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Discussion Starter #1
I have some questions about a 250 Rebel crankcase breathing system, from time to time there are discussions about the breathing system and on many occasions the crankcase exhaust is referred to as the breather when it is actually the crankcase exhaust. The exhaust is directed through the separator where the moisture is directed in to the puke tube and the vapor is pulled through the air filter and returned to the engine. Firstly, why does the puke tube need the cover over it if it only collects water and why can it not just drip when any water is present?
Why could you not just design the crankcase exhaust to go to atmosphere the same as a draft tube? That way there would be no puke tube to be drained and you would not be reburning the contaminated engine vapor. Many cars had this design for over 50 years.
If the engine has a breathing system then where is the intake? Every car engine with a breathing system has a Breather and Crankcase exhaust, some use the draft tube and some use the PCV system which is similar to the Rebel breathing system. The only issue I can determine with the cover over the puke tube is to avoid dripping under the bike a few droplets of water under the bike would not be an issue if you new what it was, however this may be undesirable to some people.
 

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Member Duckster, an engineer, explained all this in an earlier post I can't find. I'll do my best to paraphrase what I recall. As the pistons move, it results in positive and negative pulses of air. The breather tube allows these pulses to vent. If there was no way to relieve the positive pressure, it could blow out the left side oil seal. Similarly, if the puke tube is left open, it could pull outside, unfiltered air into the engine.
 

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Member Duckster, an engineer, explained all this in an earlier post I can't find. I'll do my best to paraphrase what I recall. As the pistons move, it results in positive and negative pulses of air. The breather tube allows these pulses to vent. If there was no way to relieve the positive pressure, it could blow out the left side oil seal. Similarly, if the puke tube is left open, it could pull outside, unfiltered air into the engine.
If the air does exhaust from the crankcase then there must be a supply to create a positive pressure, the only way a positive pressure could develop would be past the rings if there is no other inlet. I can see in a new tight engine this would be very minimal however as an engine ages this leakage would increase. If the positive pulse vents out in to the air intake there must be means to restore the negative void and the only way that I can see is past the rings.
Some high performance engines force air into the crankcase to rid of the vapors thus maintaining a positive pressure. The draft tube design creates a negative pressure at the end/tip which will form a scavenging effect and draw the crankcase vapors out to atmosphere while the vehicle is in motion, however this has no effect while stopped idling. If this system is capable of creating sufficient pressure to blow out a seal then there has to be means to create the positive pressure. If the air just pulsed positive and negative it would be like an elastic band going back and forth. A positive pressure must be created for the air to pass through the system. I don't think we are talking a hurricane here, just a slight difference in pressure maybe only ounces.
 

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What about the pistons moving up and down in the cylinders? Wouldn't that change pressure? Since the breather is attached to the airbox, there's plenty of air available for positive/negative pressure changes.
 

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What about the pistons moving up and down in the cylinders? Wouldn't that change pressure? Since the breather is attached to the airbox, there's plenty of air available for positive/negative pressure changes.
I believe there is a check valve in the separator (PCV valve) to prevent a backfire from re-entering the crankcase. PCV valves are built that way to prevent a backfire and exploding things to pieces. I have seen the results of a wild explosion in the crankcase and the breather cap and dipstick were both airborne After several explosions the manufactures created the check valve to prevent such explosions ie one-way valve. This would mean that air could not return via the crankcase exhaust piping it can only go in one direction, out. There would have to be a difference in pressure for the gases to vent through the PC valve. The only way that I can see a difference in pressure is leakage past the rings. If there was a ventilation system on top of the valve cover that would be a different story but it is not there. Therefore the only way the gases can move through the system is a difference in pressure the crankcase would have to be greater than the atmosphere for it to flow outward. The only way that could develop is leakage past the rings. Since engines began crankcase pressures have developed via ring leakage and must be exhausted, on early engines and for over 50 years the draft tube was used to exhaust these gases. Only after several catastrophic explosions a check valve was developed to prevent the re-entry of the gases, and they gave it a fancy name PCV in reality it is only a check valve.
 

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I've taken apart a first generation separator, and it was simply a small box with open cell foam in it. I really don't think there is a valve, or it would show on the parts diagrams.
 

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I've taken apart a first generation separator, and it was simply a small box with open cell foam in it. I really don't think there is a valve, or it would show on the parts diagrams..
I have to agree with you Flight on this one. With a comprehensive review of the crankcase exhaust system there is no check valve. The system is designed as a closed system, with a cap on the drain tube and the crankcase exhaust gases moving directly into the air cleaner and mixing with fresh air the system is self maintaining. The design diagram indicates the exhaust gases when generated in the crankcase flow through the exhaust tube through the separator then in to the air intake where they are reburned. The only required maintenance would be to regularly drain the water (puke) tube and occasionally replace the air filter and that is about it. The diagram is open as to how the crankcase pressure is generated in the first place, the only way I can see is leakage past the rings. On a new engine this would be negligible, however as an engine wears with age this becomes more of an issue and the maintenance interval would become more frequent
 

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I have to agree with you Flight on this one. With a comprehensive review of the crankcase exhaust system there is no check valve. The system is designed as a closed system, with a cap on the drain tube and the crankcase exhaust gases moving directly into the air cleaner and mixing with fresh air the system is self maintaining. The design diagram indicates the exhaust gases when generated in the crankcase flow through the exhaust tube through the separator then in to the air intake where they are reburned. The only required maintenance would be to regularly drain the water (puke) tube and occasionally replace the air filter and that is about it. The diagram is open as to how the crankcase pressure is generated in the first place, the only way I can see is leakage past the rings. On a new engine this would be negligible, however as an engine wears with age this becomes more of an issue and the maintenance interval would become more frequent
 

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