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Old Nov 26, 2012, 03:40 PM
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DarZeelon's Avatar
Israel, Ramat HaSharon
Joined Nov 2003
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Quote:
Originally Posted by earlwb View Post
Well the classic air leak problem is usually through the carburettor somehow...
Earl,


More often an air-leak results from excessive clearance between the crankshaft journal; and the front of the crankcase in which it spins (whether through the bearing, or the bronze bushing).

Some people who encounter this condition within a ball-bearing engine, try to blame the front bearing's seal for the leak... This is nonsensical, since some engines use open front bearings that have no such seal (Webra and Fox, to name a couple).

The seal is actually formed between the crankshaft journal; and the front of the crankcase in which it spins, aided by fuel-borne oil; and it is this seal which prevents air from leaking into the the (average) vacuum within the crankcase.

Excessive clearance can result from wear, or from mediocre production quality.
A contact-sealed bearing (2RS or similar) can reduce the leak somewhat; possible making throttle shutdown doable.

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Old Dec 17, 2012, 02:53 PM
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United States, MI, Dearborn
Joined Jan 2010
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I had a similar problem with my Irvine 40 a couple of years ago. What caused it was that I had replaced the o-ring seal at the base of the carb with a thicker o-ring. It did not seal well. I went back to the original-sized o-ring and pushed down the carb hard onto the crankcase while tightening the carb hold-down bolt. That solved the problem for my engine.
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Old Dec 18, 2012, 12:17 AM
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Joined Jun 2011
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The front end of a two cycle glow engine is sealed by the crankshaft to crankcase fit. Nothing else. Most engines just use a metal shield that keeps crap out of the bearing or a rubber seal that doesn't seal against pressure. All that rubber seal does is keep grease in the bearing.Many engines actually have no seal or shield on the front bearing. Super Tigre is one of them.
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Old Dec 18, 2012, 02:25 AM
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DarZeelon's Avatar
Israel, Ramat HaSharon
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Actually, DatsunGuy, the seal is between the crankshaft and the crankcase, behind the carburettor.

That 'leak' from the carburettor socket forward can feed oil to the front bearing, but if a double sealed bearing is used it is not really necessary.
If the seal between the crankshaft and the crankcase behind the carburettor, is good (i.e. quality parts machining); no excessive discharge of oil will happen ('runny nose' syndrome).
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Old Dec 18, 2012, 07:25 AM
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Canada, ON, Windsor
Joined Apr 2008
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We all seem to be circling the same issue, what I posted originally as a bad fit between the crank and case, caused by low QC or bad or worn bearings. It all comes back to air and fuel getting into the main case through the nose. With some engines the ONLY solution is sealing the front bearing as you cannot likely refit the crank to bore.

The horse is dead!
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Old Dec 18, 2012, 07:27 AM
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DarZeelon's Avatar
Israel, Ramat HaSharon
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Quote:
Originally Posted by Cougar429 View Post
We all seem to be circling the same issue, what I posted originally as a bad fit between the crank and case, caused by low QC or bad or worn bearings. It all comes back to air and fuel getting into the main case through the nose. With some engines the ONLY solution is sealing the front bearing as you cannot likely refit the crank to bore.

The horse is dead!
I would never disagree with that, Cougar.
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