Carb Diaphragm Spring

[Guest]

Somebody told that making shorter the carburetor diaphragm springs will make the bike to respond quickly to your throttle demands.
Makes sense to me because less spring pressure = less resistance to vacuum in order to raise up the piston.
If yes how much is required to remove?? and are there available some softer aftermarket springs?? My bike is a GS1100E with original carbs heavily jetted.

[Nessism]

If you shorten the spring the spring rate will increase, not the other way around.

[mikluha]

If you shorten the spring the spring rate will increase, not the other way around.

The spring rate is not the only factor that affects the force it exserts on the diaphragm slider. The preload is also important. For shorter spring the preload is less. If you do a simple math you get that if the preload compression is more than the cut then the shrter spring will put out less force.

[Steve]

You say that your carbs are "heavily jetted". The picture in your sig line shows you might have a header on the bike. Any other mods that would require such "heavy jetting"?

One of the popular ways to rejet carbs is to use a kit from Dynojet. I am not suggesting that you use one of their kits, but one of the things they have you do is drill a larger hole in the slide to allow vacuum to lift the slide quicker. Have you thought about trying this, instead of going hit-and-miss with shortening the springs?

.

[jacora]

You say that your carbs are "heavily jetted". The picture in your sig line shows you might have a header on the bike. Any other mods that would require such "heavy jetting"?

One of the popular ways to rejet carbs is to use a kit from Dynojet. I am not suggesting that you use one of their kits, but one of the things they have you do is drill a larger hole in the slide to allow vacuum to lift the slide quicker. Have you thought about trying this, instead of going hit-and-miss with shortening the springs?

.

Actually the bike has: #55 Pilot Jets, #135 Main Jets everything else in the carb is stock. In addition it has individual pod filters and 4 to 1 Vance & Hines exhaust header. The head is stock for exception of a minimal port work just to clean up the casting residues and valve seat to head match. Plus dyna green coils and dyna solid core spark plugs wires.
Also, I have a Dynojet Kit to be installed in the future plus high volume oil pump gears from schnnitz racing.
The bike runs excellent and is always asking for more, but I want to improve the low end response.
Steve; Do you have experience with the technique of making short the carb springs or know from somebody?
What I am looking is to have less restriction to incoming air thru the carb, the same principle of the flat slides.
More air = More volumetric efficiency = MORE POWER
All inputs will be appreciated.

[gearhead13]

Correct me if I am wrong, but, CV stands for 'constant velocity' which as it says makes the airflow across the venturi a constant. By changing the spring you may make the slides go up faster and it will make more noise just not anymore power unless you messed with the jetting a bunch more, even then you wouldnt get the throttle response like before. The cv is a substitute for an accelerator pump that injects more fuel into the engine when you open the throttle.

[Nessism]

The spring rate is not the only factor that affects the force it exserts on the diaphragm slider. The preload is also important. For shorter spring the preload is less. If you do a simple math you get that if the preload compression is more than the cut then the shrter spring will put out less force.

I did as you suggested and ran some calculations. Note - the distance X1 - X2 is the same for both lines and indicates the slide lifting.

As can be seen, cutting the spring will give a lower force to open the slide just off the stop(as noted by F1), but will increase the force above stock levels as the slide lifts. I don’t think this is going to help with throttle response other than maybe just off idle, and it will hurt throttle response as the slide lifts further.

[jacora]

I did as you suggested and ran some calculations. Note - the distance X1 - X2 is the same for both lines and indicates the slide lifting.

As can be seen, cutting the spring will give a lower force to open the slide just off the stop(as noted by F1), but will increase the force above stock levels as the slide lifts. I don’t think this is going to help with throttle response other than maybe just off idle, and it will hurt throttle response as the slide lifts further.

Exactly, I just want to improve the low end response and in the upper range it will make better just because it will have less restriction then the volumetric efficiency will increase and will get more power. Actually she's fine Above 5,000 rpm's.
Nessism (if I understood) based on your statement, the way to go is to cut a little bit of the spring to improve the low end response, I mean above the idle up to 4,000 rpm's? Please reply.

[gearhead13]

Exactly, I just want to improve the low end response and in the upper range it will make better just because it will have less restriction then the volumetric efficiency will increase and will get more power. Actually she's fine Above 5,000 rpm's.
Nessism (if I understood) based on your statement, the way to go is to cut a little bit of the spring to improve the low end response, I mean above the idle up to 4,000 rpm's? Please reply.

Jeez I thought these 1100's were torque monsters already, good thing you don't have a 750

[Nessism]

Exactly, I just want to improve the low end response and in the upper range it will make better just because it will have less restriction then the volumetric efficiency will increase and will get more power. Actually she's fine Above 5,000 rpm's.
Nessism (if I understood) based on your statement, the way to go is to cut a little bit of the spring to improve the low end response, I mean above the idle up to 4,000 rpm's? Please reply.

I think you are on the wrong track with cutting the spring. The slide may lift more easily just off idle, but it will be restricted from lifting at anything above this point. Dynojet kits open up the vacuum feed port lifting the slide, this may be a better choice. That said, slide springs are fairly easy to come by (I have some extras if you want to experiment), so give it a go if you want to try.

Ed

[Steve]

Actually the bike has: #55 Pilot Jets, #135 Main Jets everything else in the carb is stock. ... The bike runs excellent and is always asking for more, but I want to improve the low end response.
Steve; Do you have experience with the technique of making short the carb springs or know from somebody?

No, I have never heard of anyone shortening the spring to change throttle response, and I'm wondering if your jetting isn't just a bit TOO much? Here is a copy from the carb 'sticky' at the top of the forum, post #15:

'83 GS1100ED(Canadian model for sure)

Carb Type Mikuni BS34SS
Bore Size 34mm
I.D. No 49360
Idle r/min 1050+100r/min
Fuel Level 5+.5mm
Float Height 22.4+1mm
Main jet #112.5
Main air jet 1.2mm
Jet Needle 5D9-3rd position
Needle jet X2
Pilot Jet #47.5
By pass .8mm
Pilot Outlet 1mm
Valve Seat 2mm
Starter jet #32.5
Pilot Screw 2 1/2
Pilot air screw #160
Throttle cable play .5-1mm

Usually, I hear of going up about six sizes on the main jets when using pods and a pipe, you have gone up NINE. When a change in pilot jet is suggested, it's usually one size up, you have gone up THREE. Your lack of throttle response just might be because you are running too rich. Have you done plug chops?

More air = More volumetric efficiency = MORE POWER

This would be true as long as you introduce the proper amount of fuel to that air. More air and the wrong amount of fuel (either too much or too little) will actually reduce power.

Exactly, I just want to improve the low end response and in the upper range it will make better just because it will have less restriction then the volumetric efficiency will increase and will get more power. Actually she's fine Above 5,000 rpm's.
Nessism (if I understood) based on your statement, the way to go is to cut a little bit of the spring to improve the low end response, I mean above the idle up to 4,000 rpm's? Please reply.

What throttle position are you using to get to 4000 rpm? I ask because it's throttle position, not engine speed, that determines what circuit is used in the carb. At idle and up to about 1/4 throttle, the pilot circuit is in use. From 1/4 to about 1/2 throttle, it transitions into the needle circuit. Above 1/2 throttle the main jet has the most effect. Yes, there is plenty of overlap in the circuits, but since you are complaining about the lower end, I would point you back up to my suggestion that you are running way too rich on the pilot and main jets.

[chef1366]

In my opinion your pilot jet is too fat. I wouldn't go over 47.5. Factory pro kits come with a shorter spring but it's quite a bit "softer". They come with a special jet needle and also want you to drill the slides. They do improve the stock throttle response.
If I was you I'd ditch the CV carbs and go with some Mikuni RS Flatsides 36mm. These will get you where you want.

[mikluha]

I did as you suggested and ran some calculations. Note - the distance X1 - X2 is the same for both lines and indicates the slide lifting.

As can be seen, cutting the spring will give a lower force to open the slide just off the stop(as noted by F1), but will increase the force above stock levels as the slide lifts. I don’t think this is going to help with throttle response other than maybe just off idle, and it will hurt throttle response as the slide lifts further.

I still stick with my statement that unless the slide lift ( X1-X2 ) is on the order of the preload compression the shorter spring will give less force. From what I've seen on many bikes the preload compression on these springs is about the half of the length of the spring.

These argument is not very important since besides wider throttle opening you also need more fuel for better response. And this is not going to hapened with shorter springs anyway.

[rapidray]

I, ALSO, think the pilot jet is the issue. It is too BIG! Go back to the 47.5 & set the mixture screws 2 turns out before you put the carbs back on. Once you have it warmed up & idling, set the screws correctly to what the motor tells you it wants. I'll bet your low end picks up a BUNCH! Ray.