Technical Q and A for classic motorcycle maintenance and repair.
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, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.
Q: I have a 1981 Honda GL500 with 61,225 miles. The bike starts and runs well up to about 4,500rpm, then breaks up and runs really rough above that. It behaves the same way hot or cold. I’ve tried running it on the center stand with the same result. In terms of problem-solving, so far I’ve replaced the air and fuel filters and spark plugs, and I’ve cleaned the carburetors. None of this solved the problem. I then went on a CX500/GL500 forum where a member suggested replacing the plug wires and coils, which I changed, but this didn’t solve the problem either. I then located your article on CDI units and their function, and it seemed like the logical solution since I’m losing high-end. After changing both to a set ostensibly from a low-mileage bike, it still didn’t solve the problem. I also did a resistance check on the pickups (yellow and blue wires), with an accurate resistance reading on both sides. Additionally, I did the confirmation spark-check with the pick-up wiring harness disconnected. I find myself in a quandary wondering which way to look next and would like to ask for your help. Any suggestions? — Phil Quattrone/Pittsburgh, Pennsylvania
A: You’ve been very thorough in your pursuit of this problem from an electrical standpoint, to the point where I have to wonder if it isn’t a fuel delivery problem. The carburetors on the GL500s can be difficult to clean thoroughly, as the slow jet is pressed in on some models and difficult to clean properly. I once went through a pair of GL500 carbs three times before they began to work decently, and even then the owner claimed they improved over time with fresh gas and SeaFoam. I would consider cleaning the carbs again and, if possible, finding a shop that can run your carburetors through an ultrasonic cleaner. MC