Carburettor Rebuild

The bike came with two carbs - this was the better condition one and, on that logic alone, I chose to strip it down. It's an Amal Monobloc - pretty ubiquitous on 60's British bikes.

After an afternoon of cleaning with carb cleaner and my small ultrasonic cleaner, it looked like this.

If you are particularly observant, you might note that the float bowl cover is different - the "other" carb had this one on, which I thought looked nicer with the AMAL logo on, so I gave it a quick clean and polish and swapped it over. It is all just roughly assembled for this photo, as I decided to do a full overhaul. All of the parts are still available (including new carbs) from Burlen .

I ordered a full repair kit, and gasket set, which means that all of the jets, valves, seals and gaskets will be new. I also ordered a choke kit, which was missing from my box of bits.

It was at this stage that I realised that the two carbs I had were quite different specs. The "other" one to that shown above is a type 376 but this one is a type 389. The 376 was used on the Crusader Sports so is the "correct" one, but the 389 was used on the last of the Continental GTs (a higher performance model) and should contribute to a better performance. As far as I can see, the only difference between the cylinder heads for the two models is a bigger inlet tract to match the 3/16" larger bore of the 389 carb.

So I ordered jets to match the Continental GT spec and decided to open up the inlet on my head by 3/16" to match the bore of the bigger carb (see here). I never thought I would end up tuning this bike! I suspect that I might have to mess about a little if the main jet is a bit big, but that looks relatively straightforward compared to the Japanese multis that I have been used to.  

The rebuild kit duly arrived and I installed all of the various jets and seals etc.

 

Winding forward several months, when I came to fit the air filter and connecting tube, I found that the larger bore carburettor needs a correspondingly larger connecting hose, which is not standard as, in its Continental GT application, it just had an unfiltered inlet "trumpet". I want to keep the air filter so I have concocted an inlet to link filter and carb, although I think it might restrict the air supply a bit at full throttle openings and cause rich running, so I suspect there is some work to do in future to fine tune the main jet size.

When I came to run fuel through the carb for the first time, I found that the float bowl cover wouldn't seal, so I ended up swopping back to the original plain cover. I also found that the float wasn't seating properly as I had used a fibre washer (from the rebuild kit) on the valve seat / fuel inlet fitting which effectively raised the seat. I subsequently found that the fibre washer was for an updated fuel inlet fitting, as the original plain mating face had been found to be a common source of leaks. So I ordered a new one and also a new tickler assembly so that at least I know that pretty much everything except the body has been renewed now!