Lotus Elan

Weber float needle....

PostPost by: elanner » Mon Oct 05, 2015 5:39 pm

How critical is the needle size for the Weber float valve? I started on changing mine this weekend but only did one after I discovered that the new Viton valves I have are 2.00 size, while the worn brass originals are, per the manual, 1.75. I overlooked this when purchasing refurb kits. :-(

I assume a larger size will allow the float chamber to fill a bit quicker, but that should be harmless, right? Nevertheless, the fact that multiple sizes are provided suggests there's more to it.....

It's not a big deal, I'm sure I can purchase a couple of 1.75s quite easily.

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PostPost by: Galwaylotus » Tue Oct 06, 2015 9:18 pm

There's also a 30% increase in area so an additional 30% force trying to open the valve.
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PostPost by: RichardHawkins » Wed Oct 07, 2015 7:27 pm

That's a very good point. A 30% increase is significant, I had never bothered to do the sums, but this may explain why some people recommend setting the fuel level by measuring the distance from the top of the float cover to the the fuel surface in the float chamber. This method gives a direct measurement of the fuel level, whilst the normal method is an indirect method of setting fuel level, and I guess consistent fuel level is more important on an engine with multiple carburettors.

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PostPost by: Chancer » Wed Oct 07, 2015 7:44 pm

Galwaylotus wrote:There's also a 30% increase in area so an additional 30% force trying to open the valve.


Compared to the force closing it another 30% is still a spit in the ocean.

I dont know where my Weber book is now but I'm sure that I recall that the needle valves are sized according to maximum fuel demand and with one carb serving two cylinders the smallest will always be large enough on a twin cam engine.
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PostPost by: Craven » Thu Oct 08, 2015 10:32 am

Clearly a larger size will not reduce the fuel supply, but an effect may be over fuelling, will the floats buoyancy produce enough closing force to shut off the extra flow? Effort required will depend on pump delivery pressure.
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PostPost by: Chancer » Thu Oct 08, 2015 2:00 pm

Extra flow or extra force from the valve area being 30% larger?

Like I said the force on the needle opposing the archimedes force on the float is de minimis.

I had to use Google to find out what pouss?e d'Archim?de was in my native English :roll:
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