Unburned Fuel collection

[Pete]

quote ryanjames170:

So if I might ask a dumb one how dose the oil gas mix get into these drains? Ie with out creating a rather large pressure leak in the crank case?

The excess mostly oil mix collects at the bottom of the crankcase and with the gravity and positive crankcase pressure pulses the oil mix is forced thru a small orifice and into the lower unit housing. The orifice is small enough to cause little operation effect. On 2 cycle engines the higher the revs the less residual to be collected since most of the oil fuel mix gets to the cylinders especially at full throttle. On my 2 cylinder 70’s era Merc engines the design included an oil residual collection system that crankcase positive pressure pulses pumps the extra residual oil mix to the top bearings where extra lube is always needed, there is a check valve in the system to prevent reverse flow during the crankcase suction pulse phase. Earlier Merc and probably other brand ( :?:) engines had a small hole drilled in the lower crankshaft seal and the positive pressure pulse forced the residual out the lower unit.

[bill-mcnamara]

Wow, 55% with a Fat Fifty. I thought that around 5% in my case was substantial.

[outboardnut]

Chris
Did you take out the leafs in the case drain cover?
Would it be ok to route the little gas line from the tapped/barbed case drain cover to a t barbed connector which would be attached to a single line fuel line hose?
Or would it affect the fuel pump performance?
Is it possible to describe where to block the hole in the motor leg?

[outbdnut2]

I have a 1976 20 Mercury that had a bad lower crank bearing – I found one cheap at a local bearing house that was same dimensions but was a sealed bearing. I stupidly put it in and partway through the 1st tank of gas, it started running really bad – like on one cylinder. I haven’t taken it apart yet, but checked carb, spark, etc – all OK. If I remember right, I think the tube that scavenges gas from the bottom of the crankcase , putting it in again up higher, is picking up nothing now because gas has to get through the lower bearing that is not supposed to be sealed. So I think unburned gas is pooling in the lower crankcase until it rolls into the intake port. That was several years ago and maybe some day I’ll get to tearing it down again…I hate that motor, from the day I got it, something new broke every time I ran it – but it did have mega-hours on it before I got it. I had to replace a piston because of balls from the lower bearing being sucked into the lower cylinder and beat up the piston top pretty bad!
Dave

[chris-p]

quote outboardnut:

Chris
Did you take out the leafs in the case drain cover?
Would it be ok to route the little gas line from the tapped/barbed case drain cover to a t barbed connector which would be attached to a single line fuel line hose?
Or would it affect the fuel pump performance?
Is it possible to describe where to block the hole in the motor leg?

No you need to leave the leaves installed.

I had not thought about trying that? Worth a shot!

[chris-p]

In my second picture you plug the hole that the screwdriver is pointing to. You plug that. Not all motors are EXACTLY like this, but similar enough. Then you can see in my first picture where it would originally run through into that slot, through the hole my screwdriver is poking into, and then into the exhaust cavity where it gets dumped.

[jim-moffatt]

Sucking the unburned fuel back to a T on the fuel pump line may not work well. I tried recirculating it right to the intake manifold.
The motor lost it smooth transitions between low and high speeds. I also had to readjust the low speed needle setting. But trying it is simple enough once you have the fuel pump. I like experiments.

When you recycle it to the gas tank the oil rich recycle fuel is dispersed throughout the whole tank and that problem doesnt happen.

[vintin]

I have seen on some more modern motors where the collect fuel from the crankcase drain reeds is fed to a carb like float bowl(separator) and then pressure fed into the fuel line going to the carb to be mixed in with new fuel from the tank. These are complicated systems and expensive.

[amuller]

I think this topic is important enough to perhaps justify a separate board or topic that people could refer to and easily learn what are the "best practices" for different motors. As limitations on running 2-stroke motors tend to increase, it’s important to be seen as responsible and responsive…..

[Author]

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