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1965 Triumph Bonneville Carburetor Problem
Q: My 1965 Triumph Bonneville ran well when I put it up for the winter back in December, but hasn’t run right since. I didn’t drain the fuel, but used Sta-Bil fuel stabilizer instead (maybe not enough). Here are the symptoms: It starts easily, usually on the second try, and idles fine. It runs fine when fully accelerating, but starts to miss on the left cylinder when a steady speed is attempted. It absolutely will not hit properly at any steady speed. The right-hand cylinder runs beautifully at all speeds. After riding about two to three city blocks, I limped home and pulled the spark plug and found it to be totally black with soot (no oil, just powdery soot). It’s obviously running way too rich. I cleaned the plug, attempted to tune the carb with the air/fuel jet and rode around the block. The left bank again started missing when a steady speed was attempted. I pulled the plug and again found black soot. At this point I pulled the carburetor (a new Amal), disassembled it and cleaned it in B-12 Chemtool carburetor cleaner. I then blew out all the passages with high-pressure air. I replaced the left-hand spark plug with a new one. The problem returned immediately as I attempted to ride down the street. Once again, it would run fine at full throttle.
Do you think it possible that I may have an ignition problem rather than a carburetor problem? This bike had a new electronic ignition system (Boyer) installed by a reputable vintage Triumph mechanic approximately 1.5 years ago. At the same time it received two new Amal carburetors, new coils, new spark plug wires, new complete wiring harness and a new battery. I have not yet swapped my left and right coils to see if that might be the problem. I also don’t know if my battery was fully charged at my last attempt at riding. I am currently charging the battery with a Battery Tender. Before I go any further, I am anxious to hear what you think may be going on. — Thomas Jones/via email
A: The fact that it runs on both cylinders at full throttle but only the right at partial throttle still points to a carburetor issue. If the left cylinder wasn’t sparking, you wouldn’t see the black soot on the plug; it would just be wet. Steady throttle probably means you’re in the middle range of the carburetor, where the mixture is set by the combination of main jet, needle and throttle slide cutaway. I’d start by once again removing the carburetors and comparing their components closely. Check to make sure the slides are numbered the same on both, and that the main jet and needle jet are identical to both. Also make sure the needle is clipped in the same notch on both carburetors. After you’ve checked all this, if you find that they are identical, you should start by dropping the needle in the left carburetor by one notch. That will lean out the mixture at partial throttle, which should help with the black soot. If that helps, you are on the way to solving the problem. There’s a great online reference for carb tuning called Bushman’s Carb Tuning Secrets. Just a little down the left hand side of the page is the link to a comprehensive explanation of how to tune the Amal Concentric carburetor. MC