Yamaha SR500 Petcock Challenges

| 7/5/2018 12:00:00 AM

Motorcycle Classics tech expert Keith Fellenstein
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, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with "Keith's Garage" as the subject.

Q: My 1979 Yamaha SR500 has a vacuum fuel petcock. I've found no way to eliminate it for a simple gravity-style petcock. I installed an inline fuel shut-off in the fuel line below the petcock. When I'm done riding, since the stock petcock has no "off" position and relies on engine vacuum to allow fuel flow, I turn the inline one to "off" and let the carburetor run dry to eliminate any gas going stale inside the carburetor. My problem is, when I go to start it, it takes anywhere from 5 to 20 kicks to get it started. The engine is running a 1980 Hi-Per-Kinetics Stage Two 650 stroker with 97mm x 88mm bore and stroke, a Megacycle 5120HP cam, Mikuni VM36 and other goodies.

I've owned three Yamaha TT500s, and they all had the on/off/reserve manual petcocks and all started first kick, hot or cold. How can I eliminate the funky stock petcock without replacing the tank? Does it require a lot of kicks to get the fuel flowing from the petcock because of the engine modifications? Should I just not run the carb dry after riding? I use octane booster and fuel preservative. The bike had sat for over 25 years when I bought it in 2012. The inside of the carburetor was surprisingly clean, with no gum at all. — David Fruhling/via email

A: It is taking so many kicks because there would normally be enough gas in the carburetor to start, then engine vacuum would open the petcock and refill the bowl before it ran dry. There should be a "PRI" or prime position on the petcock that bypasses the vacuum and flows gas to the carb for those instances when the bowls are dry, such as after a carb rebuild. After a little searching I found a good option. It's an adapter that bolts directly to your tank and allows you to use a standard non-vacuum petcock. They also offer the petcock that fits the adapter, making it a one stop shop. Don't forget to plug the vacuum port on the intake manifold.

The sound and the fury: celebrate the machines that changed the world!

Motorcycle Classics JulAug 16Motorcycle 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 $5.00 and get 6 issues of Motorcycle Classics for only $29.95 (USA only).

Or, Bill Me Later and I'll pay just $34.95 for a one year subscription!

Facebook Pinterest Instagram YouTube Twitter