6. Carburetors and Inlet Stubs

It is generally accepted that Vincent Black Lightning's and Flash's were fitted with specially made clip fitting racing Amal's of 32mm bore. As far as I can tell this is generally true, but if you examine photographs of the earliest Bikes (i.e. of 1948-49, the year my bike is built around), you can see that the more standard stub fitting 10TT9's are fitted.

As you might expect, it was these earlier carburetors that I decided to fit. Reasoning behind this was as follows. First off, photographs of the '49 spec bikes showed stub fitting could have been a possibility. Secondly, trying to find a pair of original clip fitting carb bodies would prove almost impossible (I have actually seen a couple of pairs advertised over the subsequent years but, not having a large sum of available cash spare, have not even enquired as to their price!). Finally, I was intending my bike to have the original Lightning specification of heads, i.e. one front and one rear, not two fronts. I believe that stub fitting would be correct for this spec, and am almost sure that by the time Vincent's had switched to two front heads, they had also switched to clip fitting carburetors.

32mm Bores
The only problem with this idea was that, as my section on cylinder heads explains, I had managed to bore both inlet tracts out to the later used bore of 32mm, meaning I would have to attempt to bore both carburetor bodies out to the same diameter. As far as I am aware, original 10TT9 bodies were not designed to be taken out to this large a diameter and I was worried that there might be problems. Incidentally, something Phil Irving points out in his book 'Tuning for Speed', is that 1 3/16" (T10TT9) and 32mm (T10TT) were the only size of TT carburetors that had the same bore all the way through. All other (smaller) sizes were only of the stamped size on the engine side of the throttle valve. Before this point they were of a larger size, to allow for the space taken by the needle. I assume it was not possible or necessary on the 2 larger sizes to need this 'dual' size of bore.
Towards the end of the Black Lightning's era a small batch of 1 7/16" carburettors were produced for Manx Nortons and Vincents. I believe a set of these were used by Bob Burns when he set the ultimate motorcycle speed record at 185mph in 1955. I am not sure if these were T10TT's (T5TT's?) or GP's. I have never seen a factory photograph of a Black Lightning that shows conclusively that GP's were fitted. I would be interested if anybody knows.

Gunga Din with clip fitting GP1's fitted. Were thesefitted to production bikes though?

Selecting the Carburetors
As it happens, racing carburetors are something I have always been 'turned on' by, so I had a number of 10TT9's available of various sizes and types. I had already picked out 2 bodies that were in generally good condition, ready to bore. These were flange fitting versions and were 'handed', the front cylinder having the choke barrel on the left and the rear cylinder having it on the right. As can be seen from the photograph, the left handed carburetor also had paint on it, which I suspect might be original, I used this as a pattern for a new batch of paint, of which more later. Incidentally, a lot of the carbs have now gone or have been used since the photo was taken!
Neither of the carburetors were 32mm (no surprise there!), so it would be necessary for each to be bored to that diameter.
As for the floatchambers, for the rear carb I would use a direct fitting vertical chamber (ie no downdraught offset) but for the front head I would use a remotely mounted chamber. A similar arrangement can be seen on the photograph of a large tanked Lightning in the Postwar section of this site.

Boring the Carburetors to 32mm
The first step was to strip the carburetors and remove the brass Primary Choke sleeve from the aluminium main choke. These protrude into the bore and leaving them in would mean they would be damaged when I machined the new bore on the lathe. They are threaded into the main choke and only have a very small screwdriver slot (bored through the centre), with which to remove them. Over the years, the alloy bodies tend to corrode slightly and if this happens the brass choke is almost impossible to remove. This was the case with one of the carburetors and it was clear that trying to remove it would only damage it beyond repair, despite making a special tool jig to help with this task.
This was a real pain as it meant I would not be able to bore out this choke in one pass, instead I would have to make 2 passes, boring from each end into the centre, leaving a section surrounding the brass choke that would be at the original diameter.

My collection of racing carbs - take your pick!

This was pretty much how it worked in practice. For the carburetor that I had removed the brass primary choke it was slightly easier, first I pressed the aluminium main choke back in place, in the carburetor body and secured it. Then I very carefully mounted one end (the stub end) of the carburetor in the 4 jaw chuck of my lathe. I spent the next 2 hours trying to ensure that not only the end closest to the chuck ran true, but also the other end of the bore (protruding some 3 inches out don't forget) was also running true! Not only that, but I had to tighten the chuck hard enough to hold the spinning carburetor whilst it was being bored, but not enough to damage the carb!
I found that the easiest way to ensure the trueness of the end closest to the chuck, was to make up special little tool, consisting of a wheel spoke, sharpened at one end, with a fulcrum point half way along its length.
This could then be inserted into the carburetor bore, where touching the innermost point it acted like a 'trembler', indicating the highspots. Although a very laborious task, once set up the actual boring of the carburetor was quite uneventful in comparison. Though not a trained engineer (although my father was), I am well aware that setting up for a job can often take far longer than performing the job itself!

Both complete, the finished job looks quite neat. Certainly I have to say that the size of the hole (32mm) looks enormous. I only hope they work ok, especially as the centre alloy choke body is perilously thin at this bore. As a final task, I have carefully re-stamped the air choke casting with the new size (32mm), instead of the original imperial size.

Bellmouths
I did not mention it in the previous section but I had decided to fit the bellmouths before I started the boring process. By boring them in situ, it would ensure they were absolutely true to the main body bore.
There are (as far as I am aware) 3 standard lengths of bellmouths, short, medium and long. Although I had various longer lengths, which personally I have always preferred, you can see in photographs that the early Lightning's look to be fitted with short bellmouths. These are of approximately 15/8" in length. As I did not have a matching pair of these I ordered a new set from Autocycle Engineering (call 01384 -253030).

Float Chambers
Although early photographs show Lightning TT carburetors with standard TT float chambers, in about 1949 the later Type 302 float chambers were introduced. These are slightly larger than the earlier chambers and have anti-frothing baffles fitted. They are easily identified by their slightly larger bodies and the top covers are held by 2 hex bolts, rather than the earlier versions screw-in top.
Although I am not sure if this type of chamber were used with the earlier carb type, they were certainly used with the 32mm carburetors, which were the bore mine are now at. For this reason I had been saving a pair of these float chambers for many years, the rear one being a standard (vertical fitting) chamber, while the front one was a remote mounted version (see Post-War sectionfor an example of these fitted to Lightning's).

Although I had owned both chambers for many years it was only when I stripped them for rebuilding that I noticed that the standard (non-remote) chamber had a smaller hole than the normal TT main jet holder, and therefore would not fit without some modification. This was not a problem however, as it was not a big job to mount the chamber on my horizontal miller and bore the hole out to the required size. I puzzled for sometime as to why the hole was a different size to normal. It was only this year at the Founders Day Rally that I found the reason why. I spotted a similar chamber for sale and it was marked 'Triumph TR'. Of course, I then remembered that early Triumph TR's were fitted with 302 floatchambers, what I had not realised is that these must have been 'specials' with a smaller hole to fit the standard carburetors (can't remember if these were Amal 276's or SU's though).

Remaining Parts
I wanted to bring the carburetors back as close to 'as new' condition as possible. To do this I wanted to fit the original side choke mechanism fitted to TT carbs, so often removed from bikes being raced today. To be fair I have never bothered fitting them to my racing bikes either!.
The original adjusters that screw into the top of the choke casting had a very distinctive knurl and are very often damaged. Luckily I had two good examples. Some of the other choke parts were missing, but again Autocycle engineering was able to help.
TT carburetors were fitted with two types of main body screw tops, again I think they changed around 1949. The inner alloy throttle cable holder having a second hole drilled and threaded, to hold a square headed screw identifies the earlier types. The purpose of this screw was to lock the ring and stop it unscrewing. The later type was fitted with a smaller alloy centre, with only the throttle cable hole fitted. In conjunction with this type, a distinctive pressed steel spring was used to stop the knurled ring unscrewing. This spring was itself secured by the bellmouth locking ring.
I decided to elect for the later type, as I think it looks a prettier setup. I only had one full set of bits so again; had to purchase the other set new.
I had enough fittings to build up all the remaining parts. Some of these were slightly the worse for wear, so I spent some time carefully cleaning the faces up before removing as much as the original plating as possible.
As I have my own nickel plating bath, I then re-plated all the parts with dull nickel, including the original slides (which are now a very nice smooth fit). Although very time consuming, I have to say I am very satisfied with the results, not to mention the satisfaction of having plated them myself.

Painting
Finally, I went carefully round all the main castings, carefully removing any blemishes, and (of course) ensuring all mating surfaces were absolutely flat.
Many modern restorations of racing motorcycles are in my mind spoiled because the carburetors are given a highly polished finish. Although very pretty, to me does not look correct, as they did not come out of the factory like this. I had one carburetor body in my spares that looked to still have some of the original paint attached. This is an unusual yellowly-green colour, which I had also seen before on other racing carbs. I went to my local car paint specialist and had a 0.5 litre tin of automotive paint made up to as close to this colour as possible. It turned out to be a colour fitted to very early Volkswagen Beetles!
After masking and applying a 2-pack alloy etch primer to all the castings, I then used my small Devilbiss MP gun to apply the silver topcoat.
I spoke to the owner of Autocycle Engineering about this paint. He too said he knew it had been fitted originally (although he also told me he had seen many different shades over the years). However, he was not able to recommend a type of lacquer that would stop the paint being removed by petrol. Because of this, he does not paint the items he makes himself.
One of my other hobbies is flying Radio Control planes. Like on my racing Norton, the glowplug engines fitted to these planes use methanol fuel, often mixed with high percentages of Nitromethane (rocket fuel!!). To protect the engine bay of these planes I use a specialist 2 pack lacquer called Super Tufkote, which is totally fuelproof. I have now lacquered the completed carbs with this lacquer, and although they are yet to be tested I am quietly confident.

Although yet to fit the correct main jets, I have now finally assembled the carbs, complete with new washers all round. I will admit I am rather pleased with the results and am really looking forward to seeing them finally fitted to the bike.

Jet Sizes
I almost forgot, although I will not be able to verify the sizes are correct for this bike until I test the bike on the track, below I have listed the sizes I have used for the initial build (these are taken from Phil Irving's Tuning for speed and Vincent service records):
Pilot Jets

Inlet Stubs

Press on the following link to read about how the inlet stubs were made: