Has an iris shutter/valve every been tried on a carburetor?

Brad William /

Above is an industrial version for hvac.

I was reassembling a telephoto camera lens recently and had this in my hand and thought, hmm...


Many are very robust and ideally sized for someone to experiment with on a small simple two stroke. They have little levers on the side that a throttle cable could be easily attached to.

Butterfly plates, round slides, flat slides... round bores, oval bores, etc. Each with it's pros and cons. I'm surprised I can find no reference at all to experimenting with an iris.

It may simply be that it would work just fine, theoretically perhaps even better, but the tiny improvement is offset by the enormous increase in complexity, cost, and reliability.(edited)

Re: Has an iris shutter/valve every been tried on a carburetor?

Dirty30 Dillon /

Very interesting thought, that I'm surprised I never considered given ow many cameras I have taken apart.

The main issue is that the thin, hard edge of the iris/leaf shutter is going to cause violent eddying which would slow charge flow into the intake.

The other issue is that this would require a center axis mounted fail metering rod that would have to extend either along the x or y axis of the bore in order to meter throughout iris opening. This means that there is little tuning to be done with a needle to manually sync with throttle position.

As you noted, this all comes at the price of complexity. So more complex and mutliple tuning hurtles probably would render this moot.

But a really cool idea in theory, noentheless.

Re: Has an iris shutter/valve every been tried on a carburetor?

yeah i spent a bunch of time puzzling over this same idea, it transfixed me for years

there are people who have made them

in theory i think

1) don't over think it- regular carberaters work really good as-is

2) part of the way carbs work is the air always sticks to the bottom of the hole, that way all the jets and stuff can work from the bottom up, you'd have to put an annular venturii in it like a butterfly car carburetor

3) if you need more flow just make it bigger

4 there is nothing magical about an annular opening, carbs like dirty turbulent air, breaks the fuel up better and gives you better mixing.

Re: Has an iris shutter/valve every been tried on a carburetor?

Brad William /

On a similar note (my curiosity often outruns my threshold of comprehension and I get smacked down for it)..

there is a thread here on diaphragm/pumper carbs. Like 30dillon, I asked myself... "I've been tuning weedwhakers and motorcycles for years... why did it never occur to me to ask why the diaphragm carb is not suited to a moped/motorcycle."

It's a very interesting dive into the web. So it's not so novel after all? Found a few relatively exhaustive threads on motocross forums. After a few hours it seemed to come down to a bit of consensus.

Best for tiny engines like weedwhackers where low and mid power and response are not critical... ie, wide open all the time! Also these tiny engines are not optimized for fuel efficiency, but reliability and low cost parts. In most cases, a swap from float to diaphragm carb will penalize fuel efficiency regardless of any performance gains you might painstakingly tune into it.

Seemed like the majority of experimenters with motorcycle applications ended up after an often long winding path, just slapping the ol' reliable float carb back on and calling it good.(edited)

Re: Has an iris shutter/valve every been tried on a carburetor?

Kevin Bishop /

Put one between the filter and carb and report back. You could remove the slide or butterfly and operate it by the throttle cable. It may prefer a velocity stack.

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