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

[scobman]

why is there more pressure blowing air out the breather hoses than the plug holes when a motor is kicked over priming the oil in it after rebuilding it ??? we're talking twice as much pressure.

How does that happen to a carefully put together motor ?

A crack in a cylinder flange or the head ?
Malfunctioning valves ?
carefully installed rings with massive blow by ?

.........To put gas in it and try to start it, or not to put gas in it and try to start it ?
Thats a mental chew toy for you experts in here to wrap your brains around.

 

[1974jh5]

I'm tempted to say the cam's not timed right.

 

[scobman]

One tooth off on the cam/TDC alignment?

It would be getting too lucky if just the cam chain master link came loose ???

 

[scobman]

looking at the points, advance rod pin position at TDC would probably prove it.

 

[1974jh5]

It should, yeah. Off the top of my head, I can't think of anything else, other than REALLY way sloppy piston/bore clearance. I once did a leakdown test on an XR200 with stuck rings (guy ran it without an air filter) and it blew the oil dipstick gasket out.

 

[PetesPonies]

Blow by ( pressure in the crankcase ) happens when the engine is running. This engine the poster is asking about hasn't been run yet and is not running when he experiences what he is seeing. It has to be a valve open to allow air back up through the intake port. So, it would seem the cam timing is off.

 

[daroberts3]

There is a larger volume of air in the crankcase below the pistons than there is in the combustion chamber. Both pistons are moving down at the same time moving that large volume of air out the crankcase breather passage. From your statement that there is twice as much air coming from the breather than the plug holes, this seems logical to me. Have you measured this observation?
IMHO you need to verify timing one last time and then run it !
I love that feeling of panic and thrill just before starting a motor I have just built !

 

[PetesPonies]

The volume of air in the crankcase has nothing to do with the amount of air that MIGHT be pushed out. It only has to do with the volume of the cylinders that are pushing down. But the air in the crankcase is ventilated anyway.

 

[daroberts3]

The volume of air in the crankcase has nothing to do with the amount of air that MIGHT be pushed out. It only has to do with the volume of the cylinders that are pushing down. But the air in the crankcase is ventilated anyway.

So what are you suggesting is going on here?
The breather passage allows air to escape that has been displaced by the downward movement of the pistons. The spark plug holes are breathing out the combustion chamber air volume. There SHOULD be more air coming out the breather passage.

 

[Gordon]

Squirt a few tablespoons of engine oil; down into the cylinder's to help seal the new ring's. That should reduce the amount of air from the breather. You have a couple thing's going on, 1st is the ring's are not sealing well yet, have to run the engine for that. Second is as Dar roberts stated, along with the ring's. You asked if you should run it, I say yes. After several hundred miles of running, the ring's will seal as they and the cylinder's wear together. When running on the road, be sure to run the engine at varying rpm's, so the cylinders don't get 'glazed'. Around town instead of a constant speed, like on the freeway.One question, is your breather assy. assembled correctly? You have the baffle in place, and the small wire 'filter' in it? What you described is a common occurence on a fresh engine rebuild.
FWIW, I have done engines with so little compression that I had to use the oil to get them to run. And no, all the clearences were in proper spec, using iron ring's. Iron ring's are rougher then a moly ring, so the hone finish is rougher also.

 

[PetesPonies]

It will be approximately the same volume of air, but totally different pressure. What he is witnessing is pressure, not volume. I said what it must be.

 

[daroberts3]

PetesPonies is correct in stating that it COULD be a cam timing issue. It COULD also be a normal occurrence. The spark plug hole is larger than the breather hole(passage) so it WOULD have less PRESSURE.

 

[PetesPonies]

No, there's a compression ratio that is far far greater than what the cylinders do pushing into the crankcase. The pressure difference isn't even close.

 

[daroberts3]

No, there's a compression ratio that is far far greater than what the cylinders do pushing into the crankcase. The pressure difference isn't even close.

Cylinders don't push into the crankcase, pistons do. And there is no compression with spark plugs removed as they are in this case which we are trying to solve. Verify cam timing one last time and run it!

 

[PetesPonies]

First of all, I don't want to make a big deal here, but I'm trying to explain this is a short way. First, my background . . 30 years as an Auto Tech teacher., 25 years in a Restoration Business. I mention this because I have taught this for years, and taught it correctly The volume in the cylinder is what is being pushed up against the head and down into the crankcase. But above the piston is new air being brought in. The crankcase has little of this happening, no comparison. When air is expelled from the plug hole it is being compressed, thus is why it leaves. The ratio of volume at BDC to volume at TDC is what its all about. Its in the range of 8:1 etc. This is not the case when comparing the cylinder volume to the crankcase volume. da, don't make a big deal out of this. Stop with the know -it-all attitude and learn. That is what its all about learning. Luckily for you, its not too late.

 

[Tomterrific]

You guys.... sigh. The crankcase has two pistons displacement. A single cylinder has one piston displacement. Two is twice as much as one! I can see this in my imagination, can't you? Do you even try or am I a freak?

Tom

 

[PetesPonies]

You are a freak . . as you said. You don't understand pneumatics. Thats all, not a curse Doesn't work that way, sorry.

 

[Tomterrific]

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.

 

[PetesPonies]

Class is over, its Friday I'm tired I say, lets let the poster check his cam and make sure it all set up correctly or not.

 

[bened]

scobman says there air is coming out of the breather hoses. Those are in no way connected to the intake ports. Cam timing being off will in no way change the amount of air being expelled from the breather hoses. It's physically impossible.