Inlet me down gently
Since first seeing 3D printers in use at Malossi and then Lorenzo’s 3D printer running at Rimini Lambretta Centre I’ve been fascinated by these machines and what you can do with them.
As far as making prototype parts and checking for fit and alignment they have revolutionised engineering. If you can imagine something and transfer that idea to 3D drawing software then you can make it.
The aspect I struggle with is that I’d love to be able to use the parts you produce, but the most common plastic filament used – PLA – has a very low melt point. Low enough that it would warp if bolted to an air-cooled engine.
For this project the 3D printer allowed us to design and test various angles of inlet manifold which I could then get produced in a combination of plate and tube using either steel or aluminium. Steel I could weld myself, but aluminium meant phoning a friend.
After numerous 3D prints, produced with different lengths and angles so that the carburettor was both clear of the engine mount and not at risk of being knocked off while kick-starting, I came up with an alternative solution.
While browsing on eBay for rubber inlet connectors I came across some angled inlet rubbers complete with a 2-hole bonded metal flange and O-ring. A pair of these inlets for a Yamaha XVS Drag Star cost less than £10 delivered. Handily, the cheap copy of a copy of a copy of a Keihin PWK carb I bought would fit straight into the rubber. It was decided that for less than a tenner for an all-in-one manifold solution, it was worth a punt.
As it turns out, all the 3D printing was a waste of time. All we needed was a piece of flat aluminium plate with some holes in. After moving the bolt holes a little in the manifold with a Dremel it all bolted together, putting the carb in the right place. Job done adequately and for the right price. The Drag Star manifolds are ‘handed’ so only one of the pair is usable, but at a tenner for two it’s still not expensive to keep a spare on the shelf.
From a tuning perspective, it’s not ideal to have a 30mm bore manifold with a 26mm carb on the front but we can worry about performance after we’ve got it up and running.
LEAK DOWN FAIL
The rest of the engine was cobbled together with donor parts from my old box of racing spares, including an old Taffspeed close ratio gearbox and a Vespa PK XL2 clutch.
With all the parts bolted on we tried the now-viewed-as-essential leak-down test (see this SLUK article on how to do it) on the engine and it failed in spectacular style. The problem stemmed from Darrell Taylor’s attempt to carve a decent-sized inlet out of original casings which broke through, where they often do, behind the rear tyre.
In situations like this, where there is no physical force exerted on a repair, I’ve always been happy to seal up a small hole in casings with an aluminium putty such as Devcon F or JB Weld. Sam simply ground around the whole area with a Dremel to get back to clean aluminium before slapping in a load of JB Weld and letting it set hard.
A day later we tried another pressure test and the engine passed with flying colours. If we hadn’t pressure tested it we may never have found the hole and we’d be forever trying to sort the carb jetting to make up for the fact that it had a hidden air leak. I’ve got Charlie Edmonds to thank for introducing me to leak-down testing because this process is now saving people a lot of money in blown engines.
NEXT TIME – we develop a bracket of the unholy, allowing you to fit a Vespa engine into a Lambretta.
New products always in development…