. I never worked on the motor, all I ever did with it was heat-wrap the headers and exhaust pipes.
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Old 01-17-2013, 09:33 AM   #59
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So, quick update, I just found out that the new head gaskets won't be here for 2-3 weeks...
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Old 01-19-2013, 04:48 PM   #60
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So, tore the motor open again today. Rings and bore were not the issue. Ring end-gap is spot on, cylinder bore itself is great too. Looks like the MLS head gasket gave up. Had coolant and oil all over every layer of the gasket, and the head showed some scorching marks around the firing ring area on the exhaust side. Don't know how this is possible, but the pressure in the engine cases still seems really high. Apparently it's not exhaust slipping past the rings, like I thought. Piston skirt looks fine, not scorched at all.

Where else could exhaust be making its way into the cases? Between the steel gasket layers and into the timing chain tunnel, but not into the coolant passages like before?

To top it all off, it looks like it was burning a lot of oil, too. The dome in the head, exhaust runner and valve face were both coated in sticky black gunk, none of which was there when I tore it apart the first time. It cleaned off with some effort, but I don't know how that oil was making its way in there anyway...Intake valve and runner looked perfect.

Any ideas, folks?? I'm dying here, waiting for new head gaskets to arrive in 2 weeks...:facepalm:
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