Sometimes a carburettor needs a replacement part that is not available or only available as part of an expensive assembly. Take the last type of carburetor fitted to the GSXR750 on the T/V models or SRAD, these have an arrangement where the needle slides in a plain brass tube fed at it's top end by a gallery from the emulsion tube. The emulsion tube sits inside the main jet holder and is fed by a gallery from an air jet in order to mix the flow of fuel from the main jet.
This all sounds complicated, but it gets worse; the emulsion tube is a brass part permanently pressed and bonded into the carburettor body casting - big bucks to replace!
Imagine our horror (and the customer's) when we last worked on a set of these to find that one emulsion tube (on #1 carb) had been rotted at the point it entered the body casting and the resulting expanding corrosion had blown through it's wall. The part was left dangling when we removed the jet holder.
In the picture above the emulsion tube is missing from the left hand carb body, at this point it was decided to try a repair strategy that would entail removal of the remains left in the body casting and employ a section of custom made brass soldered to the remaining part of the emulsion tube in order to preserve it's dimensions.
To extract the remaining stub of the emulsion tube we wound in a suitable tap until it hit the bottom of the hole and gently eased it out. Below you can see the remains of the emulsion tube and means of extraction.
The brass had waisted quite badly at the point where the tube enters the body of the carb, so the stub was discarded and a new section manufactured on a high precision miniature lathe - the vintage Cowells 90E used in our workshop.
At this point the blank tube was attached to the existing emulsion tube and sized to match the height, internal Diameter and External diameter of the original part. One cross hole had been lost to the corrosion and was carefully drilled using a special self-reaming carbide drill bit in a cross-slide mounted milling vice. The newly restored part was then burnished and cleaned inside and out in a special solution to neutralise any by-products of the soldering process.
Once we were happy with the finish and accuracy of the repaired part it was offered up for a final check, then secured in the body with a thin smear of epoxy bonding material; the latter is the same product as used by some engine tuners to re-shape inlet ports on high performance cylinder heads and as such it is well tested in hot environments where fuel is present.
To ensure a good bond the hole in the body was cleaned out to remove corrosion and ensure that sufficient epoxy remained in the joint, the portion of the emulsion tube to be bonded was also gently knurled using a small coarse file in order to retain the epoxy and centralise the part in it's bore.
Once set the assembly was checked for concentricity and height compared to the adjoining carbs and the reassembly of the remaining parts completed.
I will let the customer's comments on the finished job complete this post:-
"All's well that ends well.... the refurbished carbs were fitted to the bike, & aside from balancing / synchronising, there were no other issues..............Thanks again, a great job & I'll definitely be recommending your service to my biker pals ;o)"