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 your subject.
1982 Yamaha XV920 Virago with Problematic Aftermarket Exhaust Pipes
Q: The 1982 Yamaha XV920 Virago I acquired last fall has aftermarket exhaust pipes. They weigh less and are a bit louder than the stock system, but I don’t know if the carbs were rejetted when this modification was made. The airbox is original and the choke works properly. I took it in for service, and the Yamaha dealership synched and adjusted the carbs using an exhaust gas analyzer and now the bike idles properly and runs OK except that it still backfires quite a bit on decelaration. Their service manager told me that “all V-twin aftermarket pipes backfire, just live with it” and that rejetting would be an expensive series of trial and error that might not even make it any better. They never even looked in the carbs to know what size jets were in there! Would I be better off putting the original, stock mufflers back on? I like the advantages of the slash cut pipes but can’t take the noise, popping off like it’s the Fourth of July. — Don/Natrona Heights, Pa.
A: You’ve discovered a pitfall of aftermarket pipes. They are less restrictive than the DOT/EPA mandated originals your bike was tuned for, but that causes a lean condition, which in turn causes the popping in the exhaust. The solution involves changing the idle mixture from the factory lean settings. Because of EPA regulations, the dealership won’t or can’t help you change that. If you want to keep the aftermarket pipes, you’ll probably have to find a custom bike builder willing to work on the jetting. It can be a trial and error method, or you can try one of the Dynojet kits. Kit 4113 is for your model Yamaha. Before you spend any money on rejetting the entire carburetor circuit, you could try enriching the idle mixture to see if that cures the backfiring on deceleration. The idle mixture screw is located on the bottom of the carburetor close to the engine intake. For pollution control purposes it is set very lean from the factory, then sealed so that you’re not tempted to adjust it. If it is still sealed you’ll need to carefully drill through the plug sealing the adjustment then, with a sheet metal screw screwed into the plug, pull it off. Once that is done you can carefully turn the screw closed, counting the turns to lightly seat the screw, then back the screw out 2.5 turns. You may have to adjust your idle speed after adjusting the mixture. If that cures your backfiring, you’re done. If it doesn’t, you’ll have to experiment with further enriching the idle mixture by turning the adjustment screw out one-quarter turn at a time. If you don’t get any improvement and you’ve turned the screw out another full turn, you’ll have to try the Dynojet kit mentioned earlier. It will provide a richer idle jet along with richer main jet and modified jet needle. MC