why is there more air blowing out the breather than the plug holes ?

[ippytattoo]

I am no expert by any means but just for peace of mind I would check cam timing just to be sure. If it is a tci ignition it should be very simple with the motor in the bike. Put the crank at TDC using your timing marks and check the hole in the cam for the advance mechanism, it should either be facing directly up towards the little tab (for lack of a better word) on the rocker cover directly above the inner points cover or directly down away from it. If it all lines up correctly I would put gas in it and start it after checking for spark, compression etc so you won't be chasing your tail if it doesn't start right away. Any experts out there correct me if I am wrong here.

 

[daroberts3]

Pete, You know we are using a 360° twin for our example? Since the spark plugs are out and the engine is turning much slower than running we can pretty much determine the intake and expelling of air in both the cylinder and crankcase is complete. Thus the argument that two pistons moving air in and out the breather and only one piston for each sparkplug hole.

Agreed ! My point exactly! BTW are we talking about a stock or rephased crank?

 

[1974jh5]

As the pistons rise and fall, yes they do compress the air in the crankcase, that's how a 2 stroke engine works and is the reason for the 4 stroke's crankcase breather in the first place. Crankcase pressure on a 4 stroke can get pretty high; back in the 80's and 90's Husqvarna/Folan engines as used in Husabergs etc used one way reed valves in the oiling system and relied on crankcase pressure to push oil to the top end, for instance.

For a 4 stroke to have twice the volume of air coming from the breather than it does the spark plug holes, it either has 1) a compression leak into the crankcase or 2) it's not drawing in enough air to properly compress in the cylinders which points to a cam timing problem or even valves adjusted way too tight. Of the two, this is more likely.

Improper cam timing won't change the volume of air from the crankcase but it most definitely can change the volume/pressure being shoved out of the spark plug hole. At kick speed, an XS should have, what? 140-150 PSI compression? Even if it's as low as, say, 100-120 PSI that should still make for a pretty sharp snort from the plug hole.

The only way for air to get from the cylinder to the crankcase is either through poorly sealed rings or a bad/not sealing head gasket (that would allow compressed air from the cylinders into the cam chain opening between the cylinders). That would probably be felt as pulses, since the leak would be worst on the compression stroke and at kickover speeds that should be pretty apparent.

The way to settle this for sure would be a leakdown test: if air is applied to the spark plug hole with both valves closed and a lot of air escapes through the breather or dipstick hole, it's a ring or head gasket problem. If the amount of leakage is below, say, 15% on a new engine it's due to the rings not being seated. If the leakdown is below this, then the rings are sealing OK as are the valves.

If a leakdown test is done with the valves both closed on the compression stroke and air escapes from either the intake or exhaust port, then the valves are not seating properly.

A really dumb question on my part, due to the mention of TCI: when I was assembling my bike it looked like it was possible to easily flip the cam 180 degrees left/right. In that case, yes it would look like it was timed correctly but in reality, no. Can someone verify that for both me and the OP?

http://www.650motorcycles.com/CamTiming.html

 

[weekendrider]

1974jh5,
Yes it can be filpped by mistake I believe havebikewilltravel even got it running.
You could search for his post.

 

[XSLeo]

Yes the bike will run with the cam backwards. Look at the pic you will see that the lobes are at the same angles, just one side up the other down.
Look closely at the ramps of the cam. Now flip the cam the ramps are the same.
Being a wasted spark ignition it will run.
On the older points ignition it won't unless you swap points or plug wires.
The end with the threads for the advancer unit go to the right side of the engine. Yes if timed right the hole on the threaded end will point up or down.
Even with the cam timed right, as the pistons move down 650 cc's of air gets pushed the breather, as the pistons rise, being a 360 degree engine 325 cc's gets pushed out one plug hole.
This means that at every revolution 650 cc gets out the breather 325 goes out a plug hole. So at kicking speed twice as much air goes out the breather vs the plug hole.
Now being the breather hole is less than half the size of the plug hole it will sound like more pressure is coming out the breather. Mostly because the smaller hole will be moving air faster, making more noise.
If you put a pressure gauge or even a finger over the plug hole and the breather hole then you will find the plug hole has much more pressure.
If the engine was assembled right then it is safe to run the engine.
Leo

 

[1974jh5]

XSLeo is right on the money about the difference in air volume being moved and how it will whistle through a smaller orifice. There is one difference between the top and bottom of the engine: the compression ratio in the crankcase (yes there is such a thing!) will be lower due to the much larger volume of crankcase (remember to add all that empty space in the transmission, the right side cover, etc) than the cylinder(s).

Like this: a cylinder has maybe 325cc with the piston at the bottom of the stroke, once at the top the remaining volume is more like 30cc. (I know this is not accurate, but bear with me.) The 650cc of air shoved into the crankcase by the pistons traveling down will be trying to raise the pressure of probably 5000cc of air, not nearly as much volume difference as the cylinder. But yeah it does pressurize the crankcase plus it will create a vacuum and suck air in as the pistons rise, that's why you need to run a filter on the breather tube. It keeps all that dust and junk out.

FWIW, race car exhausts use exhaust pulses to 'evacuate' the crankcase, if you have a slight negative pressure in there all the time there's a little 'free' power to be gained since you aren't wasting some by compressing air in the crankcase.