Technical Q and A for classic motorcycle maintenance and repair.
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1970 Triumph Bonneville Amal Carburetor Concerns
Q: My cousin gave me a 1970 Triumph Bonneville that was bordering on junk a number of years back. I restored it and it runs fine, however there is one ongoing problem. After it sits for a period of time, the carburetor idle jet stops up on the left side. After I take out the idle adjustment screw and open the orifice up with a .017 drill bit, then tickle the carb, the bike starts the first or second kick. The right side idle jet stops up, but not as often. The carbs are Amal Concentrics. The left side carb’s slow-speed jet is in the side behind the slow-speed adjusting screw. The slow-speed jet in the right side carb is screwed into the carb body, which means the float bowl must be removed to clean the jet. I am quite certain that the 10 percent ethanol is why the jets are clogging. My question is, why does the left side slow-speed jet (which is in the side behind the slow-speed adjuster screw) clog roughly four times more often than the slow speed jet in the right side carb, which screws into the carb body inside the float bowl? By the way, I use fuel stabilizer. - George W. Miller, Jr./via email
A: While I don’t have any scientific data to back this up, I’d say the reason the left carb clogs up faster is due to the horizontal layout of the idle jet in the circuit. Gas can sit behind the jet, or even in the jet, due to surface tension, and the more volatile components evaporate, leaving behind the stuff we call varnish. The right-hand carb, with its vertical jet, drains cleanly, leaving little or no gas behind to evaporate. As far as I can tell, this has been a problem since the introduction of the Amal Concentric. They were originally meant to have the screw-in jet you have in your right carburetor, but as a cost-cutting measure they ended up with the pressed in jet behind the air screw. Recently, Burlen Fuel Systems, the modern manufacturer of Amal carburetors, introduced a new version of the Concentric, called the Premier, with the idle jet on the opposite side of the carb from the air screw. Now you can remove both screws and clean the idle circuit from both sides, making it very easy to see if your idle jet is clogged. Unfortunately, there is no kit to retrofit existing Concentric carburetors. Many riders have done their own modification over the years, drilling out the blank side of the idle circuit opposite from the air screw and tapping it to the thread size of the idle air screw. They then cut off the taper of the screw and use it along with the usual O-ring to seal off the backside of the idle circuit. Once you’ve done this, you can easily check your 1970 Triumph Bonneville’s idle jet for clogging. MC