Problems with crankcase building pressure - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 17 (permalink) Old 06-10-2016, 01:37 PM Thread Starter
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Problems with crankcase building pressure

I need help. I got a new engine with 500 miles on it. Installed in and had some problems. It has the flapper mod, Pair removed with block off plates. What is happening is the crankcase is building pressure and it first blew out the seal around the front pair blocker plate. Re sealed it and i checked the breather and it was free no kinks or anything. Started ran for 30 min and it blew out the stator cover gasket and while running i can feel air coming through the hole it blew in the gasket. My question is if the o ring between the head and valve cover on that dowel is bad will it cause it to build a lot of pressure? After that second ride there was about a inch and a half of oil in the pink breather tube.
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post #2 of 17 (permalink) Old 06-10-2016, 02:02 PM
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Well the o ring could cause it, you would be blowing exhaust into the crank case. Also for the block off plates I have found that it's best to uses the factor o ring seal, I have used only them taken off the reed valve plate and I have also used it with the plate with the reeds and screws removed. The other worse case senerio that can cause it is rings not sealing and a lot of blow by. The hose is not pinched or something going into the airbox is it? Also another thing to consider on the pair is it should not be getting crank case pressure, only exhaust. So those two things should not be related unless there is a bad seal in between them.

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post #3 of 17 (permalink) Old 06-10-2016, 02:19 PM Thread Starter
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Yeah I was thinking about just using the factory seal with the screw and plate removed. When the plate blew out the first time it gushed out oil. The pair port where the plate is at shouldn't have oil coming out of it should it?
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post #4 of 17 (permalink) Old 06-10-2016, 02:42 PM
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No, it should not unless it is getting forced past the oring type seal.

Maybe some more details on the engine build would help shed some light on it.

2003 RC
1988 FZR1000R
2013 YZ-450f
1974 Yamaha RD350
1971 Honda CB500
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post #5 of 17 (permalink) Old 06-10-2016, 02:45 PM
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Sounds to me like you blocked off the crankcase breather instead of the PAIR lines
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post #6 of 17 (permalink) Old 06-10-2016, 02:55 PM Thread Starter
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No i didn't block off the breather tube. The breather tube is connected to the breather port on the front valve cover and connected to the reservoir and the reservoir was connected to the right side of the box and the reservoir has the tube coming out the left side with all the others. I blocked the port on the left side in front of the other drain tube. I know i removed and blocked the right port on the airbox. The engine isn't a build I bought it from a company off of here from a bike they parted out with 500 miles on it. All that's done to it is the Pair mod and block off plates and the flapper mod. So seeing as though when the front block of plate blew out the seal it was leaking out oil as well im guessing its a bad dowel o ring between the head and valve cover.
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post #7 of 17 (permalink) Old 06-10-2016, 02:58 PM
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Originally Posted by 14steveo View Post
No i didn't block off the breather tube. The breather tube is connected to the breather port on the front valve cover and connected to the reservoir and the reservoir was connected to the right side of the box and the reservoir has the tube coming out the left side with all the others. I blocked the port on the left side in front of the other drain tube. I know i removed and blocked the right port on the airbox. The engine isn't a build I bought it from a company off of here from a bike they parted out with 500 miles on it. All that's done to it is the Pair mod and block off plates and the flapper mod. So seeing as though when the front block of plate blew out the seal it was leaking out oil as well im guessing its a bad dowel o ring between the head and valve cover.
So are you saying it is not going into the airbox?

2003 RC
1988 FZR1000R
2013 YZ-450f
1974 Yamaha RD350
1971 Honda CB500
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post #8 of 17 (permalink) Old 06-10-2016, 03:03 PM Thread Starter
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The breather is connected to the airbox from the oil catch reservoir the short little connection off the top. I blocked the pair hole like it said on the directions. The one that i blocked is on the left rear side of the airbox in front of the little drain tube on the same side. The breather is on the right and the one i blocked is on the left
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post #9 of 17 (permalink) Old 06-10-2016, 03:05 PM
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oh got you,

Well maybe sure you can freely blow air from the breather hose. Also make sure if you take the valve cover off that you can freely blow air through the port.

2003 RC
1988 FZR1000R
2013 YZ-450f
1974 Yamaha RD350
1971 Honda CB500
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post #10 of 17 (permalink) Old 06-10-2016, 03:08 PM
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I'll take your word for it then as you may certainly be correct and you are privy to what has been done where I am not.

I will say though that I've seen my fair share of case and valve gaskets blow on a lot of different bikes (especially on the dyno over the years) and it's always because someone blocked the wrong line. One guy actually stuck a cap inside the return line completely hidden from anyone trying to diagnose the problem, but usually they just cap off the wrong line back into the airbox. When they blow at high rpms oil can seemingly come from a lot of different areas that it didn't actually exit from as well.

I look forward to hearing what the issue actually is.
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