Carburettor Lost Motion Linkage

B

Brian-Northampton

Administrator
Hi All,

I've failed to understand the concept of the lost motion linkage on the HS carburettors.

I know what it is, I know how to set it, I know that the result is that the rear carburettor will open before the front carburettor and that at full throttle, the rear carb is fully open, and the front one isn't quite .... because of the lost motion linkage.

Does anyone know why???

It makes more sense to me that the front and rear carbs are first tuned and balance independently - by releasing the lost motion linkage, then when correctly balanced, the linkage is set so that they both open simultaneously instead of having a 25 thou gap.

The Workshop manual says:
The .025 to .030 in. clearance in the lost motion linkage is extremely important and is designed to allow the rear carburetter to operate by a pre-arranged amount before the front carburetter, to allow individual setting of the carburetter.
Click to expand...

So, why not set them individually and then tighten it up with zero lost motion.....

I'm sure someone out there knows what it's for.

Cheers,
Bri.
 
I have never read or followed the manual in this aspect. I always set them up to work simultaneously.
The idle and pick up are perfect.
I can see no reason why i should do otherwise.
 
After reading your post I had to have a look in the book just so I could tell you that you were talking rubbish, it was never going to say that in the book............Oh dear, it would seem that it does say to set the carbs so that the rear one is opening first. But it only says that in the section applicable to early cars fitted with emission controls, and is only done so that emissions can be reduced by effectively having the engine running on one carb, which like all of the half-cocked emissions set-ups doesn't work anyway.

So, in conclusion, just set the carbs up as you would normally with both opening at the same time, just as it says in the book in the section for carbs without emission controls, and disregard the whole section that has got the part about uneven settings.

The clue was in the bit that that you failed to include which was "thus reducing emissions from the exhaust"
 
Hi Harvey,

Thanks for taking the time to look through the book :)

I looked in my book for the 2000/2200 Workshop Manual printed in 1977.

Any setting of the lost motion linkage to anything other than zero will result in the rear carb opening first.

This is described for the HD8 on page 54N (8 - 10 thou)
HS8 emission control on page 57N (25-30 thou)
HS8 on page 60N (25-30 thou) - here is your reference to exhaust emissions. This is section (N-4-B) and confusingly doesn't say if it's emission control or not, but it must be as it is sandwiched between two emission control section.
HS8 on page 67N - THIS ONE is the one I was reading and quoted. I don't believe that this is an emission control section.

A clue as to what it might be for is on page 65N, Operation N-4-E, part B section B2 where it says:
.........The lost motion linkage, when correctly set, gives enough clearance to enable balancing to be carried out without disturbing the coupling clamps.
Click to expand...

So maybe the whole point is that it enables DIYers and lazy mechanics to tinker with the adjustment screws without releasing the linkage. But surely there must be a tradeoff in performance, and also having the engine slightly out of balance by having the rear two cylinders driving the car harder than the front two.

What do you think?

Bri.
 
Lost motion isn't the same thing as adjusting the lost motion link so that one carb opens before the other.

The way I was taught, and have done ever since is to set the carbs individually so that they are balanced, and then set the lost motion link so that the carbs are opening at the same time. You can do this and still have a small amount of movement on the link before both carbs start opening. (Together) That bit of lost motion then allows you to carry out minor adjustments to keep it in tune after the initial set-up, without resetting the link.

Regardless of what the book says I won't be changing any time soon. With regard to emissions, if the engine is set correctly I'm quite happy with that, and providing that "Global Warming" (Or "Climate Change" as they like to call it now because they really haven't a clue what will or won't happen) doesn't cause the world to end within the next 50 years or so then I don't care, after that I couldn't really give a toss. It won't be a problem for me, and while the likes of China keep polluting the way they do I don't see why I should be penalised just to make up for them. (Oooh, bit of a rant there.....very controversial...)
 
Is this similar to the V8 system where you are supposed to use a feeler gauge in a "fork and pin" type set up ? I thought it was to soften the initial throttle response to safeguard the transmission
 
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