What Carb for a 18TCS?

[norman_john]

Hi,
I am in the process of fitting a nice Norton Oil Pump all alloy engine to my 18CS rep bike which is coming along nicely. There seems to be little hard info available on the TCS, or at least I have not stumbled across it yet. I'm a practical person and not necessarily looking for perfection in the originality stakes, but want something that works on a daily basis. Its time to look at getting a suitable carb.

The inlet is 35mm so I guess choices are:
* An Amal GP, but they are selling for £450 and that's without a float chamber, and dont have a great reputation for ease of starting
* A 1-3/8" Monobloc (but they are not available).
* A 34mm Concentric (but they are not available)
* A 35mm MKII Concentric (but really they would look out of place)
* A 35mm modern round slide carb - any suggestions for something that at first glance has a period look?

I know a bad choice of carb has the potential to spoil the experience of riding what I'm hoping will be a brilliant bike to ride, has anyone any good advice arising from their experience?

Thanks folks
John

[g80csp11]

I have modified a 1" mono block to 1.1/16 by boring out the inlet . all other dimensions were exactly the same ie slide etc..
I'm not sure if you could do this to get a 1.3/8
Plenty of second hand carbs about to be modified

[Les Howard]

Hi John. Fitting a smaller carb than the manifold diameter works perfectly OK and you will not notice any detrimental effect in normal use. The only time the difference will manifest itself will be when you are at absolutely full bore, where the slight increase in diameter will not allow the ultimate power to be achieved from the engine. But how many times will that be?
What you have to consider with slide type carbs is that it is only at full bore that there is a smooth flow through it (Not quite correct as the needle is still in the air flow apart from the GP/TT type) The rest of the time , (which is probably 99% of the time!) you will be riding on normal roads at way below full throttle and there will be a great slug of metal disrupting the gas flow in your carb.... this is the throttle slide! So, you can see the slight jump up in diameter with a smaller carb fitted makes very little difference to the normal flow in real terms, unlike fitting a larger carb than the inlet manifold where a sharp step is created causing quite a bit of turbulence.
Coming back to your actual head, the inlet bore does seem to be on the large size at 35mm, and many would consider this size far too large for normal use even for racing. The large diameter will reduce the inlet gas speed at all revs and throttle openings and can actually reduce power by reducing the vital swirl speed inside the combustion chamber. As said, most times you will be riding the bike on the road at around 1/3rd throttle opening so with a 35mm inlet diameter the gas speed will be very low and lead to poor efficiency.
My suggestion is that you make up an alloy sleeve and reduce the inlet tract to 32mm. This will allow a Mk1 Concentric to be fitted which is quite big enough for very good performance and you will have the benefit right through the throttle range of better and more efficient cylinder filling. You might even reduce the size to 30mm which will allow the Monobloc 389 1 3/16” carb to be fitted which I believe was standard, you will then know the original jet sizes to be used.
As far as I remember the 500 single inlet tract is quite short and usually tapers near to the valve bowl. The new reducer adapter tube can be easily be made internally tapered to fit the smaller diameter at the valve end and have the correct diameter at the carb end so there will be no sudden jumps or lips created.
Coming right back to my first point, fitting a smaller carb without the reducer sleeve will still work perfectly Ok but obviously fitting a sleeve is better still.
So feel free to use the carb of your choice. Just my opinion though...Les
PS...I have assumed you will be using the bike on public roads mostly rather than just track usage?

[norman_john]

Hi Les,
Yes I appreciate your thoughts on it being OK to use a mis-matched carb for us mere mortals who chug and only occasionally thrash about the open road. Yes 35mm is on the large side, even for a 600cc and yes that tract is (too) short as well, although its no problem to add a spacer and longer studs and there is plenty of room to do that.

I don't mind playing with jet sizes - I recently had fun on a high compression B44 with tuning in a JRC and have gained enough knowledge through the years on other bikes.

I was hoping for some wisdom from members who have a TCS to know what carb they are using (the original 1-3/8"?) and how well their carburation works. Anyone out there?

Cheers
John

[Les Howard]

Hi John...I'm sure you would like feedback from actual owners and hear of their carb choices so would I.
What I would say is that my reply was in response to your ambition to achieve:

what I'm hoping will be a brilliant bike to ride..

Fitting a carb suitable to road use rather than track racing will certainly help loads, making it far more tractable, responsive, easier to start and more fuel efficient, all for the loss of say 1-2 BHP at the top end!...no need to reply John.
ATB...Les

[Don Madden]

All the classic AMC bikes were built to Imperial, not metric standards, the nominal bore & stroke were given in metric to agree with international standards notwithstanding. The inlet tract is 1-3/8in As we Colonials here in the USA still use the Imperial standards, I don't bother to convert most measurements to metric. 1959 Typhoon, as all AMC engines differ from later. The typed up spares list calls for the carb to be the same as the G80CS, an Amal 389/12 monoblock with 1-5.16in choke. Although most 1959 Typhoons were badged as AJS 18TCS, and I have inspected several as will as the later Typhoons. No Typhoons were badged AJS after 1959.

For 1960-62 the engines were fitted with part no: 026031, an Amal 1-3/8in clamp mount GP type with manifold that bolts to the head. As an option, the 389/12 was offered. There are many differences from the 1959 engine, cylinder, (1mm larger bore), piston, head, frame, etc. so not all Typhoons were created equal. The new Amal Company makes both types of carb to original specification, although they are not cheap. I have seen Amal 32mm concentrics fitted to CS & TCS engines & seem to work well if jetted correctly. I have a 1961 G80TCS which starts well when cold but does not want to start when hot. As the mag is in good condition, the old 389/12 is likely the culprit so am tweeking it to cure the problem. Maybe I will have to replace my 80yr old kicking leg, although my 500cc G80CS & BitSA 441 bike s start well, hot or cold, with the correct technique.


Cheers! Don.