AMAL Type 76 Carburettor Fault Finding and Repairs
- Andy Marks
- Jun 24
- 3 min read
Updated: Jun 28
After carrying out cylinder head repairs on my 1928 Norton CS1, the engine began running extremely rich, fouling plugs and producing excessive smoke. This article follows the fault-finding process, from checking fuel level and float condition through to identifying a hidden defect inside the AMAL Type 76 carburettor that had likely been causing problems for years.
Although all of the recent work had been on the cylinder head, this appeared to be a carburation problem so, to confirm this, I took the carb off of my 1951 Norton ES2 and ran the bike on that. It wasn't perfect but it sounded better, didn't smoke and I was able to ride it round the block.
Checking the Float and Fuel Level
There is no point in trying to jet a carburettor if the fuel level is completely wrong, or the float is sinking. So my first job was to submerge the float in some warm water and check for bubbles.
There were none so I re-fitted it and checked the fuel level. I have written about how we check this in more detail in a technical page. Click on the image or the heading below to read about that:
Jet Block and Union Nut
After confirming that the fuel level was correct I removed the jet block to check all air and fuel passages were clear.
Upon removing the union nut I was convinced I had found the problem. The nickel plating was peeling off the inside and several of the small parts were sitting in the passages in the jet block.
I cleaned the nickel off completely and used an airline to blow the lumps out of the jet block. I reassembled the carb and was somewhat disappointed to find I had not solved the problem.
Discovering the Real Fault
After stripping the carb again, I put the body and jet block in the ultrasonic cleaner. The rest of the carb was already clean from the last time I took it apart. I would usually clean the rest off in some engine degreaser or petrol.
Whilst rinsing the jet block off under a warm tap, I noticed another potential issue. The two halves of the jet block are soldered together with the central tube is also soldered in as a third part. This one had been soldered badly and if you look at the image below you can see a wide gap in the solder which perfectly lines up with the pilot drilling. Fuel had an easy passage, bypassing the small drilling, to flood far more fuel in than is required.
My suspicion is that this had been happening for some time. The extra fuelling was masked by the air leaking past the inlet valve guide. It had probably been getting worse and, with the freshly overhauled cylinder head the bike would no longer run. It would also explain why previous changes to carb setting never seemed to do very much, and why the bike had never been as easy to start as other Nortons.
I used a large soldering iron to run a fillet of solder around the joint. I wondered if I would need to clear solder out of the pilot drilling afterward but it was OK and I was ready to reassemble the carb.
At this point I was reasonably confident I had finally found, and fixed, the root cause of my problems. With the carb re-assembled and refitted the bike was transformed. I spent the first few rides checking the plug colour and steadily making it richer. During a subsequent week in Wales the bike ran better than I have ever known it to. It's amazing what you find wrong with a bike which is almost 100 years old!
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Very good analysis and eventual solution of the problem. Thanks for sharing. Steve Zerafa, Malta.