Exhaust Cam Decompression

[PaulW]

Interesting to see the centrifugal-driven decompression on the exhaust camshaft. I wonder if that will eliminate the need for ORS instructors to carry jump leads around with them ?? It should be good for cold winter starts for us all year round bikers

 

[roundincircles]

Interesting to see the centrifugal-driven decompression on the exhaust camshaft. I wonder if that will eliminate the need for ORS instructors to carry jump leads around with them ?? It should be good for cold winter starts for us all year round bikers

Help me here. What should we be looking at? I can't see any difference in the exhaust camshaft but I do not understand what I am looking at

 

[PaulW]

Help me here. What should we be looking at? I can't see any difference in the exhaust camshaft but I do not understand what I am looking at

On the inlet valve (top) you can clearly see the cam and the cam follower (pointing down from the 11 o'clock position) - valve timing is fixed in the design.

For the exhaust valve (bottom) the cam follower is partially obscured by the 'centrifugal mechanism'. This is what you should be looking at and acts as a valve lifter when the engine turns over, reducing compression and allowing the engine to spin easily without loading the battery excessively. Crudely and as I understand it, the cam lifts the valve before TDC at very low cranking speeds. Once the engine fires, proper valve timing is restored as the centrifugal force 'locks' the cam in place.

The above is only my interpretation (I am only an electronics engineer who has always hated the load placed on the battery by the boxer twins ) Maybe some of our mechanical experts can confirm or explain better than I.

My comment about the ORS (Off Road Skills) was that they all carried jump leads behind the windscreen so they could start bikes which had flat batteries after too many restarts.

Hope this helps, paul

 

[Magnon]

On the inlet valve (top) you can clearly see the cam and the cam follower (pointing down from the 11 o'clock position) - valve timing is fixed in the design.

For the exhaust valve (bottom) the cam follower is partially obscured by the 'centrifugal mechanism'. This is what you should be looking at and acts as a valve lifter when the engine turns over, reducing compression and allowing the engine to spin easily without loading the battery excessively. Crudely and as I understand it, the cam lifts the valve before TDC at very low cranking speeds. Once the engine fires, proper valve timing is restored as the centrifugal force 'locks' the cam in place.

The above is only my interpretation (I am only an electronics engineer who has always hated the load placed on the battery by the boxer twins ) Maybe some of our mechanical experts can confirm or explain better than I.

My comment about the ORS (Off Road Skills) was that they all carried jump leads behind the windscreen so they could start bikes which had flat batteries after too many restarts.

Hope this helps, paul

Sounds reasonable but fcuk it looks dry up there lets hope the oil gets round once she fires up!

 

[roundincircles]

OK all sounds logical, hope it works in the middle of the Sahara! Or Bradford.

 

[Bendy toy]

A big Ballistic battery would give more starting punch. Its costly, but got to be cheaper overall than all those decompressor gubbins.

 

[roundincircles]

A big Ballistic battery would give more starting punch. Its costly, but got to be cheaper overall than all those decompressor gubbins.

But heavier maybe?

 

[richtel]

Wouldn't an exhaust decompression system risk pushing unburnt fuel down the exhaust and into the catalyst?

 

[mcinlb]

I'm sure BMW thought of that

 

[Deleted account 190920001]

A big Ballistic battery would give more starting punch. Its costly, but got to be cheaper overall than all those decompressor gubbins.

Until it gets cold and then you big ballistic is just a paper weight until you heat it up.

 

[Bendy toy]

With at least double the cold cranking power of a standard battery they can afford to go off a bit. Unless the adverts and reports are lies. However cost is a big issue and they need a balancing charger even more cost.

The cat will cope with some unburnt fuel. But the small amount let away by the exhaust decompressor has to be better than a few cylinders full when the engine is cranking but not fast enough to start.

 

[Phil Thomas]

All big singles of a certain generation had a de-compressor... operated by the rider, kick start only of course. So any method of achieving this automatically to ease the strain on the battery should be good.

Does anybody know if this operates on both cylinders?

 

[(RIP) Mark H]

As a result of the decompresser not so much cranking energy is required and BMW will use a lighter battery and starter motor to save weight

 

[cookie]

As a result of the decompresser not so much cranking energy is required and BMW will use a lighter battery and starter motor to save weight

spot on.

 

[Deleted account 190920001]

With at least double the cold cranking power of a standard battery they can afford to go off a bit. Unless the adverts and reports are lies. However cost is a big issue and they need a balancing charger even more cost.

The cat will cope with some unburnt fuel. But the small amount let away by the exhaust decompressor has to be better than a few cylinders full when the engine is cranking but not fast enough to start.

It would appear they are. There are 2 long threads on advrider.com on how these batteries perform or don't as the case may be in the cold. These are guys like you and me who have spent their hard earned with real world experience.

 

[Bendy toy]

That's a shame with the Ballistics. My BMW Exide is still ok but the starter never feels - shall we say - eager to get it going.

This one (post #19) talks about Li Po Cobalt batteries being dangerous in a bike. Very true, but these are not LiFePO4 as used by Ballistic and Shorai, etc. Most folk dont seem to understand that 1 x 12V batteries in parallel without a split charge circuit will always over-work the smallest.
http://advrider.com/forums/showthread.php?t=860702&highlight=ballistic+battery&page=2

This seems to be a well considered review -
http://advrider.com/forums/showthread.php?t=757934&highlight=battery

 

[codcutter]

I have these sort of cams (S&S 640 ) on my hardly movinson, which at 2ltrs and approx 11.5-1 compression would be a struggle for the stock starter and battery. Work a treat

 

[Joseph]

Exhaust cam decompression

Hi there can anybody tell me if the decompression valve fails will the battery and starter be big enough to start the bike ?? I have a brand new GS and it broke down after 120 miles . Not enough power to crank bike over ?? New bat fitted and still not the power to crank it over! Any ideas guys ?

 

[Joseph]

Exhaust cam decompression valve

Forgot to say I managed to bump start the bike and it runs fine just won't start on the starter ! Battery is fine and tried a new one off an identical gs and still no cranking just a slight turn over movement !