Needle valve to meter air flow - plans?

[V22]

I am building Jan Ridders Simple 2 stroke (Debbie). The engine runs but takes very careful fuel metering. This is rather difficult given the vibrations. I am thinking of replacing the screw air-bleed with a needle valve in an attempt to get smoother running.

Can anyone point me in the direction of a simple plan to build a needle valve? I can adapt to my needs. If one does not exist I will just build what's in my head :noidea:

 

[kiwi2]

Hi,

I used to need to regulate low flow air supplies to measure liquid levels by measuring the back pressure when the air was discharged from the end of a submerged pipe. I found needle valves were not particularly reliable as the gap between the needle and the valve body was generally small and liable to blocking. Needle valves are particularly susceptible to temperature changes as well.
I found the best way to get a stable air flow was to use a ball valve. Take the handle off so the vibration doesn't affect the opening and set the flow you want using a spanner.

Regards,
Alan C.

 

[Wizard69]

No plans but needle valves are dead simple. They are nothing more than a threaded shaft with a long tapered "needle" on one end. The "needle" moves in and out of an orifice thus metering flow.

 

[gbritnell]

Hi V22,
If what I'm reading from your post you want to replace the air bleed screw, which generally is blunt, with a needle type, correct? If so then I think you don't understand that the air bleed screw is actually a coarse adjustment. By that I mean it only regulates the air when the carb is at the idle or slow speed position. For that purpose all that's necessary is a blunt needle because it only has to uncover the air bleed port. If you're having problems with maintaining your fuel needle position due to vibration then just make the needle valve longer and put a jam nut on the threaded jet part. Once you get the adjustment where you want it run the jam nut up against the needle valve and this will hold it in place. You can even put a small piece of rod sticking out of the jam nut to act as a lever. This way you won't have to use a wrench on it.
gbritnell

 

[V22]

Hi V22,
If what I'm reading from your post you want to replace the air bleed screw, which generally is blunt, with a needle type, correct? If so then I think you don't understand that the air bleed screw is actually a coarse adjustment. By that I mean it only regulates the air when the carb is at the idle or slow speed position. For that purpose all that's necessary is a blunt needle because it only has to uncover the air bleed port. If you're having problems with maintaining your fuel needle position due to vibration then just make the needle valve longer and put a jam nut on the threaded jet part. Once you get the adjustment where you want it run the jam nut up against the needle valve and this will hold it in place. You can even put a small piece of rod sticking out of the jam nut to act as a lever. This way you won't have to use a wrench on it.
gbritnell

The Simple 2 Stroke has a "screw" (threaded tube with 2mm slot and air control via a screw sleeve) with a coarse air bleed. My aim is to replace this with a needle valve. The issue is that the air bleed is too coarse and has to be pretty much fully shut to run the engine. When opening the vibration means it can be tricky to get it to position in a small enough increment to get it running nicely. By adding a fine air bleed system I am hoping to get a better fuel mixture and much more granular control over the idle. There is no fine bleed adjustment on this engine at all. It runs a vapor carburetor with only a coarse adjustment.

An alternate to the needle valve is a smaller slot - say 1 or 1.5mm and to spring load the sleeve nut..

I hope that makes a bit more sense. I was trying to keep my post as simple as possible, but in the process probably dropped too much information out.

An example of the same engine with custom needle valve (I assume) is on this video by the late AussieJimG: [ame]https://www.youtube.com/watch?v=o-zo52flDbI[/ame]

 

[gbritnell]

Sorry about that. I didn't realize it was a Jan Ridders vapor carb. I know how they work but have never fooled with one so I can't give you any advice on how to fine tune it. Hopefully one of the members like Chuck Fellows can help you out. He and some of the others use this type of carb.
gbritnell

 

[V22]

Gbritnell, no need to apologize. My question was too vague. I have immense respect for your work and the contribution you make to model engineering. Your input is valued. Thank you for the tip on Chuck. I will send him a private message.