Influence of the spring in the carbs

[georgesgiralt]

Hello !
The R65 has had a couple of springs fitted in the carbs above the pistons.
A supple one used up to end of 1982, then a stiffer one up to the end of production.
So we have 3 possibilities : no spring, the supple spring and the stiffer spring.
What one should expect from going from one possibility to the other ?
I have found many controversial information on this subject, so I think that restricting the discussion to the R65 is important  :-?
Locally, I get advices going from "put the springs in the bin" to "get the stiffer springs you can get"....  :-/
So your help will be invaluable !
Thanks in advance !

[steve hawkins]

Don't think I have any in my 79 flat tops.

[Barry]

I have flat top Bings which have no springs at all so I can't speak from experience. I have read a couple of accounts of springs being changed. Never on an R65 but a CV carb is a CV carb and I doubt the effects vary with engine size.

Thinking through the theory of what happens when a spring is added or the spring rate is increased is very confusing to say the least.

Assuming a constant throttle and revs what will happen when the spring rate is increased ?

The vacuum in the carb venturi is communicated to the top of the diaphragm via the hole in the bottom of the slide and the piston rises until the force applied by the vacuum is balanced by the combination of piston weight and spring pressure.  

If the spring rate is increased the piston must fall which will reduce the venturi area and increase the air velocity. This increased velocity must produce a higher vacuum and draw more fuel from the needle jet.  So at first it looks like a stiffer spring will cause a richer mixture but wait it's not that simple.

This increased vacuum is also acting on the diaphragm which will counteract the spring pressure and raise the piston higher again.  So where does the piston end up?  Is it back in the same position or lower than it was ?  

It seems counter intuitive but I believe the piston ends up pretty much back in the same position. The main effect of a stiffer spring is transient so a stiffer spring will improve throttle response by momentarily richening the mixture but it will not substantially change the steady state mixture.

I'd love to hear what other opinions you have already received Georges and how they reasoned the effect.

[georgesgiralt]

Frankly, Barry, all I've read is so confusing that I'm totally puzzled...
If I had more time, I'll try the stiffer spring and try to devise something.
If I do, I'll report back.
But the stiffer spring are associated with the revised air filter cover, and I wonder what the mod is...
And, as you know, changing two things at a time is worthless...  :-?

[tvrla]

I tried putting the stiffer springs in one of my bikes. I think the only thing I noticed (it's been quite a while ago, so this isn't fresh in my head) was a steadier idle and maybe better acceleration. It's been so long ago I did the mod I don't recall much about it. But the effect was minor as I recall.

[Dizerens5]

Like Barry's my 79 has the flat top carbs. There's no spring and no place for one. It does not seem to matter at all.

[georgesgiralt]

So,
Today, I thought I could do a rough experiment ! At lunch I removed the springs with the idea that a rough change could give an indication about the expected result the stiffer springs could have.
Alas, after work, once on the bike and after a few meters ride, the rain started to drop. Heavily. So I drove home as fast as I could playing no care at all about the bike handling nor the throttle response. Just trying to keep myself alive because French people leach my rear wheel with their car even if they will be unable to stop if something arose....
Maybe tomorrow morning I will be able to ride without fear and hear and look at her behaviour ....  

[georgesgiralt]

Rode the bike a lot today.
I can't tell if it changes anything at all.
As I have to dismantle the carbs this week-end to return them to the stock BMW setting, I'll put back the supple springs or maybe try the bike without any springs at all just to see if it changes something...
I talked about that to a friendly BMW mechanics and he said that the diaphragms sold now by BMW are more rigid than the one used in the eighties. So his advice is that, now, the spring force is not relevant any more. Maybe he is right....