carburator piston spring

[jakebass12]

 driving my 81 r65 and after a long drive i noticed that the rpm's would stick at 2000 and slowly come down so i asked a friend of mine (that knows more about carbs than i do) about it and he said that it was probably running to lean and that i should adjust the fuel air mixture so i adjusted the idle in conjunction with the fuel air mixture but i just couldn't get it to run right. so i decided to take the tops off and noticed there wasn't a spring to push the piston back down. so i looked up on max bmw and looked at there fiche and in there diagram there was a spring. so i did a little more research about the carb springs and came across http://www.restat.de/bing/bing-federn.htm this . if you have google chrome it will translate it for you. so i was wondering if anybody has tried this and what your opinion is on it. if you cant translate it i can just copy and paste.

thanks for any replies and help.

[Barry]

Here's an English version covering the same topic.  

http://jhau.maliwi.de/mot/r100tic.html#carb

I have flat tops with no springs and have often wondered what difference springs would make but have never tried fitting them.  In theory they should stabilise the piston and produce some initial enrichment of the mixture on opening the throttle. Essentially an accelerator pump effect. The later slightly stiffer springs may have been fitted to compensate for overall weaker mixture settings.

[jakebass12]

thanks Barry for the english link.

[Speyman]

JakeBass12,
Are you sure it is the carburettors? My 1981 R65 had the same symptoms until I lubricated the advance mechanism in the "bean can". Now it idles sweetly at about 900 rpm.

[jakebass12]

JakeBass12,
Are you sure it is the carburettors? My 1981 R65 had the same symptoms until I lubricated the advance mechanism in the "bean can". Now it idles sweetly at about 900 rpm.

 ill have to give that a try. but ill have to get those springs since mine are not there.  does your carburetors have the return springs in them?

[georgesgiralt]

Hello !
The springs in the carbs are here to make the pistons "look" heavier.
This serves two purposes :
Prevent the pistons from rising at idle (some depression can go further the butterfly and rise the piston) enriching the mixture too much for idle.
The second use if to smooth the transition from idle to partly accelerated. If there are no springs the piston rise instantly and, then the mixture gets somewhat richer than it should.
This is a very controversial subject and each sees it's own pleasure here.
Some people around me remove the springs and drill more holes under the pistons to make them go up faster. Other, like me, have put the stiffer springs to ease the life in the traffic jams I experience here...
So I suggest you clean the carbs, change the rubber and put new needles, needles jets and tune them up properly. Then, try the carbs without springs, with the "weak"springs then with the stiffer springs. And decide for yourself.

[jakebass12]

thanks georgesgiralt

[georgesgiralt]

You're welcome !

[tvrla]

I think the most common cause of a high idle is tuning/adjusting the carbs before the engine is completely warmed up. A 20-30 mile ride is best. Warmed up and off the choke after idling five minutes isn't the way to do it!

[Motu]

If there are no springs the piston rise instantly and, then the mixture gets somewhat richer than it should.

No, it gets leaner.   Air moves faster than liquid fuel, so when you open the throttle quickly there is an instant increase in air flow, and a decrease in air speed so less fuel is sucked out...it will lag behind.  Springs, and in the case of SU and CD Stromberg carbs the addition of oil in the damper rod slows the rise of the piston, increasing air speed across the jet (bigger depression - CD, Constant Depression) and so sucking more fuel out.

With an SU carb, the damper rod is the accelerator pump - thicker oil, richer mixture.   Stiffer spring, richer mixture on acceleration only.

[Barry]

I agree.

The springs produce some initial enrichment of the mixture on opening the throttle. Essentially an accelerator pump effect.

The later slightly stiffer springs were fitted to increase that effect so that the engine would tolerate an overall weaker mixture setting at steady throttle openings.

You could argue that Bings are deficient in design in that they lack any damping mechanism other than the size of the vacuum transfer hole in the bottom of the slide.  That's also true though of most motorcycle CV carbs.

I have a fondness for SU carbs. They were really very good in their day.

I'm not sure I entirely agree with this but I once read that Strombergs were a cheap copy of the SU and Bings were a cheap copy of the Strombergs. That they were a copy seems likely given the common diaphragm dimensions.

[georgesgiralt]

Hello Motu and Barry,
I agree with you on the transition phase of the process.
But without spring the piston is lighter, so the same parameters will promote a higher position of it once the air flow has stabilized. Thus, as the piston is higher, the needle will be less in diameter and give more fuel by the increased dia. Giving richer mixture.
As I said, some people bore more holes or bore out the existing holes to "lighten" the piston as much as they can, and thus, give it the maximal velocity providing more fuel as fast as they can.
On properly maintained carbs, it is easy to experiment (without springs, with weaker springs and with stiff springs) this change the behaviour of the bike very much, and decide for yourself  what suits you best.
If you do this properly, you do not have to re-synch the carbs so it takes only a few minutes to switch.

[Barry]

Hello Georges

We had a discussion about this a while back. Although we did not reach a definitive conclusion as to the effects of the spring in a steady state, I think most responses in the thread suggested the effects if any were small. http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1360779442/2#2

This is what I wrote:

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.

[Lucky_Lou]

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]

I think that as we are dealing with what is effectively a single cylinder/ single carb the only advantage must be stabilizing the diaphragm at lower rpm in what must be an erratic airflow, i am left wondering if there are any comparisons anywhere on MPG flat top v domed(sprung). If it (sprung) enriches more you would expect the MPG to be lower.
 [smiley=2cents.gif] Lou

[georgesgiralt]

Hello
I remember the thread and thought about it since.  :-/
Think about this like that :
A definite air flow cause a definite depression above the piston. This depression represent a certain force, say equal to 20 grams for example. The pistons' weight and rubber resistance take maybe 10 grams out of that force. If you have no spring, you've 10 grams left to lift the piston if you've some spring, you need to trade some part of these 10 grams to counter act the spring force. So the piston won't raise as high as it would without the spring.
Of course this is not as simple as I describe it because when the piston lift it modifies the dimensions of the air passageway and thus change the air flow and depression value.   :-?
So my understanding of this matter is that removing springs is like making bigger holes or grinding the piston and result in a quicker response to get to the equilibrium point which will be richer because the piston will go somewhat higher
Putting a stiffer spring or reducing the holes size or having stiffer diaphragms force the piston to stay lower so give a somewhat leaner mixture (air flow is more restricted around the needle jet ).
Of course, every other parameter being kept equal.
My explanations are too simplistic of course because there are a lot of transient responses and the oscillation of the air column in the carb to take into account so getting a model of what happens really into the carb is wayyyy above our heads IMHO.  
This is why I suggest to buy the two set of springs and try for yourself, without, with weak or with stiff springs. It is not that costly and fun. It is also totally reversible.
And it shows.