Technical Q and A for classic motorcycle maintenance and repair.
Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
Q: I am approaching 95 percent completion in restoring my 1975 Triumph T160 Trident, and I have encountered a unique problem. With valves set, timing correct and all three Amal carbs synched, I drive slowly and she runs great. When I push her harder, as soon as I hit third gear she dies completely and every time I coast to the roadside. Initially, I was suspect of my Tri-Spark ignition failing. But I wait one minute and she restarts. As I ride her home and shut her down inside my silent garage, my freshly sealed and painted stock fuel tank is “whistling,” equalizing a partial vacuum. Did my new 5/16-inch clear plastic fuel filters excessively restrict my gravity fuel flow? Does my new chrome OEM fuel tank cap require a vent hole? Or should I mount my aquarium air pump on my dashboard and plumb it to my fuel tank for a “positive pressure” fuel flow? What did the OEM college boys design to relieve the fuel tank vacuum? I simply cannot believe our 91 octane is so poor that the 750cc mill is consuming fuel at a rate the lines cannot keep adequately supplied. I have drilled both tank cap holes to 1/16 inch and still — same vacuum. Should I drill both holes to 1/8 inch and set the float levels higher? Have you ever heard of such a unique malady? — Yukon John/via email
A: Does it go completely dead? No coughing or stumbling first? That would point to an electrical problem. But your “whistling” tank points in another direction entirely, as you know. Since you mention it’s a newly painted tank, before you try this next test you should drain out about half the gas so you don’t slop out wholesale amounts of gas on your new paint. While you’re doing that it would be a good idea to measure the flow out of the taps to be sure they are flowing enough gas to keep the carbs full. I’ve not had to do this myself, but some quick searching indicates you should get close to 300ml per minute. Once you’ve done that, loosen but don’t remove the gas cap and go for another ride. The owners’ manual suggests that for sustained high speed running you should have both taps open, so if you’ve been doing that, continue. If not, try the loose cap with just one tap open and see if it improves, and again with both open to see if that is any better.