Engine: 2.1 Carb B21A: Backfiring Redblock - Help Please

[Scott Stephen]

My '75 244 does this backfiring thing very occasionaly. The flame trap in question is no longer connected due to a bolt on K & N. What exactly does the flame trap do???

[CTCNetwork]

Hi,

The flame trap is part of the crank case vent system and prevents oil splash getting from the crank case to the inlet manifold...

It also, reputedly stops flame/ignition of the oil vapours and such ignition going back to the crank case.

Des. . .

[malb]

Originally Posted by Scott Stephen

My '75 244 does this backfiring thing very occasionaly. The flame trap in question is no longer connected due to a bolt on K & N. What exactly does the flame trap do???

With the engine running, there will always be a pressure build up in the crankcase due to piston ring bypass and oil and contaminants heating and vapourising.

This pressure buildup used to be released to atmosphere through a vent, often containing a loosly packed steel wool. The wool would develop an oil film coating which caught any dust in air returning to the cranckcase as the engine and oil cooled after shutdown. The vent was frequently integral to the oil filler cap and was rinsed in petrol or another solvent when the oil was changed.

When pollution control sytems became mandated, crankcase breather vapour and fuel evaporation were amongst the first of pollution sources to be controlled.

With the B21 engine system, a secondary air line was installed from the air cleaner to the inlet manifold via a T piece connecting to the vapour port on the cam box. The T piece included a "flame trap" which came in at least 2 varieties, a rolled brass spiral or a moulded plastic unit. In either case, the trap contains a number of small passages which will allow vapour to pass, but are too small to pass a flame in the event of a backfire.

In operation, this secondary air passage allowed a metered air flow from the air cleaner to the manifold, and by venturi effect drew any vapour being emitted from the crankcase into the inlet manifold with the fuel air mixture. The vapour was subsequently burnt in the engine and its byproducts were emitted into the exhaust system.

When the engine stopped and the oil and engine cooled, the oil and vapour in the engine would reduce in volume and filtered air is drawn into the vapour port so maintain atmospheric pressure in the crankcase. Unfortunately, any vapour in the system beyond the flame trap also tended to condense and travel back to the air cleaner by gravity, where it tended to contaminate the filter element.

With the B23 and later engines, a different system was used, with an oil seperator to collect any condensing oil and direct it back to the sump.

If you have an aftermarket air system installed, and have removed the flame trap system, how are you venting the crankcase and filtering the air bleed system?

[JimLasVegas]

This really struck a chord. What kind of carburetors are fitted? Are they still the type that has an oil-filled dashpot with a piston that damps the movement of the moving part (can't remember what it's called) to limit the movement of the mixture needle? I had some work done years ago and when they put the carbs back on, they neglected to fill the oil chambers which caused an instant rich mixture on acceleration. It's just a thought, but sometimes really simple things are overlooked. Also, even though you just had a tune-up, it wouldn't be the first time that I've heard of a condenser being bad right out of the box... or maybe even a loose hold down screw on the condenser. Start out with all of the simple stuff first. Who knows...? It might even be carbon tracking on the inside of the cap that was caused by overly-zealous use of oil while lubricating the points rubbing block or the top bearing of the distributor which would, in extreme instances cause the firing of more than one plug. Like firing when the valves are on overlap in the neighboring cylinder.
Good Luck, from The Colonies.

[MissDMeanor]

Thank you for all your posts guys and I hope the answers are helping others.

OK, she's still mucking about, albeit not as violently as before, so I shall endeavour to play with the mixture screw myself this weekend.

Could anybody explain where it is please?

[minesa240x3]

Originally Posted by malb

With the engine running, there will always be a pressure build up in the crankcase due to piston ring bypass and oil and contaminants heating and vapourising.

This pressure buildup used to be released to atmosphere through a vent, often containing a loosly packed steel wool. The wool would develop an oil film coating which caught any dust in air returning to the cranckcase as the engine and oil cooled after shutdown. The vent was frequently integral to the oil filler cap and was rinsed in petrol or another solvent when the oil was changed.

When pollution control sytems became mandated, crankcase breather vapour and fuel evaporation were amongst the first of pollution sources to be controlled.

With the B21 engine system, a secondary air line was installed from the air cleaner to the inlet manifold via a T piece connecting to the vapour port on the cam box. The T piece included a "flame trap" which came in at least 2 varieties, a rolled brass spiral or a moulded plastic unit. In either case, the trap contains a number of small passages which will allow vapour to pass, but are too small to pass a flame in the event of a backfire.

In operation, this secondary air passage allowed a metered air flow from the air cleaner to the manifold, and by venturi effect drew any vapour being emitted from the crankcase into the inlet manifold with the fuel air mixture. The vapour was subsequently burnt in the engine and its byproducts were emitted into the exhaust system.

When the engine stopped and the oil and engine cooled, the oil and vapour in the engine would reduce in volume and filtered air is drawn into the vapour port so maintain atmospheric pressure in the crankcase. Unfortunately, any vapour in the system beyond the flame trap also tended to condense and travel back to the air cleaner by gravity, where it tended to contaminate the filter element.

With the B23 and later engines, a different system was used, with an oil seperator to collect any condensing oil and direct it back to the sump.

If you have an aftermarket air system installed, and have removed the flame trap system, how are you venting the crankcase and filtering the air bleed system?

WOW And WOW again!Very well described and very informative.
Credit were credit is due Wish I could put across answers like what you do!!
Hehe But really it's an art in itself explaining such complex thing's
Cheers Brian.

[LankyTim]

You could swap the complete dizzy for an electronic one from a later car, no need to worry about points ever again then!

Could the backfiring be a perforated diapragm in the carb I wonder?

[MissDMeanor]

Originally Posted by LankyTim

Could the backfiring be a perforated diapragm in the carb I wonder?

Hi Tim

I opened my bonnet and this was gaping at me:



The hole was so large it started talking to me.
It said 'Pedro Fandango kicks me when you're not looking'. Well I never.
Anyway, I'm hoping to fiddle about with the mixture screw this weekend and once she's running well again I can tackle the other issue

[CTCNetwork]

Hi,

Judging by the tape repair on that pipe, it been speaking to other people before.. :rawr:

Des. . .

[Pedro Fandango]

Originally Posted by MissDMeanor

The hole was so large it started talking to me.
It said 'Pedro Fandango kicks me when you're not looking'. Well I never.

lmao as if i would do a thing like that, i'd kick her while you were looking. You know how much Mildred tries to stitch me up so wouldn't wait till your back was turned lol