Gas Engine Needle Valve

[Chris VParticipant @chrisv]

I would like to make a finely adjustable gas needle valve and would value some pointers as to the design. Its for a small gas engine, Stuart 800.
The mixer shown borrowed from another project seems to work OK , even if its lacking in its visual refinement.

In this instance though I only need a needle valve, not a mixer.

I need a fine thread, BA, BSB or ME no problem.
Now should the threaded brass part be bottoming out when screwed fully in, or more likely the needle itself??
In this example the needle is 1.8mm dia, so drill a hole that diameter with a standard drill bit ?, or should the tip of the needle seat into a matching tapered hole?
Should the needle have a longer taper? Ideal material for this?
Is this a good basic design, or are there published designs/ sketches available?
Any help gratefully received.

[bernard towersParticipant @bernardtowers37738]

I would definitely go for a finer taper than the one in your picture, they are not difficult to make. you cando it in two pieces like the photo by solderinng togetyher or loctite

[Howard LewisParticipant @howardlewis46836]

IMO, definitely 40 tpi ME for a fine adjustment, and as suggested a slow taper, again to make the adjustment less sensitive.

Howard

[Ramon WilsonParticipant @ramonwilson3]

The needle point should have a reasonable taper – say about 5-10 degrees incl. – the exact angle is not that important as long as the taper is concentric.  It should bottom out in the hole with the adjuster not quite home on the thread – screw the adjuster fully home, back off 1 -2 threads and push the needle through until it seats – solder the needle in place at that point. If the hole in the adjuster has a deep centre it will allow for a good solder fillet and maximum hold.

6 – 4BA thread is suitable as is 40tpi but there’s no need for finer

[Chris VParticipant @chrisv]

Thank you Bernard, Howard & Ramon, some good helpful tips there, cheers.

[Rick HannParticipant @rickhann79631]

I have good luck by looking in my wife’s sewing box and finding a needle that looks about right.  Just break off a section near the end and loctite it into the adjustment screw.  Of course you have to measure the tapered section and drill the proper sized hole in the venturi tube.  Rick

[Chris VParticipant @chrisv]

Thank you Rick!

[Roger HartParticipant @rogerhart88496]

Reminded me of a tale from my apprenticeship.  A much older engineer related the starting procedure for an old gas engine pre WW2 in an English country house.  Circa early 1930s.

Engine,  dynamo and batteries located in an old coach house.  When the engine needed starting three servants grabbed the long leather belt – lined up in order of precedence.  Then when the engineman indicated all was ready the butler blew a whistle and the servants shuffled along pulling the belt until the engine started.

Thus were the batteries charged.

 

 

[Chris VParticipant @chrisv]

Love that tale Roger, clearly this is where I’m going wrong, need to get myself some servants LOL (-;

[Neil LickfoldParticipant @neillickfold44316]

Some, used to use old gramaphone needles. A common thread for the needle if made in 1 piece, like the Super Tigre or Rossi needles, uses the M2.5 thread. I think is 0.4mm pitch.

The needle insert, or pin can be made from 1mm piano wire or wire size slightly larger than the inlet hole. Then I use a piece of balsa wood as a support block, and use a Dremel to grind the taper on the pin. Anywhere from 5-7 deg per side works really well. I use 5deg mostly. For a Ø1.2 mm needle, it is about 6mm long the taper.

A small collet chuck like an ER11 makes them easier to make.

On my F2A NVA assemblies, they do not close to zero. That way, the needle taper can not be damaged by the edge of the inlet hole in any way. Damage to the taper seat or to the needle can make it inconsistent for its repositioning.

Neil

[Chris VParticipant @chrisv]

Thank you Neil!

[Howard LewisParticipant @howardlewis46836]

Starting large stationary engines can be difficult.

The 9″ bore Wilson at The Water Works Museum, requires someone standing on the spokes of the big flywheel to bring the engine to the point where compressed air can be applied for starting.

The 5 cylinder Blackstone is much easier to bar round to the starting position

Fortunately, both are fairly ready starters

The single cylinder Blackstone needs a lot of pre heating, before some brave soul swings it over vigorously, and another shifts the cam from the “Start” to “Run” position. Everyone hopes that a second try won’t be needed!

Howard

[Author]

Posts