Norton Commando Q and A with Keith
Hello, Keith, maybe you can help me. I bought an all original 1975 Norton 850 Commando from a woman whose husband had passed away. The bike has 2,750 miles and is a one-owner machine. It appears to be 100% original and was last inspected in 1995. I have cleaned it up, replaced the fluids and some gaskets, cleaned the carburetors and more. I did all the things that I thought should be done to a bike sitting for so long. It was in very good condition for its age, sitting in a dry garage with little heat. I cleaned the gas tank, put a new battery in it and it fired right up. All the lights work. That was a month ago. It idles great, but above that, it misses constantly with dry sooty plugs, both are exactly the same condition every time. I must have pulled the carbs 5 times just to check again and again. It has new coils, condensers and points, and the timing is dead on. I ran it without the mufflers on and there was no difference. I must have read the shop manual a couple of times. I was about ready to give up until I saw your posts. Please help.
Mike Glessner/Levittown, Pennsylvania
Well you’re probably not going to like hearing that I think you need to pull the carburetors off, yet again, just to check that the jets and needle haven’t been changed over the years. The recommended settings changed from 1974 to 1975 and the main jet and needle clip were specified leaner. Make sure you have the correct needle. The Commando needle has 4 small grooves above the 3 needle clip grooves and the needle clip should be in the top of those 3 grooves (leanest setting). The main jet went from a 260 in 1974 to a 220 in 1975, again a leaner jet. The reasons for these changes were to meet emissions standards and to accommodate the more restrictive airbox and exhaust.
Hi, Keith, I’m stumped and thought maybe you could offer some guidance. I have a 1972 Norton Mark V 750 Commando. The bike runs great with me aboard, but not as well with a passenger. Adding a passenger causes the engine to sporadically “tug” coming through first and second gear. It almost feels like the engine is all the sudden not getting a steady flow of gasoline. It takes a half mile or so of riding for the phenomena to show itself and begin coughing and shuddering. It actually continues to get worse the further we ride. Within 3-4 miles of stop-and-start neighborhood riding, it’s time to head home and drop the passenger. Off I go again (alone) and the bike is back to running great. The tank has plenty of gas, the rider’s foot is not on the brake or brake cable, the chain tension is good, loaded or unloaded, and the tire does not come in contact with the rear fender. I’m stumped. Thanks!
Pat Long/via email
We may as well make it a mostly Norton column this time around. It sounds to me without testing that your bike is running too rich. The old adage is running worse when hot is too rich, running worse when cold is too lean. If you have a patient passenger, drag along your spark plug wrench next time and when you hit that less than magic moment in your ride, find a safe place to stop and read your plugs. If they’re black and sooty you’ll have your answer.
Using a Twinmax
Hi, Keith, I was reading the subject article about fitting Don Pender’s carb gantry in Motorcycle Classics with interest. In the article for carb synchronization a TwinMax was used. Would it be possible for someone to explain how this is done?
The Twinmax was originally marketed for use on BMW airhead twins, but I’ve found a use for it on many other machines. It measures the difference in vacuum between two carburetors, a good indicator of throttle opening. To use it, you first have to have a vacuum port on either the carburetor or the manifold. On the Norton Commando, I remove the balance tube and plug the Twinmax tubes in there. Turn on the Twinmax and adjust it to the most sensitive setting, then zero out the meter. Turn the sensitivity knob back to least sensitive. Start the engine and see if the needle is deflected to one side more than the other. Adjust the sensitivity knob higher to highlight any deflection. The needle deflects to the side with less vacuum. Using the throttle slide stops, adjust until the needle is centered. You may then have to reduce or increase the idle by adjusting the throttle stops equally. Once done, your carburetors are now idle synchronized. With the gantry, you synchronize the slides with the nut and locknut on the top of the carburetor. The old throttle slide stop serves no purpose. If you have cables instead of a gantry you’ll need to check to make sure the cables are also synchronized, pulling the slides up simultaneously. Slowly increase the throttle to about 2,000-3,000rpm via the twist grip and check for needle deflection to one side over the other. Again the deflected side is being pulled up sooner and you’ll need to adjust the cable slack on one or the other cable to balance the carburetors. When you’re done you should be rewarded with a smoother idle and acceleration.
Send questions to Motorcycle Classics, Keith’s Garage, 1503 S.W. 42nd St., Topeka, KS, 66609, or email firstname.lastname@example.org
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