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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

2-Stroke Machines: Yamaha YDS3

Motorcycle Classics tech expert Keith Fellenstein

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, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Two-stroke exhaust

Q: Except for my first bike (a Puch), I’ve always been a 4-stroke guy. I now find myself in possession of two 2-stroke machines — a Montesa Cota and a Yamaha YDS-3. As you might expect for an old trials bike, the Montesa was pretty rough, but it was running so I took it out for a spin and promptly set the exhaust pipe on fire. The Yamaha isn’t running but is in very good shape, and I would rather not burn it up. A stranger at a motorcycle meet said I needed to “burn the pipe out” but had no further information. Can you fill in the blanks? — Bruce Waters/Cotopaxi, Colo.

A: The stranger had the right idea, even if he didn’t know how to explain it. Two-strokes by their very nature produce more exhaust deposits than 4-stroke engines. The partially burned fuel/oil mix that shows up as smoke from the tail pipe also gums up the inner workings of the exhaust system. Most vintage 2-strokes have removable baffles. There is usually a screw or bolt holding them in at the end of the muffler. Remove the bolt and pull the baffle out. The old method was to put the baffles in a large metal pan, douse them in flammable liquid and throw a match at them. This is frowned upon these days, both for air quality and safety issues. A slightly better method is to bypass the gasoline and match and use a propane torch to burn off the oil buildup. The goal is to turn the gummy mess into flaky carbon that can be brushed off. It’s still not very emissions friendly, but then neither is the 2-stroke itself. You can minimize the buildup by using a good synthetic 2-stroke oil and paying close attention to the mix ratio. Vintage bikes with oil injection are more problematic; most of them are set to inject at anywhere from 24:1 to 32:1. Modern synthetic 2-stroke oils work well at 40:1 and some even at 50:1. Obviously, the less oil in the mix the less gum in the exhaust. Too little oil, though, and you’ll seize up the piston. My favorite way to minimize the buildup is to make sure the engine and exhaust get good and hot every time you ride. This is best accomplished by a nice long ride, say 20 miles or so. MC