Needle membrane differences

Basic

Basic

Joined
Nov 13, 2012
Messages
45
Location
Netherlands
Bike
ST1100N '92
Whilst exchanging my carbs set, I compared them, searching for any differences causing my extreme fuel consumption.
I found that the gas needle membranes differed considerably regarding stiffness.
In the picture below, on the left is a stiff membrane from the 2000 replacement carb, whilst on the right the old '92 one that is very flexible.
Note that the "new" ones can easily "stand" on the membrane, whilst the old ones immediately collapse.
Both carburettor sets have done some 75k km / 47k miles. The 2000 one was known to be very fuel efficient.
membrane.jpg

The exchange of the carburettors resulted in fuel economy increasing considerably. Just checked the spark plugs after using nearly one tank of gas and they look near perfect.
Is it possible that the stiffness of those membranes has influence on fuel economy, or should there be some other cause?

Best Regards,
Rob
 
Hi Rob,

The best way to answer your question would have been to put the newer sliders in the old carbs and run a couple of tanks of gas. Isolate the single variable, otherwise it's just conjecture.

John
 
Hi John,
You are right that just exchanging the membranes would have shown whether this has indeed any influence or not.
Carburettors are complex thingies and setting them for optimal performance is a bit over my head.
So, for me the cheap (100?) solution was just replacing the set with a known good runner after I proved unable to pinpoint the problem with the old ones.

It is just that when checking and comparing both sets, I stumbled upon this clear mechanical difference between them.
The difference between those membranes is not something anyone would notice, unless you are able to compare the sets.
The more flexible old ones might actually proof to be the better in combination with the correct jets, but that is just speculation.

The bike is currently running great, so the question was more theoretical.
The actual flexibility of those membranes may or may not have an effect on needle position and it might be wise to replace those membranes as a set if one is broken.
When exchanging the set, it might be wise to keep an eye on your fuel economy, though I suspect that newer (stiffer) ones could be more fuel efficient.

Regards, Rob
 
No pin holes. They are 100% intact and look almost new.(Very careful and thorough inspection) The only difference with the newer carbs is their flexibility. (Measured the thickness of the rubber. It was exact match.)
I even thought about putting the old ones in the new carbs because of that, but decided to keep them as they were, not taking the risk of changing things.
The "new" carbs are running fine, so I have no regrets about the decission.

Regards, Rob
 
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