Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Add to My MSN


Keith Fellenstein

Popping exhaust

Q: I just bought a 1969 B44 BSA 441 Shooting Star. My initial ride on the bike was fine, but I was getting just a bit of a pop out of the exhaust when I roll off the throttle. I ran it for a while, hauled it home, and then took it for another ride. This time it started popping a lot every time the throttle was shut. This is my first British bike (except for an AJS Stormer motocross bike I rode in the ‘70s), so I have no experience with them. Before I start fiddling about, I figured I should ask an expert. Does this sound like a timing problem? Or do I have fueling issues? The bike appears to be completely stock. It’s using an Amal carb, and the bike has about 9,000 miles on it. — Rod Jackson/via email

A: Popping on the overrun could be due to a too lean idle mixture from the carburetor. You can test this by turning the idle mixture screw in one-quarter to one-half a turn to richen the mixture. You may have to adjust the throttle speed screw to get your steady idle back. Then go for a ride and see if things are better. Another possibility is an exhaust leak at the head to exhaust pipe junction. Air sucked in there will cause popping in the tailpipe, too. MC


Keith Fellenstein

Tune up

Q: I figured it was time to do a tune-up on my 1970 Yamaha XS650. I had a tune-up kit a friend gave me, purchased in 1981 at Seattle Yamaha, and the “How-To” article from the January/February 2011 Motorcycle Classics  in hand, proper tools and experience. I timed the right cylinder, lining up the “F” on the engine cover with the timing mark and tightened down the backing plate. When I went to time the left-hand cylinder, the timing mark was advanced (to the left of) the “F” and there was not enough movement in the half-plate to bring it back (it was up against the stops/screws). I left it with the right-hand cylinder properly timed on the “F” and the left, slightly advanced. Do you have any suggestions on how to correct the timing of the left cylinder? — Terry Zeri/Bellingham, Washington

A: There are a couple of ways you can pull the left cylinder into correct timing. You can go back to the right cylinder and close the points gap by a thousandth or two. That will have the effect of advancing the timing for that cylinder. Then you rotate the full plate to retard the timing enough to bring the right cylinder back to the correct setting. That may be enough to allow you to use the half plate to correct the left cylinder timing. If that doesn’t quite get it, you can then open the points gap on the left cylinder by a thousandth or two, which will have the effect of retarding the spark for that cylinder. If the left cylinder is not too far advanced you may just be able to open the gap as mentioned and have that correct the timing. Good luck! MC


Keith Fellenstein

Losing spark

Q: I have a 1980 Honda CX500C. It starts easily and runs great until it gets warm. Once warm, it loses spark on one cylinder. Any suggestions as to what is wrong with my bike? — Mitchell Smith/via email

A: It is likely you have a bad coil that is opening up when it gets warm. You could try switching the coils from one side to the other and see if the problem follows the coil. If it does, you can bet the problem is with the coil. MC


Keith Fellenstein

Blown clutch

Q: I blew the clutch on my 1975 Kawasaki H1 500. It had been bad for a while. When it finally went, I couldn’t downshift and the engine lost power and died. I finally got it back into neutral and tried to restart it. There is now very little resistance felt in the kick-starter, but it doesn’t feel like it’s broken. I drained the oil and it had plenty, but it was black and smelled like burnt clutch. I pulled the spark plugs and the pistons don’t move when I kick the engine over. Could this possibly just be a blown clutch or do I have more serious problems? — Clark Connelly/via email

A: If the clutch had been getting progressively worse I would assume it finally failed. It’s unusual for one to fail so spectacularly that you can’t turn the engine over with the spark plugs removed, but I guess it’s possible. I think your next step should be to remove the clutch side cover and disassemble the clutch. If it isn’t grabbing at all the wear on the friction plates should be quite obvious. The steel plates might still be good unless they have been overheated and blued from clutch slippage. If you’re going in to inspect the friction plates, it’s probably a good idea to just replace them. They’re not that expensive and since you’re in there anyway you may as well renew them. Let us know what you find. MC


Keith Fellenstein

Triumph stumble

Q: I have a 1965 Triumph Bonneville T120 with 1,000 miles on it since it was rebuilt. This engine ran fine for approximately 600 miles, but then began to stumble and lose power after about six miles of riding. When I stop for a minute or two, it will start and run again, but only for a half mile, then it stumbles and loses power again. It will not generally get above 20mph once it begins to act up, but it does stay running. It will stall once at idle. It is running one Amal carburetor with a Pazon ignition. The battery is charging 12.7-13.2 volts (approx.). I replaced the coil. It ran strong for about 200 miles after that, but then started the symptoms all over again. The plugs are evenly colored with no carbon build up to speak of, nor any indication of running lean. Popular opinion is there is an overheating problem affecting ignition performance. It acts as if it is being starved of fuel, but there appears to be no reason to suspect the carburetor. I have good fuel flow from the tank. I even checked the cap to ensure that the venting wasn’t plugged. Thanks for any assistance you can offer. I’m stuck. — Jonathan P. Young/Fort Drum, New York

A: It could be a compression issue. Since the problem started soon after a rebuild, I have to ask if you’ve retorqued the head bolts. Depending on the type of head gasket used, you may have to do this a few times. You can check for low compression with a compression tester or a leak down tester. Another possibility is overheating of the electronic ignition trigger in the points cavity. You can quickly rule this out by pulling off the points cover and going back out for a ride. That will keep the trigger assembly cool. If the problem disappears you’ll know to replace the trigger assembly. Or it could be a problem with valve clearances; everything runs OK until the engine warms up and valve clearances start to close up. Once that happens your valves won’t seat as they should, leaking compression and causing the engine to lose power. Double check your cold valve clearances. They should be 0.002 inches for intake and 0.004 inches for exhaust. As a test, if they check out OK you could add 0.001-0.002 inches to each setting and see if that makes a difference. If it does, then you know there is a problem with the valves. If it’s a problem with the head gasket, you’ll probably have to pull the head and replace it if it’s composite, or reanneal it if it’s copper. Then be sure to do the retorquing to keep it sealed. Good luck. MC


Tech Corner

Losing idle

Q: My 1973 Honda 450 K7 doesn’t idle down properly after it gets good and hot. According to info I’m finding online, this might be the result of an air leak around the carbs or a weak advance return spring. I have done the “spray with carb cleaner” around the carbs with no noticeable change. How can I determine for sure that my return spring is truly the problem? — Lynn A. Metzger/Lawrence, Kansas

A: This can be tested using a timing light. Hook up a timing light and shine it on the timing marks on the alternator rotor. Rev the engine up and watch the timing marks move to the fully advanced mark. Release the throttle and they should settle back to the initial timing mark pretty quickly. This is not fool proof though. If there is a reason why the idle isn’t coming back down, such as too much fuel for some reason feeding the engine, the timing will remain advanced as long as the engine speed remains high. MC


Tech Corner

KZ750 advice

Q: I’m a woman looking to purchase my first bike and I’m looking at a 1981 Kawasaki KZ750 for sale here in Monroe, Washington. I found your information while researching maintenance issues with the Kawasaki KZ750. I was wondering if you would be willing to offer advice on anything I should be particularly concerned with or specific questions I should be asking of the dealer before considering buying it? — Reis Pearson/via email

A: I can’t think of anything about that particular model that raises any red flags, so I would suggest the usual questions: How new are the tires? Have the carburetors been cleaned recently? How old is the battery? Has the bike been dropped or wrecked? That said, I would strongly recommend a smaller bike as a first bike. Many times people get off to a bad start in motorcycling by buying a bike more powerful than their experience level warrants. I’m old enough to remember when a 750 was a Superbike, something you got after you learned to ride, and fall off of, a 125, 350 or 400. I’d also advise that you take a motorcycle safety riding course, often offered through local dealerships. I hope this helps. MC

The sound and the fury: celebrate the machines that changed the world!
First Name: *
Last Name: *
Address: *
City: *
State/Province: *
Zip/Postal Code:*
(* indicates a required item)
Canadian subs: 1 year, (includes postage & GST). Foreign subs: 1 year, . U.S. funds.
Canadian Subscribers - Click Here
Non US and Canadian Subscribers - Click Here

Motorcycle Classics is America's premier magazine for collectors and enthusiasts, dreamers and restorers, newcomers and life long motorheads who love the sound and the beauty of classic bikes. Every issue  delivers exciting and evocative articles and photographs of the most brilliant, unusual and popular motorcycles ever made!

Save Even More Money with our RALLY-RATE plan!

Pay now with a credit card and take advantage of our RALLY-RATE automatic renewal savings plan. You save an additional $4.95 and get 6 issues of Motorcycle Classics for only $24.95 (USA only).

Or, Bill Me Later and I'll pay just $29.95 for a one year subscription!