Tech Corner
Technical Q and A for classic motorcycle maintenance and repair.

Norton Commando Roadster Idling Problem

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

Norton Commando idling problem

Q: About two years ago I bought a 1974 Norton Commando Roadster that was  in excellent original condition, with one exception: The left cylinder doesn’t fire regularly at idle. I had the original ignition upgraded to a Pazon electronic ignition, but the problem remained. I replaced the original Amals with new ethanol-resistant Amal carburetors, and still there was no improvement. I also tried reversing the coil wires to see if the problem changed sides, and the left cylinder remained anemic at idle. I recently replaced both coils, plug wires and plugs, but the problem remains. The old plugs indicate rich running on the left, which I think may be due to no fire at idle. The bike runs great otherwise, and is a joy to ride. I’ve compensated by using the idle mixture set screws to maintain a slightly higher idle, but the right cylinder is carrying the load at idle. Do you have any ideas why this is happening after installing new Amals and the Pazon ignition? Emmett Fox/via email

A: Having chased this problem through all new equipment with no resolution must be maddening. It also makes it hard to diagnose, as you have already taken all of the easy answers out of this equation.

The first check is compression: If that cylinder is low, that could be the issue. The only other easy fix that I can think of would be to run a separate ground wire from the cylinder head (under a head steady bolt perhaps), directly back to the battery. I’ve read of poor grounding resulting in one cylinder firing in a wasted spark ignition where you’d think they would both have to fire due to the nature of the system. Adding a direct ground fixed that problem. Now the other possibility is that for some reason the left side isn’t getting the right mixture at idle to fire properly. Does the left side kick in if you pull the slide up on that carburetor only? If so, then you have a problem with fuel, not fire. I’ve also read enough about the new Amals to know that you can’t take it for granted that they are free of machining swarf as delivered from the factory. It’s worth your time to make sure the idle passages are clear and working. MC

Rear Brake Master Cylinder for a 1983 Kawasaki GPz750

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

Finding a master cylinder

Q: I own a 1983 Kawasaki GPz 750, and I am in need of a rear brake master cylinder. I’ve already tried to find a rebuild kit; however, there is no part number listing for one from Kawasaki or the aftermarket. I’ve called quite a few salvage yards and have not had any success. Each time I ask what will work instead from either Kawasaki or the aftermarket, no one seems to know. So I pose the question to you: What from Kawasaki will work or what from the aftermarket will work? I’ve spent a number of years re-doing this bike and this is the last hurdle. I hope you guys can offer a suggestion or point me in a direction. Dennis Ventura/via email

A: The GPz was produced from 1982 to 1985, and yet there are no replacement kits for the rear master cylinder. I checked several sources including K&L and Kawasaki. Any readers out there with parts substitution advice? You may have to find a master cylinder you can adapt and install in place of the original. This is one of the many challenges of owning old bikes. MC

Rebuilding Ewarts Gas Taps

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

Rebuilding fuel taps

Q: I’m trying to re-cork some old Ewarts push-pull gas taps. How do I get the old pins out? — Jim Koenig/via email

A: The best advice is probably to replace them with something else, but if you want to keep the originals, the first thing you want to do is buy several corks so you have a chance to select some good smooth ones and also in the event you split one installing it. The brass rod that holds the cork in place is peened over into the chrome tap pull, so you have to drill through the peening. Center punch the brass and use a small bit to drill just barely into the rod. Press or punch out the rod from the pull. While you are cleaning up the brass, boil a little water and sink the corks (I know, right?) in the water to soften and swell them. Once you have the brass cleaned and the corks soft, push a new cork onto the brass rod. Take it easy pushing it over the diameter change; this is where the cork will split if it has a mind to. If you haven’t drilled off too much of the brass rod, you can re-peen the end into the chrome pull, but a little Locktite bearing fit will also help hold the two together. A very little goes a long way, and you don’t want any on the cork. Carefully fit the cork back into the tap body. I’ve been warned against greasing them, but a very small amount of silicone grease on the very edge of the cork entering the tap helps ease the two parts together. MC

1980 Yamaha XS850 SG Heat Problem

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

A heat-related problem

Q: My 1980 Yamaha XS850 SG runs well until it warms up, but then it starts to miss, uses more fuel, and eventually stops running. Once it cools down it will do the same again. Back in 1980, I had one of these that did the same thing, and an adviser told me that the wiring harness had some added feature to suppress radio “noise” to improve radio reception. I think that he replaced the harness and everything was OK. Is this memory possibly correct or am I losing it?! Robert O’Regan/via email

A: A few things are possible.  Generally the ignition wiring either uses suppressor wire or suppressor caps. You can tell by pulling the cap off the coil and looking at the crimped-on connector at the end. Visible wire means the spark plug cap will be the noise suppressor, while a flexible ribbon or carbon string indicates suppressor wire. Sometimes with suppressor caps, the resistor contained in the caps goes bad, but this is rare. Suppressor wires can get brittle and the inner core string breaks down. Either of these things are simple and inexpensive to replace, so that would be a good starting point. A heat-related problem that affects all the cylinders also points to a central system problem like the igniter box. Those usually are only testable with factory tools, and often tested by swapping out a known good unit. If that doesn’t help, you might consider checking the fuel/air mixture. Put a new set of plugs in and go for a spirited ride on a lightly traveled road. Take along a plug wrench. When the bike starts to miss, hit the kill switch and coast safely to the shoulder. Pull the plugs and inspect for a rich condition. I suspect rich because you say it’s worse as it warms up and the old rule of thumb is, “Worse cold, lean condition; worse hot, rich condition.” Another good check would be valve clearances. If they are set too tight, when the engine warms up it is possible the clearance disappears and the valves don’t close properly. MC

1966 Honda CL77 305 Charging Troubles

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

Charging troubles

Q: I have a 1966 Honda CL77 305. We all know the charging system setup wasn’t the best on these 12-volt bikes. From the factory they were weak at best. I’m trying to figure out why the stator isn’t producing 60 volts to the rectifier? I have installed two different stators and replaced the rotor as well. I also installed a new rectifier and new coils. The bike now starts with one kick, but when I turn the lights on, I start losing the DC current, as shown using my voltmeter. I have checked the red wire, the brown wire and the yellow wire going to the rectifier, as shown in the manual. I have also replaced the headlight switch, and I’ve added a new wiring harness, too. I have also installed the electronic ignition from Charlie’s Place. The new battery is simply not getting up to 14 volts when lights come on. Lights on or off, the battery is losing voltage. I could go insane over this. Bruce A. Ferguson/via email

A: We’ve been fighting a losing battle with these marginal charging bikes ever since full-time headlight use became law. It used to be that the bike would mostly be used during the day, long enough to barely get the battery to full charge. Then if you had some night riding, it would drain the battery, but hopefully not so much that you couldn’t make up the deficit the next day. I’m not sure those machines ever managed to charge a battery to 13 volts, much less 14 volts. I think your best bet is a battery tender, since you’ve replaced all the components with no improvement. MC

1968 Honda CL450 Cylinder Problems

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

Cylinder problems

Q: I recently purchased a 1968 Honda CL450 with just 782 miles on it. The cylinders are out of round, apparently. I had the lower end inspected and the shop told me it was nice and tight, and the pistons are fine, also. My question is, should I bore it, which will then require a new set of piston kits, or should I sleeve it, in which case I could then leave the top end alone except for new rings? Tim/via email

A: Either of those options would work, but I think boring the cylinders and fitting oversize pistons may be easier to do and probably less expensive than resleeving. Plus you get the added benefit of a little more oomph from more CCs. What makes you think the cylinders are out of round? Is it smoking excessively? You could try the test I suggest in the first letter to confirm the diagnosis. MC

1965 T100 Triumph Smoking Cylinder

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

Lots of smoke

Q: I am 63 years old. I have had my own little shop under my house for about 43 years. I have fixed just about every brand in those years. I bought a 1965 T100 Triumph 500 from a friend of mine about seven years ago. When I had a little extra cash I would throw it in the bike. I finally got it running. When I fired it up the left side cylinder smoked very badly. I rebuilt the whole engine. I had the head done by E & V Engines, new valve guides and valves, new pistons and rings at 0.20 over. I cannot figure out why it is smoking only on one side. I even put another set of new rings in it and it’s still smoking. I don’t know everything, but I think I put the engine together right. I hope you can help me. Larry Petras/Ohio

A: After talking with Larry, it seems as though he’s done about all he can to eliminate the common causes of smoking. All I could suggest was that maybe the left cylinder was not concentric and that was keeping the oil scraper from cleaning off the excess oil. I suggested he back off the valve adjusters so that they wouldn’t open, and then do a leakdown test at TDC, mid-stroke, and BDC to see if there was any change in the amount of air getting past the rings. Another possibility would be a crack in the head letting oil leak down from the overhead valves. Do any of our readers have other suggestions as to what the problem may be? MC