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
In the May/June 2013 issue, a reader asked a question about his BMW K100RS. He was having problems with his fuel pump not staying on after initial engine cranking. Without much experience with that model, I offered the only solution I could come up with after looking at the wiring diagram, which was a possible faulty ignition switch. A reader with more experience than me recently replied to this article and his advice is probably closer to right than mine. Here’s the original question and the updated answer.
Q: I have a 1985 BMW K100RS, which uses L-Jet Bosch fuel injection. It had been running pretty well, though with an occasional miss. I filled it with gas the other day and now it won't start. The fuel pump in the tank is not running. I took the fuel pump relay out and it bench-tested good. I can jumper the relay socket terminals and run the pump, so the wiring is good. I can hot-wire the left side of fuse no. 6 and the pump will run, proving that the pump is getting both power and ground, and the connector through the tank is good. If I hit the starter button while the pump is hot-wired, the bike will run and rev, proving the ignition controller is good and the injection computer, also. What’s left? I seem to have proven that all the components in the fuel supply side are good, and also that the bike will run when the fuel pump is hot-wired. Something's missing and I don't know what. There has to be some little thing I’m overlooking?
A: James Bayles of Bayles Tuning and Electric in West Palm Beach, Fla., writes: “This ignition system was used in BMW automobiles as well as motorcycles. A set of contacts in the air flow meter keep the field coil of the fuel pump relay energized after the release of the starter button. Possible causes of this circuit not closing are an air leak down stream of the air flow meter, a door stuck in the air flow meter or bad contacts in the air flow meter. The owner’s manual wiring diagram is fully comprehensive. Find a BMW car mechanic to look at it. It would only require one hour of diagnostic time at my shop. P.S. It’s not the ignition switch. MC
A bigger tank
Q: I have a 1979 Triumph Bonneville Special T140D. I am interested in using it for touring and am pondering an upgrade to the U.K.-spec gas tank, which has about twice the fuel capacity of my stock U.S.-spec tank. Would I have to modify my seat or anything else? Also, do you have any suggestions on locating a U.K. tank or a well-made reproduction? — Boyd Smith/via email
A: The parts book for the 1979 T140D model shows two different gas tanks, the 3.6-gallon U.S. version and the 4.8-gallon (4-gallon Imperial) British version. And yes, you will have to change out your seat assembly to accommodate the larger tank. The tank badges are different for the different tanks, too, so either get a complete tank with badges or make sure you can get badges for the larger tank. I wasn’t able to find a U.K.-spec tank in my searches, but you might email Burton Bike Bits and see if they can offer any guidance. MC
Using pod-type air filters
Q: I own a 1980 Suzuki GS450, which I bought from a guy who had started to “rehab” the bike. He installed pod filters on the carbs. While they look good, he changed nothing on the carb setup, resulting in a typical flat spot in midrange on/off throttle response. He said he threw away the original air filter and airbox, which he said taught him not to throw away original parts from an older bike. The bike has the original Mikuni BS34s carbs, and due to EPA regs, the air screw is set and capped from the factory. I’m not looking to make a screamer out of this bike, but I like the look of the pod filters. If I pull the plastic cap off the air screw and tighten it up a little, wouldn’t I be able to get back to the correct air/fuel mix without re-jetting the carburetors? — Eric/Cincinnati
A: Your problem’s probably not in the air/fuel mixture. The GS450 had a reputation for poor carburetion when new, running very lean right out of the box (see A 400 on Steroids: Suzuki GS450) — and those pod filters are likely just making it worse. I’ve never been inside a GS450 carb, but Suzuki supposedly installed 1mm shims above the slide needles, lowering them for a leaner mixture. Removing the shims may be all that’s needed to fix the stumble. If that doesn’t fix it to your satisfaction, MikesXS.net has richer needle and needle jet setups for that carburetor. I can’t offer any exact specifications; those will vary with altitude, fuel type, engine condition, etc., and have to be found by experimentation with your machine. MC
Q: I have a 1983 Honda CB1100F. The carburetors are overflowing. I have replaced the float needles. I was told the floats were white when new; they are now dark brown. If I replace them will it cure my carb problem? The floats are $65 each, and I wanted a second opinion before I spend that much money. — Mike Gregg/via email
A: Without seeing the floats, I can only guess as to their condition. It depends on the type of float you have. Are they hollow or solid? If they are hollow, the color of your carburetor floats has no effect on how well they work. The important consideration for hollow floats is whether or not they are still sealed, with no fuel inside them weighing them down, which would cause overflowing. If they are sealed, then they should still be working as designed. If the floats are a solid foam type, it’s hard to tell if they are still good without a new set to test against. If you have a new set to test against you can try floating both sets in gasoline and seeing if they both bob to the same height or if the older set sits lower in the fluid. MC
Q: I am in the process of restoring a 1970 Yamaha AT1 125. Can you give me some advice on setting the fuel/air mixture screw on a Mikuni carburetor? — Phill/via email
A: There are two screws to adjust when setting the idle on a carburetor. One adjusts the slide height and is called the idle speed screw, while the other adjusts the air-to-fuel ratio and is called the idle mixture screw. The manual for the AT1 says start the idle mixture screw at 1-1/2 turns counterclockwise from fully screwed in (lightly seated; don’t screw it tight), then adjust either way in 1/2 turn increments to find the fastest, smoothest idle. If the fastest idle is too fast, turn the idle speed screw out to slow it down. Again turn the idle mix screw a 1/2 turn clockwise and counterclockwise seeking the fastest, smoothest speed. Once you determine which direction helps, refine it to 1/4 turns either way to fine-tune the mixture. If you find that the bike stumbles a bit when you open the throttle, 1/4 to 1/2 turn tighter will richen up the mixture a bit and compensate for the added air in the mix from the opening slide. MC
Q: I’m trying to take the gas tank off my 1970 Triumph Bonneville and am having trouble. I want to have the tank and fenders repainted back to original Triumph color specs, as well as rebuild both carburetors. — Jeff Sniadach/Moline, Ill.
A: If it’s the original tank there should only be three nuts/bolts holding it on; one at the rear of the tank under the front of the seat and two up front under the tank. The front nuts may be hidden behind the factory reflectors if those are still on the bike. I don’t have a service manual for that year U.S. model, but looking at the parts book it seems the only access is through the back of the reflector housing with a ring spanner/box wrench. Now I see why you hardly ever find one of these with the reflectors still on it! It may be possible to remove the reflector and access the tank nut from the front. If those three fasteners and the gas lines are removed, the tank should just pull right off, watching to make sure you don’t scratch the paint on the handlebar mounts. Hope this helps. MC