DC Wasp Rebuild

[William ChithamParticipant @williamchitham75949]

I have a DC Wasp .049 glow plug engine which I would like to recommission but it is missing a couple of key components, to whit the rear crankcase cover and the needle valve. As far as I can remember the rear cap screwed in to leave fairly minimal clearance to the crankpin, I think I can reverse engineer that. The thing that has me stumped is the needle valve. I have the tube it screws into (the spraybar?) but the threaded end is broken so I will have to remake that. I have probed the through hole with various drill bits and established that there are two steps. From the threaded end the first few mm is around 1.5mm diameter which I imagine is just clearance. Then it narrows to approx 1.4mm which extends to near the fuel pipe end where it steps down again to approx 1.1mm. I suppose this second step is the “valve seat” where the needle regulates the fuel flow. What I’d like to know is what sort of clearance there should be around the parallel shaft of the needle to allow the passage of the metered fuel to the spray hole and what sort of angle should the point of the needle be? Also a material recommendation for the needle would be much appreciated, as far as I remember the original was a just a steel needle with the end bent around 45° to turn it and an aluminium shroud with the internal thread for adjustment.

[William ChithamParticipant @williamchitham75949]

Might need a new prop as well…

[bernard towersParticipant @bernardtowers37738]

try adrians model engines or motor boys for some inspiration on you valve and needle. it is fairly basic and the finer the needle taper the finer the adjustment.

[Ramon WilsonParticipant @ramonwilson3]

I’ve made quite a few spraybars/needle valves over time. It’s something that can be made fairly easily. Firstly the taper is not that important as long as it is concentric and reasonably long. Using piano or music wire of 18-16swg I grind them by hand on the side of an off hand grinder wheel to give an approximate taper then finish them off using a needle file by spinning in the lathe with the very point supported on a piece of wood held in the tool post – crude, simple, but effective. Usually if the taper is long enough then there is sufficient clearance for the fuel flow. I would think that you would be fine with 1/16″ music or 16swg piano wire. Once the needle and housing are made push the needle in the spray bar until it seats then screw the thimble on fully home then back off one turn. Solder the needle to the thimble with it all assembled – this will ensure that the needle closes when fully home – surprising how that will affect starting/running if it doesn’t! Centre the outer end of the thimble to create a reservoir for the solder to give a good join.

The back plate obviously needs clearance but again is not important save that the deeper it goes the less crankcase volume. If you made it with say 5 thou clearance and fit the backplate with a (copy) paper gasket that should be fine.

Finally very small glow motors such as this require a fuel with a reasonable nitromethane content – I would say a minimum of ten percent but preferably 15.

Hope you get it running okay

 

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Allternativly, take the easy way out and purchase a standard NV from PAW Engines, 01625423891. Cost about £10 inc postage. You might have to make a small bush to fit it in the intake.

Keith.

[Andrew TinsleyParticipant @andrewtinsley63637]

Keith beat me to it! Support PAW in these days of electric flight!

Andrew.

[William ChithamParticipant @williamchitham75949]

“We do these things not beacause they are easy but because they are fun” to misquote JFK. Love the PAW site though thanks for the steer.

I have had a few goes at the backplate and have one now that screws in but I’m not convinced I have the right thread form. I’m at 5/8″ x 36tpi but I’m using a 60° tool, maybe should be 55°?

Thanks Ramon and Bernard, I have seen those sites but yet to find a drawing that clarifies how the needle valve actually works. I have made a drawing from the remains of the original and attached a section. What I don’t understand is whether the needle meters the fuel by it’s position in the reduced diameter “A” or by obscuring the jet as at “B”. Ramon’s suggestion “…push it in till it seats..” makes me think “A”.

 

 

[bernard towersParticipant @bernardtowers37738]

Definitely “A”

[Ramon WilsonParticipant @ramonwilson3]

William,

The needle always meters the fuel at it’s point of contact at the far end ‘A’ – the hole (or holes) is just to let it out! if the needle is very close to the same diameter as the bore then the taper has to be long enough to allow fuel to reach the hole or the needle slightly reduced in diameter to do the same.

Just in case you are not certain – If the spray bar has a single hole then it (the hole) should point downwards into the venturi – if two holes then it should be set so these are at 90 degrees to the venturi

[William ChithamParticipant @williamchitham75949]

Ok great. The original needle must have been smaller than the bore, otherwise the point would have protruded out of the end. Seems to me better to ensure there is always a parallel section at the jet, other wise the taper would influence the fuel flow twice. Presumably the annular space around the needle needs to be at least equal to the area of the jet.

I’m expecting to have to experiment with the orientation of the jet hole, I’d read somewhere that 90° might be best but not that that applied to a two hole set up.

Thanks for the help, just need to make a sensitive drilling tailstock attachment now!

[petetwissellParticipant @petetwissell]

I remember the DC wasp – it was the first engine I had when I started out in model flying.

As I remember, the engine was almost worn out by the time it was run in!

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