Is this an air leak?

[bashman]

Hi everyone. Hyper7 .21 3 port fires up ok but revs go high and clutch shoes fly out and it tries to run away. I pulled the engine and sealed up the carb and exhaust then blew air down the fuel inlet through some fuel pipe. Submerged in a bucket of water, the only air bubbles are coming out of the front crankshaft bearing. No fuel leaks out so could air be leaking in when it is running. This is what is confusing because if the crank is not sealed, surely fuel will leak out. Any thoughts on this please?

[bashman]

Should add, the back plate seal and all the carb seals are new. It is a new carb.

 

[Anthoop]

The front bearing will almost always leak when you do that test unless it is brand new (and filled with grease)....this is because the bearing is not a seal....the seal for the front of the crankshaft is only made when the engine is running......when the engine is running there is oil between the crankshaft and the engine casing and also oil in the bearing, that is what forms the seal.

When the engine is tuned correctly then there should be a balance of positive and negative crankcase pressure pertaining to the amount of oil around the crank that will prevent fluid leaking out and air getting in....note- when running a new engine really rich it is common to see fluid out of the front bearing.

 

When an engine has an air leak it will almost always run inconsistently....so one minute it is idling nice and then the next the idle is up and down, also the engine will be very hard to tune making it seem rich one minute and lean the next.

The front bearing (both crank bearings really) can cause air leaks but that is when they are worn and have excessive play allowing the crank to wobble around displacing the oil (that is doing the sealing).

 

Is this engine new....has it run about a half a gallon from new....is it near the end of life?

If it is new then often the setting for the LSN is too lean on the Hyper 21 and Macstar 28.....I find that if you set the head of the LSN around 0.5mm out from flush it provides a nice rich starting point with a 1mm slide gap.

If it has run around half a gallon then it has loosened up and needs to be tuned.

If it is near the end of life then you will probably find that it will play up and stall/be hard to tune once it has properly warmed up. 

 

Also if you have had the engine running before and it was fine and none of the options above fit....has the car sat with any fuel left in it?

When fuel is left in the car then the main parts of the fuel evaporate leaving behind the oil....sometime this can lead to blockages....thus a lean mixture...thus high rpm.

Oh and as the weather is on the turn then bear in mind that you will need to adjust the mixture settings to suit.

 

When I start an engine and it revs high then I have two immediate options and can combine the options.

1- Reduce the slide gap....the idle screw.....anticlockwise to make the gap smaller and reduce the idle speed.

2- Richen the LSN....again anticlockwise....putting more fuel in will reduce the rpm.

Edited October 21, 2015 by Anthoop