Brass needle valves

[mrwhittle]

One of my customers wants me to make some brass idle mixture screws for small gas engines. Does anyone know what alloy I should use, or is it just plain old 360 brass?

Thanks,
Martin

[vulcom1]

Before I would comment I need a clarification on the location. Is the screw for adjusting the plate(buttefly) in the venturi or the screw for adjusting the low end that screws into the jet in the body? Also are you asking about as Walbro pump type carb?
John

[mrwhittle]

It's the screw for adjusting the low end that screws into the jet in the body.

<Also are you asking about a Walbro pump type carb>
I don't know the brand, but I doubt it's a pump type.

Thanks,
Martin

[vulcom1]

Martin,
The needle's are steel for all the gas engines I have seen. Don't know the grade but brass would wear fairly quick when you bottom it for adjusting. I have some for small rc glow engines that the threaded portion is brass with the needle pressed in and they would work also. Make it easier to cut the threads.
The pump type carb has a membrane under a flat plate held by 4 screws. The plate has a tiny hole in it for breathing when the pump is working.
Hope this helps.
John

[mrwhittle]

vulcom1
I appreciate the input! The customer is sending me some samples, so that should simplify things.

Thanks,
Martin

[Mcgyver]

i've used free cutting stainless when making needle valves, albeit not specifically for a carb. when i;ve done it, its usually stainless into brass seating, that way it will seat properly, i wonder if brass/brass would deform/gall

[vulcom1]

Martin,
Would be interesting to see an example as it would make it easier to comment. Using the brass for the needle or jet would be asking for trouble. I can see it now when some ham fisted guy decides to bottom it to start to adjust.
John

[vladdy]

took a quick look into the carb pile in the garage...and..
older auto carbs run mostly steel needle valves for idle mixture..
snowmobile carbs are mostly brass [yellow, no red tinge like bronze]
motorcycle are mixed, older is brass, newer are steel
snowmobile are mostly brass, mid 90's are larger precentage steel [or similar]
On the seating surface, brass seats seem to use mostly steel needles,
aluminum seats [machined integral to carb] use mostly brass needles..

seems like the preference is to a dissimilar match between seat and needle to reduce possible galling from 'enthusiastic' tuners..

enjoy..

[mrwhittle]

vladdy

I appreciate the research!! I still haven't seen the samples from my customer,
but I think you're right on about the dissimilar metals.


vulcom1

<I can see it now when some ham fisted guy decides to bottom it to start to adjust.>

I can see that same guy looking for a drill bit to drill out the broken needle.

Martin

[HPbyGD]

Along these same lines, I am looking for a needle with some thing like a 10/32 thread to use in a adjustable jet I want to make. Does anyone have a source for somethiing around that size? Martin I hope that you don't mind me hijacking your thread.
Gary

[vulcom1]

I don't think he will mind as it is for doing the same thing.
It all depends on what you want to make it out of as 10/32 is very coarse for a needle valve. Also how small of an angle (ie sharp or blunt tip) There are quite a few variables needed to know before an answer. If it is one off to try I would turn down a 10/32 screw.
Martin,
Let us know when you get one.
John

[HPbyGD]

I want to make a adjustable jet for a carb that I use on my dyno. It would be a lot easier to get the jetting dialed in as I would not have to even stop running the engine to adjust the fuel mixture. I have a needle that I don't know what it came out of that has about a 10 degree angle on it. That is 10 degrees on each side. I was hoping to find something that I could modify instead of making it from scratch. I could always take a brass screw and tune it up. Like I said I wanted to find something close to start with if I could.
Gary