Exhaust coming out from vent tube?>

[banshee]

Bike is a 77 GS750. I checked the compression and it was great. The vent tube is the one that runs from the head, right above the airbox and is connected to the frame. Bike seems to run fine? What could be the cause.

 

[1948man]

Oil over-filled??

 

[banshee]

I ran out and checked it. Not overfilled.

 

[Guest]

Sounds like blowby. What causes this?

 

[DanTheMan]

That isn't exhaust, at least it shouldn't be. That vent tube is the crankcase vent, and it's venting blowby from the crankcase (which is normal). If there's a large amount of blowby your bike is burning oil due to rings or valve guide seals. A small amount of blowby is normal. If you have an airbox on your bike it's channeled into the airbox. If you don't then it's venting to the atmosphere.

 

[allojohn]

That vent tube should run into the airbox. There will be some venting, how much are you getting?

 

[banshee]

If I plug the hole with my finger I can hold it for 5 seconds or so before the pressure builds.

 

[Guest]

Run it to the airbox like it's supposed to be and forget about it.

 

[850 Combat]

If the bike hasn't been used much lately, there is a reasonable possibility that the blowby may diminish with renewed use. Rings sometimes gum up after sitting.

Some venting from the crank case is to be expected. That is why every motor has a vent. The flow from the vent isn't supposed to be oily. It isn't supposed to be smokey.

Hook it to the airbox and ride the bike. If it is excessive, oily, or smokey, Hook it to the airbox and ride the bike. A repair to fix excessive blowby is too much trouble unless it is super horrible. There is suposed to be some mesh in the valve cover to collect oil mist in the blowby. In my experience, it is sometimes missing.

 

[chuck hahn]

You can pull all the plugs and put an ounce or so of a 50/50 acetone and ATF mixture in each cylinder and let it sit overnight. This will eat any carbon making the rings stick. Be mindfull that its gonna smoke like a locomotive till the juice burns out of the cylinders...so start it up OUTSIDE. If you dont have the stock box, like was said it just vents to the atmosphere and many put a K&N looking little filter on the end of the tube.

 

[allojohn]

If you dont have the stock box, like was said it just vents to the atmosphere and many put a K&N looking little filter on the end of the tube.

Not only is it K&N like, this one is K&N.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&vxp=mtr&item=370590535355

 

[Guest]

Not only is it K&N like, this one is K&N.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&vxp=mtr&item=370590535355

You can buy those little things at the parts store way cheaper.

 

[AJ]

It's not just blow-by from the combustion chambers. That line equalizes the pressure underneath the pistons and the crankcase. As the pistons rise and fall, they decompress and compress the air underneath them in the crankcase. For most 4-cylinder engines, the two outer pistons rise, creating a partial vacuum, while the two inner pistons fall, pressurizing the crankcase, but since the inners and outers move oppositely at the same time, the air more or less moves back and forth under the pistons. Since there is stuff in the way (rods, crank, cam chain and slippers), this airflow is not even and local pressure variations occur. That breather line lets the crankcase breath so that the pressure never gets too far in either direction, with air sometimes going out, sometimes going in. Since the rubber line is cool, oil mist condenses in the line and the line will slowly "drip" oil, even in a perfect engine. It's a good idea to use a filter as allojohn suggests to keep the engine from sucking dust back into the crankcase.

Of course, piston blowby does cause the overall flow of air to be net out of the engine crankcase through this line.

Harley engines tend to be real air pumpers in and out of the engine because both pistons share a single crank and the pistons are not 180 degrees apart as is done in a BMW boxer engine.

Most modern engines run that crankcase line to the air cleaner to capture and burn any escaping crankcase fumes through the engine. The main air filter also ensures that any air sucked into the crankcase is filtered as well.

 

[Guest]

Ajay, do you think one of those Umbrella Valves like the Hayden Crank vent + would help an inline 4 at all. They are designed for the older Harley Evos to keep them from blowing gaskets by maintaining a vacuum in the crankcase. About 99% of the time I don't burn any oil, but at random times I do loose some and was wondering if it was through the opened filtered vent. Hayden claims that the valve would keep 3 to 5 lbs. of vacuum in the crankcase on our bikes and the umbrella valve is also ported underneath so it doesn't exceed that amount of vacuum. They also say that it will stop the misting you see coming out of the breather on stock motors.

 

[AJ]

Ajay, do you think one of those Umbrella Valves like the Hayden Crank vent + would help an inline 4 at all. They are designed for the older Harley Evos to keep them from blowing gaskets by maintaining a vacuum in the crankcase. About 99% of the time I don't burn any oil, but at random times I do loose some and was wondering if it was through the opened filtered vent. Hayden claims that the valve would keep 3 to 5 lbs. of vacuum in the crankcase on our bikes and the umbrella valve is also ported underneath so it doesn't exceed that amount of vacuum. They also say that it will stop the misting you see coming out of the breather on stock motors.

I wasn't familiar with that Hayden valve, but will look it up. Pulling a partial vacuum under the pistons would probably save some pumping losses and even boost power a tiny bit. But I don't think it would do much good for an inline four cyl engine, since the pistons do such a good job offsetting each other.

The Harley single-crank pin design results in the pistons sort of rising and falling together, but out of phase by the amount of the V-angle. Worst would be a vertical twin with the pistons going up and down together. Best twin would be a flat boxer twin where one piston shoots away from the centerline at the same time the other piston moves towards the center. Like the inline four, the volume under the pistons stays pretty constant as the pistons move. This is why the Harleys and some vertical twins pump so much air in and out of the crankcase. Your Hayden valve would reduce the pressure in the crankcase, reducing friction losses of the flowing air, although the pistons then might lose power to the vacuum they pull underneath.

FWIW, most piston airplane engines are a flat boxer design, but, because they are built to run 75% power full time and for extremely long service life (2000 hrs), they tend to run with a lot of blow-by as they get old. You can install a special oil trap for that line that lets the water vapor out, but catches the oil and feeds it back to the crankcase. That makes a big difference when your engine is blowing about a quart of oil per hour of operation.

In summary, I don't think you'd see much advantage for a 4 cyl Suzuki motor. Hope that didn't bore everybody.

 

[7981GS]

The main air filter also ensures that any air sucked into the crankcase is filtered as well.

It also creates vacuum to pull the crankcase by-products out.
That is why it is important to mount the vent tube under and behind the engine to draw them out if you do not run it into the airbox.
It is then referred to as a "draft tube"

Draft tube:
From 1928 until the early 1960s, car and truck petrol engines vented combustion gases directly to the atmosphere through a simple vent tube.
Frequently, this consisted of a pipe (the 'road draft tube') that extended out from the crankcase down to the bottom of the engine compartment.
The bottom of the pipe was open to the atmosphere, and was placed such that when the car was in motion a slight vacuum was obtained, helping to extract combustion gases as they collected in the crankcase.
The vacuum was satisfied by a vent, typically in the valve or valley cover, creating a constant flow of clean air through the engine's air volume.
The oil mist would also be discharged, resulting in an oily film being deposited in the middle of each travel lane on heavily-used roads.
The system was not positive though, as gases could travel both ways, or not move at all, depending on conditions. (Most modern diesel engines still use this type of system to dispose of crankcase fumes.)

Daniel

 

[AJ]

It also creates vacuum to pull the crankcase by-products out.
That is why it is important to mount the vent tube under and behind the engine to draw them out if you do not run it into the airbox.
It is then referred to as a "draft tube"

Draft tube:
From 1928 until the early 1960s, car and truck petrol engines vented combustion gases directly to the atmosphere through a simple vent tube.
Frequently, this consisted of a pipe (the 'road draft tube') that extended out from the crankcase down to the bottom of the engine compartment.
The bottom of the pipe was open to the atmosphere, and was placed such that when the car was in motion a slight vacuum was obtained, helping to extract combustion gases as they collected in the crankcase.
The vacuum was satisfied by a vent, typically in the valve or valley cover, creating a constant flow of clean air through the engine's air volume.
The oil mist would also be discharged, resulting in an oily film being deposited in the middle of each travel lane on heavily-used roads.
The system was not positive though, as gases could travel both ways, or not move at all, depending on conditions. (Most modern diesel engines still use this type of system to dispose of crankcase fumes.)

Daniel

That's very cool, Daniel. I hadn't really thought about it before. On my airplane, the "draft tube" exits underneath the cowling in a low pressure area. The tube itself is angled to create a venturi draft pulling gases out of the tube. This leads to an "oily belly" as the oil condenses on the bottom of the fuselage, and this is the main reason airplane owners use the oil scavenger system I mentioned in my previous thread. It also points out how much engineering, even of the "trial-and-error" kind there is in modern vehicles, and how aware we need to be as we modify these machines.

I always assumed feeding the crankcase vent back into the air cleaner (plus the PCV valve) was for environmental concerns, but it had the added benefit of getting oil off the roads, which are much cleaner today than I remember as a youngster.

Thanks for posting that! Cheers.

 

[7981GS]

It also helps to draw the pressure out of the crankcase allowing the engine to rev quicker.
And aids in piston ring sealing. I.E.: the piston is allowed to send pressure down after combustion with less pressure pushing back up.

Daniel