Replacing the carburettor mounting inlet manifold

The carburettor manifold was missing from this bike. Its a cunningly designed tube of aluminium that connects the carb to the cylinder, adapting for the height difference between carb outlet and barrel inlet port via a bend. It also changes the shape from the round Carb mouth to the squared off shape of the port. Finally it does all this with smooth curves so as not to obstruct the flow. My fabrication skills are not up to making one of these so I searched for a replacement. The fb group told me that a Puch inlet manifold will fit the barrel although it's angle is wrong, the cutouts don't match and it will need longer mounting studs. I'll have to see if I can make it work. Currently my plan is to modify the Puch part making an adapter from the barrel. 

This is the Puch part as supplied- it's rubber covered aluminium and the bulk of the angle is created in the rubber extension.

I cutback the rubber

and removed as much of the extension as I could.

The fit is close but the angle is clearly still mismatched. This could be adjusted with a cunning rubber tube union but its asking  lot to find one with different diameters at each end let alone the bend required for angle adjustment.

The only answer was to remove more of the Puch part to try and improve the angle match. Eventually I had no alternative but to slice practically of the manifold off the mounting flange and then insert a new tube to connect to the carb.

This is the stub of the Puch part. Unfortunately it's a tapered opening the cylinder side being a larger hole than the carb side.

I started with a length of 22mm aluminium tubing and turned it down over a 5cm length to the smaller opening diameter. I could then taper this dimension (8 deg taper) to the larger diameter so the the tube would slip snugly through the Puch manifold and engage in the tapered hole. All I needed to do then was reduce the length of the full width section to fit flush in the manifold and lie against the cylinder. Finally I turned down the projecting end until it would fit into the silicone tubing I have available.

Final turned aluminium tube note flare at left hand side to fit into the puch manifold and the reduced diameter at the right to fit into the tubing available to connect to the carburettor.

Turned tubing inserted

..and machined flush at the rear.

Assembled connector. I fixed the tube in place (hopefully airtight) using high temperature Loctite joining compound.

The silicone hose I have available is left over from a former project (Lotus Excel). It usefully features two different sizes joined by a 60 degree bend. I can trim the narrow end to join to my new adapter whilst the wider end should fit the carburettor outlet. This tubing even has a bend and so should fit the joint geometry very well. However- there is no avoiding the fact that building the adapter in this way has reduced the area and thus the amount of mixture able to enter the cylinder. This will reduce performance but at this stage, when a replacement part is not available, it should at least enable me to get the motor running.

Silicone tubing- narrow end already trimmed.

Trimmed and installed. Need to take it off to fit the gasket and hose clips. 

Hopefully in the fullness of time I will find an original part and if not then I'll eventually need to revise this adapter but use wider aluminium tubing TIG welded to the union plate. Currently this is beyond my expertise.