Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

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10/29/2014

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



10/15/2014

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



10/8/2014

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



9/24/2014

Tech Corner

Using lead additives

Q: Should I use a lead additive in my 1970 Triumph TR6? Chances are the head has never been off. Also, can you explain to me how the choke system works on this bike? It won’t run when I pull the lever toward me. It only runs when the lever is away from me.

A: Most of the information I’ve read states that in normal use, other additives in gasoline provide the same protection that lead used to provide. During the initial switch from leaded to unleaded gasoline, there was concern that rapid valve seat wear would occur without the lubricating effect tetraethyl lead provided. Hardened valve seats were to be the solution. Experience has shown that the concern was unfounded except in the case of engines that were run very hard, i.e., racing engines. On your choke issue, the Amal chokes are off when the cable is tight and on when loose. MC



9/17/2014

Tech Corner 

Bonneville carburetors

Q: Can you give me any advice on carburetors for my 1970 Triumph Bonneville? Right now the bike is using Amal 930/43-930/44 carbs, which I believe are worn out. Do you know which new ones will fit the bike? Would it be the 930/9 or will the 930/300-930/301 premium be better? — Graham Rashleigh, U.K./via email

A: I’m a big fan of using original carbs on these bikes. That way someone else has taken the time to figure out the jetting and slide cutaway. With that in mind, the Amal carb website shows either the 930/43-44 standard carbs or the 930/43-44 premium carbs for your model Bonnie. ACK145 is the carb set for the standard and PACK145 is the premium set. I like the features of the premium carbs — improved metallurgy, improved idle circuit and stay-up floats — but you do have to consider the extra cost. A less expensive option I’ve used here in the U.S. is having the carbs sleeved. I don’t know of any firms offering that in the U.K., however. Here it costs about $100 per carb to have the bodies bored out and the slides turned down and sleeved with stainless steel. That brings them back to original tolerances and removes the sloppy fit of slide to body. Having dissimilar metals in the body and slide then reduces the rapid wear you get from the original setup. MC



8/27/2014

Tech Corner

Triumph clutch troubles

Q: I have a question for you regarding troubleshooting the clutch on my 1973 Triumph Trident 750. I have recently replaced the clutch plate. When riding the bike I cannot shift into neutral, and when stationary with the clutch pulled in the engine is still engaged. I have adjusted the clutch lever and the main clutch nut under the circular clutch plate cover. Should the clutch nut be loose enough to turn by hand? How much should the clutch arm move up and down inside the clutch case? — Nick/via email

A: Adjusting the clutch on a Trident is one of those things that can take all day: Finding the sweet spot for three things at once can be difficult. You want the clutch to fully disengage when you pull in the lever, fully engage when you release the lever, and not load the bearing buried deep in the clutch basket when engaged. The original manual calls for the clutch nut to spin freely with 0.005in clearance, but the collective wisdom of Trident owners is that this is too much clearance. Setting it to the factory clearance usually results in difficulty finding neutral and creeping in gear with the clutch pulled in. The generally accepted method is to adjust until you have almost no clearance, then lock it with the locknut. If you can still turn the big nut by hand when the clutch is relaxed, you have it properly adjusted. Start by adjusting the cable so that the arm on the actuating mechanism inside the transmission outer cover is between 3 and 4 o’clock when the clutch lever is released. Make sure the arm isn’t touching the case stop. Tighten the pull rod through the big adjusting nut until it is just tight. Keeping the adjusting nut steady, loosen the pullrod about 30 degrees. Lock it down using the locknut without moving either the adjusting nut or the pullrod. It helps to have three hands for that part of the procedure. Try to rotate the big adjusting nut with your fingers. If you can then the clutch is adjusted properly. Any minor adjustments should now be possible from the adjustment screw at the clutch lever. MC



8/20/2014

Tech Corner 

Honda oil pressure

Q: I have just finished rebuilding a 1970 Honda CB750K. It is my first attempt at rebuilding this type of engine. I have rebuilt other types with good success.

I now have about 480 miles on the engine and it is running great. It sounds good and doesn’t leak a drop. My problem is the oil pressure. Lately I find that after running the engine for about 30 minutes the oil pressure light comes on at an idle. I suspected the oil pressure switch. So I pulled it and have installed a pressure gauge. When cold the pressure starts at about 65lb. After running for about 15 minutes it drops to 60lb and remains there no matter how long I run it. — Daniel Pensyl/League City, Texas

A: According to the shop manual, the oil pressure bypass valve is set to 56.9psi at 4,000rpm for an engine temp of 176 degrees Fahrenheit. It looks like your engine has the correct oil pressure. An old rule of thumb was 10lb of pressure for every 1,000rpm, which makes your setup exactly right at about 6,000rpm. If your oil pressure light continues to light, it could be the switch that’s faulty. They’re still available for about $35. If you look for it on a parts schematic, it’s part no. 37240-P13-013 on the starter motor schematic. MC





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