Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.


3/26/2014
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Tech Corner

Keith’s notes

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



3/19/2014
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Tech Corner

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



3/12/2014
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Tech Corner

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



3/6/2014
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Tech Corner

Carburetor floats

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



2/26/2014
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Tech Corner

Mixture setting

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



2/19/2014
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Tech Corner

Triumph tank

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



1/22/2014
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Tech Corner

Leaky seal

Q: I have a 1981 Honda CB750K with 54,000 miles on it. I took the bike to the Honda dealer to have the carburetors overhauled and was told that the cam chain needed to be replaced, so they did a complete engine overhaul. Four months and many parts problems later, I got the bike back. It runs great but has an oil leak at the drive sprocket. They have put two more seals on and they have both leaked. They glued the second and third seals on. The bike was not leaking oil when I brought it in. Everything is tight. The seal only leaks when the transmission is in gear and the shaft is turning. The people at the Honda shop say they do not have a clue. I was an engineman in the Navy for five years. I have never seen anything like this. — John K. Thompson/via email

A: If you hadn’t mentioned the top end overhaul I would suggest you have a burr on the final drive sprocket that is destroying the seal. Since you mention that the problem only showed up after the overhaul, I’m going to suggest a different cause. I wonder if, during the top end rebuild, somehow the engine breather became blocked? There shouldn’t be much pressure buildup in your engine, but with blow-by from the cylinders plus a little pumping loss it might be enough to produce some internal pressure in the engine. Since the engine and gearbox share an oil supply, the easiest place to relieve this pressure might just be the big seal at the final drive sprocket. Ask the Honda shop to test the engine breather and see if it is still working as designed. There is also the possibility that the automatic chain oiler is doing its job too well. There’s supposed to be a felt element built into the hollow end of the output shaft that is oiled by internal circulation. The drips from that are spun off by centrifugal force to supposedly lubricate the chain. If the felt is missing, or otherwise damaged, I can see where the volume of oil would increase drastically.

Note: I managed to get the schematics confused between SOHC and DOHC Honda CB750s, which made part of my answer to this question meaningless. There is no automatic chain oiler on the DOHC 750s. If the oil is coming from behind the drive sprocket it must be coming through the large seal. If the oil is coming from in front of the drive sprocket, it may be that the small O-ring seal behind the large washer holding the sprocket in place has failed or is missing. MC



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